# "Speed of Light"



## Bullgrit (Feb 3, 2014)

In reading some stuff recently because of ideas from a couple other science threads here, I just learned something that surprises me and explains a lot.

The "speed of light" -- 186,000 miles per second -- is not the physics law I always thought it was. It's not the "speed of light" that is the limit, it is the 186,000 mps that is the limit. That is, the ultimate speed is not limited to the speed of light, but rather the speed of light is limited to the ultimate speed.

I'd always thought it was the properties of light that limited speed, but it's the speed that limits light and everything else. I was always told, "Nothing can travel faster than light." But actually it's, "Nothing can travel faster than 186,000 mps." This limits light, as well as other things. It could be called the "speed of gravity" just as accurately.

This may be an obvious concept to some, but it actually puts things into more clear understanding to me to know that the speed limit is the reverse of what I thought for all these years. Calling it the speed of light put more emphasis on light than the physics speed law should have.

Am I understanding the speed of light ultimate speed correctly, now?

Bullgrit


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## Morrus (Feb 3, 2014)

Yes. Light moves at the speed it does because that's the fastest anything can move.  Other things move at that speed, too - radio waves, the whole EM spectrum, lots of stuff.

The speed of light's a bit of a misnomer - light moves at various speeds, up to the universal speed limit. In different mediums, it slows down.  In a vacuum, though, light moves at c.


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## Dioltach (Feb 3, 2014)

So perhaps it should be called the "universal speed limit"? That might encourage some people to break it.
(I agree with you, by the way: it's an interesting factoid that completely changes the way I look at the issue.)


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## Morrus (Feb 3, 2014)

Dioltach said:


> So perhaps it should be called the "universal speed limit"?




It often is!

Although technically its name is "c".


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## Dungeoneer (Feb 3, 2014)

Dioltach said:


> So perhaps it should be called the "universal speed limit"? That might encourage some people to break it.
> (I agree with you, by the way: it's an interesting factoid that completely changes the way I look at the issue.)



While that phrase does give a better idea of what 'the speed of light' means then the original phrase, it's not strictly accurate. IANAPhysicst, but my understanding is that there are some known exceptions to the 'speed limit' such as when particles engage in quantum tunneling, and we hope to find more.

'The speed of electromagnetic radiation in a vacuum' is more accurate, although admittedly not as catchy.


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## Umbran (Feb 3, 2014)

Bullgrit said:


> In reading some stuff recently because of ideas from a couple other science threads here, I just learned something that surprises me and explains a lot.
> 
> The "speed of light" -- 186,000 miles per second -- is not the physics law I always thought it was. It's not the "speed of light" that is the limit, it is the 186,000 mps that is the limit. That is, the ultimate speed is not limited to the speed of light, but rather the speed of light is limited to the ultimate speed.




Technically, it is the speed of any and all massless things in a vacuum - they cannot move more slowly.  And all things with mass must move more slowly.  

At the time the limit was discovered, the only massless particle known was the photon.  We've expanded the known massless particles to be all the gauge bosons (so, the photon and the gluon, the latter of which we cannot observe directly, but whose action we can verify indirectly).  The graviton is theoretical, but gravity's action in quantum has not been observed by experiment - so they may not exist.

So, in terms of things humans see, "speed of light" is still pretty accurate.


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## tomBitonti (Feb 3, 2014)

More precisely, "c" is "the speed of light in a vacuum", not "the speed of light".

Thx!

TomB


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## Morrus (Feb 3, 2014)

Morrus said:


> In a vacuum, though, light moves at c.






Dungeoneer said:


> 'The speed of electromagnetic radiation in a vacuum' is more accurate






Umbran said:


> Technically, it is the speed of any and all massless things in a vacuum






tomBitonti said:


> More precisely, "c" is "the speed of light in a vacuum", not "the speed of light".




Has anybody mentioned that c is the speed of light in a vacuum yet?  If not, maybe I should mention it!


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## Kramodlog (Feb 3, 2014)

I'm pretty sure _c_ is a grade some people get in physics class.


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## tomBitonti (Feb 3, 2014)

Morrus said:


> Has anybody mentioned that c is the speed of light in a vacuum yet?  If not, maybe I should mention it!




Wait, as in, speed, and in, like, a vacuum?

I wrote a _little_ more: The focus is on _precision_: Writing just "the speed of light" is imprecise, leading to issues such as in the original post.  One issue of interest (to me) is the consequence of the lack of precision.  Or, when is the extra precision (as seems to be in this case), necessary.

Thx!

TomB


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## PigKnight (Feb 3, 2014)

goldomark said:


> I'm pretty sure _c_ is a grade some people get in physics class.




I _C _wut u did there.

But, for realsies, why is it called _c_? Latin? Kicks-n-giggles? Discovering scientist?


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## Umbran (Feb 3, 2014)

PigKnight said:


> But, for realsies, why is it called _c_? Latin? Kicks-n-giggles? Discovering scientist?




Back in the mid to late 1800s, folks working on electromagnetism variously used "c" or "V" to stand for the speed of light (or, in some cases, a constant that happens to turn out to be related to the speed of light) - basically to stand for "constant" or "velocity".  Over time, scientists just sort of drifted to using "c" as a standard notation, in large part because that's what a few of the Big Names of the time used.


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## The_Silversword (Feb 4, 2014)

Umbran said:


> At the time the limit was discovered, the only massless particle known was the photon.




Are photons really massless though? I mean theyre all the time getting sucked into black holes, if they had no mass a black hole wouldnt affect it, right?


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## tomBitonti (Feb 4, 2014)

The_Silversword said:


> Are photons really massless though? I mean theyre all the time getting sucked into black holes, if they had no mass a black hole wouldnt affect it, right?




Hi,

I'm finding two (in the end, equivalent) explanations:

1) Gravity works on the total energy of an object.

