# ask a physicist



## freyar (Aug 3, 2015)

This will be an off-topic AMA; I thought it would be fun to take any and all physics questions, since there seems to be genuine interest here on EN World.

My background: I am a physicist who works in the broad subject of "high energy physics," which these days includes subatomic/particle physics (think Higgs boson/LHC experiment/dark matter), mathematical aspects of these types of theories, string theory, and other aspects of quantum gravity (stuff like Hawking radiation/black holes as well as non-stringy quantum gravity theories).  My own work deals with particle physics models of dark matter and (separately) string theory, often with respect to extra dimensions.

Some guidelines for the thread, just to keep things organized:

I'm willing to try to answer questions outside my specialty, but there are some I may just say I don't know. An example might be about detailed workings of electronics or something if I can't look up something easily.
I won't necessarily go in order that the questions are posted since I may need to postpone a more involved answer due to my work schedule.  
If any of the other EN World physicists want to answer questions, please feel free.  I may add something, or I may just quote your answer to highlight it.
If you want to discuss an answer more than just a short follow-up question, please copy information over and start a new thread in the Miscellaneous Geek Talk & Media Lounge forum.
Please be patient, as my work often spills past normal business hours.  This is something I'm just doing for fun.

An "advertising" or "introduction" thread is over here.


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## Janx (Aug 3, 2015)

What's the layman's explanation for String Theory these days?


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## Morrus (Aug 3, 2015)

Has anybody ever told you that [someone like] Einstein was wrong, and they knew why?


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## Janx (Aug 3, 2015)

If you could unHiggs me, what would happen?  Would I be less massive?  What would that mean?


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## Janx (Aug 3, 2015)

What's the deal with Dark Matter?  Is it just a placeholder for the math (until they find the real thing and name it Umbranium, or is there really funky stuff out there?


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## Janx (Aug 3, 2015)

Can I affect the results of the 2-slit experiment* such that the photons favor one side?

*https://en.wikipedia.org/wiki/Double-slit_experiment


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## Janx (Aug 3, 2015)

What's the coolest physics question you've answered?


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## Janx (Aug 3, 2015)

What's the coolest Physics problem you've solved or contributed to?


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## Umbran (Aug 3, 2015)

Janx said:


> Can I affect the results of the 2-slit experiment* such that the photons favor one side?
> 
> *https://en.wikipedia.org/wiki/Double-slit_experiment




Sure.  Cover one slit.  

You may need to be a little more specific.  Do you mean, "Can I look at the apparatus funny, and make the results different?"  Or, "Can I affect the results with _the power of my Mind_?"  Or, "If I make one slit narrower, are the results different?"  Or what?


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## AlexM (Aug 4, 2015)

OK. Electrons. Where do they get the energy to stay in orbit around the nucleus?


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## Joker (Aug 4, 2015)

A common trope in science fiction is using a type of propulsion which "bends" spacetime. Some have the idea that a ship could move at normal speed while contracting the space in front of it. Effectively reducing the distance it has to travel.

Is this theoretically possible (with exotic matter)?

What would happen to any objects in or near the space being contracted?

Cheers.


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## freyar (Aug 4, 2015)

Just going to start with a couple of the fast ones to get rolling, then I'll see if I can get back to this toward the end of the day.  Keep the questions coming!  These are great so far!




Janx said:


> Can I affect the results of the 2-slit experiment* such that the photons favor one side?
> 
> *https://en.wikipedia.org/wiki/Double-slit_experiment






Umbran said:


> Sure.  Cover one slit.
> 
> You may need to be a little more specific.  Do you mean, "Can I look at the apparatus funny, and make the results different?"  Or, "Can I affect the results with _the power of my Mind_?"  Or, "If I make one slit narrower, are the results different?"  Or what?




Pretty much this.  

But I'll try to answer some of Umbran's specific questions.  To get it out of the way, you're going to have to physically do something to the apparatus --- "looking at it funny" isn't going to do it.  But, yes, making the slits different sizes is one way to do it.  You have more options if you send electrons through the experiment rather than photons, since electrons are charged.  For example, put a solenoid parallel to the slits between the slits and the screen.  Then turn on the solenoid, so there's a magnetic field in it.  That will shift the interference pattern around _even if_ the electrons can't enter the region of the solenoid with the magnetic field in it.  So, in other words, electrons are barred from entering the solenoid but can still "know" whether there's a magnetic field in the solenoid or not.  This is known as the Aharonov-Bohm effect and is a classic example of electromagnetism being used in quantum mechanics.  This wikipedia article is a bit technical but may be interesting.


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## freyar (Aug 4, 2015)

One more fast one...



Morrus said:


> Has anybody ever told you that [someone like] Einstein was wrong, and they knew why?




This or something similar happens pretty much all the time, even now that I'm at a fairly small and not terribly prestigious school.  I get several phone calls a year from people wanting help with their personal theory of physics, though sometimes they are too worried I'll steal it to tell me exactly what they think they've done.  At my last job, there was one guy who came around the entire department wanting to explain how our understanding of light was wrong (security eventually had to ban him from the building).  I once received a book of poetry in the mail from somewhere in Scandinavia (consider that I was a postdoctoral scientist, not even a professor, in North America at the time) which was billed in the cover letter as a theory of everything.  Famous physicists tend to get stranger/more threatening stuff.  My PhD supervisor once received someone's model of string theory, which was actually made of wooden balls and rubber bands -- yes, sent through the mail -- and another time was threatened with a law suit basically for once having worked with another famous physicist.

Anyway, this happens a lot.  It requires pretty sensitive handling from the physicist, though the questioner isn't always respectful or may not realize how much time they're actually taking since they're typically more concerned about self-promotion than learning anything.  But it does lead me to appreciate a forum like EN World, since the conversation here is respectful and genuinely curious.


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## garnuk (Aug 4, 2015)

How does Einstein's theory of bent space for gravity work, when there is no "gravity" acting on the object to keep it in the well created by the bent space-fabric?


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## Umbran (Aug 4, 2015)

AlexM said:


> OK. Electrons. Where do they get the energy to stay in orbit around the nucleus?




There are two answers to this.  One is classical (Newtonian) mechanics, the other quantum mechanical.  The classical answer is incorrect, but reveals what might be a bit of misunderstanding implicit in the question.  The quantum mechanical answer is weird, and hard to understand and put in just text, because it is quantum mechanics.

So, I'll take a stab at both.

Classical:

Well, where does the Earth get the energy to stay in orbit around the Sun?  Or the Moon around the Earth?

In classical mechanics, once you have a stable orbit, you don't need to continue to add energy to stay there.  The orbiting object is pulled towards the center, but is moving fast enough sideways that it always misses the thing at the center.  So long as nothing acts to slow down or speed up the sideways motion, it just keeps falling toward the center and missing.  Very Douglas Adams - throwing itself at the ground and missing is the essence of being in orbit.

Quantum mechanics:

It isn't appropriate to think of the electron as a particle in an orbit.  On the scale of an atom, the electron is a wave.  Just for an idea, you can sort of think of it as a wave running in a circle around the nucleus, and it can be stable at distances that make the orbit an even number of wavelengths of the wave.  This need to be an even number of wavelengths means that the electron can only sit at specific distances from the nucleus, so there are "energy levels" it can sit at.  In order to become part of the atom, the electron sheds any excess energy, and then can just sit in a particular energy level.  

In reality, we don't think of its position in its orbit, so much as the probability of finding the electron at a particular point near the nucleus.  The electron is sort of smeared out in a cloud (sometimes called the "electron cloud") around the nucleus, until we directly observe whether it was at a given place.  The equation to calculate the probability distribution is a differential equation, so it requires some calculus to get it right, but it falls out that the "orbitals" that result have some interesting shapes - they aren't just circles.  These shapes give rise to the shapes of molecules.

https://en.wikipedia.org/wiki/Atomic_orbital#Orbitals_table


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## Janx (Aug 4, 2015)

Umbran said:


> Sure.  Cover one slit.
> 
> You may need to be a little more specific.  Do you mean, "Can I look at the apparatus funny, and make the results different?"  Or, "Can I affect the results with _the power of my Mind_?"  Or, "If I make one slit narrower, are the results different?"  Or what?




I would say all those but covering one slit as that is no longer the 2 slit experiment. 

I was mainly thinking if there were any kind of field or something I could use to weight it.  What if I ran the slits horizontally instead of vertically (thus not left/right, but top/bottom)?  Would gravity pull some photons toward the bottom?  Stuff like that but more sciency.


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## AlexM (Aug 4, 2015)

I get it. But since it's a wave, where does it get the energy to stay in the wave state? Photons have characteristics of a particle and a wave. I'm just trying to understand gravity I guess. Is there gravity at the atomic level or just energy states?


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## tomBitonti (Aug 4, 2015)

I thought an electron stayed "in orbit" because its location can't be fixed.  That is, the electron wants to zip into the nucleus of the atom, but that fixes the location and the momentum too precisely to satisfy the uncertainty relationship.  What ends up is an electron buzzing around the nucleus in a manner which minimizes its energy.  The closer the electron gets, the less potential energy it has, but the greater kinetic energy it has, because, as the electron's position becomes more certain, the less certain the electron's momentum must become.  A balance is reached which minimizes the sum of these energies.

Feynman covers this in his lecture notes.  See:

2–4 The size of an atom

http://www.feynmanlectures.caltech.edu/III_02.html#Ch2-S4



> We now consider another application of the uncertainty relation, Eq. (2.3). It must not be taken too seriously; the idea is right but the analysis is not very accurate. The idea has to do with the determination of the size of atoms, and the fact that, classically, the electrons would radiate light and spiral in until they settle down right on top of the nucleus. But that cannot be right quantum-mechanically because then we would know where each electron was and how fast it was moving.
> 
> Suppose we have a hydrogen atom, and measure the position of the electron; we must not be able to predict exactly where the electron will be, or the momentum spread will then turn out to be infinite. Every time we look at the electron, it is somewhere, but it has an amplitude to be in different places so there is a probability of it being found in different places. These places cannot all be at the nucleus; we shall suppose there is a spread in position of order a. That is, the distance of the electron from the nucleus is usually about a. We shall determine a by minimizing the total energy of the atom.




The notes are wonderful to read, especially for learning about quantum mechanics.

Thx!

TomB


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## Umbran (Aug 4, 2015)

AlexM said:


> I get it. But since it's a wave, where does it get the energy to stay in the wave state?




Being a wave isn't a state that requires extra energy to maintain.  It is just how the thing is, really all the time.  We think of it as a particle or a wave based on what math is more convenient to describe what's going on. 



> Photons have characteristics of a particle and a wave.




*Everything* has characteristics of a particle and a wave - including you.  The larger (really, more massive) the object, the shorter its "wavelength".  So, for the stuff we think of as objects - humans, baseballs, and so on, have such a short wavelength we can't notice it.  Electrons have much, much less mass than you or I, so it's wave nature is far more important to understanding its behavior.



> I'm just trying to understand gravity I guess. Is there gravity at the atomic level or just energy states?




The force holding an electron in an atom isn't gravity.  It is electromagnetic - the nucleus as a positive electric charge, the electron has a negative charge, so they are attracted to one another.  

Now, technically, the nucleus and the electrons have mass, so there should be gravity - but they have so *little* mass, and the force of gravity between them is so weak, that it might as well not be there.  For our intents and purposes, we can ignore it.

In any case, this is a Q & A thread, not a discussion thread - if it requires more discussion, we should break it out to elsewhere.


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## freyar (Aug 4, 2015)

AlexM said:


> OK. Electrons. Where do they get the energy to stay in orbit around the nucleus?




I'm just going to say that Umbran's given some really nice answers to your questions regarding this and emphasize one point that's appeared implicitly, which is that the electron actually has *less* energy than one that is separated from a proton.  

And as Umbran has said in response (to AlexM and tomBitonti), it would be great to move any further discussion of this question to a new thread in the Misc. Geek Talk forum to keep this thread a bit easier to navigate.  Thanks!


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## freyar (Aug 5, 2015)

Janx said:


> What's the deal with Dark Matter?  Is it just a placeholder for the math (until they find the real thing and name it Umbranium, or is there really funky stuff out there?




This is a popular topic on EN World, so I'll try to give a thorough answer for it.  Here's the tl;dr version: there's very good evidence that normal matter isn't enough to account for the gravity needed to hold things like galaxies together (or get them to form in the first place), so there must be something there with certain characteristics.  We call this dark matter.  At a basic level, it's already incorporated in the math of our understanding of cosmology (history of the universe), but we don't know its detailed properties.

_Long version_
First, why we know that there's something new there (ie, why we have a known unknown):

The first accepted evidence came from looking at the orbits of stars in other galaxies.  Basically, if what we see is all the matter there is, the orbital speeds should decrease far from the center of a galaxy, but instead they stay the same. That means there has to be a lot of mass distributed even at large distances from the centers of galaxies, a total of 5 to 6 times as much as we can see.
If you look at clusters of galaxies, you also find that the galaxies in the clusters are moving far too fast to stay together unless there's more mass than we can see.  This was actually the first evidence discovered for dark matter, but it wasn't widely accepted, perhaps because people suspected the observations just didn't see a lot of the gas between galaxies (which I think ended up to be the case).
We can also look at light from galaxies farther away passing those clusters.  That light is gravitationally lensed by the mass in the cluster, and we can use the lensing as a measure of the mass, which is again more than the normal visible matter.
The cosmic microwave background (aka CMB) shows us the clumpiness of matter in the very early universe.  Up till that time, normal matter was a plasma, and the pressure of the plasma strongly resists clumping.  It turns out that the amount clumping we see is consistent with having a large amount of pressureless matter (not normal stuff).
It's worth mentioning that all these observations require about the same amount of extra matter.

What are the possibilities for explaining this?

Gravity/mechanics don't work how we think.  The most popular option is called "modified Newtonian dynamics" (MOND), which takes the idea that Newton's second law (F=ma) should be changed for very small accelerations.  This works pretty well for stars in galaxies, less well for clusters (and see below) and not really at all for the CMB.  The relativistic version of MOND is very messy, and it just doesn't seem like you can explain the CMB data without extra matter unless you say the gravitational force didn't point toward mass but in an apparently random direction in the early universe.  It's very hard to make sense of that, and all but the most hard core MOND supporters (like the people who invented it) say as much.
There could just be lots of "dark" normal matter, like gas clouds, super-Jupiter planets, etc.  (The planet-like option used to go by the name of massive compact halo objects, or MACHOs.)  There was a big search for MACHOs in the 1990s, which didn't find enough, and CMB data now tells us the excess mass can't be made of normal matter.
There is some new kind of elementary particle, generically called "dark matter."  There are several possible types of particle that could be dark matter, but the simplest to describe is a weakly interacting massive particle or WIMP (MACHOs vs WIMPs, get it?  Weren't those physicists so clever?).  This is basically a very heavy kind of new particle (usually considered to be at least as heavy as a Higgs boson, though not necessarily) that doesn't interact with normal stuff very much.  There are actually a number of reasons to have this type of particle from a theoretical point of view --- they arise in solutions to other problems in particle physics, for example. With the right amount, this fixes all the observational problems above and helps to explain the formation of structure (like galaxies) in the universe.
So most physicists prefer particle dark matter, and WIMPs are the most popular, though that's partly because they're easiest to look for compared to other particle alternatives (like looking for your keys under the lamppost rather than in the middle of the dark street).

How are we looking for dark matter?  3 ways, primarily:

Colliders: dark matter or perhaps other new particles related to dark matter could be produced at a particle collider experiment, like the Large Hadron Collider that discovered the Higgs boson and more recently pentaquarks.
Direct detection: the earth is passing through a cloud of dark matter (if the dark matter particle mass is the same as for a Higgs boson, there would be about one dark matter particle per liter here).  Every once in a while, a dark matter particle could hit an atomic nucleus and cause a recoil.  So there are lots of experiments set up to look for those extremely rare recoils. The problem is, until we do the measurement, we don't know how rare the recoils will be!
Indirect detection: dark matter may have rare interactions out in space, which we might see just because there's enough dark matter that rare events still happen a lot!  The most common thing to look for is dark matter annihilating with anti-dark matter (every particle has an antiparticle, after all).  Presumably, this would eventually produce "normal" Standard Model particles that eventually produce photons we can see.  [Most of my work has been on this area.]
These are the main experimental means of searching for WIMPs, and there are variations for other types of particle dark matter.  These are very difficult experiments, so there have been a number of controversies and false alarms.  Still, the experiments are reaching into very interesting parts of "theory space," so there is hope that the next decade will tells a lot more about what dark matter is.

One other point to make is that dark matter, being most of the matter in the universe, has a lot to do with how the structures of our universe (galaxies, clusters of galaxies, etc) formed.  By and large, simulations of dark matter particles moving under the influence of each others' gravity give results similar on a large scale with observations of our universe.  On a smaller scale --- like the center of a galaxy like ours or like the dwarf galaxies that orbit our Milky Way galaxy --- some people have noted discrepancies (and some EN World posters have asked about these previously).  This is a point that MOND adherents like to bring up, since MOND does an ok job explaining some of them (at least according to abstracts of some papers I've seen).  There are, though, some reasons not to get worked up about this:

Doing gravitational calculations of many dark matter particles is ridiculously hard.  Even solving for the motion of the planets in the solar system is very difficult if you include the gravitational effects of the planets on each other (this problem is generally called the "N-body problem").  Even low resolution simulations require the gravitational interactions of billions of "dark matter particles" each with mass many times that of the sun and take years and years of CPU time to run.  These have improved a lot, so the point is that there is still work to be done in understanding the theoretical prediction.
A lot of the "problems" noticed were in comparing the real world to simulations that involve only dark matter.  People are only in the last few years incorporating normal matter in with the dark matter in their simulations --- things like star formation, supernovas, etc.  These can have some big and complex effects.
When you use good resolution and include normal matter, at least some current simulations resolve a number of the "problems" people have talked about.
Very small interactions of dark matter particles with each other also sometimes fix some of the problems.
So the jury is out, but it's relatively encouraging.  At the very least, its' way too early to say there is a crisis for the dark matter paradigm.

I think that's basically it.  Hope that gives you something to read for a while.


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## freyar (Aug 6, 2015)

After that wall of text yesterday, I think I'll go with a couple short ones today.



Janx said:


> What's the coolest physics question you've answered?




Given your other question, I'm going to assume you mean "question" as in from the public or a student.  This is a pretty hard question in that I'm not quite sure how to quantify the coolness of a question.  For me, it's pretty cool when a student asks a question that shows he/she is really getting something or thinking about what they're learning.  I'm also very impressed with a lot of the questions at EN World --- in this thread and elsewhere --- because they are well-informed and curious.  And generally on cool topics like wormholes, transporters, etc.  

One that stands out (for all the above reasons) was from what I think was a high-school or early college student at a public panel I did on dark matter last fall.  In the question period, he asked about dark energy and multiverses, which was certainly off-topic for the panel.  But it was on a cool topic, showed enthusiasm (ie, this was clearly someone who'd been reading about science), and took advantage of a pretty rare opportunity in Winnipeg.




Janx said:


> What's the coolest Physics problem you've solved or contributed to?




Geez, this is like asking me to choose my favorite pet or something.   More seriously, if you're in this line of work and don't find yourself thinking you're working on something cool all the time, you're either getting burnt out or aren't in enough control over your work.

I think in terms of "wow" factor, the work I did on "tachyon condensation" in string theory sounds the coolest.  In a less exotic context, tachyon condensation is really similar to a ball rolling down a hill into a valley, and the Higgs field of the Standard Model of particle physics does this in the early universe.  Yes, it would be pretty nasty to live through, but it's a comprehensible enough process.  About 10 years ago, some of my colleagues showed that certain string theory tachyons actually destroy entire dimensions of space when they "roll down the hill."  My work was about applying this in the context of the Big Bang and understanding some details --- one of the results was that the story is actually a bit more complicated, and parts of the original calculations may not be 100% reliable (though that isn't necessarily a surprise when something that dramatic happens).

But I'm also very excited about some work I'm doing right now, which definitely also is cool.  This is about black hole formation in "anti-de Sitter" spacetime, or AdS, which is important in string theory for reasons I'll mention in another answer.  What's interesting is that AdS has a gravitational potential even though it's "empty." In normal flat spacetime, if you throw a ball away, it will just keep going.  In AdS, if you're at the center, a ball you throw away will come back.  So, in flat spacetime, matter that tries to collapse into a black hole but isn't concentrated enough will just fly away, but in AdS, it will fly off and then fall back and get another chance to form a black hole.  And over and over. (As usual, I'm oversimplifying a bit, but hey.)  To track how this works and when a black hole forms takes fairly heavy duty computation, and this is my first time doing real computational physics, so that's personally exciting.  And it's a relatively hot topic.  As for the coolness, besides the work itself, the calculations lend themselves to making animations.  Simple ones, but still --- MOVIES!  So that's cool.


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## Umbran (Aug 7, 2015)

A nice little video on Dark Matter and Dark Energy.  Doesn't tell you much that wasn't already said, but these folks do a nice video series more people should know about.

[video]https://youtu.be/QAa2O_8wBUQ[/video]


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## freyar (Aug 7, 2015)

Umbran said:


> A nice little video on Dark Matter and Dark Energy.  Doesn't tell you much that wasn't already said, but these folks do a nice video series more people should know about.
> 
> [video]https://youtu.be/QAa2O_8wBUQ[/video]




Thanks for the link!  It's worth saying that there are lots of great video and blog resources out there.

It's also worth saying after the big dark matter post I did that most of the energy in the universe is really just energy, not particulate matter, which people have named "dark energy."  We know very little concrete about it, but the simplest idea for it is that it's the cosmological constant, which is another way of just saying the constant energy of the vacuum (completely empty space).  I'm of course willing to entertain questions about that too.


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## Janx (Aug 8, 2015)

Thanks for answering my generic questions.  I figured I could at least find some questions you liked answering by asking for what you liked


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## freyar (Aug 8, 2015)

There are several questions related to the general theory of relativity/Einsteinian gravity, so I'll do a series of answers over the next couple of days.



garnuk said:


> How does Einstein's theory of bent space for gravity work, when there is no "gravity" acting on the object to keep it in the well created by the bent space-fabric?




I'm going to assume that you're asking about, say, how the earth moves in orbit around the sun, why a dropped tennis ball accelerates downward here on earth, etc, in the context of general relativity (rather than how spacetime gets bent in the first place).  One way to think about it is to go back to one of Newton's laws that "an object at rest stays at rest and an object in motion stays in motion unless acted upon by an outside force" like you might have learned in high school or introductory university physics.  I want to phrase it a little differently: *objects stay in the same state of motion unless acted on by an outside force.*

In normal Newtonian mechanics (which is flat spacetime in relativistic terminology), staying in the same state of motion means motion at constant speed in the same direction --- no acceleration.  In general relativity, though, as you mention, spacetime is curved.  If you go through the math, you find that "no acceleration" means something different in curved spacetime. What it means is "freefall."  In other words, astronauts in the International Space Station float because they (and the ISS) are in a constant state of motion --- freefall around the earth. (In Newtonian gravity, you'd say instead that the gravitational force accounts for the centripetal acceleration, so the space station doesn't have to push on the astronauts).  So that's it, really: curved paths like orbits are really the "no acceleration" paths in curved spacetime.

This really does change how we have to think about gravity.  In Newtonian gravity, we say that we stand on the earth with no acceleration because the downward force of gravity is balanced by an upward push from the ground.  In general relativity, we say that we are actually accelerating upward compared to our natural state of motion (falling toward the center of the earth) due to an unbalanced upward push from the ground.  It's remarkably different if you stop and think about it.  But it's also a pretty simple statement that has many consequences that have been verified experimentally many many times.


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## freyar (Aug 8, 2015)

Janx said:


> Thanks for answering my generic questions.  I figured I could at least find some questions you liked answering by asking for what you liked




Not a bad idea.  The truth is, I like pretty much all my research.  As I said, in this line of work, you should find anything you do either important, exciting, or cool (even if it's just because it's simple enough to have undergraduates work on it).  But the coolness of some projects is much more obvious to a non-physics audience than other projects.


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## garnuk (Aug 9, 2015)

freyar said:


> There are several questions related to the general theory of relativity/Einsteinian gravity, so I'll do a series of answers over the next couple of days.
> 
> 
> 
> ...




Thanks! Learn something new everyday...
However, my question just now shifts to what causes the "freefall".  I imagine that if you take a ball and funnel into space, and push the ball around the edge, that without some other force, (like blowing on it), the ball would just spin out of the funnel all together.


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## freyar (Aug 9, 2015)

garnuk said:


> Thanks! Learn something new everyday...
> However, my question just now shifts to what causes the "freefall".  I imagine that if you take a ball and funnel into space, and push the ball around the edge, that without some other force, (like blowing on it), the ball would just spin out of the funnel all together.




If you recall the idea of inertial motion from introductory physics like I mentioned in my answer --- the idea that objects move in a straight line at constant speed unless acted on by an outside force --- that's all freefall is.  In space, very far from a large mass, there's "no gravity" (in relativity, spacetime is flat), and a free object will just move along at constant speed in a straight line.  It's just that near a large mass (like a star or planet), the curviness of spacetime means that "inertial motion" includes behavior like falling toward a planet or orbiting it.  I don't think you can really say there's a cause of freefall or inertial motion.  It's just how things work, and I think it's really important to remember that "freefall" in general relativity is the same idea as Newton's laws, just in a broader framework.

In your example, the ball and the funnel are pushing (exerting force) on each other, so that's how the ball can roll around the edge of the funnel.  But, yes, eventually it would run out of funnel and move off inertially --- in empty space, that would be a straight line, but, near a planet, that might be an orbit.

Good questions.  Maybe if there are more follow-up questions, it would be good to post a new thread in the Misc. Geek Talk forum.  Thanks!


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## Evenglare (Aug 9, 2015)

Always good to see another Physicist here. My study was astrophysics, specifically exoplanet detection with the Kepler Satellite data as well as modeling the planet and it's system.


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## Umbran (Aug 9, 2015)

garnuk said:


> However, my question just now shifts to what causes the "freefall".  I imagine that if you take a ball and funnel into space, and push the ball around the edge, that without some other force, (like blowing on it), the ball would just spin out of the funnel all together.




I will try one small bit that wasn't put so clearly that might clarify it, and then respect the "take it to another thread" request...

Consider space away from anything large.  It is open and flat, right?  Say you have an object.  It got a push some time ago, and is now just cruising along.  If nothing else pushes on it, it'll keep cruising along in a straight line, right?

Well, now put that object cruising along through a _curved_ spacetime.  Imagine, what does a "straight" line look like in a "curved" space?  

Think about driving in your car, in a straight line down the road.  Now, zoom out in your head.  Way out, until you see the curve of the Earth - and you realize that "straight" line you were driving in is, from teh point of view of someone not on the globe, curved!

This is what is happening in your example - you keep thinking that the spacetime is flat, and straight lines are straight - but near a massive body, the spacetime is curved, and the "straight" lines near it - the lines that are parallel and wont' intersect - are *curved* from the point of view of a distant observer.


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## garnuk (Aug 9, 2015)

Umbran said:


> I will try one small bit that wasn't put so clearly that might clarify it, and then respect the "take it to another thread" request...
> 
> Consider space away from anything large.  It is open and flat, right?  Say you have an object.  It got a push some time ago, and is now just cruising along.  If nothing else pushes on it, it'll keep cruising along in a straight line, right?
> 
> ...




Ok thanks. I'll just attribute it to some sort of ethereal "friction" and "surface tension" then.  I'm trying to understand what keeps things attached to the flat or curved space, that they stick to the contours at all.


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## Umbran (Aug 9, 2015)

garnuk said:


> Ok thanks. I'll just attribute it to some sort of ethereal "friction" and "surface tension" then.  I'm trying to understand what keeps things attached to the flat or curved space, that they stick to the contours at all.




Oh, I see.  Here's the thing you aren't getting - it isn't "attached to" the space.  Despite the analogies, the curved space is *not* a 2-dimensional sheet that the thing has to stick to.  The thing isn't resting on a sheet - it is embedded in a volume all around it.  All three dimensions of space, and time, are *all curved together*.    If it went "up", it'd still be in the space.


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## freyar (Aug 9, 2015)

Evenglare said:


> Always good to see another Physicist here. My study was astrophysics, specifically exoplanet detection with the Kepler Satellite data as well as modeling the planet and it's system.



Neat stuff!  Feel free to chime in with any appropriate answers if you like.



Umbran said:


> Oh, I see.  Here's the thing you aren't getting - it isn't "attached to" the space.  Despite the analogies, the curved space is *not* a 2-dimensional sheet that the thing has to stick to.  The thing isn't resting on a sheet - it is embedded in a volume all around it.  All three dimensions of space, and time, are *all curved together*.    If it went "up", it'd still be in the space.




Thanks, Umbran!  That does seem to have gotten to the question!


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## freyar (Aug 10, 2015)

Joker said:


> A common trope in science fiction is using a type of propulsion which "bends" spacetime. Some have the idea that a ship could move at normal speed while contracting the space in front of it. Effectively reducing the distance it has to travel.
> 
> Is this theoretically possible (with exotic matter)?




Yes, this is the way the "warp drive spacetime" discovered by Miguel Alcubierre works (along with an expansion of space behind the ship).  In fact, the shop doesn't have to move at all --- the bending of space does everything!  But don't get your hopes up: the exotic matter necessary is has much more unusual properties than, for example, dark matter (which is in most models like normal matter in many respects).  Specifically, it needs negative energy density.  That is possible, but most hypothetical forms of exotic matter with negative energy density end up causing mathematical inconsistencies in physics.  From a quick literature search today, I'm not finding anything more specific than that for the case of the warp drive spacetime.

Also, to give you something from a true authority on the subject, I saw a talk by Professor Alcubierre a couple of months ago.  On his slides, he wrote "please, please, please don't believe the hype" about NASA's Eagleworks lab's work on warp drive.  In short, he doesn't find that work terribly promising.



> What would happen to any objects in or near the space being contracted?




That's a little more complicated, and I haven't seen anything analyzing this directly in the literature.  But here are my thoughts on it.  Generally, objects are ok with being in expanding or contracting space, as long as the expansion or contraction is gentle enough.  For example, our universe is expanding, but galaxies are able to remain as coherent objects.  On the other hand, one paper I did find argues that the "warp bubble" (contracting region) has to be really very thin.  In that case, an object passing through the bubble would be contracted (and perhaps stretched) different amounts in different places.  That can certainly destroy the poor object, most likely by crushing part of it.  Slightly more technically, I'd expect the victim to feel strong tidal forces that stretch or crunch it because there's a different gravitational action on different parts of the object.  In a much less extreme form, the gravitational pull of the moon is more on the close side of the earth than the far side, which causes tides in the oceans.  Another example of tidal forces that's come up on EN World before is near the event horizon of a black hole --- essentially your feet want to move toward the black hole so much faster than your head does that you stretch out like spaghetti.  But for large enough black holes, the stretching of space is actually gentle enough to leave you alive until you're well inside the event horizon.


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## fuindordm (Aug 10, 2015)

freyar said:


> One more fast one...
> 
> Anyway, this happens a lot.  It requires pretty sensitive handling from the physicist, though the questioner isn't always respectful or may not realize how much time they're actually taking since they're typically more concerned about self-promotion than learning anything.  But it does lead me to appreciate a forum like EN World, since the conversation here is respectful and genuinely curious.




I had several long conversations with one such author about general relativity and cosmology. The man had a law degree but no science degree. Eventually I realized that he was fixated on an antique cosmological theory (I forget which one--a deSitter variant, I think) as the basis of his new theory, not because it was better but because it had precedence. In his mind, the early cosmologists simply had more weight than the late cosmologists, and even if observations had disproved the early theory then it was a sounder foundation than general relativity.  I was never able to convince him to drop that mindset. 

My colleagues in biology tell me that they get similar submissions, especially alternative theories of evolution.

Anyway, thanks freyar for starting the thread. As a former observational cosmologist, I can say that your answers are spot on.


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## freyar (Aug 10, 2015)

fuindordm said:


> Anyway, thanks freyar for starting the thread. As a former observational cosmologist, I can say that your answers are spot on.




Thanks for stopping in!  Feel free to chime in if you like!


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## freyar (Aug 10, 2015)

Janx said:


> What's the layman's explanation for String Theory these days?




Pretty much the same as it's always been. 

More seriously, string theory starts out as studying the behavior of a quantum string moving in spacetime --- you can think of the string as something like a very tiny rubber band.  In quantum mechanics, the string can only vibrate in particular ways.  If you look at those vibrations closely, they look like certain types of particles, including photons (as you know, particles of light) and gravitons (hypothetical particles of gravitation).  Remarkably, if you study the motion of the string even more carefully, you can (among other things) derive Einstein's equations for gravity.  So, string theory predicts at least one thing: gravity!

The advantage of string theory vs normal particle physics lies in the detailed math.  String theory is typically more complicated (more below, though), but it is complete.  With normal particle physics, we are typically forced to say that there's some energy level beyond which we have no knowledge, but string theory in principle can explain everything.  That's part of the appeal.  Of course, part of the complexity includes things like extra dimensions, etc, etc.  And most of the distinctively stringy effects are at such high energies we don't really have prospects for testing them, so string theory gets criticized a lot for that.  On the other hand, other theories of quantum gravity which are less mathematically rigorous also don't make unique predictions, so we just have to accept that if we want to think about quantum gravity at this point.  It's very hard for any quantum gravity theory to make currently testable predictions, and many of the predictions can be mimicked by more standard physics.

So string theory is, in principle, a theory of all physics, including quantum gravity.  It is simple in its basics but extremely rich, and that richness comes along with complications (extra dimensions, for example, give us the chance to figure out where the Standard Model of particle physics comes from but at the cost of some very hairy math and a lot of other possibilities to choose from).  But string theory is also very important in other ways.  In 1997, Juan Maldacena made the startling discovery that string theory in certain spacetimes is actually the same thing as certain theories of normal particle physics which happen to be very similar to theories of the strong nuclear force, which are notoriously difficult to solve otherwise.  This has been extremely well tested and in fact provided the first remotely reasonable explanations for some results of the RHIC experiment (which smashes large nuclei together).  Many nuclear theorists have learned string theory for this reason.  In more recent years, this correspondence has been generalized to other theories, and some physicists are using string theory to try to understand problems in materials science, such as superconductivity.  So string theory is also very much another way to understand more normal theories of particle physics.


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## Umbran (Aug 11, 2015)

Okay, so, let's go with a hard one...

What do you think of pilot wave theory?

For those who haven't heard of it - "plot wave theory" is an old interpretation and formulation of quantum mechanics.  Instead of having wave-particle duality, you have a real physical particle, and a real physical wave.  The two are connected, so that the movement of the wave impacts the particle, and vice-versa.  The ultimately important bit is that this interpretation tosses out quantum indeterminacy - the universe is deterministic in pilot wave theory.


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## freyar (Aug 11, 2015)

Janx said:


> If you could unHiggs me, what would happen?  Would I be less massive?  What would that mean?




Well, I don't think you'd enjoy it very much.

But let's first deal with a misconception I see lurking under your question, since it's been popularized by the media who are either mis-quoting particle physicists or have been the victim of physicists who've gone for a sound bite without bothering to get things right.  I imagine you've heard that the Higgs field is responsible for creating mass or some such thing.  In a sense, that's true: all the *fundamental* Standard Model particles that have mass do have their mass because of their interaction with the Higgs*.  So electrons, for example, gain mass due to the Higgs field.

However, matter is made of protons and neutrons as well as electrons, and protons and neutrons are about 2000x heavier than electrons.  Protons and neutrons are *not* fundamental but are made up of quarks.  While the quarks get their mass due to the Higgs, nearly all the mass of a proton or neutron is due to the energy of the quarks whizzing around relativistically inside (E=mc^2!).  

What this means is that you wouldn't actually lose much mass if you "unHiggsed" (this is not a good diet plan!) since only your electrons would become massless, and they make up only about 0.05% of your mass (actually less; that's the maximum amount, which would be the case if you were pure hydrogen).  The mass of your protons and neutrons wouldn't change much.  

What would happen, however, would be rather nasty to you.  If your electrons became massless, the electromagnetic force would no longer be able to bind them to protons.  In other words, all your atoms would fly apart.  Your electons would suddenly leave your body at the speed of light, leaving behind a cold plasma of atomic nuclei.  In other words, chemistry would cease to exist for you.  

Fortunately, it would take a ridiculous amount of energy to reset the Higgs field to zero ("unHiggs") a person-sized part of space, so you don't have to be worried about your electrons flying off.  You'd probably be vaporized by the necessary high-energy particle beams first. 


*except for neutrinos. In the Standard Model, they are massless, but we know that they must (in reality) have very small masses.  We don't know what mechanism gives them mass yet.


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## fuindordm (Aug 11, 2015)

Umbran said:


> Okay, so, let's go with a hard one...
> 
> What do you think of pilot wave theory?
> 
> For those who haven't heard of it - "plot wave theory" is an old interpretation and formulation of quantum mechanics.  Instead of having wave-particle duality, you have a real physical particle, and a real physical wave.  The two are connected, so that the movement of the wave impacts the particle, and vice-versa.  The ultimately important bit is that this interpretation tosses out quantum indeterminacy - the universe is deterministic in pilot wave theory.




Doesn't that just push the problems of QM down a level? Instead of matter particles having a wave nature, this postulates a new field in nature that can supply the waves, and then you have to think about what this field is made of and how it interacts with different kinds of matter particles...and in the end you don't do any better than standard QM because the predictions are still probabilistic and you still can't explain how the wave collapses to an eigenstate at the point of measurement.

It's kind of like dark matter: as long as you don't understand something, you might as well make the theory as simple as possible with one unknown rather than a hodgepodge of alternative explanations for the different types of observations that call for dark matter. QM is not simple, but the pilot wave seems like a big complication with no real payoff--the particles might have a deterministic veneer but the nature of reality described by the theory is no more deterministic than before.

But maybe I'm not remembering enough details, and the theory is better than I think. Did any of its proponents have a good candidate for the pilot wave?


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## garnuk (Aug 12, 2015)

I just saw the movie Ant-man, and I have to ask.  Does the idea of a "molecule" which "reduces the amount of space between atoms", sound like nonsense to a physicist as much as it did to me?


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## Umbran (Aug 12, 2015)

fuindordm said:


> Doesn't that just push the problems of QM down a level?




This is a Q&A thread, not a discussion thread.  So, while I have thoughts on the matter, I'll leave it to the OP if he chooses to address it.


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## Umbran (Aug 12, 2015)

garnuk said:


> I just saw the movie Ant-man, and I have to ask.  Does the idea of a "molecule" which "reduces the amount of space between atoms", sound like nonsense to a physicist as much as it did to me?




It is a superhero movie.  Of course the physics us hokey as anything.

I just saw the film last night - I recall them repeatedly referring to it as a "Pym particle" (what they call in the comics).  I don't recall them calling it a molecule, but I may have missed it.

That said... I can imagine an as-yet undiscovered fundamental particle that alters the electromagnetic force could change bond distances (and thus shrink the space between atoms), or alters spacetime (say, through gravitation) having such an effect.  The latter allows for funny gravity/quantum effects down when you reach the Planck scale for distances to get us to the Microverse!  Woot!  Micronauts!


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## was (Aug 12, 2015)

This may be a bit silly, but is it theoretically possible to harden glass to the point that it could physically break through a brick wall?  a.k.a. the Kool-Aid Man theory...


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## freyar (Aug 14, 2015)

Sorry for a short delay, everyone --- had a couple of busy days.



Umbran said:


> Okay, so, let's go with a hard one...
> 
> What do you think of pilot wave theory?
> 
> For those who haven't heard of it - "plot wave theory" is an old interpretation and formulation of quantum mechanics.  Instead of having wave-particle duality, you have a real physical particle, and a real physical wave.  The two are connected, so that the movement of the wave impacts the particle, and vice-versa.  The ultimately important bit is that this interpretation tosses out quantum indeterminacy - the universe is deterministic in pilot wave theory.




It should tell you something about my answer that I had to look things up to make sure I wasn't forgetting about a new variant or something.   Short answer: not a big fan.  I don't like the loss of locality vs other interpretations (more below), and it's also pretty ugly.

Longer answer with explanations: quantum physics is very weird compared to our intuition from daily life or even a very rigorous understanding of Newtonian mechanics.  Many brilliant physicists of a century ago (including Einstein) had real problems coping with it.  A famous example of that weirdness is the Schrodinger's cat thought experiment, in which a poor feline's fate is tied to a quantum event like a nuclear decay and ends up 50% dead/50% alive until someone looks at it.  In a standard view of quantum physics, that cat is *neither and both* dead/alive until you look at it --- we say that standard quantum physics lacks "reality" in that .  In classical physics or a pilot wave version of quantum mechanics, that cat *is* either dead or alive, but you just don't know which.  The thing about the pilot wave version is that, just like normal quantum mechanics, if you perform the experiment 100 times, about 50 of the cats will come out dead and 50 alive.  But don't do this experiment, because then you should be arrested for cruelty to animals (quite justly IMO).

Anyway, the pilot wave theory is a way to explain the probabilistic nature of quantum mechanics as a measure of our ignorance rather than as a fundamental thing.  It seems to have the benefit that it allows a derivation of the Born Rule (named for Max Born) that tells us how to assign probabilities based on a wavefunction.  However, it has a couple of big flaws.  One is that it does require quite a bit of extra machinery to it that seems unnecessary and is pretty ugly.  What's probably worse in most physicists' minds is that it is written in terms of mathematics that violates locality (there looks like there is instantaneous communication in the math), and it's pretty tricky to avoid that instantaneous communication.  I'd also say that it looks like it would be difficult to describe a quantum field theory with pilot waves (though apparently it has been done).

Of course, this raises the question of the "right" way to think about quantum mechanics.  There are quite a few ideas. The standard presentation of quantum mechanics is the "Copenhagen Interpretation" since Niels Bohr, one of the earliest quantum physicists and promoter of this interpretation (and his collaborators) worked in Copenhagen.  This interpretation says that the cat turns alive or dead when someone "measures" it.  That leads to a lot of questions about what counts as a measurement and requires breaking the world into a "quantum part" and a "classical part."  This is of course silly, since all physics is supposed to be quantum.  But this is still a popular view, since it was how the mathematics were developed, and most physicists learn to "shut up and calculate"  (yes, that's a quote often attributed to Feynman but probably is due to David Mermin instead).  

I personally prefer something like the "many worlds approach," in which the whole universe is quantum and branches into many possibilities any time an interaction happens between two different systems.  Apparently the Born rule can be derived in this approach, as well, though that's a recent result that may not have been scrutinized a lot yet.  I'm also intrigued by the "consistent histories" approach, which basically says that interactions with the (quantum) environment is important for the evolution of a quantum system (like Schrodinger's cat).  But I can't explain that one in too much detail because the explanations I've seen are philosophically very heavy and honestly hard for me to decipher.  I do have a sense that it's somewhat related to the "many worlds" approach.

Anyway, as fuindordm and Umbran have indicated, there's a lot to say on this subject, and I think a thread just devoted to this topic might be really interesting if someone cared enough to start one.

Incidentally, as of the last informal poll taken, a plurality of physicists prefers the Copenhagen Interpretation.


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## fuindordm (Aug 14, 2015)

freyar said:


> Incidentally, as of the last informal poll taken, a plurality of physicists prefers the Copenhagen Interpretation.




Interestingly, though, Bohr himself did not favor this interpretation. He considered the wavefunction to describe an inseparable relationship between the system being measured and the experimental apparatus. He never took steps to try to formalize that idea with mathematics, but he expressed this view several times in letters as a way of interpreting the wave function without insisting that it has to "collapse" non-locally; in effect, when you make a measurement the relationship between the observer and the observed changes.


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## Umbran (Aug 14, 2015)

freyar said:


> However, it has a couple of big flaws.  One is that it does require quite a bit of extra machinery to it that seems unnecessary and is pretty ugly.  What's probably worse in most physicists' minds is that it is written in terms of mathematics that violates locality (there looks like there is instantaneous communication in the math), and it's pretty tricky to avoid that instantaneous communication.  I'd also say that it looks like it would be difficult to describe a quantum field theory with pilot waves (though apparently it has been done).




As for locality - a number of folks have issues with standard quantum entanglement for similar reasons.

And, for the reader, I'd like to comment on "ugly".  I am guessing freyar is using it in the way I would - and to a physicist the word has two related meanings.

"Ugly": the math is inelegant.  There has been a striking tendency for math that turns out to be a good model to have a kind of simplicity of expression that is aesthetically pleasing to a mathematician.  

"Ugly": The math is difficult to actually do anything with.  The model may be simply expressed, but as soon as you actually try to calculate things, it blows up in complexity to the point that you snap your pencil in frustration.  Doing the orbital mechanics of the solar system in spherical coordinates centered on the Sun isn't child's play, but doing the same in geocentric rectangular coordinates is pretty darned ugly.

Pilot wave QM is certainly ugly in the second sense.  Using it to calculate even fairly simple things is a pain in the chalk.


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## freyar (Aug 14, 2015)

Umbran said:


> As for locality - a number of folks have issues with standard quantum entanglement for similar reasons.




Well, there is a difference in "nonlocality" in standard quantum mechanics (either Copenhagen or many-worlds or whatever) vs pilot waves.  In standard quantum mechanics, the wavefunction covers the whole universe, so there's nonlocality in that sense.  But there are absolutely no nonlocal interactions (in the Hamiltonian, for the technically-minded).  Nothing *happens* faster than the speed of light.  The reason weird things can happen is due to the fact that quantum properties of an object just don't have any operational meaning or reality unless there's an interaction between that object and something else.  From a common-sense point of view, that's much more profoundly disturbing than nonlocality (after all, we'd all love a FTL space ship!), but it causes fewer problems with physics.

The nonlocality in pilot waves is nastier in the sense that there is an instantaneous interaction between objects at a distance from each other explicitly in the equations describing the theory (ie, in the so-called guiding equation which supplements the Schrodinger equation, if I understand it correctly).  So, even though in the end the results have to be the same as standard quantum mechanics, you have to work hard to make sure that the nonlocalities don't show up.  And it would be easy for something to break, leaving you with time travel paradoxes and such.

And, yes, the math for pilot waves looks ugly in both senses you (Umbran) describe.


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## freyar (Aug 14, 2015)

garnuk said:


> I just saw the movie Ant-man, and I have to ask.  Does the idea of a "molecule" which "reduces the amount of space between atoms", sound like nonsense to a physicist as much as it did to me?






Umbran said:


> It is a superhero movie.  Of course the physics us hokey as anything.
> 
> I just saw the film last night - I recall them repeatedly referring to it as a "Pym particle" (what they call in the comics).  I don't recall them calling it a molecule, but I may have missed it.
> 
> That said... I can imagine an as-yet undiscovered fundamental particle that alters the electromagnetic force could change bond distances (and thus shrink the space between atoms), or alters spacetime (say, through gravitation) having such an effect.  The latter allows for funny gravity/quantum effects down when you reach the Planck scale for distances to get us to the Microverse!  Woot!  Micronauts!




Haven't seen the movie, but that does sound pretty hokey!  But, just for fun, let's look at Umbran's suggestion that there's some kind of undiscovered particle that can change electromagnetism (and shrink atoms/molecules).  Actually, if it were a particle, you'd probably have to stick one inside every atom, which wouldn't be convenient.  Fortunately, every fundamental particle really comes from a field (like electromagnetic fields go with photon particles), and fields fill space.  So we need a field that controls the strength of electromagnetism.  This is not so far fetched, actually --- remember that the value of the Higgs field controls the masses of many fundamental particles in the Standard Model of particle physics.

It turns out we have to look beyond the Standard Model for a field that can control the strength of electromagnetism (really, we're talking about changing the value of the electric charge).  But then it's not too hard to find something appropriate.  Most extra-dimensional models (whether in string theory or not) as well as "supergravity" models include fields called moduli, which can control the strengths of different forces, etc.  So you just have to find a way to adjust the value of the appropriate moduli fields for the person you want to shrink (but not anything else, I guess).  That, of course, is the hard part; like the Higgs particle, corresponding moduli particles have to be heavy, which means it takes a huge amount of energy to change the value of the field, so you'd probably actually vaporize the person you're trying to shrink.  Ooops!

The fact that moduli control the strength of fundamental forces is a very appealing part of extra-dimensional physics/string theory/etc.  In normal particle physics, the strength of a force is just a number, and you just have to go measure it.  There's no way to figure out why it takes a particular value.  On the other hand, if the interaction strength is controlled by a field, you can in principle ask about what kind of physics determines what value that field should take.  Of course this is a hard problem, but it means that at least theoretically you can try to predict from first principles the strengths of fundamental forces.  It also opens up the possibility that maybe the field value and therefore the strength of the force was different in the very early universe.  There have actually been a number of studies that looked at galaxies very far away to determine if the electromagnetic force strength changed even a tiny amount over the age of the universe.  I think they're finally settling on the answer "no" (though it's not an easy measurement and I believe there have been claims in the past of "yes"), but, from a particle physics point of view, you'd expect the field value to be very well settled at its present day value long before galaxies could form.


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## fuindordm (Aug 15, 2015)

Ok. Quick question for you:
One of the big problems with the dark energy/cosmological constant component of the universe is explaining its value-- when I was in grad school the best models from particle physics proposing a nonzero vacuum energy were still 30 orders of magnitude off from the observed cosmological value. 
Has string theory made any progress in the past 10 years on this problem: a reasonable value for the vacuum energy density?
Ben


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## freyar (Aug 16, 2015)

was said:


> This may be a bit silly, but is it theoretically possible to harden glass to the point that it could physically break through a brick wall?  a.k.a. the Kool-Aid Man theory...




I'm going to answer this one the best I can in terms of more garden-variety physics in contrast to the Ant-man question.  This is really more of a materials science or engineering topic than I usually deal with, but I'll do my best. 

The answer, actually, seems that you don't really have to do much to make glass tough enough to break through brick. The property of a material to resist breaking (from a pre-existing microscopic crack) is called fracture toughness, and normal glass actually has a fracture toughness in the same ballpark as brick or concrete (see wikipedia, for example).  So I think you'd just need thick enough glass, which actually does seem pretty hard to break to me.  If you google "glass breaking brick," you can find a couple of youtube videos of martial artists breaking thin cinderblocks with drinking glasses (assuming they're not faked somehow), too.

Fortunately, someone else already did the math to figure this out.  Amazing what kind of research people will do.

Of course, the Kool-Aid-Man-shaped pitchers I've seen have tended to be plastic, and I don't think that would break from hitting a brick wall, either.  But then I guess he'd bounce off instead of breaking through...


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## Scott DeWar (Aug 16, 2015)

Ok, Ask you anything, huh?

I have seen this subject come up every now and then, tht of the EM Drive. I read this article about it, but now I would like to hear of how you guys react on it. Can it work, why or why not.

If this requires a separate thread, so be it.


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## Scott DeWar (Aug 16, 2015)

freyar said:


> . . . . *lots of stuff* . . . . .  Another example of tidal forces that's come up on EN World before is near the event horizon of a black hole --- essentially your feet want to move toward the black hole so much faster than your head does that you stretch out like spaghetti.  But for large enough black holes, the* stretching of space is actually gentle enough to leave you alive until you're well inside the event horizon*.




re: Highlighted portion in quote . . . . Baring lack of survivability of hard vacuum or massive doses of hard radiation, of course.


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## Umbran (Aug 16, 2015)

It only requires a separate thread if it goes into a repeated back-and-forth.

I'll refer you to something more like a reliable science blog on the topic:  http://www.iflscience.com/no-em-drive-will-not-lead-warp-travel-any-time-soon

To summarize:  "The EM Drive is a hypothetical form of propulsion that uses microwaves in an enclosed chamber to create forward thrust."  There have been several experiments that have reported *tiny* amounts of thrust.  And by tiny, I mean (if my envelope-back calculation is correct) enough force to hold up one-tenth of one grain of rice against the force of Earth's gravity.  And that's not "the device + that bit of rice".  It is *just* the rice.

The most recent experiment that I know of didn't verify the effect, so much as eliminate some of the possible sources for erroneous measurements of effect.

Can it work?  I shrug.  I don't know of any solid analysis saying it should.  What I have seen includes some pointed hand-waving at particular areas, which doesn't sit well with me.  There is a basic reason it should not work - conservation of momentum.  

Thus, I am skeptical.  If they've found some new form of interaction that makes it possible, that is awesome.  But, I have severe doubts, and will wait until experiments confirm the effect is real before I worry all that much about it.


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## Scott DeWar (Aug 17, 2015)

conservation of momentum, would that be similar to "For every action there is an opposite and equal reaction" as thus for the claimed thrust, where is the energy to equal it . . . or something like that?


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## freyar (Aug 17, 2015)

Scott DeWar said:


> Ok, Ask you anything, huh?
> 
> I have seen this subject come up every now and then, tht of the EM Drive. I read this article about it, but now I would like to hear of how you guys react on it. Can it work, why or why not.
> 
> If this requires a separate thread, so be it.






Umbran said:


> The most recent experiment that I know of didn't verify the effect, so much as eliminate some of the possible sources for erroneous measurements of effect.
> 
> Can it work?  I shrug.  I don't know of any solid analysis saying it should.  What I have seen includes some pointed hand-waving at particular areas, which doesn't sit well with me.  There is a basic reason it should not work - conservation of momentum.
> 
> Thus, I am skeptical.  If they've found some new form of interaction that makes it possible, that is awesome.  But, I have severe doubts, and will wait until experiments confirm the effect is real before I worry all that much about it.




I almost put in "ask me anything except about the EM drive" in the OP but decided not to.   Anyway, like Umbran, I am skeptical on general grounds, as follows: this violates one of the oldest principles of physics, that of conservation of momentum, which is another way of saying Newton's laws.  Before I'm willing to believe that could happen, I'd need very very good experimental evidence ("extraordinary claims require extraordinary evidence" in the words of Carl Sagan).  I think it's worth saying that the experimental groups who've looked at this haven't yet felt their evidence was strong enough for submission to peer review yet as far as I can tell (certainly the most recent group has not).  Another good discussion of the results on this can be found here: http://io9.com/no-german-scientists-have-not-confirmed-the-impossibl-1720573809

The problem with the EM drive is that the predicted thrust is so small as to be nearly unmeasurable with current techniques. While there have been measured "thrusts," those thrusts also behave in ways consistent with a heating effect (see the link I gave; apparently the measuring devices don't work well at the temperatures reached by the apparatus).  Furthermore, there is no credible theoretical work to motivate testing the drive in the first place.  

I think it's instructive to compare to another recent episode when an experiment possibly indicated a violation of a important physical law: a few years ago, the OPERA experiment, which was timing the flight of neutrinos over hundreds of kilometers, found them arriving something like 60 nanoseconds faster than the speed of light would allow.  In that case, the experiment's press release was a touch more cautious --- inviting the physics community to find problems with their experiment rather than saying their results warranted further investigation --- but just a touch.  The reception by the physics community was also very cautious, but the consensus was that there was less of an obvious problem (both the recent EM drive experiments have been criticized for not analyzing the effects of heating on the apparatus).  In the case of the OPERA experiment, there were a number of theory papers, either showing that FTL travel wouldn't look like what OPERA saw or trying to figure out a way neutrinos could move FTL, even though it would be fair to say that everyone would have been shocked if the OPERA results stood up.  In the end, the problem was a faulty cable connection.  

And, in the end, these EM drive results are probably due to something like a faulty cable or more likely measuring devices that can't take the heat.  But the EM drive experiments haven't given as much a reason to be interested as OPERA did.

Back to more normal physics tomorrow....


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## freyar (Aug 17, 2015)

Scott DeWar said:


> conservation of momentum, would that be similar to "For every action there is an opposite and equal reaction" as thus for the claimed thrust, where is the energy to equal it . . . or something like that?




Yes, conservation of momentum is equivalent to the statement "For every action there is an opposite and equal reaction."  What that means is that, if there's a thrust on an engine, there has to be a push on some kind of exhaust going the other way as well. The EM drive claims not to have any exhaust to be pushed the other way.

And perhaps further discussion on the EM drive can go to a new thread in Misc Geek Talk, unless it's a very short follow-up question.


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## Umbran (Aug 17, 2015)

Scott DeWar said:


> conservation of momentum, would that be similar to "For every action there is an opposite and equal reaction" as thus for the claimed thrust, where is the energy to equal it . . . or something like that?




Pretty much.  If the drive feels a thrust, it needs to be pushing off something.  In a conventional rocket, it is the rocket exhaust.  But, the EM drive has no known exhaust.


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## Scott DeWar (Aug 17, 2015)

I see how that makes no sense whatsoever. Ok, Next questioner!


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## freyar (Aug 17, 2015)

fuindordm said:


> Ok. Quick question for you:
> One of the big problems with the dark energy/cosmological constant component of the universe is explaining its value-- when I was in grad school the best models from particle physics proposing a nonzero vacuum energy were still 30 orders of magnitude off from the observed cosmological value.
> Has string theory made any progress in the past 10 years on this problem: a reasonable value for the vacuum energy density?
> Ben




There's lots and lots to say on this subject, but I'll try to avoid writing a book and just stick to the idea of vacuum energy.

Here's the background for the question: The universe is expanding, and through the mid '90s, everyone expected that the expansion should be slowing down due to the gravitational attraction of all the stuff in the universe.  Then two different research groups independently showed that the expansion is actually speeding up.  This requires some kind of unusual physics to explain, and the simplest explanation is that there's a cosmological constant term in the Einstein equations of general relativity.  This term is also how vacuum energy, or the energy of empty space, would appear in the Einstein equations.  Vacuum energy is interesting because, as space expands, the density remains constant, so the total energy goes up.  This property makes the expansion of space speed up.  Of course, there are other ways to get the universal expansion to accelerate (which as a whole are given the name *dark energy*), and I'm happy to talk about those, but I'm keeping to vacuum energy here.

The problem with vacuum energy (known as the cosmological constant problem), as fuindordm indicates, is that rough estimates of it from particle physics calculations give a ginormous number, around 120 orders of magnitude too big (that is, 1 followed by 120 zeros times the actual number).  Until the discoveries of the accelerating expansion, most physicists believed there must be an unknown physical principle that sets the cosmological constant to exactly zero, since making something so much smaller than it "should" be but not actually zero seems like quite a tall task (indeed, many people who believe in other forms of dark energy at least implicitly believe the cosmological constant is zero even now).  This latter, more difficult situation is the one we ended up with.  How do you make something so much smaller than it "wants" to be?

So, what ideas do we have (especially from the last decade or so) from string theory?  One possibility is a "string inspired" idea that uses extra dimensions.  The essentials are that all the normal matter (and probably dark matter too, I'm not that sure of those details) are stuck to objects called branes (short for membranes), which have the usual 3 spatial dimensions.  But there are also 2 extra dimensions perpendicular to the branes.  There can be a large vacuum energy on the branes, but, instead of causing our space to expand faster and faster, it causes the extra dimensions to curve (into a shape like an American football or rugby ball).  There are some people who really really love this idea, but not a lot of people are sold on it (at least, not a lot of people have worked on it).  Part of the reason is that it seems like it shouldn't work in the end for some technical reasons.  In fact, I saw a paper the other day arguing that this idea is fatally flawed.

The main development on the cosmological constant problem in recent times came up in 2003 and involves the (weak) anthropic principle.  The anthropic principle says that, if a universe can be observed, it must be capable of supporting intelligent life that can observe it.  The point is that the vacuum energy must be incredibly tiny for stars and galaxies (and presumably therefore life) to form. In fact, the maximum possible value for the vacuum energy is not much higher (maybe a factor of 100) than that observed, and Steven Weinberg (a Nobel-winning particle physicist) actually predicted the discovery of the cosmological constant based on these anthropic arguments back in the 1980s.  

Where string theory comes in is to provide a mechanism to work.  You see, for the anthropic principle to make sense as a physical principle, you need a universe where there are lots of different regions of different effective vacuum energies; this is often called a *multiverse*.  To get this, you need a theory with a lot of different states of different vacuum energies and a way for the universe to transition in between them.  About 12 years ago, string theorists working on problems about moduli fields (see the Ant-man answer above) discovered that string theory apparently satisfies both of these properties.  In other words, it looks like string theory automatically gives you a multiverse.  Since then, there's been a lot of work on understanding what the probability is that we'd live in a part of the multiverse with our value of the vacuum energy.  This is kind of tricky, since it's hard to come up with a mathematically rigorous definition of probability that applies to chunks of an infinite universe.

This idea is pretty polarizing.  A lot of physicists think the use of the anthropic principle is a cop out.  Some of them have argued that there are technical (read as "highly mathematical") reasons to think that the calculations suggesting a string theory multiverse are subtly wrong.  On the other hand, as witnessed by the amount of work in this area, a lot of people think this use of the anthropic principle makes sense and seems to fall out of the mathematics of string theory.  The divide is partly, but not entirely, by age, with younger physicists a bit more in favor of anthropic arguments.  Full disclosure: I think anthropic arguments are perfectly legitimate and have worked on multiple parts of this story.  I'm also fairly sensitive to the technical questions raised by "anthropic opponents" and am perfectly willing to admit that there could be a subtle reason some of the calculations don't work the way they appear to. In any case, I think if you polled string theorists, you'd find a pretty significant split on whether the multiverse is a good/correct solution to the cosmological constant problem or not.  And that's where we stand on vacuum energy (leaving aside other models of dark energy).


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## fuindordm (Aug 17, 2015)

Thanks for the update!  I'm interested in hearing more about the string theory version of a multiverse.

As I recall, another possible multiverse model was 'eternal inflation', which only requires a vacuum field and not string theory. 

(Aside for the benefit of others) Inflation was a very short segment of time prior to what we usually think of as the Big Bang, when the universe expanded at an exponential rate rather than just a polynomial rate. This was a clever idea proposed by Alan Guth in 1981 which solves several problems of observational cosmology, such as "why is the universe flat" and "why is the cosmic microwave background the same temperature in every direction, even though wide angles were too far away from each other to come to thermal equilibrium when the background was created?" If you accept inflation, then the next natural question is "why did inflation stop", and the answer is that the high-energy field that causes inflation is still subject to random fluctuations in temperature, and eventually it cools enough to undergo a phase transition into a more "normal" set of particles and fields.

But why stop there? If we can imagine a pre-universe that was undergoing exponential inflation, it makes just as much sense to imagine that only small "bubbles" of the field undergo the phase transition to normal space.  So the eternal inflation model is one where a real infinitude of "space" is filled with this inflationary field, and is eternally expanding at an exponential rate, but small regions of it are eternally cooling down through random fluctuations into regimes where field transitions to normal space. Each bubble undergoes its own Big Bang, but since a Big Bang only expands at a polynomial rate, the new universe never takes over (or even catches up with) the inflationary field that spawned it. Viewed from the expanding reference frame of the inflationary field, these bubbles quickly shrink down to nothing. Unfortunately, it's a multiverse model that really offers no hope of ever meeting our evil twins. 

I don't think this model requires string theory, but it is similar enough in outline that we might be talking about the same thing.  It is certainly a model that supports the weak anthropic principal, and I agree with you that the principal is reasonable provided we can make a strong case for a multiverse.

So is this the kind of multiverse that string theory supports? There are also cyclical multiverses (I vaguely remember a scale inversion principal of string theory, which implied that a universe that expands too far is equivalent to a universe that had shrunk too small), dimensional multiverses (different branes?).


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## Umbran (Aug 18, 2015)

fuindordm said:


> Inflation was a very short segment of time prior to what we usually think of as the Big Bang




There is no spacetime to inflate prior to the Big Bang.

Everything else works, though - inflation happens after the BB (from 10^−36 seconds after the Big Bang to sometime between 10^−33 and 10^−32 seconds), and there's a phase transition... but not everywhere.  If the phase transition happens in bubbles, you get islands of what we consider normal space, separated by *vast* stretches of nothingness.

Though, with an infinite space, you don't really need that.  In an expanding universe, you have bubbles of visible universe that'll never communicate anyway.


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## freyar (Aug 18, 2015)

More on the multiverse later, but I quickly wanted to clear up a confusing bit of terminology.



fuindordm said:


> Inflation was a very short segment of time prior to what we usually think of as the Big Bang






Umbran said:


> There is no spacetime to inflate prior to the Big Bang.




There are actually two slightly different meanings of "Big Bang" in physics, which are often not carefully disambiguated (to borrow a term from wikipedia).  

One common one, which Umbran seems to be using, might be specified as the "Big Bang Singularity," meaning the beginning of space and time.  There is literally no time before that.  This is a singularity in general relativity but might be smoothed out in some theory of quantum gravity, like string theory.  There is some but not a lot of research on fixing the Big Bang Singularity in string theory.  I've done a little bit of work related to that.

The second common definition, which I think fuindordm is using, is often called the "Hot Big Bang."  This is the period in the early universe (*after* the period of inflation fuindordm talked about) when there was a dense hot plasma of stuff.  This is, for example, when the different isotopes of the light elements are formed (this is called Big Bang Nucleosynthesis).  

Anyway, I hear and use "Big Bang" both ways, and it's an understandable confusion.

More of a substantial answer either tonight or tomorrow on the multiverse.


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## Umbran (Aug 18, 2015)

freyar said:


> The second common definition, which I think fuindordm is using, is often called the "Hot Big Bang."




Yeah.  I never liked this terminology, *because* of the ambiguity it creates.  It confuses laymen.


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## fuindordm (Aug 18, 2015)

You're right, I was referring to the beginning of the hot dense phase, because anything prior to that doesn't have direct observational support. The singularity is an extrapolation, and most astrophysical cosmologists (as I was) do not include it in polite conversation.


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## freyar (Aug 19, 2015)

Umbran said:


> Yeah.  I never liked this terminology, *because* of the ambiguity it creates.  It confuses laymen.






fuindordm said:


> You're right, I was referring to the beginning of the hot dense phase, because anything prior to that doesn't have direct observational support. The singularity is an extrapolation, and most astrophysical cosmologists (as I was) do not include it in polite conversation.




Well, the problem is that the phrase was never defined precisely in the first place and was in fact coined as a derogatory for the idea by Fred Hoyle, who believed in steady state cosmology.  The real issue is that, until the '80s or '90s, inflation wasn't part of the standard cosmological picture.  So the Big Bang Singularity was immediately followed by the Hot Big Bang phase.  It was all mushed together, and both uses stuck.  And, as fuindordm notes, the singularity isn't really something we even really know is there.


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## freyar (Aug 19, 2015)

Back before that sidetrack on nomenclature, we were talking about the multiverse and such...



fuindordm said:


> Thanks for the update!  I'm interested in hearing more about the string theory version of a multiverse.
> 
> As I recall, another possible multiverse model was 'eternal inflation', which only requires a vacuum field and not string theory.
> 
> ...




We can start with another nomenclature issue, I guess.  There are some people who use "multiverse" to refer to different types of particles living on different branes in different parts of the extra dimensions, but that doesn't fit the usual definition of multiverse because these different particles interact with each other gravitationally.  While you're also right that there can be cyclical models of cosmology, I'm not aware of one that really qualifies as a multiverse.  The typical definition of multiverse is a "large" universe where there are different (separated) regions with *different physical laws* (ie, different field/particle content, though basic principles of quantum mechanics still apply everywhere), large parts of which are out of (causal) contact with each other.

In any case, the "string theory multiverse" I talked about before is of this eternal inflation type that fuindordm describes.  What he describes is a multiverse/universe where most of space keeps expanding exponentially fast with "pockets" of normal expansion, but all those pockets have the same physics, in particular the same vacuum energy.  If we want to explain the small value of the vacuum energy, though, we need pockets of normal expansion with different physics than each other.  In other words, the overarching theory needs (very very very) many states with different values of vacuum energy in each one.  You certainly don't *need* string theory to have this structure --- this is a complicated but reasonable model just to make up from scratch --- but any model you do make up should eventually come from some theory of quantum gravity.  The fact that string theory, which is a theory of quantum gravity (and everything else), apparently does give you just this structure of many states of varying vacuum energies is a strong motivation and is what led to the recent interest in these multiverse/anthropic models.