For a detailed discussion, see:

http://en.wikipedia.org/wiki/Mass–energy_equivalence



> Relation to gravity
> 
> In physics, there are two distinct concepts of mass: the gravitational mass and the inertial mass. The gravitational mass is the quantity that determines the strength of the gravitational field generated by an object, as well as the gravitational force acting on the object when it is immersed in a gravitational field produced by other bodies. The inertial mass, on the other hand, quantifies how much an object accelerates if a given force is applied to it. The mass–energy equivalence in special relativity refers to the inertial mass. However, already in the context of Newton gravity, the Weak Equivalence Principle is postulated: the gravitational and the inertial mass of every object are the same. Thus, the mass–energy equivalence, combined with the Weak Equivalence Principle, results in the prediction that *all forms of energy contribute to the gravitational field generated by an object*. This observation is one of the pillars of the general theory of relativity.
> 
> The above prediction, that all forms of energy interact gravitationally, has been subject to experimental tests. The first observation testing this prediction was made in 1919.[30] During a solar eclipse, Arthur Eddington observed that the light from stars passing close to the Sun was bent. The effect is due to the gravitational attraction of light by the sun. The observation confirmed that *the energy carried by light indeed is equivalent to a gravitational mass*. Another seminal experiment, the Pound–Rebka experiment, was performed in 1960.[31] In this test a beam of light was emitted from the top of a tower and detected at the bottom. The frequency of the light detected was higher than the light emitted. This result confirms that the energy of photons increases when they fall in the gravitational field of the earth. *The energy, and therefore the gravitational mass, of photons* is proportional to their frequency as stated by the Planck's relation.




*Bold* added by me.

2) Light always travels in straight lines.  Or rather, since on a curved surface, "straight line" requires modification, light always travels along the shortest possible line between points.  Then, gravity causes space-time to be curved, and the straight lines of light in gravitationally flat space are modified to the apparently curved lines in the curved space near a mass.

Here is a ho-hum explanation:

http://hubblesite.org/reference_desk/faq/answer.php.id=58&cat=exotic

Shortest lines in a curved space are called *geodesics*, with a basic definition, as an adjective "of, relating to, or denoting the shortest possible line between two points on a sphere or other curved surface."

In general relativity, there are special equations to describe geodesics, for example:

http://en.wikipedia.org/wiki/Geodesics_in_general_relativity

Lot's of heavy math, so be forewarned.

There are also definitions of "geodesic curve", for example (and also with heavy math):

http://math.stackexchange.com/questions/28690/which-is-the-proper-definition-of-a-geodesic-curve

"A shorted possible line between two points" works well as a common definition.

Thx!

TomB


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## GMMichael (Feb 4, 2014)

Anyone else feel like the strange complications in this light theory are a hint at why we can't figure quantum physics out?  Like, light is massless, yet light carries mass?  Energy always travels at the same speed in a vacuum, even to observers moving at different speeds?  "Energy carried by light," when light itself is supposed to be energy?  Or if it's not energy, it must be matter.  Even though light doesn't always behave like matter...



Morrus said:


> Yes. Light moves at the speed it does because that's the fastest anything can move.  Other things move at that speed, too - radio waves, the whole EM spectrum, lots of stuff.
> 
> The speed of light's a bit of a misnomer - light moves at various speeds, up to the universal speed limit. In different mediums, it slows down.  In a vacuum, though, light moves at c.




Does light actually slow down in different media, or does it simply take a longer path (by, say, reflecting off multiple particles resulting in a wandering, yet relatively straight, path)?


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## The_Silversword (Feb 4, 2014)

tomBitonti said:


> Hi,
> 
> I'm finding two (in the end, equivalent) explanations:
> 
> ...




Ahh ok, #2 makes sense, #1 is a little harder for me to wrap my brain around.


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## The_Silversword (Feb 4, 2014)

DMMike said:


> Does light actually slow down in different media, or does it simply take a longer path (by, say, reflecting off multiple particles resulting in a wandering, yet relatively straight, path)?




Oooh, oooh, I can answer this one!

Yes, they have managed to slow down light


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## GMMichael (Feb 4, 2014)

The_Silversword said:


> Oooh, oooh, I can answer this one!
> 
> Yes, they have managed to slow down light




I read the article, and while I'm no physicist, I have to say that I'm not impressed.  

Talking about the "speed" of light ties into another one of our quantum-physics problems: we don't much understand what Time is, either.  We get a lot of great examples of the watch that doesn't turn as many times when it's close to the speed of light, or the person who doesn't age as much.  But if the position of a watch's hands are what determines how much time has elapsed, then I can make time speed up or slow down with my pinky.  Or if it's time that ages people, why all the anti-aging focus on anti-oxidants and other nutrients?

I'm sure the physicists are talking about the age of subatomic particles, not people and watches.  But - does a photon have an age?  Can a graviton be a spring chicken?


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## Umbran (Feb 4, 2014)

The_Silversword said:


> Ahh ok, #2 makes sense, #1 is a little harder for me to wrap my brain around.




Yeah, both of tomBitonti's descriptions are reasonable ways to look at the situation.

For #1, we can try this.

Einstein told us E=mc^2.  Energy and mass are interchangeable.

Most things we call physical matter have what's called a "rest mass". This is the mass of objects in your everyday experience - the mass you'd measure them having when they look to you like they are standing still (you are in their "frame of rest" or "rest frame").  By Einstein, then, an object at rest still has some energy just from being - we call that the object's "rest energy".

But Einstein's E is not particular.  It applies to _any and all_ energy the object has.  If it has kinetic energy from moving, say, we add that to the bucket of total energy, and recalculate the mass.  So, the faster the thing moves, the more effective mass it has.  

Now, photons are screwy.  When we say they don't have mass, what we really mean is that they don't have a rest mass.  However you may be moving, the photon looks like it is moving at _c_ relative to you.  You *cannot* be in the rest frame of the photon, because you cannot move at its speed.  But, it still has well-defined energy, and E=mc^2 still applies, so we can still think of it as interacting with gravity using that mass-from-energy.


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## Umbran (Feb 4, 2014)

DMMike said:


> I read the article, and while I'm no physicist, I have to say that I'm not impressed.
> 
> Talking about the "speed" of light ties into another one of our quantum-physics problems: we don't much understand what Time is, either.




No, we have a pretty good handle on what Time is - at least as good a handle as what "space" is.  Now, yes, there's some contention on what that really is down on what is known as the "Planck scale" - the very, very, super tiny down below the sizes of subatomic particles - but for most purposes if you grasp Relativity, you grasp what Time is.



> We get a lot of great examples of the watch that doesn't turn as many times when it's close to the speed of light, or the person who doesn't age as much.  But if the position of a watch's hands are what determines how much time has elapsed, then I can make time speed up or slow down with my pinky.  Or if it's time that ages people, why all the anti-aging focus on anti-oxidants and other nutrients?




Make the clock an atomic one, where the ticks are based on the ocillations of electrons in an atom, and no, you can't move the hands with your pinky any more.  And, well, all those anti-aging creams don't work anyway, so why mention them? 



> But - does a photon have an age?  Can a graviton be a spring chicken?




No.  And actually, _THAT'S THE ENTIRE POINT_!