There is one other small difference compared to what fuindordm describes.  In modern/stringy multiverse models, the transitions are not just from inflationary expansion to normal expansion, but mostly between regions with inflation at different rates.  Of course, there are additional technical details --- for example, in this type of model, our pocket of the universe is created as a bubble in the eternal inflation that then undergoes a more standard type of inflation that heats up.  But now, we see that the expansion of space in our region is starting to accelerate, so we might be entering another phase of inflation (just a very slow one).  So we also experienced a transition from inflation to inflation, just with some more complicated intermediate stage in between.


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## rknop (Aug 20, 2015)

Re: the name "Big  Bang", here is an article I wrote nearly a decade ago back when I was an active blogger about why it's a bad name for the theory.  Here, I'm talking about what freyar calls the "Hot Big Bang".  I come from an observer's bias, so that's what I tend to think of as "The" Big Bang nowadays, rather than the initial singularity that comes out of General Relativity (while ignoring quantum mechanics).


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## Scott DeWar (Aug 22, 2015)

What would happen if you shot a gun in space?


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## Scott DeWar (Aug 22, 2015)

For every particle, there is supposedly an anti particle. So where is all of the anti-matter and why isn't matter obliterated by the anti matter?


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## Umbran (Aug 23, 2015)

Scott DeWar said:


> What would happen if you shot a gun in space?




Okay, so let us assume you're floating out there in a space suit.   Let us also assume you're using a pretty modern firearm.  First off, yes, the gun will fire - cartridges these days are loaded with a charge that contains its own oxidant, so it doesn't need to be fired in air to burn.

So, you have gun, you pull the trigger.  A small explosive goes off in the chamber.  A bullet flies... and so do you.  This is "conservation of momentum".  A "standard human" is about 65 kilograms.  A bullet from a .45 caliber round is about 13 grams.  If my quick math is right, if the bullet takes off at a speed of 260 m/s, you take off at a speed of 0.05 m/s - about 2 inches per second.

If you are holding the gun up at shoulder level, you'll begin to spin as well, as the force of recoil is away from your center of mass.  

The bullet will go on i a straight line until it runs into something.


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## Scott DeWar (Aug 23, 2015)

And so the shooter will just spin on and drift at .05 m/s in the opposite direction from equal force. I was actually not sure of the bullet working in space, but yu have confirmed the chemical needs.


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## Umbran (Aug 23, 2015)

Scott DeWar said:


> For every particle, there is supposedly an anti particle. So where is all of the anti-matter and why isn't matter obliterated by the anti matter?




To our first best guess, the Big Bang should have created just as much matter as antimatter - particles are produced in matter-antimatter pairs.  So, yes, we should see a lot of antimatter around, but don't.  Why?

The answer is:  We don't know yet.   There are a few hypothesis.  

One is that, back in the very early universe, when matter and anti-matter should have been made in equal amounts, there was some asymmetry we don't currently see that led to more matter than anti-matter being created.  Then, there was a frenzy of matter and anti-matter annihilating, leaving us with the slight excess of matter we see now.

Another is that there's a *slight* difference in the decay rates of matter and anti-matter.  If anti-matter decayed a bit more quickly than matter, we might then see an excess of matter today.

A third is that the Big Bang did create equal amounts of matter and anti-matter, but they are in regions widely separated - this leaves us with a problem of why stuff was created in uneven bubbles, instead of evenly.  Part of that may be answered by the Anthropic Principle - in order for us to see a universe, it must be a universe we can live in.  If the universe had just as much of both, they'd all annihilate, and leave nothing for us to be made out of!  So, while move of the entire universe may have been created with a pretty even distribution, maybe there was at least one statistical anomaly (in an infinite universe, there *will* be some anomalies) where the matter and antimatter were created slightly separated, and we live in one.  If we didn't, we wouldn't live at all....


https://en.wikipedia.org/wiki/Baryon_asymmetry


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## freyar (Aug 23, 2015)

Scott DeWar said:


> What would happen if you shot a gun in space?




Well, there's something in common between firing a gun in space and on earth: the bullet (and "exhaust" or whatever you call the byproducts of the exploding gunpowder) would come out of the muzzle in a straight line, and you'd feel the recoil and start moving in a straight line in the opposite direction but with equal momentum to the bullet.  That's all that happens in space.

On earth, quite a few other things happen. First off, the bullet falls toward the ground, so it doesn't really quite move in a straight line after it leaves the gun.  Air resistance also slows the bullet down, so it doesn't move quite as fast as it would in space.  And that exhaust (and potentially turbulence of the bullet in the air) will create sound.  If the bullet is moving fast enough, it will make a sonic boom. And, of course, you still feel the recoil, but unless you're on wheels, chances are that your friction with the earth means that the recoil affects you and the earth together; since the earth is so massive, the recoil speed is effectively zero.  And then, after the flight of the bullet, all the bullet's momentum gets transferred to the earth and atmosphere, so the recoil stops.

So actually, firing the gun on earth is a lot more complicated than in space, I think.

EDIT: also, what Umbran said.


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## GMMichael (Aug 23, 2015)

Umbran said:


> there was some asymmetry we don't currently see that led to more matter than anti-matter being created. . .
> while move of the entire universe may have been created with a pretty even distribution, maybe there was at least one statistical anomaly (in an infinite universe, there *will* be some anomalies) where the matter and antimatter were created slightly separated, and we live in one.  If we didn't, we wouldn't live at all....
> https://en.wikipedia.org/wiki/Baryon_asymmetry



Well, this more-or-less answers my "why isn't the universe composed only of perfectly symmetrical spheres" question.  It sounds like a pretty important physics question though: if the universe had/has an asymmetrical anomaly, what caused that anomaly???

I'd like to answer myself, and blame it on the quantum field, which was basically subatomic chaos, pure randomness, if my last reading was interpreted correctly.  So...

1) How's quantum field theory coming along, and is the field(s) as chaotic as I've understood?
2) Could the quantum field exist before/during the big bang, and explain why the universe isn't spherically symmetrical?

and just for fun...

3) I've read that time isn't just some simple measurement that keeps going steadily forward.  Does Time show up as a variable in current physics math, and how does it vary from the Time that we plug into high school math?  Or getting-to-work-on-time math?


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## Umbran (Aug 23, 2015)

DMMike said:


> Well, this more-or-less answers my "why isn't the universe composed only of perfectly symmetrical spheres" question.  It sounds like a pretty important physics question though: if the universe had/has an asymmetrical anomaly, what caused that anomaly???
> 
> 2) Could the quantum field exist before/during the big bang, and explain why the universe isn't spherically symmetrical?




I wasn't speaking of a spacial asymmetry, but a lack of symmetry in physical laws - like matter and antimatter were not exactly opposite in all ways. 

But, the spherical symmetry question raises a point. It assumes that space is finite, and has an origin, a middle. This is probably not correct. If space is infinite, it cannot have a center.

Space is still spherically symmetric, if you choose yourself as the center, mostly. But only in a sort of statistical sense,  on the large scale.


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## Scott DeWar (Aug 23, 2015)

Re: Matter/anti-matter question

All very interesting theories, but I had to laugh at the blunt honesty of 


> The answer is: We don't know yet.
> 
> Read more: http://www.enworld.org/forum/showthread.php?466337-ask-a-physicist/page3#ixzz3jgDWsIde


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## freyar (Aug 24, 2015)

Scott DeWar said:


> For every particle, there is supposedly an anti particle. So where is all of the anti-matter and why isn't matter obliterated by the anti matter?




Umbran does a pretty good job breaking down the logical possibilities.  I'd like to elaborate, though, especially because that wikipedia article is a bit disappointing compared to most of the physics wikipedia articles.

The most important point to remember is that we really don't know the answer to this question.  It's also quite likely we won't for a long time, since there's very little way to test any of the possibilities --- there are some potentially related effects we can test experimentally, but those are really only good at ruling out options as opposed to pointing toward the correctness of one.



Umbran said:


> To our first best guess, the Big Bang should have created just as much matter as antimatter - particles are produced in matter-antimatter pairs.  So, yes, we should see a lot of antimatter around, but don't.  Why?
> 
> The answer is:  We don't know yet.   There are a few hypothesis.
> 
> One is that, back in the very early universe, when matter and anti-matter should have been made in equal amounts, there was some asymmetry we don't currently see that led to more matter than anti-matter being created.  Then, there was a frenzy of matter and anti-matter annihilating, leaving us with the slight excess of matter we see now.




This is what people think happens for the most part (I'll explain why below), though how this happens is completely up in the air.  What we do know is that there are several criteria that have to be met (called the Sakharov conditions) and that they are *not* satisfied (enough) by the Standard Model of particle physics.  In fact, the Standard Model can almost cause a predominance of matter over antimatter, but it would require a certain behavior of the Higgs boson field in the early universe --- and in the Standard Model, the Higgs doesn't behave that way.  To get it to work, you have to add other ingredients to change the behavior of the Higgs.

Of course, there are other possibilities.  There could be an entirely new, undiscovered group of particles that are responsible for the excess of our matter. The roots of the excess could happen during inflation.  Or another possibility is that dark matter is also somehow unbalanced between matter and antimatter, and that imbalance gets generated at the same time as the imbalance of normal matter.  There are many different theories.

I should also mention that the excess amount of matter is *very* tiny.  In the early universe, the amount of matter and antimatter was essentially equal.  For roughly every 10 billion matter/antimatter pairs of particles, there was one extra matter particle.  Then all the 10 billion or so pairs annihilated each other away, leaving behind the one matter particle.




> Another is that there's a *slight* difference in the decay rates of matter and anti-matter.  If anti-matter decayed a bit more quickly than matter, we might then see an excess of matter today.




There's a way this is correct and a way it's not correct, so I want to be very careful here.

If we're talking about the decay rates of "everyday" particles and their antiparticles (like protons/antiprotons, neutrons/antineutrons, and electrons/positrons), this is a logical possibility that just doesn't work out.  Based on cosmic ray measurements, the lifetime of the antiproton is at least a million years, which wouldn't leave enough of an imbalance between protons/antiprotons.  Furthermore, there's a mathematical theorem in particle physics that says the total decay rates of particles and their antiparticles must be the same, and any decay of a proton/antiproton generates the same amount of matter/antimatter.  It doesn't work (incidentally, a violation of this theorem would be a super-big deal, meaning we'd have to redo basically all of subatomic physics).

On the other hand, it is possible that some very heavy undiscovered particle and antiparticle decay differently into matter and antimatter.  That can happen and can create the imbalance we need.  Of course, we've not discovered such a particle yet.



> A third is that the Big Bang did create equal amounts of matter and anti-matter, but they are in regions widely separated - this leaves us with a problem of why stuff was created in uneven bubbles, instead of evenly.  Part of that may be answered by the Anthropic Principle - in order for us to see a universe, it must be a universe we can live in.  If the universe had just as much of both, they'd all annihilate, and leave nothing for us to be made out of!  So, while move of the entire universe may have been created with a pretty even distribution, maybe there was at least one statistical anomaly (in an infinite universe, there *will* be some anomalies) where the matter and antimatter were created slightly separated, and we live in one.  If we didn't, we wouldn't live at all....




This is an interesting idea, though I confess I've not seen any work related to it.  I also can't immediately think of a way to implement it, either, since, if it's just a "local" fluctuation, it would have to be created during inflation.  Anyway, suffice it to say that I'm not sure if there's a way to get this to work off the top of my head.


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## freyar (Aug 24, 2015)

Scott DeWar said:


> Re: Matter/anti-matter question
> 
> All very interesting theories, but I had to laugh at the blunt honesty of




It really is one of the big questions!


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## Umbran (Aug 24, 2015)

freyar said:


> On the other hand, it is possible that some very heavy undiscovered particle and antiparticle decay differently into matter and antimatter.  That can happen and can create the imbalance we need.  Of course, we've not discovered such a particle yet.




Yes.  I left this nuance off - the imbalance would arise in the time when very heavy and energetic particles were far more common - and one of those super-heavy particles could be the culprit that led to the very small imbalance in matter.



> This is an interesting idea, though I confess I've not seen any work related to it.  I also can't immediately think of a way to implement it, either, since, if it's just a "local" fluctuation, it would have to be created during inflation.  Anyway, suffice it to say that I'm not sure if there's a way to get this to work off the top of my head.




The most intriguing idea I saw actually linked it to the inflation-multiverse concept discussed above.  That regions of the universe fell out of the inflation mode, and into regions of universe we think of with "normal" rate of expansion *because* they had developed a predominance of matter or antimatter, by mere statistical variation.  Regions where matter and antimatter were balanced continued in full inflation mode, giving rise to isolated "bubble" universes of one form of matter or the other, in a sea of vast inflationary universe.

I no longer have the math supporting this suggestion on hand.  I'll see if I can find a reference.


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## freyar (Aug 24, 2015)

Umbran said:


> The most intriguing idea I saw actually linked it to the inflation-multiverse concept discussed above.  That regions of the universe fell out of the inflation mode, and into regions of universe we think of with "normal" rate of expansion *because* they had developed a predominance of matter or antimatter, by mere statistical variation.  Regions where matter and antimatter were balanced continued in full inflation mode, giving rise to isolated "bubble" universes of one form of matter or the other, in a sea of vast inflationary universe.
> 
> I no longer have the math supporting this suggestion on hand.  I'll see if I can find a reference.



If you can, that would be great.  I've seen some possibilities including inflation but nothing credible that sounds quite like what you say or that uses the anthropic principle in a significant manner.


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## freyar (Aug 24, 2015)

DMMike said:


> Well, this more-or-less answers my "why isn't the universe composed only of perfectly symmetrical spheres" question.  It sounds like a pretty important physics question though: if the universe had/has an asymmetrical anomaly, what caused that anomaly???
> 
> I'd like to answer myself, and blame it on the quantum field, which was basically subatomic chaos, pure randomness, if my last reading was interpreted correctly.  So...
> 
> ...




Umbran has already addressed the spherical symmetry issues quite well, so I'll just talk about the quantum field theory issue a bit.

One thing I'd like to make clear is that there's not just one "quantum field."  Quantum field theory is the name for a framework in physics.  This is like how quantum mechanics isn't a single theory (like the theory of the hydrogen atom is) but rather a set of rules that should apply to theories of physics.  So when we say that the Standard Model of particle physics is "a" quantum field theory, we mean that it obeys the rules and framework of quantum field theory with a particular set of fields and interactions --- each field corresponds to a type of particle.  In any case, this framework is going along very well, thank you.   I mean that it's a well-developed framework with lots of very precise results and quite a few Nobel prizes.  But most of the developments have been for theories where the interactions between fields are weak, so there is a lot of work right now in trying to understand principles that apply to quantum field theories with strong interactions.

It is true that, as in quantum mechanics (which quantum field theory is part of), the results of measurements are probabilistic.  And, in quantum field theories, that applies even to measurements of empty space.  So I don't know if I'd actually use the word "chaotic," but it is true that there's "quantum activity" even in empty space (virtual particles, etc).  And we definitely don't understand it --- there is a quantum contribution to energy, which should show up as the energy of empty space (the vacuum).  I've already talked about that in one of the answers here, but the thing is we certainly don't understand the value of that (though there are ideas).

Quantum fields (or maybe some quantum gravitational generalization) certainly existed back to the Big Bang (Singularity), except that in the true theory of quantum gravity, we don't expect there is actually a singularity at the Big Bang.

If you're interested in this stuff, I recommend the blog "Of Particular Significance" by Matthew Strassler.  He has a number of articles up on "what is quantum field theory," including a nice introduction to fields and particles.

I think I'll come back to your question on time tomorrow.


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## Scott DeWar (Aug 24, 2015)

I am going to fork this as I seem to have a few more questions


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## freyar (Aug 26, 2015)

DMMike said:


> 3) I've read that time isn't just some simple measurement that keeps going steadily forward.  Does Time show up as a variable in current physics math, and how does it vary from the Time that we plug into high school math?  Or getting-to-work-on-time math?




Well, getting to work on time and the time you'd use in high school physics are pretty well the same.  The way we think of time in our daily lives and the way physicists did until Einstein is as an absolute quantity that passes at a fixed rate.  But Einstein changed that.  First, the theory of special relativity showed that time passes at different rates for objects moving with respect to each other.  Generally, we don't move very fast, so this doesn't impact us much.  But if, say in the future, future you wanted to do your daily commute from earth to mars in half an hour, you'd need to move at a significant fraction of the speed of light, and the amount of time that passes in the clock on your space ship would be noticeably different than the time that passes on a clock that stays on earth while you make your round trip.  Then, the theory of general relativity says time is affected also by gravity.  So, for example, for every one second measured by a clock on earth, a clock in space ticks a bit more than one second.  The extreme example is that, while a clock on the event horizon of a black hole ticks one second, an infinite amount of time passes on a clock far away from the black hole.

This does have some practical impact.  If you have a smart phone, it most likely has a GPS unit, which you might use to track your location.  GPS works by figuring distances from a set of satellites orbiting earth, and it figures distances by finding the length of time a radio signal takes to travel from the satellites to your phone.  That takes very accurate time-keeping, and it actually has to be accurate enough to notice both of those "time dilation" effects I just mentioned.

This "flexibility" in the behavior of time is a very important and fundamental part of gravitational physics.  It also causes some complications in our calculations, though the details are a bit complex.

Of course, there are lots of other speculative theoretical ideas about the role of time in a quantum theory of gravity.  A popular thought is that time (and space) is not really a fundamental concept but emerges from something else.  So maybe time is just an approximate notion altogether.


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## Scott DeWar (Aug 26, 2015)

freyar said:


> The way we think of time . . .passes at a fixed rate.  But Einstein changed that.  First, the theory of special relativity showed that time passes at different rates for objects moving with respect to each other.  . . . .  Then, the theory of general relativity says time is affected also by gravity.  So, for example, for every one second measured by a clock on earth, a clock in space ticks a bit more than one second.  The extreme example is that, while a clock on the event horizon of a black hole ticks one second, an infinite amount of time passes on a clock far away from the black hole.
> 
> This does have some practical impact.  If you have a smart phone, it most likely has a GPS unit, which you might use to track your location.  GPS works by figuring distances from a set of satellites orbiting earth, and it figures distances by finding the length of time a radio signal takes to travel from the satellites to your phone.  That takes very accurate time-keeping, and it actually has to be accurate enough to notice both of those "time dilation" effects I just mentioned.
> 
> ...




first, edits are for saving tie and space. [heh heh heh]

second, regarding the effects of gravity on tme

     are you suggesting that perhaps the very gravity of 'big blue' earth can effectually alter the time it takes for a signal to traverse in a round trip, moving a  velocity of 'C' from a gps to a satellite and back to the gps?  if so, then would it be possible, but not necessarily plausible, that gravity has an affect on RF signals of any frequency? Is there a way to test this, olor has it already been tested?


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## Scott DeWar (Aug 26, 2015)

freyar said:


> The way we think of time . . .passes at a fixed rate.  But Einstein changed that.  First, the theory of special relativity showed that time passes at different rates for objects moving with respect to each other.  . . . .  Then, the theory of general relativity says time is affected also by gravity.  So, for example, for every one second measured by a clock on earth, a clock in space ticks a bit more than one second.  The extreme example is that, while a clock on the event horizon of a black hole ticks one second, an infinite amount of time passes on a clock far away from the black hole.
> 
> This does have some practical impact.  If you have a smart phone, it most likely has a GPS unit, which you might use to track your location.  GPS works by figuring distances from a set of satellites orbiting earth, and it figures distances by finding the length of time a radio signal takes to travel from the satellites to your phone.  That takes very accurate time-keeping, and it actually has to be accurate enough to notice both of those "time dilation" effects I just mentioned.
> 
> ...




first, edits are for saving tie and space. [heh heh heh]

second, regarding the effects of gravity on tme

     are you suggesting that perhaps the very gravity of 'big blue' earth can effectually alter the time it takes for a signal to traverse in a round trip, moving a  velocity of 'C' from a gps to a satellite and back to the gps?  if so, then would it be possible, but not necessarily plausible, that gravity has an affect on RF signals of any frequency? Is there a way to test this, or has it already been tested?


----------



## Scott DeWar (Aug 26, 2015)

freyar said:


> The way we think of time . . .passes at a fixed rate.  But Einstein changed that.  First, the theory of special relativity showed that time passes at different rates for objects moving with respect to each other.  . . . .  Then, the theory of general relativity says time is affected also by gravity.  So, for example, for every one second measured by a clock on earth, a clock in space ticks a bit more than one second.  The extreme example is that, while a clock on the event horizon of a black hole ticks one second, an infinite amount of time passes on a clock far away from the black hole.  This does have some practical impact.  If you have a smart phone, it most likely has a GPS unit, which you might use to track your location.  GPS works by figuring distances from a set of satellites orbiting earth, and it figures distances by finding the length of time a radio signal takes to travel from the satellites to your phone.  That takes very accurate time-keeping, and it actually has to be accurate enough to notice both of those "time dilation" effects I just mentioned.  This "flexibility" in the behavior of time is a very important and fundamental part of gravitational physics.  It also causes some complications in our calculations, though the details are a bit complex.  Of course, there are lots of other speculative theoretical ideas about the role of time in a quantum theory of gravity.  A popular thought is that time (and space) is not really a fundamental concept but emerges from something else.  So maybe time is just an approximate notion altogether.




  first, edits are for saving time and space. [heh heh heh]  

second, regarding the effects of gravity on time       are you suggesting that perhaps the very gravity of 'big blue' earth can effectually alter the time it takes for a signal to traverse in a round trip, moving a  velocity of 'C' from a gps to a satellite and back to the gps? 

 if so, then would it be possible, but not necessarily plausible, that gravity has an affect on RF signals of any frequency? Is there a way to test this, or has it already been tested?


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## AbdulAlhazred (Aug 26, 2015)

Scott DeWar said:


> are you suggesting that perhaps the very gravity of 'big blue' earth can effectually alter the time it takes for a signal to traverse in a round trip, moving a  velocity of 'C' from a gps to a satellite and back to the gps?  if so, then would it be possible, but not necessarily plausible, that gravity has an affect on RF signals of any frequency? Is there a way to test this, olor has it already been tested?




Depends on what you mean by 'alter the time', from every frame of reference that photon always moves at the speed of light. However, a photon emitted by a GPS satellite will cover a different amount of distance and take a different amount of time to arrive at its destination on the surface from the viewpoint of the satellite and from that of the ground-based receiver. 

Gravity does have an effect on ALL RF signals, it doppler-shifts them. A photon coming down to Earth will, relativistically, be received by a receiver with a slower clock, making its frequency appear higher, so it will be blue shifted. A classical interpretation would be that the photon gained energy, but classical pre-relativistic mechanics doesn't really have a way to deal with the fixed speed of light...


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## Scott DeWar (Aug 26, 2015)

AbdulAlhazred said:


> Depends on what you mean by 'alter the time', from every frame of reference that photon always moves at the speed of light. However, a photon emitted by a GPS satellite will cover a different amount of distance and take a different amount of time to arrive at its destination on the surface from the viewpoint of the satellite and from that of the ground-based receiver.
> 
> Gravity does have an effect on ALL RF signals, it doppler-shifts them. A photon coming down to Earth will, relativistically, be received by a receiver with a slower clock, making its frequency appear higher, so it will be blue shifted. A classical interpretation would be that the photon gained energy, but classical ore-relativistic mechanics doesn't really have a way to deal with the fixed speed of light...




as for Doppler shifts, I know of Doppler radar shifting for radio navigation of aircraft, I guess it works for gravity force as easily as magnetic.


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## freyar (Aug 27, 2015)

Well, that's quite a few duplicate posts! No saving time or space there... 



Scott DeWar said:


> second, regarding the effects of gravity on time       are you suggesting that perhaps the very gravity of 'big blue' earth can effectually alter the time it takes for a signal to traverse in a round trip, moving a  velocity of 'C' from a gps to a satellite and back to the gps?
> 
> if so, then would it be possible, but not necessarily plausible, that gravity has an affect on RF signals of any frequency? Is there a way to test this, or has it already been tested?






Scott DeWar said:


> as for Doppler shifts, I know of Doppler radar shifting for radio navigation of aircraft, I guess it works for gravity force as easily as magnetic.




There are two different effects at work.  The first, which is what I mentioned before, is about gravitational time dilation.  It's really pretty simple though counter-intuitive: suppose you take two precise atomic clocks (or whatever kind of perfect clock you want to imagine).  You make sure in a lab that they are running at the same rate when they are sitting side by side.  Then you put one on the surface of the earth and one on a rocket that holds position above the earth (ie, not moving compared to the earth).  While the one on the ground ticks off one second, the one in the rocket will tick off more time according to general relativity.  If you put the clock in an orbiting satellite, you also have to worry about special relativity since the earth and satellite are moving relative to each other, so that would make the clock in the satellite tick off less time. The special relativity and general relativity effects don't quite cancel.  The way it effects GPS is that the GPS satellite signal is basically broadcasting the satellite's time, so your phone can compare to its own time and figure out how far the signal travelled.  If GPS did not account for how relativity affects the running of time, it just wouldn't work.  So this is not only tested but used all over the world every day.  But just to be clear, this isn't altering the time for the signal to travel a certain distance, it's really altering how time flows in different places.

The other thing you stumbled on, the change of frequency, as AbduAlhazred says, is the gravitational Doppler shift, which is also due to this time dilation effect but can be thought of as the photon gaining kinetic energy as it falls.  This has been measured, first by Robert Pound and Glen Rebka, who "dropped" a photon off the Harvard University physics building.  It's actually quite a clever experiment, but I won't get into the details or else this post will go on for too long.  But note that the normal Doppler shift is due to the relative speed of emitter and receiver while this is again due to location in a gravitational field.


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## Scott DeWar (Aug 27, 2015)

freyar said:


> There are two different effects at work.  The first, which is what I mentioned before, is about gravitational time dilation.  It's really pretty simple though counter-intuitive: suppose you take two precise atomic clocks (or whatever kind of perfect clock you want to imagine).  You make sure in a lab that they are running at the same rate when they are sitting side by side.  Then you put one on the surface of the earth and one on a rocket that holds position above the earth (ie, not moving compared to the earth).  While the one on the ground ticks off one second, the one in the rocket will tick off more time according to general relativity.  If you put the clock in an orbiting satellite, you also have to worry about special relativity since the earth and satellite are moving relative to each other, so that would make the clock in the satellite tick off less time. The special relativity and general relativity effects don't quite cancel.  The way it effects GPS is that the GPS satellite signal is basically broadcasting the satellite's time, so your phone can compare to its own time and figure out how far the signal traveled.  If GPS did not account for how relativity affects the running of time, it just wouldn't work.  So this is not only tested but used all over the world every day.  But just to be clear, this isn't altering the time for the signal to travel a certain distance, it's really altering how time flows in different places.




Could you explain further on this, as in Why does the time in a satellite or rocket differ from time on earth? Would time b3 different on Mars? I a guessing yes, and on the moon as well.

What happens? Does time bend in different places? To quote #5,"Need more input!" [movie: short circuit]


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## AbdulAlhazred (Aug 27, 2015)

Scott DeWar said:


> Could you explain further on this, as in Why does the time in a satellite or rocket differ from time on earth? Would time b3 different on Mars? I a guessing yes, and on the moon as well.
> 
> What happens? Does time bend in different places? To quote #5,"Need more input!" [movie: short circuit]




You can understand it in terms of Special Relativity like this: When you stand on the surface of the Earth its indistinguishable from being on an accelerating rocket ship. You can easily equate acceleration and time dilation in SR (I will leave it as an exercise for you, but most intros to SR will present it fairly succinctly). Its effectively the same thing, you'd experience the same time dilation on the surface of the Earth, or on a rocket ship accelerating at 1G, relative to an observer who's not accelerating.


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## Umbran (Aug 27, 2015)

AbdulAlhazred said:


> You can understand it in terms of Special Relativity like this




You can, but I wouldn't advise it.  While you can deal with accelerating reference frames in special relativity, it is a hassle.  And, in fact, the better and more accurate answer comes from general relativity anyway.


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## Umbran (Aug 28, 2015)

Scott DeWar said:


> Could you explain further on this, as in Why does the time in a satellite or rocket differ from time on earth?




We know that it does.  The effect is measurable, and we have a model that can calculate how it changes, to high degrees of accuracy.  We can describe, as a sort of physical interpretation of that model, what is happening.

But *why*?  I don't think we can actually answer that question yet.  To answer a why, we must understand the universe one level down from the thing we are describing.  Perhaps a string theory, or whatever quantum gravity we come up with (if we ever come up with one) may answer the why.  For now, we can more talk about what happens...



> Would time b3 different on Mars? I a guessing yes, and on the moon as well.




Yes.  



> What happens? Does time bend in different places? To quote #5,"Need more input!" [movie: short circuit]




#5 is alive!

Special Relativity, and by extension, General Relativity, are the inevitable fallout of one simple basic observation - to all observers, everywhere, light travels at the same speed.

Say I am sitting on a train that is moving at 50 mph.  Say I pick up a baseball, and throw it to the front of the car.  And I measure the throw to be going at 50 MPH, relative to me and the car.

My friend, sitting beside the track as the car and I go by, would say the ball was moving at 100 mph, relative to her and the ground outside.  for her, the ball was moving at 50 MPH before I even threw it, after all, so my arm only adds to that speed.

That's not how light works.  If I fired a laser toward the front of the car, I'd measure it at 186K miles per second, and my friend would measure it going the *same* speed.  The speed of the train doesn't add to the speed of the light.

How is that possible?  How can we both see the same speed?  There's only one way to do that.  Speed is the distance something covers over some period of time.  If the speed is immutable, then distance and time must not be!  Time and space with bend, fold, and mutilate such that everyone, everywhere, sees light moving at the same speed.  Time and space measurements are not absolute and objective for all, but are *relative* to the observer - thus "Relativity".

So, one answer to "Why?" is "Because light moves at the same speed for all observers."   But, why does light travel at the same sped for all observers?  Um, well, it just does!  

(And people tell us quantum mechanics is weird.  Piffle!)

Anyway.  So, Special Relativity is called that because it is the special case - describing the alteration of time and space in frames that are not accelerating with respect to each other.  They can be in motion - like my friend says she's sitting still on the ground, and I'm moving by on the train, but they aren't accelerating.

General Relativity is the general case, more complicated, that also handles frames that are accelerating with respect to each other.

Newton tells us that Force = Mass * Acceleration.  Therefore, if you feel a force, you are under an acceleration.  So, if you feel your weight, you are under acceleration.  So, on the planet, you are under acceleration.  If the ground wasn't under you, you'd start to speed up going downwards, right?  Thus, General Relativity gives us the answers here.  Unfortunately, to really do General Relativity requires some really heavy math - tensor calculus - which I can't even write properly in plain text of these boards.  So, I'll go to being descriptive....

You, on the ground, and the satellite in orbit, are under different accelerations - you are accelerating more than the satellite is.  So, for both of you to see light move at the same constant speed, you must have different clocks and rulers.

Note that I mentioned clocks *and* rulers.  It isn't just that time bends - space bends too.  In relativity, space and time are dealt with in largely the same way, which is why you hear us refer to "spacetime", as a unit, inseparable.  We can think of a duration as merely a distance traveled through the time dimension, just as a separation between two points is just a distance though a space dimension.

So, you and the satellite are both accelerating.   If you compare clocks and rulers, they'll be different.  So, somewhere between you and them, space is bent.  It is bent because each of you feels the force of gravity.  The force of gravity is there because the Earth has mass.  Ergo, mass bends spacetime!    

Really big masses bend spacetime so much that it curves back in on itself - this is a black hole.  Things fall in, and never come out, because all the paths out are bent back around to be paths in!  Smaller masses bend spacetime less.  So, the clocks on Earth, on the Moon, and on Mars will all be different, as the masses of the bodies they are on are different.


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## fuindordm (Aug 28, 2015)

Umbran said:


> Newton tells us that Force = Mass * Acceleration.  Therefore, if you feel a force, you are under an acceleration.  So, if you feel your weight, you are under acceleration.  So, on the planet, you are under acceleration.  If the ground wasn't under you, you'd start to speed up going downwards, right?  Thus, General Relativity gives us the answers here.  Unfortunately, to really do General Relativity requires some really heavy math - tensor calculus - which I can't even write properly in plain text of these boards.  So, I'll go to being descriptive....




Just to add a gloss to Umbran's very nice answer:

Special Relativity can be derived from the assumption that light travels at the same speed in every reference frame.

General Relativity can be derived from the same assumption, plus a second assumption that inertial mass and gravitational mass are identical (or at least proportional).  That second assumption is called the Equivalence Principle, and it can be expressed in several other ways having to do with the nature of reference frames, but that's the simplest and my personal favorite. The tensor calculus required by general relativity stitches together lots of local, small-scale, and essentially flat coordinate systems into global coordinate system with curvature that satisfies both assumptions. It is rather analogous to modeling a curved surface as a patchwork of flat tiles, like what game consoles do to render 3D graphics.

The equivalence principle is one of my favorite mysteries to introduce to first-year physics students. They all learn F=ma and F=mg in short order, but there is nothing in Newtonian physics to explain why the same physical property plays two such different roles.  The first one resists a change in motion due to any external force; the second actually creates a force on other objects.  Lots of very clever experiments were done before and after Einstein to find out whether the two masses really had the same value in all cases.

Maybe string theory has something to say about that as well? I know it does some very clever things with gravity.

Ben


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## Umbran (Aug 28, 2015)

fuindordm said:


> The tensor calculus required by general relativity stitches together lots of local, small-scale, and essentially flat coordinate systems into global coordinate system with curvature that satisfies both assumptions. It is rather analogous to modeling a curved surface as a patchwork of flat tiles




I think it is important to note that they are an infinite number of infinitely small tiles!  As opposed to a few, or some large but finite number.


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## freyar (Aug 28, 2015)

Some very nice answers from Umbran and fuindordm here.  I can't say much as I'll be at a workshop all day today, but I'll try to comment on string theory tonight.


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## fuindordm (Aug 28, 2015)

Umbran said:


> I think it is important to note that they are an infinite number of infinitely small tiles!  As opposed to a few, or some large but finite number.




Indeed! Hence the calculus. The local reference frames are infinitesimals, and solving Einstein's equation for general relativity yields a large number of coefficients that fully describe the geometrical properties of the 4D space-time. I had to do this to trace photon trajectories around a black hole for a post-doc projects, and I can testify that the math is quite painful.


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## Umbran (Aug 28, 2015)

fuindordm said:


> I had to do this to trace photon trajectories around a black hole for a post-doc projects, and I can testify that the math is quite painful.




Yah.  I walked myself through the details of Hawking's "The Large Scale Structure of Space-time" (which is basically "A Brief History of Time" but with all the math) for fun.

But then, I read the Silmarillion for fun, too.


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## Scott DeWar (Aug 28, 2015)

Umbran said:


> But then, I read the Silmarillion for fun, too.



 there is a pill for that, you know. I now a few Psychiatrists to help you.


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## Scott DeWar (Aug 28, 2015)

Re: post 95

I am going to have to re-read that a couple of times.


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## Umbran (Aug 28, 2015)

Scott DeWar said:


> Re: post 95
> 
> I am going to have to re-read that a couple of times.




Because it is dense, because it is hard to wrap your head around, or because I wasn't writing very well?  I can try to fix the last, if that's the case.


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## Scott DeWar (Aug 28, 2015)

Umbran said:


> Because it is dense, because it is hard to wrap your head around, or because I wasn't writing very well?  I can try to fix the last, if that's the case.




it is dense. lots of new information for me to consider

*processing . . . . processing . . . . processing . . . . *


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## Scott DeWar (Aug 30, 2015)

A new question:  is this plausible? What makes it work??

https://www.youtube.com/watch?v=shkFDPI6kGE


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## Morrus (Aug 30, 2015)

Scott DeWar said:


> A new question:  is this plausible? What makes it work??




The fact that the title of it seems to think it makes Obama quake in his bed is entertaining!


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## Scott DeWar (Aug 30, 2015)

two different inks on a ceramic square. add a fuel source and it produces electricity. There is more to it, what would that be?


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## freyar (Aug 30, 2015)

Scott DeWar said:
			
		

> it is dense. lots of new information for me to consider
> 
> *processing . . . . processing . . . . processing . . . . *



There's also a shorter version of some of the same content in post 26.

Anyway, I was going to say something about string theory and the equivalence principle, that is, the idea that inertial mass and gravitational mass are the same.  So, as has already been discussed, this is a feature of any theory where gravity is really just the geometry of spacetime.  That includes general relativity, extensions of general relativity, and also string theory.  String theory is interesting since you can start with strings moving on a flat spacetime, but you end up with (an extension of) general relativity (in more dimensions). In that sense, the equivalence principle is "derived."

But it is also possible to find many discussions of violations of the equivalence principle in string theory and elsewhere. The context is that you get a lot more than gravity in string theory and really in most other theories beyond the Standard Model of particle physics.  These new types of particles can introduce new forces.  If one of these forces interacts differently with, for example, protons than neutrons, then different elements will experience this force differently, so we might mistakenly think that gravity acts differently on different materials. This is sometimes called a "violation of the equivalence principle."

The problem I have with that wording is that no one says that electromagnetic forces violate the equivalence principle. Or the strong or weak nuclear forces.  Or even the very short range Higgs force which also must exist.  That's because these forces aren't gravity.  Well, any other new force wouldn't be gravity either.  But I think this is one of those cases where there's a phrase that gets stuck in usage, even though it isn't really appropriate.  There are some others in physics I don't like much either.


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## freyar (Aug 30, 2015)

Scott DeWar said:


> A new question:  is this plausible? What makes it work??
> 
> https://www.youtube.com/watch?v=shkFDPI6kGE




I believe I've found some news articles about this thing, and it just seems to be a fuel cell. You can kind of think of fuel cells as batteries that require, well, fuel.  This is not really new technology, but I guess they claim to be able to make them smaller and cleaner.  I can't say whether they have or not, of course --- part of how "clean" or "green" a fuel cell is depends on the fuel you give it --- but I don't think there's anything in principle that says they can't.


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## Umbran (Aug 30, 2015)

Scott DeWar said:


> A new question:  is this plausible? What makes it work??




Yes, it is plausible.  It is a fuel cell, and the basic idea dates back to 1838 - https://en.wikipedia.org/wiki/Fuel_cell

Now, when they say early in that video that there's no burning - that's technically inaccurate.  The fuel cell takes some specific fuel, and combines it with oxygen to get energy.  What do you think "combine fuel and oxygen" is?  Oxidation, aka "burning".  A fuel cell does it without an open flame, mediated by a catalyst on a substrate that takes a spare electron out of the process and feeds it into a wire.

Now, note something - _you need fuel for this to work_.  In order to put one of those in your yard to power your house, you need it to be next to a fuel tank, or fed by a municipal fuel line, or something.  So, yes, you can remove power lines and electrical distribution grids, but then you need to have fuel distribution instead.  My house already gets natural gas, so I could install one of these in my basement.  But for rural areas that don't have gas service?  Carting around big bottles of flammable liquids to remote areas is not itself cheap, energy-wise. 

Also note that whether the energy is "clean" depends on the fuel used.  If you use pure hydrogen, then you get water and a bit of heat out - that's not bad, but pure hydrogen is actually pretty expensive.  If you use a fossil hydrocarbon, you get CO2 out, which is not so good.  If you use a biofuel (say, alcohol), then at least the carbon is coming from the biological carbon cycle - but typically you have to watch biofuels because their production processes are often not very clean.


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## Scott DeWar (Aug 30, 2015)

freyar said:


> I believe I've found some news articles about this thing, and it just seems to be a fuel cell. You can kind of think of fuel cells as batteries that require, well, fuel.  This is not really new technology, but I guess they claim to be able to make them smaller and cleaner.  I can't say whether they have or not, of course ---* part of how "clean" or "green" a fuel cell is depends on the fuel you give it -*-- but I don't think there's anything in principle that says they can't.






Umbran said:


> Yes, it is plausible.  It is a fuel cell, and the basic idea dates back to 1838 - https://en.wikipedia.org/wiki/Fuel_cell
> 
> Now, when they say early in that video that there's no burning - that's technically inaccurate.  The fuel cell takes some specific fuel, and combines it with oxygen to get energy.  What do you think "combine fuel and oxygen" is?  Oxidation, aka "burning".  A fuel cell does it without an open flame, mediated by a catalyst on a _substrate_(1) that takes a spare electron out of the process and feeds it into a wire.
> 
> ...




first of all, the *highlighted portions* were the source of my concern. My question of how green / clean this is considering the use of hydrocarbon fuels. what is the exchange rate of say cu ft of natural gas to watt vs natural gas to watt of a nat gas generator?

second of all my points of interst as numbered

(1) I am guessing the two "inks" mentioned are the catalysts, the ceramic of silicone sand the substrate

(2)rural areas often are fueled by liquid petroleum gas: propane. They are usually in taks of various sizes sitting on the same side as where the kitche sits and brought into the house with copper tubing.

rural Missouri is about 99 percent run by this gas for heating and cooking.

another couple of hydrocarbon fuels you see is found  on farms typically: agricultural diesel and ag gasoline.Chemically the same except for a green dye put in it. this fuels is not for road travel use as road taxes are not charged in this fuel and there are very heavy fines in finding the green die in  your car's tank as a very convincing deturrant.

(3) water: I have seen a few water to fuel conversions out there, bu none are in the market yet. Where are they. Why are they not in use??


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## Umbran (Aug 31, 2015)

Scott DeWar said:


> what is the exchange rate of say cu ft of natural gas to watt vs natural gas to watt of a nat gas generator?




If you mean, what is the efficiency of the Bloom Box as compared to the traditional generator of the same type?  I don't know.  That's not a matter of theory, such that I can ballpark a guess.  That's entirely dependent on the engineering details.

Something to note when considering the cost of a generation system - to honestly compare, you need to compare end-to-end.  For traditional generation, you have losses from power transmission through wires to your home.  However, there's a cost to distributing fuel in teh Bloom Box as well - say we are using those big propane tanks - you *drive a truck* to deliver it, so the gas used in that truck counts against the Bloom Box.



> (3) water: I have seen a few water to fuel conversions out there, bu none are in the market yet. Where are they. Why are they not in use??




The only straight water -> fuel I know of is taking water and splitting it into hydrogen and oxygen.  You can do this just by running a current through it - a normal high-school science experiment.  The basic problem is that it takes a lot of electricity to do this, so it isn't terribly cost-effective - you don't get more Hydrogen energy out than you put electrical energy in.  Another difficulty is that, when you take it to an industrial scale, impurities in the water matter.  Since the water bubbles off as H2 and O2, any impurities are left behind, and they tend to gunk up production - getting on the electrodes, making them less efficient, and so on.


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## freyar (Aug 31, 2015)

Yeah, Umbran said what I was going to say here.  Water is the opposite of what you want from a fuel: its atoms are tightly bound, so you have to work pretty hard to get anything out of it.


*An administrative note for the thread:* Fall term will be starting soon at my university, so that means teaching classes, attending committee meetings, and generally having more students around to ask questions, etc.  That's all in all a fine thing, but it does mean I'll have less time for posting here.  So, this is kind of a "last call" for new questions --- I'll try to answer everything that comes in by Weds or Thurs this week, but I can't promise to answer anything after that.  When I close the thread, I'll try to leave it so people can still comment, assuming I can figure out the administrative features.


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## Janx (Sep 17, 2015)

I just thought of a new question, based on seeing a science news article title about 2 black holes colliding with each other.

How "fast" is the speed of gravity?

This is a flexible question, I'm not talking about how fast an apple falls from a tree, that's a googleable formula.

In the case of those 2 black holes, they are far away.  What astronomers are watching now, happened a long time ago, and thus possibly even their collision and resulting changes in gravity (due to the new combined mass) has also already happened.

How long does it take for a new gravity shift to to be measured/or have impact at a set distance away?

Consider Star Trek: Generations where the crazy guy blew up stars in order to affect gravity to shift the travel path of the Nexus.  For movie purposes, that was pretty fast.


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## freyar (Sep 17, 2015)

Janx said:


> I just thought of a new question, based on seeing a science news article title about 2 black holes colliding with each other.
> 
> How "fast" is the speed of gravity?
> 
> ...




Since you're not talking about the acceleration of an object due to gravity (the apple falling), I gather you mean the speed of gravitational waves.  In general relativity or any similar theory of gravity, gravity waves travel at the speed of light in vacuum.  So when those black holes collide, the gravitational waves ("ringing of spacetime") travel outwards at the speed of light.  Incidentally, the Advanced LIGO (that's the Laser Interferometer Gravitational-Wave Observatory) is just coming on line this month and has a very good chance of detecting gravitational waves from colliding black holes within the next few years.  That would be the first direct measurement of gravitational waves.  We have indirect evidence from watching two pulsars which are orbiting each other; their orbit is decaying precisely according to the prediction of energy loss to gravity waves (and the observations have won a Nobel prize).

So the timing in ST: Generations is a bit fast.  It would really take a few minutes for light or gravitational waves to get from an exploding star to a planet at an earth-like distance.  That's not the only issue, either.  To create a gravitational wave, the explosion of the star couldn't be spherical but would have to be quite asymmetric.  It's been a while since I've watched the movie, but I don't think that explosion was really odd-looking enough.


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## Janx (Sep 17, 2015)

freyar said:


> Since you're not talking about the acceleration of an object due to gravity (the apple falling), I gather you mean the speed of gravitational waves.  In general relativity or any similar theory of gravity, gravity waves travel at the speed of light in vacuum.  So when those black holes collide, the gravitational waves ("ringing of spacetime") travel outwards at the speed of light.  Incidentally, the Advanced LIGO (that's the Laser Interferometer Gravitational-Wave Observatory) is just coming on line this month and has a very good chance of detecting gravitational waves from colliding black holes within the next few years.  That would be the first direct measurement of gravitational waves.  We have indirect evidence from watching two pulsars which are orbiting each other; their orbit is decaying precisely according to the prediction of energy loss to gravity waves (and the observations have won a Nobel prize).
> 
> So the timing in ST: Generations is a bit fast.  It would really take a few minutes for light or gravitational waves to get from an exploding star to a planet at an earth-like distance.  That's not the only issue, either.  To create a gravitational wave, the explosion of the star couldn't be spherical but would have to be quite asymmetric.  It's been a while since I've watched the movie, but I don't think that explosion was really odd-looking enough.




gravity comes in waves?

I'm picturing the bowling ball on a mattress example to reflect how it impacts a marble rolled across the bed past the indentation made by the heavier ball.  That's pretty static (no waves), which also doesn't likely trigger my question of speed.

I imagine, gravity as waves being applicable in my blowing up a star example.  That is more akin to tossing a pebble into a pond and the ripple of waves traveling outward having a certain speed.  This sudden change in the presumably static system takes time to propagate the effects.

Is this closer to what you meant by gravity waves or is there some kind of graviton thing like photons?


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## Umbran (Sep 18, 2015)

Janx said:


> gravity comes in waves?




We think so.  *EVERYTHING* comes in waves.  *YOU* come in waves.  For you, the wavelength is very, very, very short, so you look like a solid object.

But, we can consider light to be particles (photons) or to be waves in an electromagnetic field.  We can consider gravity to be particles (gravitons, not yet observed, and we don't have a good quantum theory of gravity to tell us how gravitons act), or as waves in a gravity field (also not yet observed).  The gravity field determines the curvature of spacetime, so waves in the gravity field mean waves in the curvature of spacetime.



> I'm picturing the bowling ball on a mattress example to reflect how it impacts a marble rolled across the bed past the indentation made by the heavier ball.  That's pretty static (no waves), which also doesn't likely trigger my question of speed.




Imagine it is a water bed.  Now imagine the bowling ball is a little lopsided, and spinning.  It'll wobble as it spins, and that will produce ripples in the water bed.


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## freyar (Sep 18, 2015)

Janx said:


> gravity comes in waves?
> 
> I'm picturing the bowling ball on a mattress example to reflect how it impacts a marble rolled across the bed past the indentation made by the heavier ball.  That's pretty static (no waves), which also doesn't likely trigger my question of speed.
> 
> ...



Umbran gave a nice answer already, but as I thought of some things I wanted to say before I saw it, I'll go ahead and add them. 

There are really two ways to look at your question(s).  We can think about things in terms of classical "old-fashioned" physics, as if we'd never heard of quantum physics.  What is gravity like?  A good analogy is electricity and magnetism.  You can have static electric and magnetic fields.  If you have ever sprinkled iron filings around a magnet and watched them line up with the field, that's an example of a static magnetic field.  If you've ever seen someone hold on to a van de Graaf generator, their hair stands on end because of a static electric field.  Those fields are like the basically static gravitational field that holds us to earth or hold the earth in orbit around the sun (or, in relativistic terms, the static but curved spacetime).  On the other hand, electromagnetic waves are also familiar --- good classical examples include radio waves, which are created by moving charges.  Similarly, moving masses create gravity waves, which, yes, mean ripples in spacetime.

As Umbran says, in quantum physics, everything is a wave and a particle, simultaneously.  When we think of a photon as a "particle" of light, we really mean it is a very tiny wave that moves at the speed of light.  Those radio waves I mentioned before are really conglomerations of lots and lots of individual photons.  Similarly, a gravity wave would be a conglomeration of lots and lots of gravitons particle/waves (we do have enough understanding of quantum gravity to say that --- they are a common feature of all quantum gravity theories I know of).  Those static fields, like the gravitational field of the earth or a star, are created by what we call "virtual particles."  These are really more little waves, but they're waves that don't look like waves moving at light speed.  They're more like localized ripples.  So the gravity of the earth that we feel is due to lots and lots of little graviton ripples.


One other neat thing about gravity waves: like I said before, we're hoping to observe them from the collision and merger of black holes within the next few years.  The fun thing about these gravity waves is that their frequency is expected to be in the same range humans can hear.  So rather than having to plot things about the waves, you can just run the signal from the detector to a speaker and listen to the gravity waves (well, you'd want to take the experimental noise out first).  Black hole mergers should sound like a "chirp" that starts out a low pitch and goes to a high pitch over the course of a few seconds or less.  I've heard output from a number of simulations. This site has some audio for you.  The LIGO experiment site also has some, but I'm having trouble getting that to play.


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## tomBitonti (Sep 20, 2015)

There is a, I would imagine, very usual demonstration of relativity which puts a flash at a center point and sets it off with one observer moving past while another is at rest relative to the flash.  The invariance of the speed of light is then used to determine the equations for transforming between the frames of the two observers.  Along the way the notion of simultenaity is shown to be limited, and other results.

The issue, and my question, has to do with assumptions which are made in regards to each of the frames, in that these are assumed to be flat and that measurements can be reliably made, leading to the calculations and so forth.

My problem is that the calculations show that measurements are, generally, not quite as easy to make, as it would seem, and, because of this, the validity of those seems to depend on some additional assumptions (I'm guessing some statement about asymptotic flatness or thereabouts) but this part of the reasoning doesn't ever seem to be addressed.  Can any insight in this regards be provided?

Thx!
TomB


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## Umbran (Sep 20, 2015)

tomBitonti said:


> There is a, I would imagine, very usual demonstration of relativity which puts a flash at a center point and sets it off with one observer moving past while another is at rest relative to the flash.  The invariance of the speed of light is then used to determine the equations for transforming between the frames of the two observers.  Along the way the notion of simultenaity is shown to be limited, and other results.




That example is frequently used to describe some of how relativity impacts the idea of simultaneity, yes.  But nobody actually physically performs that demonstration. It is done in the imagination, a "_gedankenexperiment_" (thought experiment - a term first coined by Einstein, precisely because he couldn't actually do physical experiments at the appropriate speeds)



> The issue, and my question, has to do with assumptions which are made in regards to each of the frames, in that these are assumed to be flat and that measurements can be reliably made, leading to the calculations and so forth.




Overall, the universe *is* extremely flat.  Yes, there's some minor curvature around massive bodies, but we can correct for that.



> My problem is that the calculations show that measurements are, generally, not quite as easy to make, as it would seem, and, because of this, the validity of those seems to depend on some additional assumptions (I'm guessing some statement about asymptotic flatness or thereabouts) but this part of the reasoning doesn't ever seem to be addressed.  Can any insight in this regards be provided?




We don't usually go deeply into the assumptions in discussions with laymen, as they tend to clutter up the conversation.  Let us remember that Einstein developed his theories in the first part of the 20th century (the first publication of Special Relativity was in 1905).  Actually doing the experiment described above was out of the question.

But, the assumptions of special relativity are quite simple.  And they have their own wikipedia article!

https://en.wikipedia.org/wiki/Postulates_of_special_relativity

Einstein assumed very few things:

1) The speed of light in vacuum is a constant in all inertial frames of reference.

2) The laws of physics are the same in all inertial frames of reference

Special Relativity then goes on to consider motion in those inertial frames of reference.  An "inertial frame" is one in which space is described homogeneously (it has the same properties at every point), isotropically (it is the same in every direction we look), and as time-independent.  It turns out that all inertial reference frames are in uniform linear motion with respect to each other.

You can hack out some interesting things from special relativity when considering uniformly accelerating frames, which are not inertial, but the base assumptions are frames that are moving in straight lines at constant velocities with respect to one another:  So, for example, I am sitting on the ground, and a train goes by on a straight track at a constant speed.

Which all boils down to:  Einstein assumed the outright simplest case he could for special relativity.

For general relativity, we consider spaces that are not necessarily outright inertial frames, but in which the curvature of spacetime isn't too big, so that we can think of it as being made of an infinite number of frames in which space is locally flat*.  This is like finding the area under a curve by cutting it into an infinite number of rectangles and adding together their areas.  In other words, special relativity is the arithmetic of spacetime, while general relativity is the calculus.



*This is why black hoes are such a big pain in the relativistic neck, because they are the cases where, at the singularity, the curvature of spacetime finally gets too big, and GR falls apart.


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## tomBitonti (Sep 21, 2015)

Hi,

Yes, I was using demonstration loosely, more meaning by doing some equations on a blackboard.

I follow most of the rest which you have written.  What I'm looking for is more about what physical assumptions are allowed at the beginning of the demonstation.  Also, what means of measuring time and distance are taken as safe to use.  Given the very slight effects at slow speeds, everyday means of measurement don't seem safe to take as reliable, at least not to the level of accuracy which are sufficient to detect the effects.

I ask because of difficulties that I have in recounting the basic demonstration, a difficulty not in doing the math, which is quite simple, but rather in knowing what assumptions are made at the beginning.  What is the physical basis for the reasoning which follows?

Thx!

tomB


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## freyar (Sep 21, 2015)

tomBitonti said:


> Yes, I was using demonstration loosely, more meaning by doing some equations on a blackboard.
> 
> I follow most of the rest which you have written.  What I'm looking for is more about what physical assumptions are allowed at the beginning of the demonstation.  Also, what means of measuring time and distance are taken as safe to use.  Given the very slight effects at slow speeds, everyday means of measurement don't seem safe to take as reliable, at least not to the level of accuracy which are sufficient to detect the effects.
> 
> I ask because of difficulties that I have in recounting the basic demonstration, a difficulty not in doing the math, which is quite simple, but rather in knowing what assumptions are made at the beginning.  What is the physical basis for the reasoning which follows?




OK, this is a somewhat different question that I thought you were asking before (or what Umbran thought, I guess, but I should say that I like his answer a lot).  But the assumptions Einstein made are really just the two that Umbran said: (1) Physics is the same in all inertial reference frames and (2) there is some speed, called c, which is the same in all inertial frames [and also happens to be the speed of light in vacuum].  The math all follows from that.  You do seem to be a bit worried about how we think about measurements of distance and time in the actual mathematical derivation --- the answer is that we don't.  We're concerned about the underlying physics, not difficulties with measurement, when we're doing a derivation like that.  So we imagine that each reference frame has a grid of clocks and rulers spreading throughout all of space.  And since we're worried about two different frames moving with respect to each other, the clocks and rulers are ghostly in the sense that they can pass through each other.  The point is that we're getting to the idealized measurement, what some perfect measuring device would see.  That's why relativity is about the actual properties of space and time, not measurement issues.


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## Umbran (Sep 21, 2015)

tomBitonti said:


> What I'm looking for is more about what physical assumptions are allowed at the beginning of the demonstation.




For the _gedankenexperiment_, clocks are rulers are idealized.  The physical assumptions of clocks and rulers don't change the result - they only change whether or not you could detect the result if you actually performed the experiment.



> Also, what means of measuring time and distance are taken as safe to use.  Given the very slight effects at slow speeds, everyday means of measurement don't seem safe to take as reliable, at least not to the level of accuracy which are sufficient to detect the effects.




When we are discussing the demonstration, for purposes of establishing the logic to create those equations, we don't worry about the details of clocks and rulers.  It is an experiment _in the imagination_, not in the real world.


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## AbdulAlhazred (Sep 21, 2015)

Umbran said:


> For the _gedankenexperiment_, clocks are rulers are idealized.  The physical assumptions of clocks and rulers don't change the result - they only change whether or not you could detect the result if you actually performed the experiment.
> 
> 
> 
> When we are discussing the demonstration, for purposes of establishing the logic to create those equations, we don't worry about the details of clocks and rulers.  It is an experiment _in the imagination_, not in the real world.




Although in all fairness there are pretty accessible modern instruments (modern being meant rather loosely actually) that will do the trick. The interferometer used by Michaelson and Morley  in 1887 was quite sensitive for instance, and laser interferometers can quite easily be built today using off-the-shelf components which achieve results good enough to detect differences in the range of something like 1000's of KPH at least. I don't think anyone can detect relativistic effects at the few M/sec^2 typical of everyday human life, but you'd be surprised at how accurate these things can get. I'm not sure you can do a high school physics experiment in a classroom, but you could certainly calculate the relativistic corrections needed for GPS and by how much they change the measured positions, with a few simplifications.


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## Umbran (Sep 21, 2015)

AbdulAlhazred said:


> Although in all fairness there are pretty accessible modern instruments (modern being meant rather loosely actually) that will do the trick. The interferometer used by Michaelson and Morley  in 1887 was quite sensitive for instance, and laser interferometers can quite easily be built today using off-the-shelf components which achieve results good enough to detect differences in the range of something like 1000's of KPH at least. I don't think anyone can detect relativistic effects at the few M/sec^2 typical of everyday human life, but you'd be surprised at how accurate these things can get. I'm not sure you can do a high school physics experiment in a classroom, but you could certainly calculate the relativistic corrections needed for GPS and by how much they change the measured positions, with a few simplifications.




Yes, humans are capable of measuring the impacts.

But the point is that the example stated is a _gedankenexperiment_ - a *thought* experiment, to make the logic plain.  And, for that, we don't have to worry about the quality of the measuring implements, because they don't exist!  Einstein didn't run the experiment with some set of real-world instruments!  We can, in thought, crank up the speeds as high as we want, so the effects are large, and we can imagine them observed with simple wind-up clocks, if we want.  This is the power of thought.


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## tomBitonti (Sep 21, 2015)

Umbran said:


> For the _gedankenexperiment_, clocks are rulers are idealized.  The physical assumptions of clocks and rulers don't change the result - they only change whether or not you could detect the result if you actually performed the experiment.
> 
> When we are discussing the demonstration, for purposes of establishing the logic to create those equations, we don't worry about the details of clocks and rulers.  It is an experiment _in the imagination_, not in the real world.




This statement encompasses a lot!



> 2) The laws of physics are the same in all inertial frames of reference




That is, it pulls a notion that an initial frame of reference is a distinguished frame, and that physical properties such as the rate of passage of time (as measured by common physical processes, say, decay rates) will be measured as the same thing for different observers in the same initial frame, and similarly, that distances can be reliably measured (say, as a count of wavelengths of light as emitted, again, by common physical processes), and will yield a consistent result by different observers, and that these measurements will be stable over time.

Then, the notion of an idealized clock must be presented as physically meaningful, even if no actual ideal clock can be made.  And, it turns out that certain physical processes can be used to make very accurate clocks.  As well, one must demonstrate that if a number of idealized clocks may be created, they will be shown to record the passage of time uniformly, so that any irregularities must be shared by the clocks and by the observer.  It must also be shown that an idealized clock can be transported to various locations and remain accurate.  (Knowing the results of the experiment, we will know that the clocks will show slightly different times after they are transported, but, they will still show the same rate of passage.)

This would all seem to be overly pedantic, except that the experiment will go on to show that observers in different frames of reference will not obtain the same results, so these sorts of considerations are not outside of the realm of the experiment which is being done.

Thx!
TomB


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## AbdulAlhazred (Sep 21, 2015)

Umbran said:


> Yes, humans are capable of measuring the impacts.
> 
> But the point is that the example stated is a _gedankenexperiment_ - a *thought* experiment, to make the logic plain.  And, for that, we don't have to worry about the quality of the measuring implements, because they don't exist!  Einstein didn't run the experiment with some set of real-world instruments!  We can, in thought, crank up the speeds as high as we want, so the effects are large, and we can imagine them observed with simple wind-up clocks, if we want.  This is the power of thought.




Sure, I just get the impression that tomBitonti was looking for some sort of "hey, you can actually play with this instrument and SEE some sort of relativistic effect".

There ARE actually relativistic effects that are apparent in everyday life, they're just so common that we don't really recognize them as such. The color of gold for instance is the result of relativistic effects within the large electron shell structure of gold atoms. Its just not something we normally think about in those terms.


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## tomBitonti (Sep 21, 2015)

AbdulAlhazred said:


> Sure, I just get the impression that tomBitonti was looking for some sort of "hey, you can actually play with this instrument and SEE some sort of relativistic effect".
> 
> There ARE actually relativistic effects that are apparent in everyday life, they're just so common that we don't really recognize them as such. The color of gold for instance is the result of relativistic effects within the large electron shell structure of gold atoms. Its just not something we normally think about in those terms.




That would be nice, but, that's not quite what I'm looking for.

What I'm looking for is more along of these lines:

The thought experiment envisions an ideal clock.  How do we know that such a notion is even for an idealized device at all reasonable?  If transporting a clock affects what time it measures, even returning to the starting point, what assurance do I have that transporting a clock to a distant point and using it to measure time _there_ for an observer _here_ is valid?

Similarly, built into the experiment is a statement that, in a rest frame, different observers in that frame can reliably measure distances.

The point is that the experiment relaxes one common assumption -- that durations and distances are the same for all observers -- but allows to stand other common assumptions.  If one takes that notion from the experiment, of relaxing a usual assumption, but doesn't have guidance on what should be assumed and what should not be assumed, working through the thought experiment gets to be difficult.

Thx!
TomB


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## AbdulAlhazred (Sep 21, 2015)

tomBitonti said:


> That would be nice, but, that's not quite what I'm looking for.
> 
> What I'm looking for is more along of these lines:
> 
> ...




What do you mean by 'reasonable'. We can imagine a gridwork of clocks associated with each point in space-time where they all start out with their 'hands' measuring exactly the same and then move forward in time and see what they look like as we do so. In REALITY we'd have to pick some points, figure out what to set each clock to based on our theory of relativity such that we'd SEE the same hand positions on each once they got to their assigned locations and velocities. There simply is no other way to define 'what time do I think it is over there' except 'what am I seeing on the face of a clock which I perceive to be at that location now'. Note how different this is from classical mechanics where one could at least posit infinite velocities and some sort of notion of 'absolute time' existed. 

This is why it is said that Relativity stems from 'Machian' principles, there simply are no absolutes, except one, that every observer will see a consistent time line under which the same laws of physics will apply, even if no 2 of them can agree on what specific events happened in what order or which one caused the other.


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## Umbran (Sep 21, 2015)

tomBitonti said:


> The thought experiment envisions an ideal clock.  How do we know that such a notion is even for an idealized device at all reasonable?




Oh, that's easy - because we don't actually care about the clock.  

You see, clocks are made by people.  They don't exist naturally.   Therefore, the nature of the universe *must be independent of clocks*!  Time and distance exist whether or not we have instruments to measure them!  



> If transporting a clock affects what time it measures, even returning to the starting point, what assurance do I have that transporting a clock to a distant point and using it to measure time _there_ for an observer _here_ is valid?




Well, that is one of the base assumptions - that the basic laws of physics are the same in all inertial frames of reference.  If you want to consider that it is incorrect - have at you.  Find the place it isn't true, and maybe the Nobel Committee will give you an award.

However... go out on a clear night (hopefully away from a city) and look up.  What do you see?  Stars.

If we accept that the laws of time and space vary from place to place, we accept the likely need to come up with a unique explanation for each and every star.  When we see so many very similar phenomenon, everywhere we look, it rather strains credulity that the laws aren't the same all over the place.  Moreover, we can see very well that the laws of the universe hold pretty uniformly over this rock that we shall, for convenience, call "Earth", and, in fact, over the space of the solar system - we have sent out devices with measuring equipment, and they continue to operate as expected, without huge anomalies that can't be explained.  

So - the laws are all the same in our neighborhood.  And in our galaxy.  If there's some place in our sight that's substantially different, we should be able to see the difference when we look out into space.  But we don't.  So, the observable universe seems to have the same laws.

That's not enough for you?  Wow.  Tough crowd.


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## freyar (Sep 21, 2015)

tomBitonti said:


> That is, it pulls a notion that an initial frame of reference is a distinguished frame, and that physical properties such as the rate of passage of time (as measured by common physical processes, say, decay rates) will be measured as the same thing for different observers in the same initial frame, and similarly, that distances can be reliably measured (say, as a count of wavelengths of light as emitted, again, by common physical processes), and will yield a consistent result by different observers, and that these measurements will be stable over time.



The fact that inertial frames have a special name means they are distinguished!  But the second part of your sentence is the content of the assumption.  That's not something additional --- it's exactly what the postulate is saying.



> Then, the notion of an idealized clock must be presented as physically meaningful, even if no actual ideal clock can be made.  And, it turns out that certain physical processes can be used to make very accurate clocks.  As well, one must demonstrate that if a number of idealized clocks may be created, they will be shown to record the passage of time uniformly, so that any irregularities must be shared by the clocks and by the observer.




I think you're getting a bit confused or hung up on something.  An idealized clock is one without irregularities.  It is a perfect clock.  This is really not special to relativity but is in fact how physics is done.  We think of perfect systems, stripped of worries about experimental noise, etc, etc.  Theoretical physics is like what the world is in our imagination; experimental physics is about making measurements as close to that idealized imagination as possible.




> It must also be shown that an idealized clock can be transported to various locations and remain accurate.  (Knowing the results of the experiment, we will know that the clocks will show slightly different times after they are transported, but, they will still show the same rate of passage.)



  Just transport them infinitely slowly.  We're just doing this in our minds, so we can take an infinite amount of time to set things up.


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## AbdulAlhazred (Sep 22, 2015)

Umbran said:


> So - the laws are all the same in our neighborhood.  And in our galaxy.  If there's some place in our sight that's substantially different, we should be able to see the difference when we look out into space.  But we don't.  So, the observable universe seems to have the same laws.
> 
> That's not enough for you?  Wow.  Tough crowd.




There's another thing even beyond this, Noether's Theorum. Much of our modern understanding of physics was built upon this theorum, which equates symmetries and conservation laws. By this means we know that either we've just by pure luck built most of modern high energy physics (too unlikely to even seriously consider) OR the laws of nature are the same in every inertial frame of reference, everywhere in space and time, and symmetric with respect to any rotation. Its really about as close to an iron-clad argument as you can get in physics.


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## CaptainGemini (Oct 1, 2015)

What impact would it have on our understanding and study of physics if it turned out that Unified Field Theory is flat-out impossible and the universe exists as a series of interactions between different sets of physics?


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## CaptainGemini (Oct 1, 2015)

AbdulAlhazred said:


> There's another thing even beyond this, Noether's Theorum. Much of our modern understanding of physics was built upon this theorum, which equates symmetries and conservation laws. By this means we know that either we've just by pure luck built most of modern high energy physics (too unlikely to even seriously consider) OR the laws of nature are the same in every inertial frame of reference, everywhere in space and time, and symmetric with respect to any rotation. Its really about as close to an iron-clad argument as you can get in physics.