Photons (gauge bosons, massless particles in general) are different from everything else in the Universe.  All of relativity ultimately falls out from that one observationally verifiable fact.  If you fire off an photon, it goes away from you at 186,000 mps.  If you fire off a photon, and then strap the biggest rocket anyone could ever make to your butt, and take off after it, no matter how fast you go, it'll still be travelling away form you at 186,000 mps.  It will also look like it is travelling the same speed away from Earth.

Relativity is the resolution of that apparent paradox - a viewer "at rest", and one moving wicked darn fast, both think the photon is moving at the same speed.


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## Bullgrit (Feb 4, 2014)

Is there a physics/mathmatical reason for the ultimate speed limit, (avoiding saying "speed of light"), to be what it is? Or even to exist at all?

I mean, if the speed limit was, say, 200,000 mps, or 1 billion mps, would anything happen other than calculations change and things go faster -- other than just the numbers get bigger? What if there was no speed limit -- light could move instantaneously? Would this "break" the universe?

Considering the size of the universe, it seems that this speed limit is really, really, really, ridiculously slow. Is there a scientific reason for this low limit?

Bullgrit


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## Umbran (Feb 4, 2014)

Bullgrit said:


> Is there a physics/mathmatical reason for the ultimate speed limit, (avoiding saying "speed of light"), to be what it is?




In a word, no.  At least, not so far as we understand things at the moment.

There are a few constants (the speed of light, the charge of a single electron, and a couple other things) that "just are", as far as accepted models go.  For some reason, or no reason, when the Universe came into being, these constants had these values.  They could have had other values, and with other values, the universe would be dramatically different.  As in "human life could not exist" different.

This brings us to the Anthropic Principle.  We see the kind of universe we see because any universe we see must be consistent with the development of our sort of intelligent life.  



> I mean, if the speed limit was, say, 200,000 mps, or 1 billion mps, would anything happen other than calculations change and things go faster -- other than just the numbers get bigger? What if there was no speed limit -- light could move instantaneously? Would this "break" the universe?




Whether or not it would break the Universe, if you vary it much, we break you.  Specifically, we break atoms as we know them, so your physical body and the brain that supports your intelligence ceases to be possible.  There might be a universe, and there might be stuff in it, but that stuff might be either a dissociated mist of subatomic particles or it all collapses into one big lump in the middle...

Also, and I have to think on this a bit, I believe that if light moved instantaneously, our concept of having causes and effects, the idea that events happen in an order, with one leading to another, goes out the window.  There is no time.  No history.  No time-ordering.  You cannot be born, live, and die, because there is no way to differentiate the moment of your birth from the moment of your death (from the moment you were conceived, or from the moment your parents met, or from the moment the Sun ignited).



> Considering the size of the universe, it seems that this speed limit is really, really, really, ridiculously slow.




Considering the size of your living room, though, the speed limit is really, really, ridiculously fast!  Considering the size of atoms, it is even faster...

"Slow" and "fast" are measures only useful when considering some human goal - there's an implicit human-centric value judgement in those words.  It seems slow, because you actually want to get over there, and the limit gets in your way.


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## Janx (Feb 4, 2014)

Bullgrit said:


> Is there a physics/mathmatical reason for the ultimate speed limit, (avoiding saying "speed of light"), to be what it is? Or even to exist at all?
> 
> I mean, if the speed limit was, say, 200,000 mps, or 1 billion mps, would anything happen other than calculations change and things go faster -- other than just the numbers get bigger? What if there was no speed limit -- light could move instantaneously? Would this "break" the universe?
> 
> ...




Having just learned what you just learned, I like NOT saying "speed of light" as well in that it un-reinforces a misunderstanding of physics to us lay people.

I couldn't tell you if changing the c constant by +/- 1 or 100 would change much.

One goofy theory I just made up based on this new understanding is that if the universe is really a simulation, then, like a computer, it operates on a cycle.  A loop of processing all elements.

It can only do that so fast.  In computers, that upper limit is equal to the clock speed of the computer.

In the universe, what if c represents the limitation of the computer's cycles per second.

This is of course, a rough, made up analogy and I have not factored in exactly how c is tied to the simulation's clock cycle. it most certainly is not "c = GHz"


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## Umbran (Feb 4, 2014)

Janx said:


> One goofy theory I just made up based on this new understanding is that if the universe is really a simulation, then, like a computer, it operates on a cycle.  A loop of processing all elements.




That assumes a physical infrastructure similar to our digital computers.  One can concieve of other hardware that doesn't work in that matter - quantum computers, for example, don't really follow that pattern.  You can make a DNA computer that also does not work on that same kind of loop.  Who knows what hardware (or operating system) The Universe runs on?


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## Janx (Feb 4, 2014)

Umbran said:


> That assumes a physical infrastructure similar to our digital computers.  One can concieve of other hardware that doesn't work in that matter - quantum computers, for example, don't really follow that pattern.  You can make a DNA computer that also does not work on that same kind of loop.  Who knows what hardware (or operating system) The Universe runs on?




True enough, but there still would be an upper limit to what the hardware the universe simulation runs on that could form the basis of c being the constant value that it is

DNA can only compute/transfer data at a certain speed.

quantum computer thingies can only define so many states, transition to another state at a certain speed, or transfer data at a certain speed. these form constraints and bottlenecks.

Also wouldn't be so sure about loops going away.  Obviously, extra-universal computers could be completely different, but the "problem" being solved remains the same.

Fewer computing resources than total processes/objects being simulated means task swapping between processes/objects in a loop/cycle.

That could be avoided if effectively every atom (or the quantum equivalent) had a CPU doing the processing on its behalf for its state managment, etc (APU?).  Thus forming a neural network of all the matter in the universe in the extra-verse running the simulation.

Beats me.  Though quite informed on Computer Science, I have not heard much on how a quantum computer is projected to work, in order to appreciate the differences in approach it might engender.  

I do envision that programmers are going to approach it the same way as existing computers for longer than they fully embrace an alternative possibility. The same as multi-threaded development isn't something most developers embrace wholly.  Instead, it gets used as needed, and many situations don't require it.


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## Umbran (Feb 4, 2014)

Janx said:


> I do envision that programmers are going to approach it the same way as existing computers...




In another thread, we were talking about being unable to really understand the motivations and ways of alien species.

Now, we are not only talking about aliens from another planet.  We are talking about aliens from another *universe*.  Not just creatures who developed on a different world that follows the same physical laws, but that could be operating under completely different laws.  

I would hesitate to assert that their methods of computing* follows how we would go about it.

*If that is what we are experiencing.  On the other hand, the "simulation" that is our universe might just be part of the digestive process of a hyper-dimensional being - our experience may be a by-product of a much larger process, for what meaning "larger" has in this context.


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## tomBitonti (Feb 4, 2014)

My understanding is that substantial changes to 'c' would have a dramatic impact on the nature of the universe.  That is, it would break the universe, as we know it.

The nearest explorations of this sort of issue that come to mind are:

Orthogonal, by Greg Egan:

http://en.wikipedia.org/wiki/Orthogonal_(novel)



> Technically, the space-time of the universe portrayed in the novels has a positive-definite Riemannian metric, rather than a Pseudo-Riemannian metric, which is the kind that describes our own univere.




And:

Raft, by Stephen Baxter, in the Xeelee sequence:

http://en.wikipedia.org/wiki/Raft_(novel)



> The novel is an elaborated version of his 1989 short story of the same title, Raft. The story follows a group of humans who have accidentally entered an alternate universe where the gravitational force is far stronger than our own, a "billion" times as strong.




Note: The Egan book (one of three), is somewhat of a hard read: You will need to work through his examples in detail to understand them, as if you were reading a text book, and the storyline which is put on top of the physics is just so-so (in my opinion).  But, as an example of a book really based on hard physics, there is no other example which is anywhere close.  The books are truly unique.

Also note: I found Raft to be mediocre, in comparison to a number of other Baxter novels, some of which I've found quite good.  My favorite is the Time Ships, which continues the story of the Time Machine.

Thx!

TomB


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## Scott DeWar (Feb 4, 2014)

Bullgrit said:


> Is there a physics/mathmatical reason for the ultimate speed limit, (avoiding saying "speed of light"), to be what it is? Or even to exist at all?
> 
> **stuff**
> 
> Bullgrit






Umbran said:


> In a word, no.  At least, not so far as we understand things at the moment.
> 
> **more stuff**.




Could there be a "terminal velocity of mass-less particles" that is some 300,000,000[+/- a few] meters/second? Just as there is a terminal velocity in Earth atmosphere that is slower then in the vacume of space based on things such as wind resistance of the object, could there be a terminal velocity of light and similar particles? Am I making sense?


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## Janx (Feb 4, 2014)

Umbran said:


> In another thread, we were talking about being unable to really understand the motivations and ways of alien species.
> 
> Now, we are not only talking about aliens from another planet.  We are talking about aliens from another *universe*.  Not just creatures who developed on a different world that follows the same physical laws, but that could be operating under completely different laws.
> 
> ...




You may not be following my tangent.

If you hand real quantum computers to real programmers today, they will initially program for it in ways similar to how they code for single or multi-processor computers we have now.  The fact that it may be able to handle 33 quantum states in stead of our traditional binary will be abstracted into a C-like language that really won't matter, except to somebody who really wants to optimize their code for the Quantum Processor's alien architecture.  Additionally, a certain set of computer science problems aren't made better by approaching them "differently".  As such, anybody designing a computer may be less likely to design outside of the common pattern than a non-computer scientist might think there would be.

You may also be assuming that the universe simulator we are in was designed by beings unlike ourselves.  it is equally possible that the simulator was designed by more advanced versions of us.  Just as we make video games to be "realistic" they may have made a universe simulator that is equally "like things used to be". We might be living in the artificial reality version of the GTA to the creators of our universe.


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## Morrus (Feb 4, 2014)

DMMike said:


> I read the article, and while I'm no physicist, I have to say that I'm not impressed.




If the universe fails to impress you, it might be that you're a tad of a tough sell.


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## Umbran (Feb 4, 2014)

Scott DeWar said:


> Could there be a "terminal velocity of mass-less particles" that is some 300,000,000[+/- a few] meters/second? Just as there is a terminal velocity in Earth atmosphere that is slower then in the vacume of space based on things such as wind resistance of the object, could there be a terminal velocity of light and similar particles? Am I making sense?




I think I understand what you mean.

"Terminal velocity" is what you get when an object has a force upon it, and it feeling some resistance.  The object accelerates to the point that the motive force is balanced by resistance, and that's its terminal velocity.

Problem is, photons aren't being pushed by a motive force, and aren't feeling a resistance.  Specifically we used to think they moved through the "aether" that might provide resistance, and experiments have been done to show there is no aether.  They don't start slow and speed up.  As far as can be told, they just *always* move that fast.  So, the analogy doesn't seem to apply.


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## GMMichael (Feb 5, 2014)

Morrus said:


> If the universe fails to impress you, it might be that you're a tad of a tough sell.




Guilty as charged.  I'm picky about everything - music, beer,  coffee, RPGs, and movies too.  You know Mikey, he hates everything.



Umbran said:


> Problem is, photons aren't being pushed by a motive force, and aren't feeling a resistance.  Specifically we used to think they moved through the "aether" that might provide resistance, and experiments have been done to show there is no aether.  They don't start slow and speed up.  As far as can be told, they just *always* move that fast.  So, the analogy doesn't seem to apply.




Aether is reminding me of something I read once: on a subatomic level, all sorts of tiny little particles are springing into, and out of, existence.  These tiny particles can lend energy to more permanent particles, so long as that energy gets paid back.

Is this theory still plausible?  Does this version of an aether not affect photons because photons are, relatively speaking, too big to be affected?


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## tomBitonti (Feb 5, 2014)

DMMike said:


> Guilty as charged.  I'm picky about everything - music, beer,  coffee, RPGs, and movies too.  You know Mikey, he hates everything.
> 
> 
> 
> ...




That swarm of particles -- virtual particles -- in not only virtual electrons, and such, but also virtual photons.  And apparently there is some interaction of virtual particles and non-virtual particles.

I found this:

http://physics.stackexchange.com/questions/76068/some-questions-about-virtual-particles



> Concerning the last question, yes, virtual particles may interfere with the real ones. For example, if we study processes in an external electric field create by many coherent long-wavelength photons, there will still be Feynman diagrams with virtual photons in them. The amplitudes from these diagrams have to be added to the amplitudes with the real classical electric field, and only the result (sum) is squared in absolute value. That's what we mean by interference.




I did find this:

http://arstechnica.com/civis/viewtopic.php?f=26&t=1220047



> Yes, photons participate in pair production. Yes it effects their speed, no the effect is not large enough to effect any of our deep space observations.