Please excuse the double post.

Here's where it gets complicated: We don't know how big the universe is simply because it's beyond our capacity to observe, but we know of around 91 billion light years.

Using the standard theory that the universe is speeding up as it expands, started at the speed of light, and has been expanding for fifteen billion years... At current, we cannot even observe one percent of one percent of the universe. An infinitesimal fraction of the universe's size could easily have a radius of three hundred billion light years. Even if only 1% of the universe obeyed the laws of physics as observable from Earth, you're still talking about an amount of space that we will likely never explore beyond at any point before the heat death of the universe. And that's even accounting for Star Trek-like FTL.

So, even if Noether's Theorum is wrong, we'll likely never know.


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## freyar (Oct 1, 2015)

CaptainGemini said:


> What impact would it have on our understanding and study of physics if it turned out that Unified Field Theory is flat-out impossible and the universe exists as a series of interactions between different sets of physics?




When you say "a series of interactions between different sets of physics," do you mean distinct forces, like the strong and electroweak forces as distinct forces in the Standard Model?  I'd like to understand the question a bit better.  My instinct is to say that it probably wouldn't have much impact in a practical sense, but I'd like to make sure I understand the question first.


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## CaptainGemini (Oct 1, 2015)

freyar said:


> When you say "a series of interactions between different sets of physics," do you mean distinct forces, like the strong and electroweak forces as distinct forces in the Standard Model?  I'd like to understand the question a bit better.  My instinct is to say that it probably wouldn't have much impact in a practical sense, but I'd like to make sure I understand the question first.




The idea that there's not one set of laws of physics, but multiple sets and that it is the interaction between them which causes the universe to exist. Kinda... Not a unified field, but multiple fields that don't necessarily get along.


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## AbdulAlhazred (Oct 2, 2015)

CaptainGemini said:


> The idea that there's not one set of laws of physics, but multiple sets and that it is the interaction between them which causes the universe to exist. Kinda... Not a unified field, but multiple fields that don't necessarily get along.




But there must be a set of principles which governs there mutual interactions, or there would be what, randomness? This set of principles then IS your laws of physics. Conceptually there either IS a set of laws, things happen consistently the same way all the time and each possible part of the whole universe must relate to each other part by some such consistent rule, or there isn't. There's really no halfway, is there?


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## CaptainGemini (Oct 2, 2015)

AbdulAlhazred said:


> But there must be a set of principles which governs there mutual interactions, or there would be what, randomness? This set of principles then IS your laws of physics. Conceptually there either IS a set of laws, things happen consistently the same way all the time and each possible part of the whole universe must relate to each other part by some such consistent rule, or there isn't. There's really no halfway, is there?




Conceptually, if you know both an object's velocity and direction of travel, you can derive its location at all times. But it's not actually true of all particles.

Just because it doesn't make any logical sense to us as we understand physics now does not preclude its capacity for existence. It merely demonstrates the limits of our capacity to understand existence with current knowledge. A halfway point that produces consistent results most of the time out of pure randomness is perfectly possible. It would also produce results that are completely unexpected, but might not be observable from Earth at this time due to current technological limitations. It might even explain the Sol System's quirks.

However, I was not suggesting the universe operates that way. I was asking how it would affect our understanding and study of physics.


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## freyar (Oct 2, 2015)

Well, I'm still having trouble following the set-up.  But, basically, if there is not any set of laws at all (ie, nothing we can understand), like if everything is at the whim of some set of battling D&D deities, then we just can't really do science.  But that's an extreme case.  Quantum physics has laws, but they are probabilistic and not deterministic, so we try to understand the aggregate statistical behavior of systems.  Or we could talk about the laws of physics changing in some well-defined way across the universe --- there are people looking for variations of the strength of the fundamental electric charge over the history of the universe, for example.  So I guess it depends on how extreme you mean.


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## AbdulAlhazred (Oct 2, 2015)

freyar said:


> Well, I'm still having trouble following the set-up.  But, basically, if there is not any set of laws at all (ie, nothing we can understand), like if everything is at the whim of some set of battling D&D deities, then we just can't really do science.  But that's an extreme case.  Quantum physics has laws, but they are probabilistic and not deterministic, so we try to understand the aggregate statistical behavior of systems.  Or we could talk about the laws of physics changing in some well-defined way across the universe --- there are people looking for variations of the strength of the fundamental electric charge over the history of the universe, for example.  So I guess it depends on how extreme you mean.




Right, when [MENTION=6801122]CaptainGemini[/MENTION] suggests for instance that "A halfway point that produces consistent results most of the time out of pure randomness is perfectly possible. It would also produce results that are completely unexpected, but might not be observable from Earth at this time due to current technological limitations." then you'd simply have to explain why we consistently "don't see" this "pure randomness" in our part of the universe. If it is totally hidden then what we see is actually consistent, and we should be able to describe that consistency, and thus there ARE then laws of physics, because that's ALL that laws of physics ARE, consistent descriptions of how we observe things to behave, they can be naught else! 

I think the point about Noether's Theorum wasn't really quite made either. It isn't something that may or may not be true. Noether's Theorum isn't a scientific result, it is a logical construct, it doesn't stand or fall, any more than the Pythagorean Theorum stands or falls. It is simply a truth. Furthermore we have a great deal of evidence that universal conservation laws and their equivalent symmetries exist because we observed conservation laws in action and then we derived symmetries from them. These symmetries were then used to derive further theories in physics which then matched observation, and this has happened MANY times. So either the observed conservation laws actually are really logically consistent and observed everywhere in our universe, or else most all of modern physics was discovered by random chance using a totally flawed process and we just got INCREDIBLY lucky.

This for instance is why we can with essentially 100% certainty rule out things like reactionless drives which violate Conservation of Momentum. If they exist, and Conservation of Momentum IS violated, ever, anywhere in the Universe, then all of our modern theories of physics are just blind luck, which we can state could only by true by chance at a level so unlikely that it is equivalent to zero. Now, maybe its possible to argue about what "anywhere in the Universe" exactly means, could it be that these things can be violated in some area of space which is causally disconnected from us (IE beyond our light cone and thus will never interact with us again for all time, and may have been causally disconnected since the start of inflation). I don't think we really know the answer to THAT, but is it even a meaningful question since we can never answer it, even in principle?


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## CaptainGemini (Oct 2, 2015)

freyar said:


> Well, I'm still having trouble following the set-up.  But, basically, if there is not any set of laws at all (ie, nothing we can understand), like if everything is at the whim of some set of battling D&D deities, then we just can't really do science.  But that's an extreme case.  Quantum physics has laws, but they are probabilistic and not deterministic, so we try to understand the aggregate statistical behavior of systems.  Or we could talk about the laws of physics changing in some well-defined way across the universe --- there are people looking for variations of the strength of the fundamental electric charge over the history of the universe, for example.  So I guess it depends on how extreme you mean.




What I am talking about is not a singular set of rules, but multiple sets of rules and the universe existing as an interaction between those sets of rules.

As an example, take quantum mechanics and general relativity. Now, let's say those _cannot_ be combined into a singular set of laws of physics.

How does that change our understanding and study of physics?



AbdulAlhazred said:


> Right, when @CaptainGemini suggests for instance that "A halfway point that produces consistent results most of the time out of pure randomness is perfectly possible. It would also produce results that are completely unexpected, but might not be observable from Earth at this time due to current technological limitations." then you'd simply have to explain why we consistently "don't see" this "pure randomness" in our part of the universe. If it is totally hidden then what we see is actually consistent, and we should be able to describe that consistency, and thus there ARE then laws of physics, because that's ALL that laws of physics ARE, consistent descriptions of how we observe things to behave, they can be naught else!
> 
> I think the point about Noether's Theorum wasn't really quite made either. It isn't something that may or may not be true. Noether's Theorum isn't a scientific result, it is a logical construct, it doesn't stand or fall, any more than the Pythagorean Theorum stands or falls. It is simply a truth. Furthermore we have a great deal of evidence that universal conservation laws and their equivalent symmetries exist because we observed conservation laws in action and then we derived symmetries from them. These symmetries were then used to derive further theories in physics which then matched observation, and this has happened MANY times. So either the observed conservation laws actually are really logically consistent and observed everywhere in our universe, or else most all of modern physics was discovered by random chance using a totally flawed process and we just got INCREDIBLY lucky.
> 
> This for instance is why we can with essentially 100% certainty rule out things like reactionless drives which violate Conservation of Momentum. If they exist, and Conservation of Momentum IS violated, ever, anywhere in the Universe, then all of our modern theories of physics are just blind luck, which we can state could only by true by chance at a level so unlikely that it is equivalent to zero. Now, maybe its possible to argue about what "anywhere in the Universe" exactly means, could it be that these things can be violated in some area of space which is causally disconnected from us (IE beyond our light cone and thus will never interact with us again for all time, and may have been causally disconnected since the start of inflation). I don't think we really know the answer to THAT, but is it even a meaningful question since we can never answer it, even in principle?




Stop and take a look at the probably of life existing even once on a planet, indeterminate of the fact it actually exists. Then take a look at how many events in human history pretty much amount to humanity surviving or succeeding simply due to blind luck.

Whether or not we like to admit it, it is unscientific to simply toss out blind luck as being part of the equation. Without it, it's quite possible science wouldn't exist in the first place.

Also, something to consider: The laws of physics do not care what human logic dictates. If there are to be multiple sets, or if it is all random chance, then that is reality regardless of what "truths" we know have to say. It is pure ego, not pure knowledge, to think that human logic has any dictates on the universe... and pure ego produces bad science. Which is why I referenced the electron in my previous post, as it does violate human logic in its simple existence.

That we have been right so far is just one thing: Pure luck. The universe could have easily been far different, or had massively different laws of physics.

Also, we can scientifically measure randomness, and even come up with ways to get consistent results our of pure randomness. Last I remember, there was at least one field of physics devoted to it.


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## AbdulAlhazred (Oct 2, 2015)

CaptainGemini said:


> What I am talking about is not a singular set of rules, but multiple sets of rules and the universe existing as an interaction between those sets of rules.
> 
> As an example, take quantum mechanics and general relativity. Now, let's say those _cannot_ be combined into a singular set of laws of physics.
> 
> ...




OK, so probability is meaningless and if something can happen (and pretty much anything COULD in principle) then we can just assume it did! You've literally removed all meaning from the scientific process. You've left the reservation and are now just another garden variety mystic who's universe simply 'just happened to be as it is'. 

There's 2 different things being conflated here actually. First is basically an invocation of a very extreme version of the Strong Anthropic Principle, when you say that "we just happen to be right by pure luck" you're just saying "well, we're here, so no matter how improbable I calculate the odds to be it says nothing about the way the universe works because we just happened to luck out and get this one". The other one is the question of the details of how things played out given a fixed set of physical laws. Nothing in our understanding of physics says that we will be able to utterly deterministically calculate the current state of the universe from its initial conditions, even if we DO perfectly understand the rules it evolves by. This is trivially true today as we know that these laws are statistical in nature, and that small quantum mechanical statistical outcomes can generate different macroscopic results. Thus knowing the exact state of the big bang at time 0 won't ever tell you if the coin I flip today will come up heads or tails. It probably won't even tell you if the Earth we live on would inevitably exist as it does. HOWEVER, it would tell you that the universe would contain many Earth-like planets and what kinds of chemistry would happen on their surfaces.


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## CaptainGemini (Oct 2, 2015)

AbdulAlhazred said:


> OK, so probability is meaningless and if something can happen (and pretty much anything COULD in principle) then we can just assume it did! You've literally removed all meaning from the scientific process. You've left the reservation and are now just another garden variety mystic who's universe simply 'just happened to be as it is'.




And what you are stating is almost, but not quite, entirely unlike what I have said. You would be a fine heir to the drink making machine on the Heart of Gold.



> There's 2 different things being conflated here actually. First is basically an invocation of a very extreme version of the Strong Anthropic Principle, when you say that "we just happen to be right by pure luck" you're just saying "well, we're here, so no matter how improbable I calculate the odds to be it says nothing about the way the universe works because we just happened to luck out and get this one". The other one is the question of the details of how things played out given a fixed set of physical laws. Nothing in our understanding of physics says that we will be able to utterly deterministically calculate the current state of the universe from its initial conditions, even if we DO perfectly understand the rules it evolves by. This is trivially true today as we know that these laws are statistical in nature, and that small quantum mechanical statistical outcomes can generate different macroscopic results. Thus knowing the exact state of the big bang at time 0 won't ever tell you if the coin I flip today will come up heads or tails. It probably won't even tell you if the Earth we live on would inevitably exist as it does. HOWEVER, it would tell you that the universe would contain many Earth-like planets and what kinds of chemistry would happen on their surfaces.




If you don't like them being conflated, then stop conflating them.

You were the one who first thought to conflate my question about a fixed set of physics being slightly different from how we think the laws of physics work with the idea that everything results from random chance. I humored you and responded to your absurdity to point out that what you say is possible, but that it would be different than what we have observed, and then pointed out the issue of my question was not related to that. I misworded my comment at the end, so I take responsibility for the confusion that resulted. It was also you who had to go on the offensive because your conflating of two entirely unlike things and my miswording confused freyar. That was when I clarified my question to freyar, so that he could see exactly what I was actually talking about, before responding further to your absurdity by pointing out that you were speaking from ego and not an understanding of science.

But, I see my not-so-gentle prodding flew over your head, so now I am speaking plainly: If you don't like the problem of conflating you complain about in your last post, *then stop causing it. *I made it a point to make my question clear after I saw the confusion caused earlier. Why you felt the need to ignore the obvious and continue on your holy crusade, I have no idea.

Also, you completely misunderstand the way probability works. It doesn't tell you something _will_ exist; it tells you the chances that something _might_ exist. There is no such thing as 100% probability unless you are talking about events that have already occurred. That was why I told you to ignore the fact life does exist when trying to figure out the chances life could exist. Probability operates under the scientific knowledge that absolutely nothing is guaranteed. Which was why I made that comment about ego leading to bad science... because science itself accepts the fact that nothing is guaranteed unless it has already happened. And as we increasingly discover planets so close to having become another Earth, but which had something go wrong to prevent it, we get even more reinforcement of exactly how lucky we are to exist in the first place.

Yes, we got lucky. No, our existence was not guaranteed, and the existence of another race in the universe or even another planet like Earth is not guaranteed. That's science.


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## AbdulAlhazred (Oct 2, 2015)

CaptainGemini said:


> And what you are stating is almost, but not quite, entirely unlike what I have said. You would be a fine heir to the drink making machine on the Heart of Gold.
> 
> 
> 
> ...




OK, I'm not looking for an argument. I think the last time I looked I was holding a degree in math and I don't think that what you're describing as my position is what I'm saying, so basically there's just some fundamental disconnect here. I don't know what your question is asking, whatever it is clearly its not what a plain reading would seem to indicate. Maybe there's someone else that's on your wavelength because I am seeing that clearly I'm not up to it. 

And to be clear, I don't really think your question is about probability at all, or statistics in any sense. Its really a metaphysical question that touches on the 'Anthropic Principle'. You could also talk about Beysian Analysis and priors etc as all such discussions tend to go at some point, and/or the rabbit hole of 'what can really exist in a literally infinite Multiverse/Universe', etc etc etc. Interesting topics in some degree but beyond a certain point it tends to leave the realm of what I would call 'physics'.


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## CaptainGemini (Oct 3, 2015)

AbdulAlhazred said:


> OK, I'm not looking for an argument. I think the last time I looked I was holding a degree in math and I don't think that what you're describing as my position is what I'm saying, so basically there's just some fundamental disconnect here. I don't know what your question is asking, whatever it is clearly its not what a plain reading would seem to indicate. Maybe there's someone else that's on your wavelength because I am seeing that clearly I'm not up to it.
> 
> And to be clear, I don't really think your question is about probability at all, or statistics in any sense. Its really a metaphysical question that touches on the 'Anthropic Principle'. You could also talk about Beysian Analysis and priors etc as all such discussions tend to go at some point, and/or the rabbit hole of 'what can really exist in a literally infinite Multiverse/Universe', etc etc etc. Interesting topics in some degree but beyond a certain point it tends to leave the realm of what I would call 'physics'.




I should not have reacted as I have. You have my apologies for that. I was just coming here to apologize no matter if you replied. You do not deserve to be an unwitting target of my stress, and someone very dear pointed out how snappish I've been lately.


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## freyar (Oct 3, 2015)

Well, I'm glad that's been settled peacefully! 



CaptainGemini said:


> What I am talking about is not a singular set of rules, but multiple sets of rules and the universe existing as an interaction between those sets of rules.
> 
> As an example, take quantum mechanics and general relativity. Now, let's say those _cannot_ be combined into a singular set of laws of physics.
> 
> How does that change our understanding and study of physics?




Well, in your example, you'd have a classical version of GR plus quantum mechanics of other stuff.  That leaves a lot of problems, so we'd have trouble sorting those things out, and that would take up a lot of the effort in theoretical physics for a while, I'd think.  On the other hand, we have an existence proof that it is possible to quantize gravity.  Namely, we have very very strong evidence that gravity in special spacetimes (anti-de Sitter spacetimes) is exactly described by a non-gravitational quantum theory of particle physics.  

Anyway, I hope that answers your question more or less.  I think the point is just that physics is about trying to quantify and codify how the universe works.  We've seen the overall picture of that change a lot more than once (particularly at the start of the 20th century), but the goal and process remains the same.


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## Umbran (Oct 3, 2015)

CaptainGemini said:


> What I am talking about is not a singular set of rules, but multiple sets of rules and the universe existing as an interaction between those sets of rules.
> 
> As an example, take quantum mechanics and general relativity. Now, let's say those _cannot_ be combined into a singular set of laws of physics.
> 
> How does that change our understanding and study of physics?




Well, in one way it changes our understanding only a little - what you describe is the way things are now - General Relativity and Quantum Mechanics are not united into a single set of laws at this moment.  

But, you speak of interactions *between* these sets.  If those interactions are not akin to the whims of gods, then those interactions may be charted.  And then we get a set of rules for the interactions, and we then have, in a sense, a unified theory.  It may be unified in the sense that a patchwork quilt is unified, instead of the way a knitted lace is all one thread, but it is unified, a complete working fabric still.

As an aside, sometimes I think we may generate issues with flowery language - "the universe existing as an interaction between those sets of rules" would be incomplete, as there are also interactions found *within* the sets.  



> Stop and take a look at the probably of life existing even once on a planet, indeterminate of the fact it actually exists.




Stop and take a look at how little we know of what that probability is!  Each of us has an *intuition* about that probability, but we *know*, very, very little.

It has been shown (by Freeman Dyson, among others) that with a supply of a relatively small set of units with slightly varying interactions (like, say, a soup containing supplies of just a few different kinds of peptides) and a bit of free energy around, that a transition from a disordered state to an ordered but dynamic state is nigh inevitable.  Self-organizing systems *happen*.  

How much of a bigger step is it to life from there?  *NOBODY KNOWS*.  You keep asserting how much blind luck it may be... but it could just as easily be nigh inevitable, and not lucky at all.


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## Landifarne (Oct 4, 2015)

Can you summarize of our present level of understanding of what's occuring in the nucleus of moderate-sized atom?


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## AbdulAlhazred (Oct 5, 2015)

CaptainGemini said:


> I should not have reacted as I have. You have my apologies for that. I was just coming here to apologize no matter if you replied. You do not deserve to be an unwitting target of my stress, and someone very dear pointed out how snappish I've been lately.




Its cool, maybe I'm annoying too! I'm SURE there are a few other Enworlder's who will agree with that, lol. I could also be thick, who knows?


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## Umbran (Oct 5, 2015)

Because it is funny and relevant...

(Okay, that was funny.  Mostly transparent image, with a black line drawing, means... a big nothing on the black forum skins.

let us try that again...)


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## freyar (Oct 5, 2015)

Landifarne said:


> Can you summarize of our present level of understanding of what's occuring in the nucleus of moderate-sized atom?




There's some useful background information if you search around upthread a bit (or maybe in another thread in the Misc. Geek Talk forum --- these threads have gotten a bit jumbled in my memory).  Basically, protons and neutrons (the particles that make up an atomic nucleus) are made up of 3 quarks each held together by virtual gluon particles.  Gluons mediate the strong nuclear force in much the same way that photons mediate the electromagnetic force.  The analogy ends there, though; the strong force is so strong that a great deal of the structure of protons and neutrons is determined by the virtual gluons (and also virtual quarks) inside them.  In the rest of atoms, the virtual photons don't really show up separately from the fact that there is an electric force binding the atoms together.

The difficulty is that the strong force is strong enough that quarks and gluons can't really exist freely in the nucleus.  They're stuck in the protons and neutrons.  What holds the protons and neutrons together is a remnant of the strong force.  The analogy here is the van der Waals force between atoms; atoms are electrically neutral, but, if you put two of them close together, their electrons arrange themselves so the atoms attract each other.  That's the short answer.  If you want a little more information: The mathematical description is much more complicated for the strong force, though, and it turns out that different approximations to the full equations work better for different sized nuclei (I'm not really an expert on this in detail, but I have heard about it).  For medium-sized nuclei, the main way to think about it is that protons and neutrons are a bit "sticky" when they bump into each other, and this "stickiness" helps hold the nucleus together.  Another part of this left-over nuclear force for these nuclei is due to virtual pions, which are unstable particles made up of one quark and one anti-quark.  The best description is different (and somewhat harder to explain) for really small or large-ish nuclei.  This is a notoriously difficult subject, by the way.


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## Landifarne (Oct 7, 2015)

freyar said:


> There's some useful background information if you search around upthread a bit (or maybe in another thread in the Misc. Geek Talk forum --- these threads have gotten a bit jumbled in my memory).  Basically, protons and neutrons (the particles that make up an atomic nucleus) are made up of 3 quarks each held together by virtual gluon particles.  Gluons mediate the strong nuclear force in much the same way that photons mediate the electromagnetic force.  The analogy ends there, though; the strong force is so strong that a great deal of the structure of protons and neutrons is determined by the virtual gluons (and also virtual quarks) inside them.  In the rest of atoms, the virtual photons don't really show up separately from the fact that there is an electric force binding the atoms together.
> 
> The difficulty is that the strong force is strong enough that quarks and gluons can't really exist freely in the nucleus.  They're stuck in the protons and neutrons.  What holds the protons and neutrons together is a remnant of the strong force.  The analogy here is the van der Waals force between atoms; atoms are electrically neutral, but, if you put two of them close together, their electrons arrange themselves so the atoms attract each other.  That's the short answer.  If you want a little more information: The mathematical description is much more complicated for the strong force, though, and it turns out that different approximations to the full equations work better for different sized nuclei (I'm not really an expert on this in detail, but I have heard about it).  For medium-sized nuclei, the main way to think about it is that protons and neutrons are a bit "sticky" when they bump into each other, and this "stickiness" helps hold the nucleus together.  Another part of this left-over nuclear force for these nuclei is due to virtual pions, which are unstable particles made up of one quark and one anti-quark.  The best description is different (and somewhat harder to explain) for really small or large-ish nuclei.  This is a notoriously difficult subject, by the way.





Lol, I know. Sorry about that.

I have a BS in physics ('94), but the nuclear physics course that I took back in '92 was just that...BS. My impression was that my professor (or anyone, really) did not have a very good idea of what was going on. However, I know that new models have been floating around that I am completely ignorant of.

Was looking for a way to visualize it, so that I could speak more clearly about the topic to my HS students. What you've said here has been helpful, given me a few things to contemplate. I'll probably get back to you on it fairly soon.

-Steve


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## freyar (Oct 7, 2015)

Landifarne said:


> Lol, I know. Sorry about that.
> 
> I have a BS in physics ('94), but the nuclear physics course that I took back in '92 was just that...BS. My impression was that my professor (or anyone, really) did not have a very good idea of what was going on. However, I know that new models have been floating around that I am completely ignorant of.
> 
> ...



No problem.  If some of that seemed a bit basic, well, I'm also trying to write for everyone on the board.  In any case, I'm far from an expert on the details of this stuff, though I've seen a few seminars on it.  I'll look around a bit more.  The thing about nuclear structure is that there are a number of approaches, all approximate, and which one(s) works well depends on the size of nucleus you're talking about.


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## tomBitonti (Oct 11, 2015)

CaptainGemini said:


> Please excuse the double post.
> 
> Here's where it gets complicated: We don't know how big the universe is simply because it's beyond our capacity to observe, but we know of around 91 billion light years.
> 
> ...




I was wondering ... Has it been ruled out that the observable universe is wrapped, with the current size much less than 91b (or even 15b) light years?

Thx!
TomB


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## Landifarne (Oct 12, 2015)

tomBitonti said:


> I was wondering ... Has it been ruled out that the observable universe is wrapped, with the current size much less than 91b (or even 15b) light years?
> 
> Thx!
> TomB




It was confirmed (two decades ago) that we live in an open, ever-expanding universe.


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## tomBitonti (Oct 12, 2015)

Landifarne said:


> It was confirmed (two decades ago) that we live in an open, ever-expanding universe.




I'm presuming this is obtained from the measurements of the energy density, which makes for a flat universe.

How do we get from that to the conclusion that there aren't joins?  Does the lack of uniformity in directions (diagonals are longer than the perpindiculars) make that physically impossible?  Or is it considered too strange?

(From a topologists point of view, a standard construction is to take a unit square and to identify the opposing sides to make a new quotient space of the original unit square.  That makes for a flat, unbounded, yet finite, space.  The space is not uniform in all respects.)

Thx!
TomB


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## Umbran (Oct 13, 2015)

tomBitonti said:


> How do we get from that to the conclusion that there aren't joins?  Does the lack of uniformity in directions (diagonals are longer than the perpindiculars) make that physically impossible?  Or is it considered too strange?




"Diagonal" and "perpendicular" are products of a human imposed coordinate grid.  The universe does not have such a grid naturally.  If you work in, say, spherical coordinates, there is no such distinction, and you find there *is* uniformity in directions.

There could be some local joins or curves (wormholes and black holes and such).  But, on a broad scale, there is a point to be made: Mass/energy causes spacetime to curve.  As far as we know, mass is the *only* thing that makes spacetime curve.  Real physical spacetime cannot have arbitrary topological configurations *without* mass to make it so.  This is why we say that the low energy density means spacetime is flat, at least within the observable universe.


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## tomBitonti (Oct 13, 2015)

Umbran said:


> "Diagonal" and "perpendicular" are products of a human imposed coordinate grid.  The universe does not have such a grid naturally.  If you work in, say, spherical coordinates, there is no such distinction, and you find there *is* uniformity in directions.
> 
> There could be some local joins or curves (wormholes and black holes and such).  But, on a broad scale, there is a point to be made: Mass/energy causes spacetime to curve.  As far as we know, mass is the *only* thing that makes spacetime curve.  Real physical spacetime cannot have arbitrary topological configurations *without* mass to make it so.  This is why we say that the low energy density means spacetime is flat, at least within the observable universe.




Yes.  But, with a simple join (side to side, top to top), while there is no local non-uniformity, there is a global one, with three axes (left-right, up-down, forward-back) being distinguished from the three diagonal axes.  And changing the join to a spherical one, that is, which identifies opposing points on the surface of a sphere, means either introducing curvature, or means there is a discontinuity of a derivative at the join.  (That is evident by pushing a curved arc across the join: The arc flips directions as it crosses the join.)

Your statement "cannot have arbitrary topological configurations" (&etc) seems to match my statement describing the joins as being physically impossible or simply too strange.  However, is that actually physically proven, or is it taken as being not pursued because there is no evidence to merit the consideration?  Ar, do the non-uniform axes create a substantial problem?

In this same topic, do we have a limit on the flatness of space, and a correlation of that flatness to a minimal size of the universe -- assuming a simple spherical topology?

Doing some searches found this, which is a nice read:

PHYS771 Lecture 20: Cosmology and Complexity
Scott Aaronson
http://www.scottaaronson.com/democritus/lec20.html

That references this, which seems to match my question, but I haven't had a chance to look through it yet to tell:

Measuring the shape of the Universe
Neil J. Cornish (DAMTP, Cambridge), Jeffrey R. Weeks (Canton)
(Submitted on 30 Jul 1998 (v1), last revised 5 Aug 1998 (this version, v2))

http://arxiv.org/abs/astro-ph/9807311

Thx!
TomB


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## Umbran (Oct 13, 2015)

tomBitonti said:


> But, with a simple join (side to side, top to top), while there is no local non-uniformity, there is a global one, with three axes (left-right, up-down, forward-back) being distinguished from the three diagonal axes.




Yes, but you are still thinking from the point of view of someone used to thinking in rectangular coordinates.  Why would we expect a rectangular coordinate join, specifically, of the universe?  



> And changing the join to a spherical one, that is, which identifies opposing points on the surface of a sphere, means either introducing curvature




All continuous joins introduce curvature.  Try to make a cylinder without bending the paper, dude 



> Your statement "cannot have arbitrary topological configurations" (&etc) seems to match my statement describing the joins as being physically impossible or simply too strange.




Let me put it another way - as mathematicians considering topologies, we can think up all sorts of spaces, without concern for why they are shaped a given way.  But, General Relativity tells us that the topology of our real spacetime is *not* independent of the material within the space.  You can only have arbitrary topology if you allow arbitrary placement of material in the space.  And, if we don't have material, then the space isn't shaped that way.



> However, is that actually physically proven, or is it taken as being not pursued because there is no evidence to merit the consideration?  Ar, do the non-uniform axes create a substantial problem?




It is proven, insofar as GR seems pretty good, and we've seen no evidence for other curvature of our spacetime.  I know of no physical phenomenon currently unexplained that calls for such curvature.  Occam's Razor, then, suggests that I not worry about the possibility too much.

I wouldn't state it as you do - it isn't "non-uniform axes create a substantial problem".  I would put it as, "curvature of the space that is not attributed to mass likely (if I recall correctly) equates to perpetual motion machines and/or time travel*."  Either of those gives us serious issues with causality and the laws of thermodynamics.  You need to have a really good reason to suggest those might exist.  





*IIRC, time travel implies perpetual motion, but perpetual motion does not necessarily imply time travel.


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## tomBitonti (Oct 13, 2015)

Umbran said:


> All continuous joins introduce curvature.  Try to make a cylinder without bending the paper, dude




The curvature is an artifact of the embedding, not of the join.

GR tells us about local topologies, not about global ones.  To make conclusions about the global topology, additional evidence is needed.

In the second link posted above, several experiments are described, with the result being, at that time, "we don't know".  However, there may be more recent results, as newer experimental data (satellite measurements) should now be available, compared with what was available to the authors of that paper.

Thx!
TomB


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## freyar (Oct 13, 2015)

tomBitonti said:


> I'm presuming this is obtained from the measurements of the energy density, which makes for a flat universe.
> 
> How do we get from that to the conclusion that there aren't joins?  Does the lack of uniformity in directions (diagonals are longer than the perpindiculars) make that physically impossible?  Or is it considered too strange?
> 
> (From a topologists point of view, a standard construction is to take a unit square and to identify the opposing sides to make a new quotient space of the original unit square.  That makes for a flat, unbounded, yet finite, space.  The space is not uniform in all respects.)




The energy density of the universe is (within measurement error) the amount that makes the universe spatially flat, yes.  Due to the cosmological constant, the expansion is accelerating, again as mentioned earlier.  [I am hesitant to use the phrase "open universe" since that had implications up until the discovery of the cosmological constant that no longer apply.]

However, even a flat universe can have a nontrivial topology. Your example of gluing the edges of a square together is a good one --- that's the mathematical definition of a 2D torus.  So (the spatial part of) our universe could be a flat 3D torus or one of several other alternatives.  (That's assuming it's actually exactly flat; there are other alternatives if it's positively or negatively curved.)  How do we know if the universe is infinite or a finite torus/other example?  We actually have to go measure.  There are several groups that have looked at the cosmic microwave background (the oldest visible light); if the universe is finite and small enough, we'd be able to see repeated patterns due to seeing the same spot in the early universe from several directions (think about different ways you can shoot the same area on the screen in the old Asteroids game).  So far, we don't see any repeated patterns like that, which means that the universe is either infinite or else finite but so big we can't see all the way around it.




Umbran said:


> There could be some local joins or curves (wormholes and black holes and such).  But, on a broad scale, there is a point to be made: Mass/energy causes spacetime to curve.  As far as we know, mass is the *only* thing that makes spacetime curve.  Real physical spacetime cannot have arbitrary topological configurations *without* mass to make it so.  This is why we say that the low energy density means spacetime is flat, at least within the observable universe.




This isn't really correct.  There are finite, _flat_ spaces with nontrivial topology that solve Einstein's equations with _zero_ mass or energy density.  A torus is an example.  And our universe could have one of those spatial topologies.  Black holes and wormholes do have curvature, though, so they require mass/energy.


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## freyar (Oct 13, 2015)

tomBitonti said:


> The curvature is an artifact of the embedding, not of the join.
> 
> GR tells us about local topologies, not about global ones.  To make conclusions about the global topology, additional evidence is needed.
> 
> ...




Yeah, just to emphasize this, TomB is exactly right here.  There are a number of flat 3D spaces with nontrivial topology, ie, which are multiply connected.  As he says, the Einstein equations, or really any field equation you want to write down for gravity, is going to be a local equation and won't tell you about the topology.  You have to go measure it separately.  So even if you want to shrink the error bars to zero and say that the universe is spatially flat (but expanding), you have to go do a separate measurement to tell if there is nontrivial topology.  As far as we can tell so far, the universe is indistinguishable from an infinite one, but that could be the same as finite but bigger than our ability to see until the universe is a whole lot older.


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## Herobizkit (Oct 14, 2015)

How might one calculate the gravitational effects, if any, of multiple moons on a theoretical Earth-like planet?  Would such moons affect said Earth in any other perceptible way (like, for example, what would tides be like with multiple moons?)


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## freyar (Oct 14, 2015)

Herobizkit said:


> How might one calculate the gravitational effects, if any, of multiple moons on a theoretical Earth-like planet?  Would such moons affect said Earth in any other perceptible way (like, for example, what would tides be like with multiple moons?)