But!

http://physics.stackexchange.com/questions/41834/speed-of-light-and-virtual-particles



> Yes, light can interact with "virtual particles". It can also interact with itself via virtual particle interactions (see Delbruck Scattering), although I believe direct observation of this effect is currently outside of our experimental capability.
> 
> Edit: Just realized I didn't address the second part. When a photon propagates, the propagation receives contributions from its splitting into an electron-positron pair which recombine etc. These processes contribute corrections to the photon propagator, but they do so in such a way that the propagator pole remains in the same place, which in turn means that the propagation speed is unaffected.




The link to Delbruk Scattering has:

http://en.wikipedia.org/wiki/Delbruck_scattering



> Delbrück scattering, the deflection of high-energy photons in the Coulomb field of nuclei as a consequence of vacuum polarization has been observed. However, the process of scattering of light by light, has not been observed.[1] In both cases, it is a process described by Quantum Electrodynamics (QED).




And ... I'm way out of my depth.  Can anyone provide a clearer answer?

Thx!

TomB


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## Mustrum_Ridcully (Feb 5, 2014)

Virtual particles as an explanation why light is experiencing resistance is still just a version of the aether. Remember the part where Umbran wrote that no matter how fast you move or how you move relatively to a light source, you will always measure the light moving at _c_? And if there are two people with two different directions and speed, they will also all agree that the light is moving at c.

If virtual particles slow provide resistance to slow down the light, you are also moving through these virtual particles, and have a movement relative to them, and so, you should also measure a relative speed difference depending on your own velocity and direction towards these virtual particle sea. But we don't.


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## Bagpuss (Feb 5, 2014)

Morrus said:


> Has anybody mentioned that c is the speed of light in a vacuum yet?  If not, maybe I should mention it!





Does it matter if it is one of those new Dyson DC54s? With the bag-less cyclone technology they can really suck, that's got to make the light travel faster surely.


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## freyar (Feb 5, 2014)

I love when these threads come up on ENWorld.  I wish I had more time available to answer them more often!  In any event, I teach a university course each year on relativity and use it every day in my physics life, so I have way too much to say on the subject for a single post.  I'll just try to mention a few things here.

1) What happens if c becomes infinite?  Well, besides breaking electromagnetism and therefore us, it does change the notion of past & future in our universe.  But it would actually change it to the notion most people have already: there would be a definite "present" that is the same everywhere, plus a definite past and future the same everywhere.  In Einstein's relativity, that's not true.  Your future is made up only of the events you can travel to at speed c or less.  The reverse is true for the past.

2) As noted, the speed of light in materials may be different than c.  This isn't due to the "ultimate speed limit" changing --- c is the same --- but due to the material affecting light.  In terms of photons, yes, you can think of the photons bouncing off of electrons in atoms.  That's not quite accurate, though, as the "photons" are large compared to atoms for visible light, microwaves, radio, etc.  It's better to think of light as waves in this case, and the change in speed is due to the response of the wave to the material's response to the wave.

3) Virtual particles do not affect the speed at which light travels.  If they did, it would violate relativity!*  It is true that we can think of virtual particles as having measurable effects, but that's not one.  Some of tomBitoni's links mention effects where you can think of virtual particles acting like matter and changing the speed at which light travels, but that's always in the presence of some other matter, like a nucleus or metal plates.  That doesn't violate relativity because of the other stuff present.

*I should note that you can make theories of physics that violate relativity in small, subtle ways that would not have yet been detected by experiments.  In those theories, you could ask about virtual particles changing the speed of light, but you could also ask about whether light always travels at speed c even before you think about virtual particles!


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## Bullgrit (Feb 5, 2014)

Does a photon have a measurable lifespan, relative to itself? I mean, a photon that travels 1,000 light years has a lifespan of 1,000 years relative to us, the observer. But the photon traveled at c, so ... instantaneous/simultaneous beginning and end, relative to itself?

It seems to me, that if it has a measurable lifespan relative to itself, time for it isn't "stopped." But if it doesn't have a measurable lifespan relative to itself, can it be said to exist?

Also, is there any proof that time reverses at speeds beyond c? Is it just theory, (can't be proved, right)? Could it be that faster than c speed won't create a paradox?

Bullgrit


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## Bullgrit (Feb 5, 2014)

Bullgrit said:
			
		

> But if it doesn't have a measurable lifespan relative to itself, can it be said to exist?



Having said this, and then thought a few moments past clicking the Submit button, I guess this is like questioning whether a black hole singularity can be said to exist.

Bullgrit


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## Morrus (Feb 5, 2014)

Bullgrit said:


> It seems to me, that if it has a measurable lifespan relative to itself, time for it isn't "stopped." But if it doesn't have a measurable lifespan relative to itself, can it be said to exist?




Well, they exist because we can observe them, and we can use them.


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## tomBitonti (Feb 5, 2014)

An aside, this has a nice historical view of our understanding of light:

http://www.olympusmicro.com/primer/lightandcolor/particleorwave.html

---

But, we don't actually observe the singularity.  Isn't observation limited to up to (but not quite including) the event horizon?

From one point of view, that could be turned into an argument that the event horizon is the entire structure of the black hole.  The history of particles which have fallen into the hole seem to be contained in the topology of the surface.

Whether the singularity exists becomes a pragmatic choice: Thinking about black holes is a lot simpler if a particular interior structure is presumed, and, generally, scientists rather prefer to keep models as simple as possible.

A question that I've had about light transmission is whether we could model this as discrete events, with the only physical events being the emission and absorption, with the absorption delayed according to the intervening distance, much as we would do if running a discrete event simulator, and sort-of what is done when rendering a scene: We compute the paths that light will follow, and render only the final point reached by light.

Thx!

TomB


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## Dungeoneer (Feb 5, 2014)

Bullgrit said:


> Does a photon have a measurable lifespan, relative to itself? I mean, a photon that travels 1,000 light years has a lifespan of 1,000 years relative to us, the observer. But the photon traveled at c, so ... instantaneous/simultaneous beginning and end, relative to itself?
> 
> It seems to me, that if it has a measurable lifespan relative to itself, time for it isn't "stopped." But if it doesn't have a measurable lifespan relative to itself, can it be said to exist?
> 
> ...



This question is even harder to answer than you think, because light is both (either?) a wave or a particle. In other words there is a quantum state such that a photon cannot be treated as a discrete particle. TomBitoni's link above is very good. I also found this clip from a BBC documentary which demonstrates the "double slit experiment" which proves this.

http://www.youtube.com/watch?v=YBxlmPHcm5c

It gets weirder though - if you setup a detector in front of the slits which measures the individual photons as they pass, the light loses its wave-like properties and starts acting like a particle once more.

td;dr - the universe is mocking us.