You can just add up the gravitational effects of the individual moons using Newton's law of gravitation.*  Assuming normalish moon sizes and distances, tides would really be the main noticeable effect on the "host" planet.  And you're right to think the tides would be more complicated; basically, you have to add the tidal effects of all the moons together at any given time.  So, as the moons change positions around the planet relative to each other (sometimes they are lined up or not), the tides will be add up or cancel out to an extent.  This is kind of like how the relative positions of the sun, earth, and moon affect our real tides --- tides are stronger then they make a line and weaker when the earth is at the vertex of a right angle.  But the pattern in time would be more complicated with multiple moons.

*I'm assuming normal-sized moons.  If your "planet" and "moons" are neutron stars, though, you'd probably have to think about general relativity.  Then things get hairy mathematically.


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## Umbran (Oct 14, 2015)

freyar said:


> This isn't really correct.  There are finite, _flat_ spaces with nontrivial topology that solve Einstein's equations with _zero_ mass or energy density.  A torus is an example.




Well, be careful there - the torus (and the 9 other such topologies that satisfy the conditions) do it by *combining* curvatures.  The torus is locally flat, globally flat, but "regionally" curved - on the inside of the torus, around the hole, you have negative curvature (you see the typical "saddle" of negative curvature around the donut hole), and the outer side shows positive curvature.  The sum total effect is net zero.

We tell ourselves that, in theory, the thing can come about by having a density parameter of 1.  But, that can be done two ways - by having it be 1 *everywhere*, or having it be a *net* 1 with it being below 1 in some regions, and above that in others.  This latter is argued to be a more reasonable expectation for how you get a universe of that shape, as it is hard to imagine how the regional curvature physically rises without the appropriate energy density.


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## freyar (Oct 14, 2015)

Umbran said:


> Well, be careful there - the torus (and the 9 other such topologies that satisfy the conditions) do it by *combining* curvatures.  The torus is locally flat, globally flat, but "regionally" curved - on the inside of the torus, around the hole, you have negative curvature (you see the typical "saddle" of negative curvature around the donut hole), and the outer side shows positive curvature.  The sum total effect is net zero.




Absolutely not.  You're talking about _extrinsic_ curvature, the curvature of the embedding of a 2D torus in 3D flat space.  The torus itself has *zero* _intrinsic_ curvature, and that's what GR cares about.  We can see the curvature of the torus easily: a 2D torus has metric ds^2 = dx^2 + dy^2 for 0<= x <2 Pi R_x and 0<= y <2 Pi R_y with x and y both identified at their edges (meaning that all functions on (x,y) have periodicity 2 Pi R_x in the x direction and 2 Pi R_y in the y direction).  As you know, that's the flat metric.  

If we're thinking of (the spatial part of) our universe as a 3D torus, we shouldn't be thinking of it as embedded in a larger-dimensional flat space, since that higher-dimensional space might not exist.  The torus defined by this type of mathematical quotient is perfectly flat _everywhere_.  This you can trust me on --- several of my research papers deal with tori.  Of course, if you want an independent source, there's always the wikipedia article.  The first section deals with the embedding of a 2-torus in 3D flat space, then you get the mathematical definitions, then generalizations, etc.


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## Landifarne (Oct 14, 2015)

Interesting question from one of my AP Chemistry kids (12th grader, dreamer-stoner type, but the guy often has intriguing ideas):

My student understands that, when discussing electron orbitals in an introductory manner, we are not terribly concerned that indivdual orbitals (and their corresponding nodes) in a polyelectronic atom ignore the effects of all the atom's other electrons/orbitals [i.e. we assume that the actual orbitals and nodes that do exist more-or-less resemble a simple superimposing of the individually calculated orbitals].

So, he asked me a month ago whether (again, assuming the above) we could gain any insight by ignoring the orbital shapes/structures and, instead, focussed on how the nodes interact/interrelate.

I paused for a while and thought about how x-ray diffraction patterns give insight into crystal lattice structures and molecular structures (interferometry). There must be an analogy there, somewhere...

Anyway, any of you have thoughts on what my kid was getting at? Is there some kind of "inverted" Schrodinger Equation that would focus more on the characterization of the nodes?


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## freyar (Oct 14, 2015)

Landifarne said:


> Interesting question from one of my AP Chemistry kids (12th grader, dreamer-stoner type, but the guy often has intriguing ideas):
> 
> My student understands that, when discussing electron orbitals in an introductory manner, we are not terribly concerned that indivdual orbitals (and their corresponding nodes) in a polyelectronic atom ignore the effects of all the atom's other electrons/orbitals [i.e. we assume that the actual orbitals and nodes that do exist more-or-less resemble a simple superimposing of the individually calculated orbitals].
> 
> ...




That is an interesting thought.  I'm not aware of anything that really just deals with the nodes, though there are a lot of different approaches in quantum chemistry, so it's possible I just don't know of one.  But I'm not terribly sure it would be useful, either.  You really need the whole wavefunction in some kind of approximation to get the energy levels.  Measurement-wise, knowing where the nodes are might help, but I don't know of a way to make that measurement in analogy to diffraction, either.

The big issue is the approximation that you can just use hydrogen-like orbitals filled up one at a time without interaction.  That generally doesn't work very well at all quantitatively.  It gives some rough guidelines for qualitative chemical behavior (hence the periodic table), but the numbers don't come out well.


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## Umbran (Oct 15, 2015)

freyar said:


> This you can trust me on




I don't need to.  It isn't like I don't have my own texts and notes - just sometimes I should double check them before writing.  I was conflating two things from a course years ago, without realizing it.  Carry on.


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## freyar (Oct 15, 2015)

Umbran said:


> I don't need to.  It isn't like I don't have my own texts and notes - just sometimes I should double check them before writing.  I was conflating two things from a course years ago, without realizing it.  Carry on.




No problem, happens to all of us from time to time.


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## Landifarne (Oct 15, 2015)

freyar said:


> That is an interesting thought.  I'm not aware of anything that really just deals with the nodes, though there are a lot of different approaches in quantum chemistry, so it's possible I just don't know of one.  But I'm not terribly sure it would be useful, either.  You really need the whole wavefunction in some kind of approximation to get the energy levels.  Measurement-wise, knowing where the nodes are might help, but I don't know of a way to make that measurement in analogy to diffraction, either.
> 
> The big issue is the approximation that you can just use hydrogen-like orbitals filled up one at a time without interaction.  That generally doesn't work very well at all quantitatively.  It gives some rough guidelines for qualitative chemical behavior (hence the periodic table), but the numbers don't come out well.





Yeah, it was interesting. Neat to think that we could characterize/fingerprint a particular atom by the nodal surfaces and the surfaces' intersections. I hadn't heard anyone ever speak of such a thing and was wondering if the resulting geometries/topologies had any significance, relation or analogy to higher level stuff.


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## CaptainGemini (Oct 19, 2015)

Something that has me curious...

Conservation of mass and conservation of energy... Is it possible those two laws of conservation prevent all forms of time travel? That you cannot travel through time because, from the laws of physics, it would be destroying mass and energy in one time and creating them in another?


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## AbdulAlhazred (Oct 19, 2015)

CaptainGemini said:


> Something that has me curious...
> 
> Conservation of mass and conservation of energy... Is it possible those two laws of conservation prevent all forms of time travel? That you cannot travel through time because, from the laws of physics, it would be destroying mass and energy in one time and creating them in another?




Modern notions of time travel all involve the construction of some sort of 'closed time-like curve', that is some sort of 'wormhole' into the past. So you would literally pass through. The question then is at what point would you say something had been created or destroyed? 

Truthfully there are vastly many things that would be broken by time travel, including every single known conservation law I believe, at least from some observer's perspective. This is why physicists are mostly pretty down on the whole concept, it seems most likely to be something that "just can't happen". Again Noether's Theorum raises its head here, if these conservation laws are broken, then their dual symmetries are also broken.

There is an 'out' here though. As best we can interpret the evidence the Universe is truly vast, and the part we can see, and ever be causally connected to, is only a tiny fraction (something on the order of one part in 10^120th power parts) of all that there is. If the Universe is really so extensive, then almost every conceivable configuration of matter must exist somewhere within it. That would include, say, a planet exactly like Earth except just like Earth was 3 weeks ago. If you could go there, would that be time travel? Opening up a wormhole to such a place and stepping through, would that violate any conserved property of the Universe? It would seem not. 

So, perhaps, we could go any place outside of the portion of the Universe causally connected to Earth. We could go there regardless of any limitations imposed by the speed of light or any other consideration of causality. You just have to find a way to build such a 'wormhole'. Of course there likely simply is no such way, but we really don't know enough to rule it out.


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## Umbran (Oct 19, 2015)

AbdulAlhazred said:


> Modern notions of time travel all involve the construction of some sort of 'closed time-like curve', that is some sort of 'wormhole' into the past.




Well, wormholes are the most popularized, but there are other solutions that have closed time-like curves that don't involve wormholes.  In these, you'd just be flying along, and when you finished flying, you'd find you landed before you took off.

And, conversely, not all use of wormholes end up with closed time-like curves.  



> There is an 'out' here though. As best we can interpret the evidence the Universe is truly vast, and the part we can see, and ever be causally connected to, is only a tiny fraction (something on the order of one part in 10^120th power parts) of all that there is. If the Universe is really so extensive, then almost every conceivable configuration of matter must exist somewhere within it. That would include, say, a planet exactly like Earth except just like Earth was 3 weeks ago. If you could go there, would that be time travel? Opening up a wormhole to such a place and stepping through, would that violate any conserved property of the Universe?




"Travel to a place that is a lot like what my world 3 weeks ago," doesn't really violate anything.  In and of itself, it is just going to another planet.  However, if you go there *faster than light could get there*, then we have the same problems as any other travel faster than light.  The nature of the destination isn't the issue, it's how fast you get there that matters.



> So, perhaps, we could go any place outside of the portion of the Universe causally connected to Earth.




Note that, as soon as you go to a place, that place is causally connected to Earth.  You came from Earth, and you can now cause events there.


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## AbdulAlhazred (Oct 19, 2015)

Umbran said:


> Well, wormholes are the most popularized, but there are other solutions that have closed time-like curves that don't involve wormholes.  In these, you'd just be flying along, and when you finished flying, you'd find you landed before you took off.
> 
> And, conversely, not all use of wormholes end up with closed time-like curves.



I didn't mean to imply that wormholes MUST have 'time-like' world lines. I'm not sure about 'other solutions', there are a LOT of different formulations of GR, including rotating space time that could produce various results. Wormholes are at least the one that I know of that is localized. In any case they are probably impossible.



> "Travel to a place that is a lot like what my world 3 weeks ago," doesn't really violate anything.  In and of itself, it is just going to another planet.  However, if you go there *faster than light could get there*, then we have the same problems as any other travel faster than light.  The nature of the destination isn't the issue, it's how fast you get there that matters.



I'm not so sure about that. Remember, we're talking about a part of the Universe which has never been in communication with the part we inhabit since the start of inflation, no photons have ever been exchanged, and none ever could be. So, if you took some action somewhere in that space time which would violate causality, etc in OUR space time's past, no observer would ever be able to tell, because they inherently cannot compare the events in the two places, there isn't any light cone anywhere that overlaps both. 

I think you would have to have some inertial frame of reference in which an actual observer could exist who would observe some violation of causality or some conservation law in order for a 'problem' to exist. Because the two areas of space time don't interact with each other or any other reference frame that interacts with either one of them, a sort of loophole is created. Such areas of the Universe, because they cannot exchange information with us, don't really exist in some formal sense as far as we're concerned. At least one COULD look at it that way. Obviously we don't know what is or isn't possible, and we haven't even a clue how one would open up a 'pathway' of some sort to such a remote location. Its just as likely such considerations are moot because it simply isn't possible.



> Note that, as soon as you go to a place, that place is causally connected to Earth.  You came from Earth, and you can now cause events there.




As to how they would become connected going forward, sure they would, but so what? There's no 'paradox' there. If you kill 'alternate grandfather' it will surely mean that 'alternate you' won't come into existence (or whatever) but 'prime you' still has a grandfather, prime grandfather. I would at least vote for their being no violations of conservation laws either, because, as you say, the two areas of space time ARE now connected. Still, for your own purposes, if you wished to 'go back in time and kill your grandfather' you can now fully enjoy the benefits of having done so, as long as you aren't concerned about y


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## Landifarne (Oct 20, 2015)

AbdulAlhazred said:


> ...
> If the Universe is really so extensive, then almost every conceivable configuration of matter must exist somewhere within it. That would include, say, a planet exactly like Earth except just like Earth was 3 weeks ago.
> ...




I'm sorry, but I don't find this argument at all convincing.

You're suggesting that an infinite number of "Similar Earths" exist out there, and that an infinite number of branches have continuously broken away from them every fraction of a second for nearly 5 billion years (or, perhaps ~14 billion years). Correct me if I'm wrong, but that's infinity to an infinite number of infinite powers...which is a tad larger than 10^120 * ("the known universe.")


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## AbdulAlhazred (Oct 20, 2015)

Landifarne said:


> I'm sorry, but I don't find this argument at all convincing.
> 
> You're suggesting that an infinite number of "Similar Earths" exist out there, and that an infinite number of branches have continuously broken away from them every fraction of a second for nearly 5 billion years (or, perhaps ~14 billion years). Correct me if I'm wrong, but that's infinity to an infinite number of infinite powers...which is a tad larger than 10^120 * ("the known universe.")




No, you're talking about a 'many worlds' interpretation of quantum mechanics. I'm talking about the sheer brute physical size of the single Universe we inhabit. If our ideas of its scale are correct, then it is SO VAST that almost every conceivable configuration of matter which could possibly exist under the laws of physics must be actualized somewhere. 

Its NOT an infinite number however. There is some huge percentage of the Universe that is outside our light cone and thus causally disconnected from us (we cannot even in principle know what exactly is happening there, and as long as the Universe continues to exist no information will ever be exchanged with those locations). 10^120th power is a truly unimaginably vast number BTW. I'm not sure exactly where that number was derived from, like many such numbers it is probably some guesstimate or other, so I wouldn't put too much emphasis on it, except in the sense of being 'really really huge'. 

I seem to recall a Sci-Am that had some authors discussing different sorts of 'Multiverse'. The terminology considered this the simplest form, physically disconnected areas of a single space-time, though you could quibble that it isn't really 'outside our Universe' in some sense. Level 2 in that scheme is IIRC your 'many worlds' or other similar concepts, then we have string theory concepts of the 'bulk', and finally one could imagine 'Universes' where the rules of logic we think by simply don't apply or apply differently, which would be well beyond string-theoretical concepts of differing fundamental constants.


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## Umbran (Oct 20, 2015)

AbdulAlhazred said:


> I seem to recall a Sci-Am that had some authors discussing different sorts of 'Multiverse'.




Yep.  By Max Tegmark, in SciAm May 2003, if no other time....

http://space.mit.edu/home/tegmark/PDF/multiverse_sciam.pdf

Other references...

https://www.youtube.com/watch?v=w3TDO1AA1Sw

http://physics.about.com/od/astronomy/f/ParallelUniverseTypes.htm

http://www.space.com/18811-multiple-universes-5-theories.html

http://space.mit.edu/home/tegmark/crazy.html


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## tomBitonti (Oct 20, 2015)

AbdulAlhazred said:


> No, you're talking about a 'many worlds' interpretation of quantum mechanics. I'm talking about the sheer brute physical size of the single Universe we inhabit. If our ideas of its scale are correct, then it is SO VAST that almost every conceivable configuration of matter which could possibly exist under the laws of physics must be actualized somewhere.
> 
> Its NOT an infinite number however. There is some huge percentage of the Universe that is outside our light cone and thus causally disconnected from us (we cannot even in principle know what exactly is happening there, and as long as the Universe continues to exist no information will ever be exchanged with those locations). 10^120th power is a truly unimaginably vast number BTW. I'm not sure exactly where that number was derived from, like many such numbers it is probably some guesstimate or other, so I wouldn't put too much emphasis on it, except in the sense of being 'really really huge'.
> 
> I seem to recall a Sci-Am that had some authors discussing different sorts of 'Multiverse'. The terminology considered this the simplest form, physically disconnected areas of a single space-time, though you could quibble that it isn't really 'outside our Universe' in some sense. Level 2 in that scheme is IIRC your 'many worlds' or other similar concepts, then we have string theory concepts of the 'bulk', and finally one could imagine 'Universes' where the rules of logic we think by simply don't apply or apply differently, which would be well beyond string-theoretical concepts of differing fundamental constants.




Whoa there Nelly.  I didn't think that the proposed scales (120b ly?) are anywhere near big enough to provide room for "almost every conceivable configuration of matter".  I don't even think that 10^120 comes close to being big enough for that, even.  How many plank units are there in a 20b ly radius?  What is 2^that number?

I do think there start to be philosophic problems, of whether it is proper to consider stuff outside of our range of interaction to be "in our universe".  That stuff would be implied because we have an easier time of mentally picturing the state of the universe as a whole, which is a psychological need more-so than a testable physical one.

As a slight tangent, something that doesn't ever seem to arise in the many-universes interpretation is whether to consider different regions which are identical as actually the same (issue number 1), and how the whole ensemble is connected (it seems multiply connected) (issue number 2), and how vastly big such an ensemble would be (as every point would be simultaneously branching to every possible outcome) (issue number 3).  Just beginning to imagine the topology of the ensemble is difficult, let alone getting a clear description of the whole.

Thx!
TomB


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## freyar (Oct 21, 2015)

Without getting into all of it, a truly infinite space/universe in the sense of infinite size geometry poses a lot of questions about how to think of probabilities (since everything is copied over somewhere).  There are people who think about that.  It's a tough problem, though.

To TomB: the many-world interpretation isn't about different regions of space.  It's about the quantum state of the universe becoming more and more complicated.  So geometric notions of "connected" and "topology" don't apply at least in the usual sense.  The state is just some vector in an infinite-dimensional space, and it continuously moves in a direction that looks complex from the point of view of separate parts of the universe.  I'm sure I'm not explaining that very well, but I didn't sleep well last night and am at something of a loss to do better right at the moment.  There are some geometrical concepts that can be adapted to entanglement, but my understanding is that they're pretty far from what you would normally think of.  But it's unquestionably *not* about almost-duplicates of the universe in different regions of space.


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## Landifarne (Oct 21, 2015)

Uh, yeah, I fully understand the magnitude of 10^120. That approximates the number of atoms in all of the galaxies in about 10^45 visible universes. One visible universe has about (~10^24 atoms/kg of matter)(10^30 kg of matter/star)(10^11stars/galaxy)(10^10galaxies in the visible universe) = 10^75 atoms in its galaxies. I can't comment on all the dark matter, but one of the pros can easily rectify that.

Despite that, not a single "Similar Earth" would have followed the same "path of Earth up to 3 weeks prior." To suggest that one would have done that is ludicrous, as the entropic variation in the positional states of the atoms of that single "Similar Earth" exceeds the 10^45 factor, alone.


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## freyar (Oct 21, 2015)

Landifarne said:


> Uh, yeah, I fully understand the magnitude of 10^120. That approximates the number of atoms in all of the galaxies in about 10^45 visible universes. One visible universe has about (~10^24 atoms/kg of matter)(10^30 kg of matter/star)(10^11stars/galaxy)(10^10galaxies in the visible universe) = 10^75 atoms in its galaxies. I can't comment on all the dark matter, but one of the pros can easily rectify that.
> 
> Despite that, not a single "Similar Earth" would have followed the same "path of Earth up to 3 weeks prior." To suggest that one would have done that is ludicrous, as the entropic variation in the positional states of the atoms of that single "Similar Earth" exceeds the 10^45 factor, alone.




I think I missed where the 10^120 came in. That's the factor between the cosmological constant and the Planck scale (roughly), but the size of the universe isn't related to that really.  When people talk about duplicate earths, etc, they're generally talking about truly infinite universes.


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## Umbran (Oct 21, 2015)

Landifarne said:


> Uh, yeah, I fully understand the magnitude of 10^120.




As an aside, that's a bold claim.  Generally speaking, when a number gets that large, humans *don't* really understand the magnitude.  They are just numbers unless you make a lot of effort to set scales.


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## AbdulAlhazred (Oct 21, 2015)

> The least controversial category of multiverse in Tegmark's scheme is Level I, which describes distant spacetime events "in our own universe", but suggests that statistical analysis exploiting the anthropic principle provides an opportunity to test multiverse theories in some cases. If space is infinite, or sufficiently large and uniform, identical instances of the history of Earth's entire Hubble volume occur every so often, simply by chance. Tegmark calculated our nearest so-called doppelgänger, is 10^10^115 meters away from us (a double exponential function larger than a googolplex). In principle, it would be impossible to scientifically verify an identical Hubble volume. However, it does follow as a fairly straightforward consequence from otherwise unrelated scientific observations and theories.



https://en.wikipedia.org/wiki/Multiverse#Tegmark.27s_classification

There is this... and the original reference http://arxiv.org/abs/astro-ph/0302131, which references 10^10^29 meters. 

Now, we don't know that the Universe spans 10^10^29th (let alone 10^10^115th) meters, but the point is even if it doesn't there could be Hubble Volumes almost indistinguishable from ours MUCH closer than that, etc. 

My only point was that there are interestingly many ways to think about things like 'time travel'.


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## Umbran (Oct 21, 2015)

AbdulAlhazred said:


> My only point was that there are interestingly many ways to think about things like 'time travel'.




Well, time travel is still implicit.  You are talking about reaching places outside the observable universe, which means outside Earth's light cone.  That implies FTL travel, and thus time travel becomes possible.


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## AbdulAlhazred (Oct 22, 2015)

Umbran said:


> Well, time travel is still implicit.  You are talking about reaching places outside the observable universe, which means outside Earth's light cone.  That implies FTL travel, and thus time travel becomes possible.




Well, that's a good question. What's the 'distance' to a point in space-time where we absolutely cannot measure? How 'fast' do you have to go to get there? Is it possible that even though FTL/time travel to areas which ARE causally connected to us is impossible (as the fact that it causes paradoxes and violations of conservation laws suggests), but travel to areas OUTSIDE that region might be technically 'legal'.

Not that I don't understand what you're getting at, nor that I necessarily disagree with you. Still, in an RPG you could still at least use it as a fig leaf


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## freyar (Oct 22, 2015)

AbdulAlhazred said:


> Well, that's a good question. What's the 'distance' to a point in space-time where we absolutely cannot measure? How 'fast' do you have to go to get there? Is it possible that even though FTL/time travel to areas which ARE causally connected to us is impossible (as the fact that it causes paradoxes and violations of conservation laws suggests), but travel to areas OUTSIDE that region might be technically 'legal'.
> 
> Not that I don't understand what you're getting at, nor that I necessarily disagree with you. Still, in an RPG you could still at least use it as a fig leaf




FTL travel by definition takes you from one spacetime point to another that's out of causal contact with the first one.  That is, it takes you out of the future light cone of the first point.


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## Umbran (Oct 22, 2015)

AbdulAlhazred said:


> Well, that's a good question. What's the 'distance' to a point in space-time where we absolutely cannot measure?




I think you are conflating practical human limitations with physical reality.  The fact that we cannot physically measure it does not imply that somehow the meaning of distance changes.  

Consider, for example, that you can measure to the edge of the visible universe.  If you go just a tad beyond that, you can then measure to the end of your new visible universe, and *add*.  Lather, rinse, repeat, and you can stepwise measure any distance.  




> Is it possible that even though FTL/time travel to areas which ARE causally connected to us is impossible




As freyar mentioned - FTL travel is, by definition, travel to a place that is not causally connected with your origin.  Causal connection is a thing of space_time_, not just space.  The Sun is about 8 light minutes away.  If I get to the Sun in six minutes, I'm at a point that is not causally connected with the point on Earth from which I left.  And "point" in this case is "point in space *and* time".


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## AbdulAlhazred (Oct 22, 2015)

Umbran said:


> I think you are conflating practical human limitations with physical reality.  The fact that we cannot physically measure it does not imply that somehow the meaning of distance changes.
> 
> Consider, for example, that you can measure to the edge of the visible universe.  If you go just a tad beyond that, you can then measure to the end of your new visible universe, and *add*.  Lather, rinse, repeat, and you can stepwise measure any distance.



Except you CANNOT DO THAT, not even in principle. I think this reasoning is incorrect because its not 'human limitations' we're talking about here. It is limitations built into the rules by which the Universe fundamentally operates. Rules that operate at the same level as these various other rules like causality and thermodynamics. I'm not saying "your wrong", but I'm saying that there's a fundamental difference between say not being able to measure the distance to the Moon because we don't know how and not being able to measure the distance to a point outside our light cone because its fundamentally disallowed.



> As freyar mentioned - FTL travel is, by definition, travel to a place that is not causally connected with your origin.  Causal connection is a thing of space_time_, not just space.  The Sun is about 8 light minutes away.  If I get to the Sun in six minutes, I'm at a point that is not causally connected with the point on Earth from which I left.  And "point" in this case is "point in space *and* time".




Yes, I understand this. There are however regions of space, presumably, which are 'causally disconnected' from us. They are FOREVER outside our light cone. If I travelled instantly to the Sun then in 8 minutes light from that location would arrive and there would be different inertial frames of reference in which causality, conservation of angular momentum, etc would be violated. If I travel instantly a googleplex lightyears from here, that will NEVER HAPPEN. So is such travel disallowed for the same reasons that travel 8 light minutes from here seems to be? I question that! Again, its a question, not a statement of some fact that I think I've uncovered or some crackpot theory that I insist must be true. It is really at this point more of a philosophical question almost than a science question, until some unforeseen time when we invent a way to do it. 

Anyway, I feel like I'm moving the thread rather far from where it was intended to be, I don't want to derail it.


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## Umbran (Oct 22, 2015)

AbdulAlhazred said:


> Except you CANNOT DO THAT, not even in principle.




In the normal universe, yes.

But you're positing a universe where _going there is possible_.  That possibility throws the principle out the window!  We cannot measure the distance only because FTL travel is not possible - as soon as you allow FTL to the spacial location, then the measurement becomes possible in principle.

Which is to say, you cannot stand by the principle, and break the principle, in the same breath and remain consistent.  



> It is limitations built into the rules by which the Universe fundamentally operates.




Which the stipulated travel breaks.



> There are however regions of space, presumably, which are 'causally disconnected' from us. They are FOREVER outside our light cone.




I think that, given the assumptions of Special and General Relativity (in which context we are talking about light cones), that the difference isn't relevant.  So long as the space between is continuous, smooth, and the same rules of physics apply in all of it, then "forever outside" and "temporarily outside" are not fundamentally different.  If those assumptions do not hold, then we have to hld the whole idea of the light cone as suspect.

Specifically, I believe you can construct the following scenario:  Point A and Z in space are so far apart, that they never fall into each other's future light cone.  There is a point M, between them, that is, at the time of your travel, is not in the light cone of either A or Z, but will eventually lie in the light cones of both.  A stepping stone, so to speak, that allows the piecewise measurement.  I think that you can always generate a string of such stepping stones, so long as the distance between A and Z is not *actually* infinite.


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## freyar (Oct 22, 2015)

AbdulAlhazred said:


> Yes, I understand this. There are however regions of space, presumably, which are 'causally disconnected' from us. They are FOREVER outside our light cone. If I travelled instantly to the Sun then in 8 minutes light from that location would arrive and there would be different inertial frames of reference in which causality, conservation of angular momentum, etc would be violated. If I travel instantly a googleplex lightyears from here, that will NEVER HAPPEN. So is such travel disallowed for the same reasons that travel 8 light minutes from here seems to be? I question that! Again, its a question, not a statement of some fact that I think I've uncovered or some crackpot theory that I insist must be true. It is really at this point more of a philosophical question almost than a science question, until some unforeseen time when we invent a way to do it.




Two quick points: 
(1) it's not at all clear that our universe has areas forever out of causal contact with earth.  For example, in a universe with a cosmological constant, people on earth will eventually be able to see the entire universe, even though it will take infinite time to see an infinite distance away.  That's just saying that we need to wait 8 minutes to see the sun, but we need to wait umpteen-gazillion years to see something umpteen-gazillion lightyears away, and we need to wait longer to see farther.  Off the top of my head, I am not thinking of an easy way to change that, either, but I admit I'd have to look at a few papers to say more than that.  (Unless you are maybe talking about disconnected components of the universe, where there is literally no space between them.)

(2) In a relativistic theory, if you can travel FTL to *anywhere*, no matter how far away (whether forever out of causal contact with your initial point or not), you can build a time machine by using FTL travel, a rocket, and more FTL travel.


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## tomBitonti (Oct 22, 2015)

Umbran said:


> I think that, given the assumptions of Special and General Relativity (in which context we are talking about light cones), that the difference isn't relevant.  So long as the space between is continuous, smooth, and the same rules of physics apply in all of it, then "forever outside" and "temporarily outside" are not fundamentally different.  If those assumptions do not hold, then we have to hld the whole idea of the light cone as suspect.
> 
> Specifically, I believe you can construct the following scenario:  Point A and Z in space are so far apart, that they never fall into each other's future light cone.  There is a point M, between them, that is, at the time of your travel, is not in the light cone of either A or Z, but will eventually lie in the light cones of both.  A stepping stone, so to speak, that allows the piecewise measurement.  I think that you can always generate a string of such stepping stones, so long as the distance between A and Z is not *actually* infinite.




My apologies for any mangling of terminology:

I *think* we are getting in trouble here, mixing notions of points in space-time with points in space.  Reaching a point seems to be shorthand for reaching a point on the geodesic of that point at time zero, with given spatial coordinates, subject to some selection of coordinate systems.  That shorthand works pretty well for everyday stuff, but not on a cosmological scale.  In flat spacetime, the light cone of a given point always intersects the geodesic of an arbitrary point (from simple geometry), but, that is not true for an expanding universe (nor, for example, for light cones originating from points inside black holes).

(Although, that is only true of geodesics; a photon which is emitted from a given point may be unreachable from another point, for example, if the photon were emitted pointing away from the second point.)

Thx!
TomB


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## AbdulAlhazred (Oct 22, 2015)

Umbran said:


> In the normal universe, yes.
> 
> But you're positing a universe where _going there is possible_.  That possibility throws the principle out the window!  We cannot measure the distance only because FTL travel is not possible - as soon as you allow FTL to the spacial location, then the measurement becomes possible in principle.
> 
> Which is to say, you cannot stand by the principle, and break the principle, in the same breath and remain consistent.



Eh, I think you'd have to take into account the mechanism by which you undertook this travel. Given that we have no such mechanisms and don't know what they would actually entail its hard to speculate, but suppose you opened a wormhole to such a remote place, you'd never be able to measure the distance there.




> I think that, given the assumptions of Special and General Relativity (in which context we are talking about light cones), that the difference isn't relevant.  So long as the space between is continuous, smooth, and the same rules of physics apply in all of it, then "forever outside" and "temporarily outside" are not fundamentally different.  If those assumptions do not hold, then we have to hld the whole idea of the light cone as suspect.



Well, this may be true, but I think its not really established. Again, questions of definitions like 'space between'. Science can only work with what is observable, what isn't observable in some fashion doesn't exist in some fashion either. Perhaps the effects of this 'continuous, smooth space' ARE significant and it is exactly that, real. I don't know. When one starts talking about places you can never go or see and things you can't ever measure the philosophical ice gets real thin for everyone . 



> Specifically, I believe you can construct the following scenario:  Point A and Z in space are so far apart, that they never fall into each other's future light cone.  There is a point M, between them, that is, at the time of your travel, is not in the light cone of either A or Z, but will eventually lie in the light cones of both.  A stepping stone, so to speak, that allows the piecewise measurement.  I think that you can always generate a string of such stepping stones, so long as the distance between A and Z is not *actually* infinite.




BUT there will demonstrably be no way to communicate any information from A to Z, even using these intermediary points, as the expansion of space will require that you establish new intermediary points at a rate that is impossible, the number of them will grow to infinity before any information passes along the chain. Any two points which are NOW not every going to be in contact again will fall into this category, you can't get around the speed of light. The distance between A and Z is not actually 'infinite', but it is growing at greater than C. 

Now, necessarily there was some point in the past where all these points A, Z, and all the ones in between WERE in communication with each other, around 10^-34 seconds after the big bang as I recall, when the Universe was in thermal equilibrium before inflation blew it up. Some points may have been in contact long after that, indeed some fall over our horizon all the time even now. Maybe that matters, maybe not so much, particularly with points we've not been in contact with since the start of inflation.


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## freyar (Oct 22, 2015)

tomBitonti said:


> My apologies for any mangling of terminology:
> 
> I *think* we are getting in trouble here, mixing notions of points in space-time with points in space.  Reaching a point seems to be shorthand for reaching a point on the geodesic of that point at time zero, with given spatial coordinates, subject to some selection of coordinate systems.  That shorthand works pretty well for everyday stuff, but not on a cosmological scale.  In flat spacetime, the light cone of a given point always intersects the geodesic of an arbitrary point (from simple geometry), but, that is not true for an expanding universe (nor, for example, for light cones originating from points inside black holes).
> 
> (Although, that is only true of geodesics; a photon which is emitted from a given point may be unreachable from another point, for example, if the photon were emitted pointing away from the second point.)




From what I read of Umbran's and AbdulAlhazred's posts, I'm pretty sure we haven't run into that confusion.  The conversation seems pretty clear to me.  And I know I've been careful on that issue (I work on this all the time, after all).  Of course, with FTL travel, there's less of this kind of distinction.  But, anyway, talking about causal connection always requires talking about the space and time coordinates of a point.


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## AbdulAlhazred (Oct 22, 2015)

freyar said:


> Two quick points:
> (1) it's not at all clear that our universe has areas forever out of causal contact with earth.  For example, in a universe with a cosmological constant, people on earth will eventually be able to see the entire universe, even though it will take infinite time to see an infinite distance away.  That's just saying that we need to wait 8 minutes to see the sun, but we need to wait umpteen-gazillion years to see something umpteen-gazillion lightyears away, and we need to wait longer to see farther.  Off the top of my head, I am not thinking of an easy way to change that, either, but I admit I'd have to look at a few papers to say more than that.  (Unless you are maybe talking about disconnected components of the universe, where there is literally no space between them.)
> 
> (2) In a relativistic theory, if you can travel FTL to *anywhere*, no matter how far away (whether forever out of causal contact with your initial point or not), you can build a time machine by using FTL travel, a rocket, and more FTL travel.




Oh yeah, I quite understand that FTL and time travel are effectively the same thing, and that ANY FTL travel results in violations of causality, conservation of momentum, and conservation of angular momentum (at least, maybe others too, I'm not sure). In fact IMHO I don't see how even things like Alcubierre Drive schemes would avoid this fact (they might work out OK for observers in the warp bubble and SOME observers outside it, but I think inevitably someone somewhere will see an inconsistency).

The question then is "if you build a wormhole to a causally disconnected location in space-time, will there be a meaningful violation of causality, etc?" In SOME sense the two points are 'in the same space-time continuum' and yet they are utterly out of touch with each other and thus any violations are effectively 'cloaked' forever by relativistic limitations of information transfer. Presumably if a wormhole works at all, the 'local channel' of information transfer doesn't really matter, since our nice closed time-like curve means we have essentially normal connectivity between the points at either end which isn't notionally different from any other such connectivity.

What gets you in trouble if you were to say build a wormhole to Alpha Centauri is that an observer somewhere observing each end of that wormhole WILL, as you point out, see something inconsistent (IE something will happen before its cause, or angular momentum won't be conserved, etc).

EDIT: Oh, yes, your first point about the future history of the Universe. This is a very good point of course. As far as we can determine today nothing will stop the Universe from expanding forever, and the rate of expansion will increase forever as well. We don't really know all that much though, and the interesting question then becomes "does forever count?" If, when you extend your light cone to eternity it overlaps with all the others, but only after literally infinite time, does that mean anything? Does that create constraints on what can happen here and now today? That's an interesting question!


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## Umbran (Oct 22, 2015)

AbdulAlhazred said:


> Eh, I think you'd have to take into account the mechanism by which you undertook this travel. Given that we have no such mechanisms and don't know what they would actually entail its hard to speculate, but suppose you opened a wormhole to such a remote place, you'd never be able to measure the distance there.




"Never" is a bit broad, especially when you have already noted that you have to take into account the mechanism.  You are positing that you can open a wormhole, but that wormholes so opened do so to *random* places in spacetime?  That sounds plausible to you?  You wouldn't instead presume that the mechanism for opening a wormhole and keeping it open would have to be known, and thus controlled?  Or that you could not infer something from measurements of the wormhole itself?

Actually, this generalizes, so that the mechanism actually doesn't really matter.



> Science can only work with what is observable, what isn't observable in some fashion doesn't exist in some fashion either.




More like, what isn't observable isn't science - if we cannot even in principle observe it, we are in the realm of the non-falsifiable.

So, in order to question the science, you bring in something unobservable - a mechanism for travel where you don't know where you are going?  

In reality, science deals with a *lot* of things that are not directly observable, but can be inferred from observations.  Nobody, for example, has ever observed a lone quark.  It is expected by many that seeing them, even in principle, is not possible.




> BUT there will demonstrably be no way to communicate any information from A to Z




If we are all limited to normal sub-light speed travel, you are correct.  There is never any way to communicate from A to Z.

But *you* are then positing what happens when you go to Z!  

I then say, any method you can use to go to Z, I can in principle use to measure the distance.  The ability to travel to some arbitrary point in spacetime implies the ability to measure the distance.


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## AbdulAlhazred (Oct 22, 2015)

Umbran said:


> "Never" is a bit broad, especially when you have already noted that you have to take into account the mechanism.  You are positing that you can open a wormhole, but that wormholes so opened do so to *random* places in spacetime?  That sounds plausible to you?  You wouldn't instead presume that the mechanism for opening a wormhole and keeping it open would have to be known, and thus controlled?  Or that you could not infer something from measurements of the wormhole itself?
> 
> Actually, this generalizes, so that the mechanism actually doesn't really matter.
> 
> ...




Well, of course these are all perfectly good points . I think there are processes which are fundamentally incalculable though, like the exact shape of a cloud which will appear over a given point on the surface of the Earth a week from now, no amount of detailed information about the current state of the Earth will let you calculate that, not even in principle (maybe a week is too little, its hard to say, but I know enough about non-linear dynamical systems to know that there are FUNDAMENTALLY incalculable results). So maybe a 'wormhole to anywhere' really is perfectly feasible. It goes SOMEWHERE, but you won't know where until after you open it, and if the only openings have to be outside causally connected space... Its sort of like the weak anthropic principle, I'm just saying "the opening is where the opening is, and that place meets the criteria for where an opening can be". 

Things like quarks of course ARE 'observable', in the same way that a book is observable, they have effects that can be sensed by our senses and thus affect changes in our mental state which represent our knowledge of their existence. The TRULY unobservable is different, and of course making statements about it is in some sense 'non-scientific', but there's that area of 'speculation' in which we use the rules we have derived from the process of science, as opposed to just say painting a dragon over there and talking about how cool the space dragon is. I think there's a meaningful difference there. So I would side with people who speculate about other Universes and etc calling those 'scientific' speculation. Not science precisely perhaps, and the difference is meaningful, but still not simple blind invention without reason.


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## CaptainGemini (Oct 23, 2015)

freyar said:


> (2) In a relativistic theory, if you can travel FTL to *anywhere*, no matter how far away (whether forever out of causal contact with your initial point or not), you can build a time machine by using FTL travel, a rocket, and more FTL travel.




To extend this to its logical conclusion, does that mean that the laws of conservation making time travel impossible would would mean that they make FTL travel impossible? I am asking to make certain I have the concept correct.

And, I apologize for having caused this long of a discussion on this thread with my question. That's twice I've caused such.


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## Umbran (Oct 23, 2015)

AbdulAlhazred said:


> Well, of course these are all perfectly good points . I think there are processes which are fundamentally incalculable though, like the exact shape of a cloud which will appear over a given point on the surface of the Earth a week from now, no amount of detailed information about the current state of the Earth will let you calculate that, not even in principle




Well, now you're getting poetic.  

You're also... quite possibly wrong.  The possibility that we live in a simulation is not one we can actually reject at this time.  

https://en.wikipedia.org/wiki/Simulation_hypothesis


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## Umbran (Oct 23, 2015)

CaptainGemini said:


> To extend this to its logical conclusion, does that mean that the laws of conservation making time travel impossible would would mean that they make FTL travel impossible? I am asking to make certain I have the concept correct.




Er.  We should be careful about how we say such things, really.  Sometimes I'm sloppy.

Technically, at the moment, we expect that those conservation laws hold strictly, and that FTL is not possible.  

We do not know for absolute certain they are both strictly and totally true.   If they are both strictly true, we do not know that one *causes* the other (and which way that causality goes), or if both are merely axioms of the universe (such that both must be true in order to have a consistent universe, and neither one really has priority), or if both are merely logical results of some even higher set of principles.

In reality, when we say, "You can't do time travel, because that violates causality/conservation, etc," what we are really saying is, if you plug in numbers faster than light, the rules as we understand them give results that do not make sense.  

But, there are ways...

As an example, we can imagine a universe with those conservation laws, in which time/FTL travel is possible, in a limited sense.  It turns out that so long as your time travel never violates causality, no conservation rules get broken.  This is equivalent to a universe in which no logical paradoxes occur as a result of time travel.  Robert L. Forward wrote a novel, _Timemaster_, that takes place in such a universe, that has a form of cosmic sensorship.  In the book, this manifests as a sort of predestination for the time traveller - he feels at every particular moment like he has free will, but once he starts time travelling, he really doesn't.

Forward was not the best at characterization, but for finding things to write about that seemed really wacky, but were technically allowed (as of the time of his writing, at least), he was pretty awesome.

We can, less easily, imagine a world in which the conservation laws do not *strictly* hold - that there are ways they may break.  Those ways must be pretty arcane, but they may be in the back corners of how things work, such that we haven't ever seen them.  But, maybe the ban on FTL still holds, for other reasons.


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## freyar (Oct 23, 2015)

CaptainGemini said:


> To extend this to its logical conclusion, does that mean that the laws of conservation making time travel impossible would would mean that they make FTL travel impossible? I am asking to make certain I have the concept correct.




In relativity (or any theory based on it), yes, FTL travel implies the ability to travel back in time.  There are also "time machine" spacetimes that don't require FTL travel, but most physicists in the field believe something prevents them from forming in a complete theory of quantum gravity (this is the chronology protection conjecture).


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## Umbran (Oct 23, 2015)

freyar said:


> There are also "time machine" spacetimes that don't require FTL travel, but most physicists in the field believe something prevents them from forming in a complete theory of quantum gravity (this is the chronology protection conjecture).




Yes, well, the alternative can make a physicist lose many hours of sleep.  A little belief instead of a lot of insomnia.  

I am not even sure it is really necessary to have the chronology protection conjecture, to be honest.  The constructions typically necessary for them are such that, really, you don't have to worry about them happening in reality.  As one of my professors long ago mentioned about Tipler's solution: "Infinite cylinders of neutronium don't just happen, and it isn't like anyone is ever going to be able to *make* one if they can't travel faster than light anyway."

Okay, if someone ever finds a cosmic string, that could be an issue.  But maybe even that's okay.


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## AbdulAlhazred (Oct 23, 2015)

CaptainGemini said:


> To extend this to its logical conclusion, does that mean that the laws of conservation making time travel impossible would would mean that they make FTL travel impossible? I am asking to make certain I have the concept correct.
> 
> And, I apologize for having caused this long of a discussion on this thread with my question. That's twice I've caused such.




Yes, ANY possible FTL travel produces the same paradoxes as (and effectively IS) time travel. If you can travel at FTL then from the perspective of some observer in some inertial frame of reference an effect will precede its cause. Beyond that you will violate the various basic conservation laws. These laws are unlikely to be just bypassable this way, as again Noether's Theorum's successful use in constructing symmetry laws from them suggests. IMHO we have effectively ruled out all forms of FTL travel to the very highest levels of confidence attainable in science. 

The only exception MIGHT be what I was suggesting, instantaneous travel to a causally disconnected location in space-time without traversing the intervening space. But of course any way of doing so is the purest fantasy at this point, requiring unobtainium of an extreme nature (negative energy mainly).


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## AbdulAlhazred (Oct 23, 2015)

Umbran said:


> Well, now you're getting poetic.
> 
> You're also... quite possibly wrong.  The possibility that we live in a simulation is not one we can actually reject at this time.
> 
> https://en.wikipedia.org/wiki/Simulation_hypothesis




And yet there are incalculable results. Lets understand what 'incalculable' means. It means "no calculating system which could be constructed within the physical universe could ever calculate this result" and obviously the universe itself can't host a machine that can simulate the same universe with perfect fidelity, so there's no proof in your statement that it can even calculate the state of a cloud next week.

Trivially most multi-body problems are not solvable. You cannot, even in principle, know where the Earth will be exactly a billion years from now, not even if you knew exactly every single gravitational interaction. Even if you had a universe that consisted of nothing but the Sun, the Moon, and the Earth, even then it would be impossible. You could, with infinite computational power and infinite time, drive the calculation to arbitrary levels of accuracy, but you cannot attain a perfect solution.

Things like the cloud are MUCH MUCH more complex than that. Its doubtful that, given relativistic and quantum limitations on basic computational power, that any conceivable machine can even calculate the shape of that cloud. 

I'm not saying these things are non-deterministic, just that they can't be calculated except by a complete full run of the entire universe from 0 to now.


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## AbdulAlhazred (Oct 23, 2015)

Umbran said:


> As an example, we can imagine a universe with those conservation laws, in which time/FTL travel is possible, in a limited sense.  It turns out that so long as your time travel never violates causality, no conservation rules get broken.  This is equivalent to a universe in which no logical paradoxes occur as a result of time travel.  Robert L. Forward wrote a novel, _Timemaster_, that takes place in such a universe, that has a form of cosmic sensorship.  In the book, this manifests as a sort of predestination for the time traveller - he feels at every particular moment like he has free will, but once he starts time travelling, he really doesn't.
> 
> Forward was not the best at characterization, but for finding things to write about that seemed really wacky, but were technically allowed (as of the time of his writing, at least), he was pretty awesome.



The real question there is, could you change ANYTHING AT ALL, or is effectively such censorship going to exclude all time travel (think for instance about the evolution of the Universal quantum wave function, you can't change anything without affecting that, and it is UNIVERSAL). The interconnections between things are vast, almost ubiquitous. Every photon is entangled with something else somewhere, etc. At best you wouldn't be able to change anything significant. Maybe you could use such a system as a 'spying device' to inform you of what happened in the past (and there was a story about doing that, but the hitch is even 1 millisecond is 'the past', so basically you can spy on anyone anywhere with such a device).



> We can, less easily, imagine a world in which the conservation laws do not *strictly* hold - that there are ways they may break.  Those ways must be pretty arcane, but they may be in the back corners of how things work, such that we haven't ever seen them.  But, maybe the ban on FTL still holds, for other reasons.




Well, the Universe, AS A WHOLE, can't be meaningfully said to obey the various conservation laws, so its definitely not clear exactly how restrictive they are. This may simply be a matter of our limited perspective or an incomplete formulation though. We just don't know.


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## Umbran (Oct 23, 2015)

AbdulAlhazred said:


> The real question there is, could you change ANYTHING AT ALL, or is effectively such censorship going to exclude all time travel




Impossible to tell.  While we can say, "every photon is entangled with something" that's a bit glib - with the statistical nature of quantum mechanics that really may not matter all that often.



> Well, the Universe, AS A WHOLE, can't be meaningfully said to obey the various conservation laws, so its definitely not clear exactly how restrictive they are. This may simply be a matter of our limited perspective or an incomplete formulation though. We just don't know.




Technically, we only have a visible universe to work with, yes.  But, there's no particular reason to say, "beyond that edge, we know *nothing*, and anything goes!"  Occam's Razor tells us that, unless we have a good reason to think the rules change out there, they probably don't.


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## freyar (Oct 23, 2015)

Umbran said:


> Yes, well, the alternative can make a physicist lose many hours of sleep.  A little belief instead of a lot of insomnia.



 So true...



> I am not even sure it is really necessary to have the chronology protection conjecture, to be honest.  The constructions typically necessary for them are such that, really, you don't have to worry about them happening in reality.  As one of my professors long ago mentioned about Tipler's solution: "Infinite cylinders of neutronium don't just happen, and it isn't like anyone is ever going to be able to *make* one if they can't travel faster than light anyway."



Well, less exotic spacetimes, like Kerr black holes, also have CTCs.  But they tend to be tucked behind horizons or where the matter that forms the black hole would be instead.



> Okay, if someone ever finds a cosmic string, that could be an issue.  But maybe even that's okay.




I think cosmic strings should be safe because they* don't rotate on their axis.  Or rather that rotation isn't physical.  *I'm assuming the cosmic strings come from a gauge symmetry breaking** since global symmetry breaking tends to make infinite string tensions and global symmetries don't seem to exist in quantum gravity.  **And my statement might be more general anyway, but I'd have to go digging through textbooks to check.


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## AbdulAlhazred (Oct 23, 2015)

Umbran said:


> Technically, we only have a visible universe to work with, yes.  But, there's no particular reason to say, "beyond that edge, we know *nothing*, and anything goes!"  Occam's Razor tells us that, unless we have a good reason to think the rules change out there, they probably don't.




What I mean is that the total energy and other conserved values are not well-defined for the Universe as a whole. Not that there is some region wherein these rules don't hold, just that you cannot apply them to the whole, at least in their currently understood forms.


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## Umbran (Oct 23, 2015)

freyar said:


> Well, less exotic spacetimes, like Kerr black holes, also have CTCs.  But they tend to be tucked behind horizons or where the matter that forms the black hole would be instead.




It comes down to a point to consider:

Do we expect that the laws of physics are *absolutely* unbreakable, or is it enough that they be such that there is no real chance of them breaking?

CTCs behind event horizons?  Who cares?  CTCs around constructs that cannot ever be built?  Again, so what?  Do we really need to make presumptions about the mechanisms that will prevent such violations?



> I think cosmic strings should be safe because they* don't rotate on their axis.




Rotation isn't necessary.  Richard Gott showed there's a solution in which two moving cosmic strings have a close encounter, and you can get a closed timelike curve around the pair.  Which makes sense, as the *pair* can have angular momentum, and that means rotational frame dragging.

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.66.1126


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## Umbran (Oct 23, 2015)

AbdulAlhazred said:


> What I mean is that the total energy and other conserved values are not well-defined for the Universe as a whole. Not that there is some region wherein these rules don't hold, just that you cannot apply them to the whole, at least in their currently understood forms.




If the rules hold *strictly* locally in all places (in no interaction do you ever violate them), then they pretty much have to sum up to conservation globally, as a result.  In order for them to not hold globally, some individual interaction would have to violate the rules.  

That is, unless you invoke some form of violation from *outside* the universe, but that's a non-falsifiable posit, and thus not really appropriate for a science discussion.


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## freyar (Oct 23, 2015)

Umbran said:


> Rotation isn't necessary.  Richard Gott showed there's a solution in which two moving cosmic strings have a close encounter, and you can get a closed timelike curve around the pair.  Which makes sense, as the *pair* can have angular momentum, and that means rotational frame dragging.
> 
> http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.66.1126




Interesting, I either didn't know or forgot about that.


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## AbdulAlhazred (Oct 23, 2015)

Umbran said:


> If the rules hold *strictly* locally in all places (in no interaction do you ever violate them), then they pretty much have to sum up to conservation globally, as a result.  In order for them to not hold globally, some individual interaction would have to violate the rules.
> 
> That is, unless you invoke some form of violation from *outside* the universe, but that's a non-falsifiable posit, and thus not really appropriate for a science discussion.




No, not really. The problem is that in GR 'total energy' of a system is not a well-defined concept, and thus the conservation of energy in the system as a whole is not defined. Even though no one observer will see a failure of conservation of energy within their reference frame the total energy of the universe as a whole need not remain the same.


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## Umbran (Oct 23, 2015)

AbdulAlhazred said:


> No, not really. The problem is that in GR 'total energy' of a system is not a well-defined concept, and thus the conservation of energy in the system as a whole is not defined.




Yes, I know that total energy is not easily defined in all systems in GR.  But, that's not actually relevant.  I don't need to know the total value to know it if is conserved.

Consider a large tank of water.  I don't need to know what volume of water is in the tank to know whether water is entering or leaving the tank.  I don't need to know how how much energy there is in a system to speak about the *change* in energy of the system.  Actually knowing the total is useful, and often makes it easier - calculate the total energy at time X, and at time Y, and compare.

But, wait - time X and time Y are not absolutes in GR!  So, I wouldn't expect this to be a useful way to find the change in energy anyway!

This is why I brought up, in effect, the differential form, rather than the integral form.


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## tomBitonti (Oct 23, 2015)

> Technically, we only have a visible universe to work with, yes. But, there's no particular reason to say, "beyond that edge, we know *nothing*, and anything goes!" Occam's Razor tells us that, unless we have a good reason to think the rules change out there, they probably don't.






Umbran said:


> It comes down to a point to consider:
> Do we expect that the laws of physics are *absolutely* unbreakable, or is it enough that they be such that there is no real chance of them breaking?
> 
> CTCs behind event horizons?  Who cares?  CTCs around constructs that cannot ever be built?  Again, so what?  Do we really need to make presumptions about the mechanisms that will prevent such violations?




Why treat the interior of a black hole different than regions beyond the observational limit due to cosmic expansion?  Wouldn't we expect the normal laws of physics to apply in either case, or to be outside of our interest and only a philosophical concern, again, in either case?

Thx!
TomB


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## Umbran (Oct 23, 2015)

tomBitonti said:


> Why treat the interior of a black hole different than regions beyond the observational limit due to cosmic expansion?  Wouldn't we expect the normal laws of physics to apply in either case, or to be outside of our interest and only a philosophical concern, again, in either case?




I'm not treating them differently.

For the Kerr black hole, or Tipler time machine, someone said, "Hm.  Look at this weirdness that the math said is possible."  Some folks in the science community react with, "Well, we should look for fundamental laws that will prevent that situation."  I question how hard you have to look for solutions to the problem that may never happen.

If someone said, "Hey, look, the math says that you can do this thing, that will only dump you outside the observable universe," I might respond similarly.  This is not sweating some particular details of a theory that otherwise works pretty darned well and has many verified predictions, because they are edge cases that may never actually occur.

But that's not what's happening here.  Someone instead went, "Well, if I claim it is outside the observable universe, I can posit anything and consider it viable, even if it goes contrary to current observation and theory."  This is basically leaning on non-falsifiability as an excuse to make stuff up.


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## tomBitonti (Nov 4, 2015)

Sorry, have been away dealing with events in my extended family.

My point was: Whatever happens outside of the observational limit in an expanding universe, why care about that any more than about what happens inside of a black hole, which is similarly unobservable?  Considerations of both invoke an assumption of universality.

A question which arose when thinking about the above: Is there an analogue to Hawkin's radiation which is implied by the disappearance of matter and light across the observational limit?  How is that different from the disappearance of matter and light into a black hole.  (Using "disappearance" to mean "placement outside of our observation".)  In both cases, entropy is carried outside of our observation.  Except that not really, for black holes.  Is there a similar "not really" for the other case?

Thx!

TomB


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## Umbran (Nov 4, 2015)

tomBitonti said:


> My point was: Whatever happens outside of the observational limit in an expanding universe, why care about that any more than about what happens inside of a black hole, which is similarly unobservable?  Considerations of both invoke an assumption of universality.




One of the basic assumptions of GR is that the rules are the same everywhere.  

So, the basic difference is this:

I was talking about some places where it looks like the rules can break - but the math tells us they only break in unobservable conditions, near unbuildable artificial constructs, or near objects whose existence is merely theorized that must be moving near each other in a very particular way while a sentient creature with a spacecraft is nearby.  The math that breaks *also* requires the situation that may never happen.

Upthread, AA posited rule breaking that does not come out of the math, but would be cool, and suggested we pull a veil of the un-observable between here and there as an afterthought because otherwise it is problematic.  Nothing in the math suggests what he wanted was possible - there's nothing mathematically or physically special about the border between the observable and non-observable universe, no real discontinuity is predicted.  So then tagging on, "But it happens outside the observable universe," doesn't somehow make it plausible.


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## freyar (Nov 5, 2015)

tomBitonti said:


> Sorry, have been away dealing with events in my extended family.
> 
> My point was: Whatever happens outside of the observational limit in an expanding universe, why care about that any more than about what happens inside of a black hole, which is similarly unobservable?  Considerations of both invoke an assumption of universality.
> 
> A question which arose when thinking about the above: Is there an analogue to Hawkin's radiation which is implied by the disappearance of matter and light across the observational limit?  How is that different from the disappearance of matter and light into a black hole.  (Using "disappearance" to mean "placement outside of our observation".)  In both cases, entropy is carried outside of our observation.  Except that not really, for black holes.  Is there a similar "not really" for the other case?




Re: your first question, I agree whole-heartedly with Umbran's answer.  Regarding your second, the answer is yes, there is an analog of Hawking radiation for cosmological horizons.  For example, in the very far future, it may be that our universe looks like a spacetime that's empty except for a cosmological constant.  If you were sitting in such a universe, there is a distance beyond which you can never see, which is a horizon (for you) much like the horizon of a black hole.  And you would indeed detect Hawking radiation from that horizon.  There are some subtle differences in comparison to the black hole, and this type of universe is actually a lot less well-understood than a black hole is, but there are some definite similarities.

I should say that this isn't something we see in our universe.  Specifically, if we look far away, we can see the opaque age of the universe in the form of the cosmic microwave background as closer (in time, really) than the horizon distance.  The universe will have to get a lot older before we can't see the CMB.


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## Landifarne (Nov 7, 2015)

Somewhat related to what you chaps have been discussing:

Even though dark matter is rather isotropic thoughout the visible universe, there must be regions where it is clumped. Also, it supposedly makes up the majority of the matter in the universe.

So, why don't we see a great deal of gravitational lensing due to it? Or, do we, and we just don't hear about it?

I can see how the lensing of objects within our own galaxy would be minimized if the dark matter is spread so uniformally within it but, if dark matter is so ubiquitous in the visible universe, shouldn't nearly all of the furthest galaxies be lensing in our scopes?


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## Umbran (Nov 7, 2015)

Landifarne said:


> Somewhat related to what you chaps have been discussing:
> 
> Even though dark matter is rather isotropic thoughout the visible universe, there must be regions where it is clumped. Also, it supposedly makes up the majority of the matter in the universe.
> 
> So, why don't we see a great deal of gravitational lensing due to it? Or, do we, and we just don't hear about it?




The thing to remember is that since dark matter interacts through gravity, clumps of dark matter will correspond to clumps of normal visible matter drawn to it - so, those clumps of dark matter are where galaxies are.  So, when they talk about lensing around a galaxy, they are implicitly talking about lensing around dark matter, too.


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## freyar (Nov 7, 2015)

Landifarne said:


> Somewhat related to what you chaps have been discussing:
> 
> Even though dark matter is rather isotropic thoughout the visible universe, there must be regions where it is clumped. Also, it supposedly makes up the majority of the matter in the universe.
> 
> ...






Umbran said:


> The thing to remember is that since dark matter interacts through gravity, clumps of dark matter will correspond to clumps of normal visible matter drawn to it - so, those clumps of dark matter are where galaxies are.  So, when they talk about lensing around a galaxy, they are implicitly talking about lensing around dark matter, too.




As Umbran says, we think that big clumps of dark matter draw visible matter into them during the formation of structure, so you generically expect a big visible structure to be sitting in a dark matter counterpart.  So each galaxy has a "halo" of dark matter around it.  By looking at motions of stars in galaxies, we can get a reasonable idea of how dark matter is spread in these galactic halos.  But it's not just galaxies.  We believe that there are smaller lumps of dark matter (called subhalos) inside the big galaxy-sized halos, though it's not clear if they'd all be associated with clumps of stars due to the non-gravitational physics that
affects normal matter but not dark matter.  Going the other direction in size, galaxies come in groups and clusters, and clusters of galaxies are sitting in really big halos of dark matter.  When we see images of very far galaxies lensed by the gravity of closer stuff, it's typically clusters of galaxies and their dark matter halos that do the lensing.  It's also possible to use the lensing of an image to map out the location of the total mass in a cluster (this is pretty common for people to do), and that doesn't always track where the visible matter is --- typically it extends out a bit.


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## Landifarne (Nov 7, 2015)

Makes good sense but, with a 5-to-1 ratio of DM to Regular Matter (RM), it seems like there couldn't be a perfect correlation. In fact, all of the previous comments on the vastness of the universe (and corresponding variety of structures) would suggest that we should observe many cases of DM without RM. Seems like a statistical model should give some idea of how many non-correlating clumps we should observe.

If there really is such a corelation, wouldn't that imply that the DM formed first? If so, have they figured out how much more quickly the DM formed?

[EDIT: Re-reading your comment on sub-halos, I'd have to say that, if such exist, then we should see a tremendous amount of lensing on every scale. Is the amount of grav-lensing that high?]


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## Landifarne (Nov 7, 2015)

I'm glad that I began reading this thread. I've learned more about cosmology over the last few weeks from looking here (and visiting Wikipedia) than I did as an undergrad. Thanks all around!

OK, here's another one:

If our universe is a false vacuum [a concept I had never heard of before], could the dark matter then be a manifestation of the true vacuum that it is "falling into"?


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## AbdulAlhazred (Nov 8, 2015)

Landifarne said:


> Makes good sense but, with a 5-to-1 ratio of DM to Regular Matter (RM), it seems like there couldn't be a perfect correlation. In fact, all of the previous comments on the vastness of the universe (and corresponding variety of structures) would suggest that we should observe many cases of DM without RM. Seems like a statistical model should give some idea of how many non-correlating clumps we should observe.
> 
> If there really is such a corelation, wouldn't that imply that the DM formed first? If so, have they figured out how much more quickly the DM formed?
> 
> [EDIT: Re-reading your comment on sub-halos, I'd have to say that, if such exist, then we should see a tremendous amount of lensing on every scale. Is the amount of grav-lensing that high?]




There ARE 'dark galaxies', halos that appear to have little or no baryonic matter associated with them. They're generally small, and like many dwarf galaxies they tend to orbit larger galaxies. Its hard to tell though, since they're practically invisible. There has been some work on mapping out nearby ones, but we still don't even have a really good count of all the VISIBLE dwarf galaxies associated with ours.


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## freyar (Nov 9, 2015)

Landifarne said:


> Makes good sense but, with a 5-to-1 ratio of DM to Regular Matter (RM), it seems like there couldn't be a perfect correlation. In fact, all of the previous comments on the vastness of the universe (and corresponding variety of structures) would suggest that we should observe many cases of DM without RM. Seems like a statistical model should give some idea of how many non-correlating clumps we should observe.
> 
> If there really is such a corelation, wouldn't that imply that the DM formed first? If so, have they figured out how much more quickly the DM formed?
> 
> [EDIT: Re-reading your comment on sub-halos, I'd have to say that, if such exist, then we should see a tremendous amount of lensing on every scale. Is the amount of grav-lensing that high?]






AbdulAlhazred said:


> There ARE 'dark galaxies', halos that appear to have little or no baryonic matter associated with them. They're generally small, and like many dwarf galaxies they tend to orbit larger galaxies. Its hard to tell though, since they're practically invisible. There has been some work on mapping out nearby ones, but we still don't even have a really good count of all the VISIBLE dwarf galaxies associated with ours.




On lensing, AbdulAlhazred is correct.  It's also true that, within a biggish galaxy like ours, we don't know if many/most of the subhalos have been smoothed out by the gravitational effects of normal matter (which are more important than dark matter in the inner parts of galaxies like ours).  So we wouldn't necessarily see much (micro)lensing in our galaxy from subhalos.  If we're talking about lensing of very far away galaxies, we're talking about strong gravitational lensing, which requires a lot of mass.  Typically the only thing large enough to make a lens we'd see is a cluster of galaxies and associated dark matter halo, and we do find that that kind of lens is very common.

Based on various types of evidence, dark matter structures did form first, and normal matter then fell into the dense regions of dark matter.  Here's a reasonable picture of how things look today: there are these really big clouds of dark matter that hold clusters of galaxies together.  Inside these cluster halos are large amounts of intergalactic gas and also smaller halos of dark matter, which by-and-large contain galaxies.  Some are reasonably big, like our Milky Way, some are dwarf galaxies (generally satellites of galaxies like ours) that may either have very little normal matter or may be separated from their original dark matter halos by tidal effects, and some are really big monster-sized galaxies formed by the collision of other galaxies.  Inside the galaxies, there are probably subhalos and other structures, but we don't have as good of a picture of that.  The cluster-sized halos of galaxies themselves form into large walls and seem to lie at the intersection of filaments of dark matter.  On the flip side, the walls surround large voids with very little matter of any kind (dark or normal).  On the average, on very very large distances, the universe is pretty smooth though, and in fact this is what we see in the cosmic microwave background light, which is uniform with fluctuations of only one part in 100,000 roughly.


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## freyar (Nov 9, 2015)

Landifarne said:


> I'm glad that I began reading this thread. I've learned more about cosmology over the last few weeks from looking here (and visiting Wikipedia) than I did as an undergrad. Thanks all around!
> 
> OK, here's another one:
> 
> If our universe is a false vacuum [a concept I had never heard of before], could the dark matter then be a manifestation of the true vacuum that it is "falling into"?




The false vacuum doesn't really "fall into" the true vacuum.  It takes a quantum decay process to get to the true vacuum, and it's quite a traumatic (read: likely destroys the universe as we know it) event.  So, fortunately, it shouldn't happen very often (like even once in our volume of the universe for many many ages of the universe).  But anyway, that doesn't have much to do with dark matter; back to that in a moment.  First I want to mention that it is possible in theories with lots of false vacua to look for evidence of the decay of another false vacuum into our false vacuum, and people are looking.  I think it's a long shot --- I agree with arguments that say any such evidence is likely pushed farther away than our ability to see it --- but it is a possibility.

There are also models that combine dark energy (general models that can explain the accelerating expansion of the universe) and dark matter.  Most of these I've seen are highly speculative, and I don't put a lot of credence in them.  There are just a lot of ways they can be sick (have subtle but important mathematical inconsistencies).


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## Landifarne (Nov 9, 2015)

"...within a  biggish galaxy like ours, we don't know if many/most of the subhalos  have been smoothed out by the gravitational effects of normal matter  (which are more important than dark matter in the inner parts of  galaxies like ours).  So we wouldn't necessarily see much (micro)lensing  in our galaxy from subhalos."

Do the gravitational attractions of DM-to-DM and DM-to-baryonic follow those of baryonic-to-baryonic? 

Your statement begs the question: How could baryonic matter smooth out DM inside a large galaxy? The result of elecroweak interactions/pressure? Or, is the ratio of DM to baryonic much smaller than 5:1 inside such galaxies, with that large overall ratio attributable to much higher % of DM strewn throughout the universe?


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## Landifarne (Nov 9, 2015)

freyar said:


> The false vacuum doesn't really "fall into" the true vacuum.




I got that; understood the context.



freyar said:


> It takes a quantum decay process to get to the true vacuum, and it's quite a traumatic (read: likely destroys the universe as we know it) event.  So, fortunately, it shouldn't happen very often (like even once in our volume of the universe for many many ages of the universe).




[Probably in my genuine ignorance] this sounds incredibly speculative. Is this so intertwined with our understanding of quantum mechanics to say it's a given? "To the extent that the following is probable" is likely the answer...



freyar said:


> First I want to mention that it is possible in theories with lots of false vacua to look for evidence of the decay of another false vacuum into our false vacuum, and people are looking.  I think it's a long shot --- I agree with arguments that say any such evidence is likely pushed farther away than our ability to see it --- but it is a possibility.







freyar said:


> There are also models that combine dark energy (general models that can explain the accelerating expansion of the universe) and dark matter.  Most of these I've seen are highly speculative, and I don't put a lot of credence in them.  There are just a lot of ways they can be sick (have subtle but important mathematical inconsistencies).