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## Umbran (Feb 5, 2014)

DMMike said:


> Aether is reminding me of something I read once: on a subatomic level, all sorts of tiny little particles are springing into, and out of, existence.  These tiny particles can lend energy to more permanent particles, so long as that energy gets paid back.
> 
> Is this theory still plausible?




Yep.  It is a standard in how we do physics these days.  Mind you, stealing energy from the vacuum like that is pretty rare, and usually only short term.  If you take and put back, what we at a distance will see is no change.



> Does this version of an aether not affect photons because photons are, relatively speaking, too big to be affected?




Not at all.  But when a photon interacts (and either gains or loses energy), it changes frequency, but not speed.  

Though, to be honest, that's not a very accurate picture.  An interaction on that level is more like, "Particle-antiparticle pair pops out of the vacuum.  Particle absorbs photon.  Particle re-emits photon.  Particle-antiparticle disappear back in to the vacuum."  The photon is a single quantum of energy - it doesn't do anything by half-measures.  



Bullgrit said:


> Also, is there any proof that time reverses at speeds beyond c? Is it just theory, (can't be proved, right)? Could it be that faster than c speed won't create a paradox?




Well, here'e the thing - by the physical laws as we currently know them, moving faster than light is... nonsense.  It cannot happen.  In order to accelerate anything with mass to the speed of light requires literally* infinite energy.  Accelerating beyond the speed of light then requires *more* than infinite energy.

The "going faster than light reverses time" is... a bit wonky, then.  Rather than it being a real, physical result, it is more a statement of how silly considering it is given how the math works out - a demonstration of how a paradox arises to show how this really shouldn't happen.

And even then, not *all* travel at FTL speeds ends up with you travelling backwards in time.  What we can say is that, if you can travel faster than light, it is possible to arrange a path such that you return to your starting physical position before you left it.  Not that *every* path results in this, but such paths do exist.  Time travel becomes possible, not mandatory.



*And not that figurative-literally popular these days, but literally literally.


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## tomBitonti (Feb 5, 2014)

Something traveling faster than light would be going backwards in time in _some_ frame of reference.

But anyways, what this would look like to the traveler, I don't know.  Watching it all happen you would have something like:

You and your buddy time traveler are having a beer.  All of a sudden, in a blaze of exotic particles, a near duplicate pops out along with a kind-of anti-sense buddy, at exactly the same time.  The two buddies (_not_ the anti-sense buddy!) chat for a bit, then the one who had been there for a while winks at you, activates his chrono-displacer, and at that very instant, he merges with his anti-sense copy and they both disappear.

I _think_ the anti-sense buddy would be made of anti-matter, from our point of view, but am not sure.

It's strange that no depiction of time travel uses a continuous movement through time, making the activity both travel backwards _and_ discontinuous.

Thx!

TomB


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## Scott DeWar (Feb 5, 2014)

*uneducated ramblingings*



Scott DeWar said:


> Could there be a "terminal velocity of mass-less particles" that is some 300,000,000[+/- a few] meters/second? Just as there is a terminal velocity in Earth atmosphere that is slower then in the vacuum of space based on things such as wind resistance of the object, could there be a terminal velocity of light and similar particles? Am I making sense?






Umbran said:


> I think I understand what you mean.
> 
> "Terminal velocity" is what you get when an object has a force upon it, and it feeling some resistance.  The object accelerates to the point that the motive force is balanced by resistance, and that's its terminal velocity.
> 
> Problem is, photons aren't being pushed by a motive force, and aren't feeling a resistance.  Specifically we used to think they moved through the "aether" that might provide resistance, and experiments have been done to show there is no aether.  They don't start slow and speed up.  As far as can be told, they just *always* move that fast.  So, the analogy doesn't seem to apply.




Ok, I am kinda picking up on what you are saying, but what if there is a general, all encompassing, here-to-fore unmeasured or even searched for, gravitational force pulling on the photons that prevent the photons from going faster. As for photons, I have to ask, that, is it because they are massless that allows them to be simultaneously created and at *c* at that same instant?



Bagpuss said:


> Does it matter if it is one of those new Dyson DC54s? With the bag-less cyclone technology they can really suck, that's got to make the light travel faster surely.



Dysons are ok, but its the electrolux that really sucks!


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## Morrus (Feb 6, 2014)

One important thing to understand is that this is all complex equations put into simple layman's terms. When we spot what we feel is a logical flaw - it's not. It's an English language flaw. The equations are good; the crappy language we try to use to explain  them conversationally does not cone close to accurately explaining these things.

If you've spotted a 'flaw' in the Fisher Price explanation, you've only spotted a weakness in the English language. You haven't spotted a flaw in the 90-page paper full of equations that that sentence is poorly attempting to analogise.


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## Bullgrit (Feb 6, 2014)

Morrus said:
			
		

> When we spot what we feel is a logical flaw - it's not. It's an English language flaw.



That's a very good point.

Bullgrit


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## Scott DeWar (Feb 6, 2014)

The anglish languij aint not got know faltz in it.


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## Morrus (Feb 6, 2014)

Bullgrit said:


> That's a very good point.
> 
> Bullgrit




It's something I see a lot. Folks say "oh, well, if x is y, I posit z!" (often in a "my logic trumps centuries of scientific theory!" kinda way) when what they're arguing against is a sentence they read somewhere which is a crappy rough attempt to translate 90 pages of equations into a simple English concept in one sentence. If there's a flaw, it's likely in that sentence; the 90 pages of equations may well have a flaw, but it won't be deduced from that English sentence - all that will be deduced is that a sentence is a crappy way to communicate 90 pages of equations.

I say this knowing full well that I understand the basic concepts of many popular scientific theories, but if you asked me to explain the equations I couldn't even begin to try. I certainly can't point at a flaw in those equations.


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## freyar (Feb 6, 2014)

Bullgrit said:


> Does a photon have a measurable lifespan, relative to itself? I mean, a photon that travels 1,000 light years has a lifespan of 1,000 years relative to us, the observer. But the photon traveled at c, so ... instantaneous/simultaneous beginning and end, relative to itself?



A photon is infinitely long-lived (it can't decay into anything), which is perfectly consistent with having time stopped. 




tomBitonti said:


> But, we don't actually observe the singularity.  Isn't observation limited to up to (but not quite including) the event horizon?
> 
> From one point of view, that could be turned into an argument that the event horizon is the entire structure of the black hole.  The history of particles which have fallen into the hole seem to be contained in the topology of the surface.
> 
> Whether the singularity exists becomes a pragmatic choice: Thinking about black holes is a lot simpler if a particular interior structure is presumed, and, generally, scientists rather prefer to keep models as simple as possible.