You're saying that the models that connect DM to DE are sick, not the existance of DE being sick? [The existance of DE is well established, I assume]


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## Landifarne (Nov 9, 2015)

Oh, and again, props to all of you guys for putting up with this. "The patience of Job" comes to mind...


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## freyar (Nov 9, 2015)

Landifarne said:


> "...within a  biggish galaxy like ours, we don't know if many/most of the subhalos  have been smoothed out by the gravitational effects of normal matter  (which are more important than dark matter in the inner parts of  galaxies like ours).  So we wouldn't necessarily see much (micro)lensing  in our galaxy from subhalos."
> 
> Do the gravitational attractions of DM-to-DM and DM-to-baryonic follow those of baryonic-to-baryonic?
> 
> Your statement begs the question: How could baryonic matter smooth out DM inside a large galaxy? The result of elecroweak interactions/pressure? Or, is the ratio of DM to baryonic much smaller than 5:1 inside such galaxies, with that large overall ratio attributable to much higher % of DM strewn throughout the universe?




The answers to these questions are kind of related.  While all matter, dark or baryonic/normal, follows the same law of gravity (in any usual type of theory), which is just Newtonian gravity (since relativistic effects are tiny).  The issue is that baryonic matter, as you say, has electromagnetic interactions and therefore pressure.  Essentially, dark matter interacts so rarely that it can just pass through everything, but normal matter acts like a fluid.  That means it's possible for normal matter to collapse into small objects.  Think of it this way: the same amount of matter in a smaller space has a lower energy (that's what you get from falling, after all), so you have to lose energy somehow to go from being spread out to being compact.  Dark matter doesn't have an effective way to lose energy.  In the end, what this means is that, even though there is much more dark matter over all, there's a lot more normal matter in the center of a galaxy like ours.  Also, outside the center, the normal matter forms a disk structure (that's where we are), but dark matter mostly stays in a large spherical halo (at least that's what a typical model would say; it's interesting to think about alternatives).  In any event, the overall behavior of normal matter is a lot more complex than that of dark matter, even though they obey the same law of gravitation.  And those complex processes can feed back into the dark matter via gravity.  It's not clear exactly what the feedback will do since there are competing contradictory effects possible, but one possibility is breaking up or smoothing out the subhalos.


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## freyar (Nov 9, 2015)

Landifarne said:


> [Probably in my genuine ignorance] this sounds incredibly speculative. Is this so intertwined with our understanding of quantum mechanics to say it's a given? "To the extent that the following is probable" is likely the answer...



Well, models of cosmology that have false vacuum decay are by their nature at least somewhat speculative, but they're easily accommodated in pretty normal models of particle physics. In any case, though there are still some arguments over the details, the basic calculation in this type of case is pretty well understood. It's not quite basic quantum mechanics, but it's an extension that's by now a part of the graduate school curriculum.  Anyway, the upshot is that, while it's possible to set up rapid decays, usual sets of model parameters will lead to very very slow decays.



> You're saying that the models that connect DM to DE are sick, not the existance of DE being sick? [The existance of DE is well established, I assume]




The former.  The acceleration of the universe, the explanation of which goes by "dark energy," is well-established enough that it won the 2011 Nobel Prize in Physics.  Incidentally, it boggles the mind that the discovery of dark matter, which is back up by more evidence and happened decades prior to the discovery of dark energy, still doesn't have a Nobel Prize.  My guess is that someone on the committee just doesn't like dark matter (an alternative is modifying Newtonian's 2nd law F=ma, though that doesn't explain most of the evidence for dark matter, including lensing in clusters), but the cynical side in me wonders if it's because they'd have to give a share of the prize to a woman.  The Nobels, especially in physics, have a truly atrocious record for acknowledging the contributions of women in science.


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## freyar (Nov 9, 2015)

Landifarne said:


> Oh, and again, props to all of you guys for putting up with this. "The patience of Job" comes to mind...




Not a problem.  It's not really part of my job, but I feel a responsibility to explain this kind of stuff to the public.  Plus it's fun!


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## Umbran (Nov 10, 2015)

freyar said:


> It's not really part of my job....




*sigh*.  That is a problem with hard sciences academia today.  The roles of research and education should be separate, as they are really separate skillsets, but the system rewards only research - for those that do education, it is only as a responsibility added on to the role of researcher, or as a really horribly crappy part time role.  

The result is some really brilliant people who couldn't teach their way out of a paper bag at the front of classrooms.


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## freyar (Nov 10, 2015)

Umbran said:


> *sigh*.  That is a problem with hard sciences academia today.  The roles of research and education should be separate, as they are really separate skillsets, but the system rewards only research - for those that do education, it is only as a responsibility added on to the role of researcher, or as a really horribly crappy part time role.
> 
> The result is some really brilliant people who couldn't teach their way out of a paper bag at the front of classrooms.




Well, what you say is true in a number of cases (especially at "research 1" universities), but that's not actually what I was saying.  For one thing, at very many universities --- certainly the large majority by numbers if not by reputation --- teaching takes primary importance over research at least in terms of job expectations.  That is certainly true at my job.  The issue, as I think you'd probably agree, is that there is little training for teaching at the university level (in science, anyway), and what exists is quite often so poor quality to be a total waste of time.  That said, there are, in addition to the brilliant researchers but crappy lecturers, many brilliant researchers who are also gifted at teaching.

What I actually meant was specifically that answering physics questions informally on an internet forum primarily devoted to RPGs isn't something I get paid for.   My job description is officially 40% teaching, 40% research, and 20% service.  In practice, during the 8-month school year, I have 10-20% of my time for research (if I work nights/weekends) in a year when I've taught everything previously, so I need to put most of my summer work into research (though I also usually teach a good number of students through research project supervision, which is more time-consuming than it sounds).  Service covers everything from university governance (ie, committee work), some professional service (like refereeing or conference organization, though some of that overlaps with research), and outreach to the public.  In a sense, this thread would go into that last category, but because it's not easily verifiable (like a public lecture would be, say) due to (a) semi-anonymity and (b) the fact that no one in my university is going to search through EN World threads for physics topics, I can't practically claim it as an outreach activity.  I do, however, generally take part in a number of outreach events each year, including lectures at libraries/retirement centers, short radio/TV spots, and even once speaking at an event for Neil deGrasse Tyson's visit to the other local university.


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## freyar (Nov 10, 2015)

Umbran said:


> ... The roles of research and education should be separate, as they are really separate skillsets...




One other thing on this point: I do largely agree, but not entirely.  Specifically, "research education," ie experiential learning through involvement in original research projects, is an important part of education in science, and I'd argue that it's become critical even for students who go directly into the workforce (without getting an advanced degree).  It has helped some of my former students get jobs, and I'd argue that the type of independent, critical thinking and problem-solving it fosters is crucial for society.  So that's one reason research is implicitly tied to education.  Another is that teaching is much more time-consuming than many people suppose, and someone with a 100% teaching position isn't likely to have the time to keep abreast of recent developments in science.  Just doing that is a non-negligible part of my (research) workday and often the only research I have time to do.  For myself, that informs some of what I choose to teach in class and certainly informs what I teach informally when I talk to students at other times.  Research and teaching are not totally unrelated.  (Not that I think you mean that, but I like to elaborate on things, as you can probably tell.)

The other thing, of course, is that universities are increasingly being run as corporations and there is already a squeeze on research/teaching positions (tenure-stream faculty) many places in favor of teaching faculty on short-term contracts (who are effectively paid less than minimum wage given the amount of work it takes to teach a single class).  That has its own problems both for those instructors and also for students, but it also raises the question of how basic science will get done if not at universities.  I think most people reading this thread would agree that basic science does provide a social good.

Anyway, I'm interested to hear your (Umbran's and everybody else's) thoughts.


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## AbdulAlhazred (Nov 10, 2015)

freyar said:


> The other thing, of course, is that universities are increasingly being run as corporations and there is already a squeeze on research/teaching positions (tenure-stream faculty) many places in favor of teaching faculty on short-term contracts (who are effectively paid less than minimum wage given the amount of work it takes to teach a single class).  That has its own problems both for those instructors and also for students, but it also raises the question of how basic science will get done if not at universities.  I think most people reading this thread would agree that basic science does provide a social good.




I hear you on the 'adjunct instructor' thing, it really is slave wages, you'd be lucky to clear $10 an hour teaching a class, unless its one you teach repeatedly, and even then its not a very lucrative career choice, having experienced this lovely little form of serfdom myself. I guess its not the worst job ever, but it surely doesn't promote quality in education. The schools really have little idea of the adjunct's teaching ability and mostly have to rely on student evaluations to decide who to keep using (if they agree to come back at all). Myself and several other adjuncts quickly learned that the students were quite aware of their power and exercised it unmercifully, so that in effect the freshman-level classes we were teaching rapidly became a joke. If everyone didn't get an 'A' on every test the instructor was out on his ass pretty quickly (happened to a couple of my friends). I still liked the teaching, but I wouldn't recommend it either to prospective adjuncts nor to any university that cares about quality.

And yes, exactly how WOULD science get done outside universities? I guess the only 2 models we have are corporate science, which has several obvious drawbacks, and 'gentleman science' as it existed in the 17th-19th Century (which was still pretty closely tied to universities and evolved into the current system). I guess we have 'government science', but that has limited appeal as well... I think we're stuck with universities!


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## Umbran (Nov 10, 2015)

freyar said:


> One other thing on this point: I do largely agree, but not entirely.  Specifically, "research education," ie experiential learning through involvement in original research projects, is an important part of education in science




I agree it is an important part of modern education.  But, being a good researcher doesn't give you, or otherwise imply you have, the skills or temperament necessary to handle the educational aspect.  

In my own experience, across several universities, considering both faculty and staff, every one who gets through or teaches the programs is knowledgeable and capable in the field - they know the science, and they know how to do research properly.  However, only about a third of these people should be teaching.  While you are correct that the standard Masters or Doctoral program in the hard sciences doesn't include a lot of training in teaching, even if you gave my classmates the proper training, most of them weren't ever going to be particularly good teachers.

That's okay.  It is also my observation that the really best teachers were not the best researchers - they were okay at it, could do it properly, as I said,  but honestly weren't going to be major grant winners, much less movers and shakers at the cutting edge.  There are some who are really stunning at both, but those are rare gems, and we should not use them as the model for the system as a whole.  We shouldn't be making competent scientists teach if that's not their proclivity, and we shouldn't be making good teachers "publish or perish".  

Quite honestly, the 40%, 40%, 20% division you mention above is inefficient and wasteful of talent.  The folks who are really good at teaching should have that has their main focus, and should have reduced research burdens, and vice versa.  



> Another is that teaching is much more time-consuming than many people suppose, and someone with a 100% teaching position isn't likely to have the time to keep abreast of recent developments in science.




I agree, but having a responsibility for doing original research is also not going to keep them abreast of recent developments, except within their narrow area of research.  Real continuing education is a separate activity from either teaching or research, IMHO.



> The other thing, of course, is that universities are increasingly being run as corporations




I am hoping recent trends will force a revision of the current, administration-heavy model for universities that seems to be largest driver of cost increases.  Specifically, the fact that college and university enrollment seems to have peaked n 2011.  Reduced revenues will require the universities to rethink their staffing.  And, well, the faculty produce the product, and the administration does not.  Eventually, they'll need to stop squeezing the productive workforce, and start cutting the admin side.


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## freyar (Nov 11, 2015)

Not going to quote everything from those last two posts since this thread has slowed down enough that I think we know what we're talking about...

AA --- you have my sympathies on the contract instruction (I just can't call it "adjunct" since "adjunct" traditionally has meant someone with another job, such as at a company or another university, who might be allowed some privileges including student supervision).  My wife taught a couple of classes on a 1-term contract last year, and we both decided the money was no where near enough to compensate for the work and stress.  

And, Umbran, I basically agree with what you're saying.  I just am not sure there's a clean separation.  I can point to my own university for an example, where there is some shifting of responsibilities.  We have quite a few instructors (teaching only), even permanent ones, and they are quite good teachers (at least the ones in physics).  But most of them don't have the perspective on the subject that seems necessary either to teach the upper-division courses or to think about planning the entire curriculum.  I do agree that a lack of current research is probably not the sole or even main factor in this, but I suspect the big issue is that these instructors (chosen for teaching ability) have *never* done research for an extended time or in significant amounts.  My experience is that you don't get how all the subjects that appear to be different when they're packaged in separate classes really fit together until you're out there trying to work out something new yourself.  So I guess my point is that good teaching of the advanced subjects seems to require a viewpoint developed by doing research.  Just to complete an earlier thought, even though the 40-40-20 split is common to all our "research faculty," the actual teaching load is reduced for those of us with external grants, so what you suggest is implemented somewhat (and this is true at other schools, too, as I'm sure you know).

Regarding continuing education, my observation (anecdotal, I guess) is that the good researchers do keep abreast of what's happening not just in their narrow specialty but at least in their somewhat broader field.  It helps generate new research ideas, after all.  Full time teaching just doesn't leave time for that, unless you want to kill yourself from lack of sleep.

I hope you're right about cutting the admin a bit.  I feel lucky that my university isn't too admin-heavy yet, but it has crept that way a bit.  Unfortunately, so far, the approach has not been to cut administration but rather to accumulate vacancies in permanent academic staff (through retirement, etc) and replace them with low cost contract instructors.  The problem is not as bad here in Canada as in the US due to stronger (a little) unions, but it is still an issue.


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## Landifarne (Nov 16, 2015)

As a high school teacher (physics and chemistry), for every hour I spend actively teaching students I spend one hour prepping, grading, fixing/fabricating equipment, responding to parents and cleaning. That equates to nearly 30 hours of instruction and 30 hours of ancillary work each week. I'm sure collegiate and university professors have a similar ratio.

The part-time junior college professors and university lecturers are the ones with really screwed up jobs, as they have zero job security and make very little.

At UCLA (which I'm familiar with, having been a student there and have a friend who was a principal researcher there for 20 years), tenured faculty teach for two quarters and have one quarter free of teaching dutiies. Summer comprises another free quarter. They teach two classes (4-5 hours each week, per class), making their jobs 50% teaching/50% research during their teaching quarters. They are also expected to spend 10% of their overall time performing departmental duties (committees, etc.)

That all seems very reaosnable for a Tier-1 university, but I was disgusted to find that undergraduate fees accounted for 35% of UCLA's overall revenue stream, while undergraduate instruction only amounted to only 3.7% of its expenditures. Undergratuates were subsidizing everything else.

That said, the tenured professors were generally abyssmal instructors, wheres the contract lecturers tended to be quite good.


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## freyar (Nov 16, 2015)

Landifarne said:


> As a high school teacher (physics and chemistry), for every hour I spend actively teaching students I spend one hour prepping, grading, fixing/fabricating equipment, responding to parents and cleaning. That equates to nearly 30 hours of instruction and 30 hours of ancillary work each week. I'm sure collegiate and university professors have a similar ratio.



If I had 30 instruction hours, I'd have to get the ratio down to 1:1, but I honestly spend closer to 2:1 prep:instruction even for classes I've taught before.  It's considerably more when I teach a class for the first time.  



> At UCLA (which I'm familiar with, having been a student there and have a friend who was a principal researcher there for 20 years), tenured faculty teach for two quarters and have one quarter free of teaching dutiies. Summer comprises another free quarter. They teach two classes (4-5 hours each week, per class), making their jobs 50% teaching/50% research during their teaching quarters. They are also expected to spend 10% of their overall time performing departmental duties (committees, etc.)
> 
> That all seems very reaosnable for a Tier-1 university, but I was disgusted to find that undergraduate fees accounted for 35% of UCLA's overall revenue stream, while undergraduate instruction only amounted to only 3.7% of its expenditures. Undergratuates were subsidizing everything else.
> 
> That said, the tenured professors were generally abyssmal instructors, wheres the contract lecturers tended to be quite good.



Funny, I spent several years hanging around UCLA a lot since my wife did her MS there (I was in school up the coast a bit but lived in LA).  I know she found some of the tenure-stream profs good and some bad with respect to teaching, but she didn't have any contract lecturers as a grad student, of course.  What she did find is that teaching assistants really had a rough deal in terms of the number of hours they were supposed to work, etc (and it's apparently gotten worse with the state budget cuts).  

As for undergraduates subsidizing everything else, that's true in I think quite a large percentage of universities, and I think it's gotten more true as public funding has shrunk.  I am curious about that 3.7% figure though --- is that only counting instructor/professor salaries for contact time or is it more inclusive, like taking into account a share of the (ballooning) administrative costs?


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## Landifarne (Nov 17, 2015)

I'd have to go and find the article again [written by a UCLA emritus professor of economics about 15 years ago] to answer whether the figure included administration costs. I don't think it did, and I believe UCLA had the lowest overall percentage spent upon undergraduate instruction. Having cycled through it, my alma mater always left a bad taste in my mouth, and, from what my students inform me, it hasn't changed much in that regard (felt like a number, just churned through...sink or swim).

My friend was a principal researcher in biology, overlapping neuroscience. He did research for the DoD on echolocation in bats. Before all the grant money dried up, he stated that the sheer paperwork involved with grant writing and keeping in compliance with federal regulations/red tape took up 80% of his time. He had a good run doing cutting edge research for 20 years, but eventually gave it up to take a less stressful job (did maintain his pension, though, unlike all of the junior, non-tenured professors).


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## gribble (Nov 25, 2015)

I've read through the thread and found it fascinating - thanks very much for giving your time to answer! 

I know this thread has technically ended, but I have a couple of questions if anyone has the time to address them. Both are probably very naive, and reflect an undergrad understanding that has been blunted by nearly 20 years of work largely outside of Science/Physics (I work in software development), so please bear with me.

1) At one point freyar said "If your electrons became massless, the electromagnetic force would no longer be able to bind them to protons". Can someone please give an explanation of why? I thought the electromagnetic force concerned the interactions of charge and was unrelated to mass - i.e.: even if the electrons became massless, the electromagnetic force would still bind them to protons as long as they didn't lose their charge, wouldn't it?

2) There was discussion about our observable universe being only a tiny fraction of the known size of the universe (a figure of less than 1% of 1% was given), but we can see the CMB... which I thought was the remnant of the Big Bang and hence the edge of the universe? What am I misunderstanding?

As I said, likely very naive/ignorant questions, but they were the two things (apart from the bits that were just totally over my head) that didn't make sense to me.


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## Umbran (Nov 25, 2015)

gribble said:


> 1) At one point freyar said "If your electrons became massless, the electromagnetic force would no longer be able to bind them to protons". Can someone please give an explanation of why? I thought the electromagnetic force concerned the interactions of charge and was unrelated to mass - i.e.: even if the electrons became massless, the electromagnetic force would still bind them to protons as long as they didn't lose their charge, wouldn't it?




I don't know for sure what freyar was thinking when he said this.  But one answer goes like this

It turns out that massless particles must move at the speed of light.  The electromagnetic force propagates at the speed of light (via photons).  Therefore, the force coming from the proton will never catch up with the electrons, so they cannot be bound.



> 2) There was discussion about our observable universe being only a tiny fraction of the known size of the universe (a figure of less than 1% of 1% was given), but we can see the CMB... which I thought was the remnant of the Big Bang and hence the edge of the universe? What am I misunderstanding?




You are misunderstanding that it isn't at the edge of the universe - it *fills* the universe.  When the universe was young, its entire volume was filled with hot, dense energy.  As the universe expanded, the entire volume cooled, so the entire volume is filled with the resulting cool microwaves.


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## gribble (Nov 25, 2015)

Umbran said:


> It turns out that massless particles must move at the speed of light.  The electromagnetic force propagates at the speed of light (via photons).  Therefore, the force coming from the proton will never catch up with the electrons, so they cannot be bound.



Ah, cool. That makes sense, thanks... although wouldn't the electron be moving perpendicular to the electromagnetic force (in a particle model), not directly away from it? So the photons should still be able to "catch" the electrons?



Umbran said:


> You are misunderstanding that it isn't at the edge of the universe - it *fills* the universe.  When the universe was young, its entire volume was filled with hot, dense energy.  As the universe expanded, the entire volume cooled, so the entire volume is filled with the resulting cool microwaves.



Right, it is an issue with my mental model then. I have a mental picture of the universe being like an inflated balloon, and the CMB being the inside of the balloon as seen by a point inside it. So the CMB (the stretch the analogy) is actually the air inside the balloon, and we can only see it as far as we can detect?


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## Umbran (Nov 25, 2015)

gribble said:


> Ah, cool. That makes sense, thanks... although wouldn't the electron be moving perpendicular to the electromagnetic force (in a particle model), not directly away from it? So the photons should still be able to "catch" the electrons?




No.  Consider an image:  there's a little guy with a photon gun standing on the proton, ready to fire the force-carrying particle at the electron.  The electron is moving quickly across his field of view, right?  So, even if his shot is moving as fast as the electron, he must aim at where the electron *will be*, not where it is.

But, in reality, the proton has now way of knowing where the thing will be.  It has no prescience.  



> Right, it is an issue with my mental model then. I have a mental picture of the universe being like an inflated balloon, and the CMB being the inside of the balloon as seen by a point inside it. So the CMB (the stretch the analogy) is actually the air inside the balloon, and we can only see it as far as we can detect?




This will probably confuse you, but... In the traditional balloon analogy, everything you know in the universe is actually on the surface of the balloon.  It is an example of what would seem to be a two-dimensional surface (the balloon material if it were laid out flat) being curved in a third dimension, and expanding.  The air in the balloon is not in the universe.  

The better way to think of it is probably like this - when you, with your eyes, look up at the sky, you see stars.  They all to your naked eye look like they are painted on a surface, the vault of the sky.  However, in reality, they are spread through a volume.  It is like looking into a fog - it all looks like it is at a distance, but really, it's everywhere.


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## freyar (Nov 25, 2015)

gribble said:


> Umbran said:
> 
> 
> 
> ...




Keep in mind that Umbran's explanation is very heuristic and just illustrates what happens.  You of course need the mathematical results.  What I was thinking about is that if you look at the mathematics describing atoms and take the electron mass to zero, you find that (1) the atomic orbitals become infinitely large and (2) the binding energy goes to zero.  So that means the atoms aren't held together.




> Right, it is an issue with my mental model then. I have a mental picture of the universe being like an inflated balloon, and the CMB being the inside of the balloon as seen by a point inside it. So the CMB (the stretch the analogy) is actually the air inside the balloon, and we can only see it as far as we can detect?




The idea that the CMB fills the universe is right.  What we see is the CMB at our location that is entering our telescope right now.  These CMB photons have been travelling through the universe since the universe became transparent, so the CMB we see and measure reflects the conditions in our universe at that moment long ago and also very far away (since the photons are travelling at the speed of light that whole time).  That's also the farthest distance we can see right now because looking a farther distance away would require looking back in time to a point when light couldn't travel because the universe was too hot and dense.


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## AbdulAlhazred (Nov 25, 2015)

Umbran said:


> No.  Consider an image:  there's a little guy with a photon gun standing on the proton, ready to fire the force-carrying particle at the electron.  The electron is moving quickly across his field of view, right?  So, even if his shot is moving as fast as the electron, he must aim at where the electron *will be*, not where it is.
> 
> But, in reality, the proton has now way of knowing where the thing will be.  It has no prescience.



I don't think this has anything to do with speed of light though. Its true of ALL force-carrying particles, one must ask how do they 'know' to appear where they are going to interact with another particle? An EM field for instance fills all of space, so does this imply that a magnet fills all of space with photons continuously? Where does the energy for this come from? Its one of those things that makes us question the nature of our views of both time and locality.



> This will probably confuse you, but... In the traditional balloon analogy, everything you know in the universe is actually on the surface of the balloon.  It is an example of what would seem to be a two-dimensional surface (the balloon material if it were laid out flat) being curved in a third dimension, and expanding.  The air in the balloon is not in the universe.
> 
> The better way to think of it is probably like this - when you, with your eyes, look up at the sky, you see stars.  They all to your naked eye look like they are painted on a surface, the vault of the sky.  However, in reality, they are spread through a volume.  It is like looking into a fog - it all looks like it is at a distance, but really, it's everywhere.




Right, there IS no 'surface of the fireball' at the big bang, we're INSIDE IT still, and always forever. Its just getting bigger and cooling off. No outside, no inside, just everywhere.


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## gribble (Nov 25, 2015)

Ok, thanks guys. I think I understand the electron thing, though more from the math than Umbran's explanation... as AbdulAlhazred points out, the "where will it be" question isn't intrinsic to moving at the speed of light. However, if the math says that without mass the orbital becomes infinitely large (which I can kind of understand from a gravitational analogy, even if that perhaps isn't strictly accurate in this case), then it also makes sense that the binding energy would go to zero (if something is infinitely far away, then it intuitively makes sense to me that the force it would exert due to charge would be zero).

Still really confused about the size of the universe / CMB thing. The best analogy I can come up with that seems consistent with what Umbran is saying is to imagine an LED on the inside surface (or even the outside surface I guess) of a balloon. The surface of the balloon represents the bounds of the universe, but the amount of the surface illuminated by the LED (much smaller than the size of the balloon) is what we see as the CMB... is that at least somewhat accurate?


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## AbdulAlhazred (Nov 25, 2015)

gribble said:


> Still really confused about the size of the universe / CMB thing. The best analogy I can come up with that seems consistent with what Umbran is saying is to imagine an LED on the inside surface (or even the outside surface I guess) of a balloon. The surface of the balloon represents the bounds of the universe, but the amount of the surface illuminated by the LED (much smaller than the size of the balloon) is what we see as the CMB... is that at least somewhat accurate?




We certainly do only see a part of the CMB, as we see a part of the universe generally. you could think of that as like the 'horizon' in your balloon analogy, space goes on, perhaps forever or perhaps not, and its all filled with these CMB photons. Anyone anywhere will see basically the same CMB, since it was nearly at thermal equilibrium when the photons decoupled from matter.


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## gribble (Nov 25, 2015)

AbdulAlhazred said:


> We certainly do only see a part of the CMB, as we see a part of the universe generally. you could think of that as like the 'horizon' in your balloon analogy, space goes on, perhaps forever or perhaps not, and its all filled with these CMB photons. Anyone anywhere will see basically the same CMB, since it was nearly at thermal equilibrium when the photons decoupled from matter.



Yeah, I guess this was what I was meaning by the LED. If we're on the outside of the balloon rather than the inside, and there is a source of light outside the balloon, then the horizon of the balloon will suffice. However if we're inside, or there isn't some external source of light, the LED serves this purpose.


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## Umbran (Nov 25, 2015)

AbdulAlhazred said:


> I don't think this has anything to do with speed of light though.




In reality, we don't think of the electron as a point object going around the protons in a classical orbit at all!  And the photon isn't a discrete object in this either - it would be virtual.  So, I was really being descritive - the quantum mechanical description doesn't make *intuitive* sense, after all.



> Right, there IS no 'surface of the fireball' at the big bang, we're INSIDE IT still, and always forever. Its just getting bigger and cooling off. No outside, no inside, just everywhere.




Exactly.  The universe may well have been created as infinite in size, it is only the *observable* universe that has an edge, and that's not a physical boundary at all.


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## AbdulAlhazred (Nov 26, 2015)

Umbran said:


> In reality, we don't think of the electron as a point object going around the protons in a classical orbit at all!  And the photon isn't a discrete object in this either - it would be virtual.  So, I was really being descritive - the quantum mechanical description doesn't make *intuitive* sense, after all.




Well, yes, this is just ONE of several ways in which the concept of 'locality' no longer makes sense. Even in the purely classical system of SR/GR the idea of 'here' and 'there' don't actually add up. At best there are 'relations' between things, which provide some localized basis from which to build a coordinate system, but in GR you certainly can't extend it. QM of course just reduces the whole concept to no more than just some state variables that tell you what might happen. Or in the case of relativistic quantum field theory one can wonder if they really tell you much of anything at all, since somehow that photon interacts with that electron no mater what. I believe this was the original impetus for the 'pilot wave' class of QM interpretations, which generally involve fun things like entities which travel backwards in time.


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## Umbran (Nov 26, 2015)

AbdulAlhazred said:


> Well, yes, this is just ONE of several ways in which the concept of 'locality' no longer makes sense.




I think you are using that term "locality" in a different way than I would normally use it.



> Even in the purely classical system of SR/GR the idea of 'here' and 'there' don't actually add up.




Well, that is probably only because "don't add up" is not a well-defined phrase, so that nobody outside your head knows what you mean. 

If to "add up" they must be objective absolutes, then yes, they don't add up.  But your experience of the color blue is not an objective absolute, either, but you and others can talk about it quite sensibly.



> At best there are 'relations' between things, which provide some localized basis from which to build a coordinate system, but in GR you certainly can't extend it.




Extend it in what sense?  And "can't" in what sense?

I mean, consider that even without GR, there are more coordinate systems that are problematic than not.  Geocentric coordinates were the first we ever used, even before Newton, and those coordinates *sucked* for just about everything other than talking about the Moon.  You'd not want to extend that to infinity and try to describe movements in the universe in it, even if spacetime didn't curve around masses.  It isn't so much "can't" as "Nobody sane would want to."



> Or in the case of relativistic quantum field theory one can wonder if they really tell you much of anything at all, since somehow that photon interacts with that electron no mater what.




It is perhaps more accurate to say that time and space are still fine, the problem is that the photon never actually exists, but we insist on using it to describe the interaction anyway.  That is not spacetime's fault.


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## freyar (Nov 26, 2015)

Umbran said:


> It is perhaps more accurate to say that time and space are still fine, the problem is that the photon never actually exists, but we insist on using it to describe the interaction anyway.  That is not spacetime's fault.




It's not really the photon's fault, either.   Really, the problem with the language is saying that "virtual photons" create/carry the electromagnetic potential/force (or really just the use of "virtual particle" more generally).  It's fine as a shorthand in jargon when people know the technical meaning, but it unfortunately describes in plain English something very different than what the physics and mathematics of quantum field theory describe.  Unfortunately, "off-shell field excitation" or "Green function" or "2-point function" doesn't do it for most people.


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## Bedrockgames (Nov 26, 2015)

I am trying to put a liquid mercury lake and river (that cycles on a square pallet chain pump) in a dungeon. What are some plausibility issues with this and how would liquid mercury behave when players do things like plunge into the lake or try to ride across it on make shift boogie boards?


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## Nagol (Nov 26, 2015)

Anything dropped on the surface will remain on top unless it is gold or platinum -- their densities are high enough that items will in fact sink.  Steel, lead. silver, copper will bob on the surface.

Plunging into the lake is going to hurt.  At about 13 times the density of water (and almost twice that of steel), people would probably be able to walk getting only their toes submerged if it offered any friction -- which it pretty much doesn't.  Its viscosity isn't much more than water (somewhere between fresh water and milk).

Simple way across would be to sit down and pole/oar with a sword.

Most metals will likely be destroyed over time (gold would go pretty quickly) through amalgamation though iron and platinum are pretty much immune.


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## Bedrockgames (Nov 26, 2015)

Nagol said:


> Plunging into the lake is going to hurt.  At about 13 times the density of water (and almost twice that of steel), people would probably be able to walk getting only their toes submerged if it offered any friction -- which it pretty much doesn't.  Its viscosity isn't much more than water (somewhere between fresh water and milk).
> .




Thanks for the information. That is quite helpful. I hadn't considered that the higher density would potentially mean injury if someone feel into it from a ledge or something. 

How hurt do think people would get? Is it equivalent to falling onto solid ground in terms of injury?


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## Nagol (Nov 26, 2015)

There will be some give but not a lot.  A 6' person somehow standing will stabilise with about 6 inches of leg under the mercury-line and the viscosity is low enough the liquid will flow away from the impact point.  I'd probably be nice and drop a d6 off the damage.


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## Displacer Kitten (Dec 20, 2015)

Could Sigil (a small torus-shaped universe with gravity pulling toward the "walls" of the universe) exist with those physical laws? Can it be modeled mathematically?


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## Umbran (Dec 21, 2015)

Displacer Kitten said:


> Could Sigil (a small torus-shaped universe with gravity pulling toward the "walls" of the universe) exist with those physical laws? Can it be modeled mathematically?




We can't say if it can exist.  It would have to have a set of laws and/or distribution of materials (matter, dark matter, and dark energy) drastically different from our own.


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## freyar (Dec 21, 2015)

Displacer Kitten said:


> Could Sigil (a small torus-shaped universe with gravity pulling toward the "walls" of the universe) exist with those physical laws? Can it be modeled mathematically?




Generally speaking, general relativity allows any sort of universe you want, but the trick is that you have to figure out what sort of matter/energy distribution would give that shape to the universe.  I'm not sure that a toroidal universe like Sigil (with a boundary) is something that can easily be made with normal matter, but we've talked a lot about wormholes and warp drive spacetimes that require exotic matter/energy anyway.


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## Scott DeWar (Dec 25, 2015)

*A funny thing*

Its me . . . . .again. This time it is not a question. This time it is a funny thing i saw on a web comic. "Eye candy ffor Christmas" If you will.

http://xkcd.com/1621/ 

Enjoy, laugh, even chastise me if you need. Bet enjoy the day regardless!


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## tomBitonti (Feb 11, 2016)

*Successful detection of gravity waves!*

This just in:

Science Magazine: Gravitational waves, Einstein’s ripples in spacetime, spotted for first time

http://www.sciencemag.org/news/2016...nstein-s-ripples-spacetime-spotted-first-time

Washington Post: Cosmic breakthrough: Physicists detect gravitational waves from violent black-hole merger

https://www.washingtonpost.com/news...ational-waves-from-violent-black-hole-merger/

Haven't had a chance to read either article yet.

Cheers!
TomB


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## freyar (Feb 11, 2016)

tomBitonti said:


> This just in:
> 
> Science Magazine: Gravitational waves, Einstein’s ripples in spacetime, spotted for first time
> 
> ...




Yes!  Very big news, though the press is missing that we've observed gravitational waves indirectly for a long time.  Still, extremely exciting and long anticipated!!  I noticed you have a thread in the Misc Geek Talk forum, so I'll discuss there mostly.


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