It's a bit trickier than that.  According to general relativity, if you are falling into a black hole, you don't notice that there's anything unusual about the horizon at all.  So, other than tidal forces, which might or might not be noticeable outside the horizon (depending on the size of the black hole), you don't notice anything weird at all until you hit the singularity.  This is part of the difficulty understanding Hawking radiation in black holes.



> A question that I've had about light transmission is whether we could model this as discrete events, with the only physical events being the emission and absorption, with the absorption delayed according to the intervening distance, much as we would do if running a discrete event simulator, and sort-of what is done when rendering a scene: We compute the paths that light will follow, and render only the final point reached by light.




That's one way of thinking about quantum mechanics (whether of light or not), but you actually have to compute all possible paths between the two events.



Umbran said:


> Though, to be honest, that's not a very accurate picture.  An interaction on that level is more like, "Particle-antiparticle pair pops out of the vacuum.  Particle absorbs photon.  Particle re-emits photon.  Particle-antiparticle disappear back in to the vacuum."  The photon is a single quantum of energy - it doesn't do anything by half-measures.



This is a common and pretty good way of explaining things.  However, a more accurate way of explaining it in simple language has been put forward by Matthew Strassler (I recommend his blog very highly if you are interested in particle physics).  Virtual particles aren't really particles.  The idea of particle/wave duality has come up in this thread already.  Virtual particles are better describe as packets of wave rather than particles, so it's more like a photon goes along, dissolves into a lump of electron wave, and then reforms.  And it does this constantly.



> Well, here'e the thing - by the physical laws as we currently know them, moving faster than light is... nonsense.  It cannot happen.  In order to accelerate anything with mass to the speed of light requires literally* infinite energy.  Accelerating beyond the speed of light then requires *more* than infinite energy.




Just to add about something weird.  Umbran is perfectly correct for anything with _real_ mass.  It is perfectly mathematically consistent to have a particle with _imaginary_ mass (as in square root of -1) that can _only_ travel faster than light.  These are called tachyons.  However, in our current understanding of particle physics, tachyons are not stable particles but instead represent an instability, like sitting on the top of a hill is unstable to falling down the hill.



> And even then, not *all* travel at FTL speeds ends up with you travelling backwards in time.  What we can say is that, if you can travel faster than light, it is possible to arrange a path such that you return to your starting physical position before you left it.  Not that *every* path results in this, but such paths do exist.  Time travel becomes possible, not mandatory.



Spot on.





tomBitonti said:


> Something traveling faster than light would be going backwards in time in _some_ frame of reference.
> 
> But anyways, what this would look like to the traveler, I don't know.  Watching it all happen you would have something like:



The time traveler would always think that he/she is traveling forward in time, despite ending up in the past of when he/she left.  It is a continuous process.  That's the weird bit.



			
				Scott DeWar said:
			
		

> Ok, I am kinda picking up on what you are saying, but what if there is a general, all encompassing, here-to-fore unmeasured or even searched for, gravitational force pulling on the photons that prevent the photons from going faster. As for photons, I have to ask, that, is it because they are massless that allows them to be simultaneously created and at c at that same instant?



All I can say is that the mathematics work perfectly if there are no forces acting on the photons.  You're precisely right that it is their masslessness that makes them move at speed c at all times, including as soon as they are created.



			
				Morrus said:
			
		

> It's something I see a lot. Folks say "oh, well, if x is y, I posit z!" (often in a "my logic trumps centuries of scientific theory!" kinda way) when what they're arguing against is a sentence they read somewhere which is a crappy rough attempt to translate 90 pages of equations into a simple English concept in one sentence. If there's a flaw, it's likely in that sentence; the 90 pages of equations may well have a flaw, but it won't be deduced from that English sentence - all that will be deduced is that a sentence is a crappy way to communicate 90 pages of equations.



Morrus, I can't tell you how much I, as a physicist, appreciate that you understand this and wrote it in this thread.  I (and many many physicists) deal with this kind of logic all the time, and it is sometimes very difficult to get people to understand that mathematics is required.  It's the same thing with the new paper by Hawking that has been a big deal in the press recently.  It's a nice idea (which is actually very similar in some ways to what other people have done), but it's right now impossible to evaluate because there is no math (though other people's similar ideas are fleshed out mathematically).  Physics can _only_ be properly understood with a grasp of the mathematics involved.  That's why Newton had to invent (or co-invent) calculus to describe planetary motion.  I think it's possible to get an idea of what's happening with an explanation in words, but a full understanding and the ability to do physics simply requires a lot of math.


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## Bullgrit (Feb 6, 2014)

freyar said:
			
		

> I (and many many physicists) deal with this kind of logic all the time, and it is sometimes very difficult to get people to understand that mathematics is required.



This is why I ask these kinds of questions. Because I want to understand how it all works, not because I think I've found a flaw. When I see a "logical flaw" in physics, I figure I just need to have it explained more/better, not that I'm the first person to see the "flaw."

And I'm very thankful to have people here who are able and willing to take the time to explain things more and better (mo' betta) to me. I love science, but I'm only a layman, and many papers and books are written above my level. Having peers, like here, who can explain things to my level without making me feel like a complete moron is quite lovely.

Bullgrit


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## Dungeoneer (Feb 6, 2014)

Bullgrit said:


> This is why I ask these kinds of questions. Because I want to understand how it all works, not because I think I've found a flaw. When I see a "logical flaw" in physics, I figure I just need to have it explained more/better, not that I'm the first person to see the "flaw."
> 
> And I'm very thankful to have people here who are able and willing to take the time to explain things more and better (mo' betta) to me. I love science, but I'm only a layman, and many papers and books are written above my level. Having peers, like here, who can explain things to my level without making me feel like a complete moron is quite lovely.
> 
> Bullgrit



I'm in the same boat as you, man.

Have you ever read anything by Paul Davies? He's a physicist who write books that make arcane concepts fantastically accessible to laymen. I never really understood what relativity was about until I stumbled across his book "Other Worlds" a few years ago. Since then I've gobbled up anything I find by him. He has an amazing gift for explaining stuff: quantum mechanics, gravity waves, dark matter, unified field theory and even a bit of string theory (although even Davies admits that string theory is pretty much pure math). Also, he's not afraid to dabble in more speculative stuff like the possibility of alien life, whether we're all living in a computer simulation, and the ever-popular question of whether god exists (Davies is agnostic).

Personally, I wish I could understand a bit of the math, but mathematics really is its own language. When someone says "Here's the basic equation behind X..." and then there is a string of Greek characters, I feel a bit out of my depth.


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## Umbran (Feb 6, 2014)

tomBitonti said:


> But, we don't actually observe the singularity.  Isn't observation limited to up to (but not quite including) the event horizon?




Nothing escapes from the singularity out beyond the event horizon to the outside world, no.  



> From one point of view, that could be turned into an argument that the event horizon is the entire structure of the black hole.  The history of particles which have fallen into the hole seem to be contained in the topology of the surface.




If scientists took "we can't see it, therefore it isn't there" as our clarion call, we'd never have discovered atoms... 



> Whether the singularity exists becomes a pragmatic choice: Thinking about black holes is a lot simpler if a particular interior structure is presumed, and, generally, scientists rather prefer to keep models as simple as possible.




Here's two things:

1) What math we have actually suggests the structure.  

2) What you say is true holds for "vanilla" black holes.  Once we start talking about holes that have angular momentum and electric charge, the space within gets far more complex and interesting.  There's no reason for you to care about it unless you've fallen inside, of course....



> A question that I've had about light transmission is whether we could model this as discrete events, with the only physical events being the emission and absorption, with the absorption delayed according to the intervening distance, much as we would do if running a discrete event simulator, and sort-of what is done when rendering a scene: We compute the paths that light will follow, and render only the final point reached by light.




On a quantum level, yes, when you're talking about the interaction of individual particles.  Well, what we do is take the initial states of the particles, imagine they go into a box, calculate all the possible interactions within the box, and come up with a probability distribution for what comes out box.  This is QM, it is all about probability distributions.

This fails when you want to talk about the action of a prism or lens, however, as the number of interactions that can occur when passing through a macroscopic chunk of material becomes too high to contemplate.


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## Umbran (Feb 6, 2014)

Scott DeWar said:


> Ok, I am kinda picking up on what you are saying, but what if there is a general, all encompassing, here-to-fore unmeasured or even searched for, gravitational force pulling on the photons that prevent the photons from going faster.




A terminal velocity is reached when a motive force (like gravity) is balanced against a drag (like air friction).  You're positing a drag, when there's no motive force.  The photon is not a rocket, with something pushing on it.



> As for photons, I have to ask, that, is it because they are massless that allows them to be simultaneously created and at *c* at that same instant?




Yep.  Massless particles are special.


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## Umbran (Feb 6, 2014)

freyar said:


> Morrus, I can't tell you how much I, as a physicist, appreciate that you understand this and wrote it in this thread.  I (and many many physicists) deal with this kind of logic all the time, and it is sometimes very difficult to get people to understand that mathematics is required.  It's the same thing with the new paper by Hawking that has been a big deal in the press recently.  It's a nice idea (which is actually very similar in some ways to what other people have done), but it's right now impossible to evaluate because there is no math (though other people's similar ideas are fleshed out mathematically).  Physics can _only_ be properly understood with a grasp of the mathematics involved.  That's why Newton had to invent (or co-invent) calculus to describe planetary motion.  I think it's possible to get an idea of what's happening with an explanation in words, but a full understanding and the ability to do physics simply requires a lot of math.




Agreed.

Take the word "schadenfreude".  It has no direct and simple translation into English.  If we are literal, it comes out to, "harm-joy".  You need a sentence of English to approximate it - It is the feeling of joy or pleasure when one sees another fail or suffer misfortune.

Now, take that same idea - that there's no direct translation - and apply it here.  All the concepts are best described by math.  We try to give you approximations in English, but they are at best translations, and we often lose something in that translation.


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## Bullgrit (Feb 6, 2014)

A camera that films at the speed of light.

A TED Talk:
http://www.youtube.com/watch?v=Y_9vd4HWlVA

The way light reflects off surfaces is very interesting. And mind boggling.

Bullgrit


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## tomBitonti (Feb 6, 2014)

Umbran said:


> Nothing escapes from the singularity out beyond the event horizon to the outside world, no.
> 
> If scientists took "we can't see it, therefore it isn't there" as our clarion call, we'd never have discovered atoms...
> 
> ...




I'm (mostly) not suggesting this is a good way to understand black holes.  I just find it very curious that there is no way to observe the internal structure without performing the irreversible step of crossing the event horizon.  My understanding is that, according to known physics, there is no way to transmit information outside of an event horizon.  (With particle histories encoded in the topology of the horizon, such that there is no information loss.)  The reason to believe in an actual singularity is that even if the singularity does not exist, the details of the description put it back in (functionally) anyways.  There is no conceptual benefit to be realized.  Except, it does avoid extreme points in the mathematics.

Spinning black holes and ones with charge get a lot more complicated ... with multiple horizons and what not.  I understand that there are conditions which could conceivably produce a "naked" singularity, but I also thought that to reach this condition the physical conditions are even more extreme than those of a "normal" black hole.

The view for photon transmission is a thought idea.  The implication is that the receiver knows (somehow) exactly when to change state to balance the change in state of the transmitter, including the time and phase differences and possible paths, which feels implausible.  It's action at a distance taken to the extreme.  I also understand that accounting for the energy of the photon as it transits gets to be a mess if the transition is not made as a continuous transmission across space.

Still, the descriptions of particle motion which talk about "sum over possible paths" and what not seem to push us in this sort of direction.  One gets curiously close to a description that might arise if the whole system were being simulated using discrete techniques.

In the end, what I've been told, is that you can use either view and obtain the same results as current explanations.  That is, in the details of the mathematics, the actual presence of the singularity is more of an interpretation type question, not one which must be answered to obtain use from the mathematics.

Thx!

TomB


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## Scott DeWar (Feb 6, 2014)

Bullgrit said:


> who can explain things to my level without making me feel like a complete moron is quite lovely.
> 
> Bullgrit




ditto this! 

By the way, just saw the you tube on the femto camera. Wow! 

Also what showed up was the video on superconductors. Something mentioned was its ability to hold a charge as long as it is kept cold. If the efficincey could be achieved that it could hold a charge during night time for solar panels or on windless days for windmills, that would turn the total power grid problem of using non-fossil fuels into reality!


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## freyar (Feb 6, 2014)

Bullgrit said:


> This is why I ask these kinds of questions. Because I want to understand how it all works, not because I think I've found a flaw. When I see a "logical flaw" in physics, I figure I just need to have it explained more/better, not that I'm the first person to see the "flaw."
> 
> And I'm very thankful to have people here who are able and willing to take the time to explain things more and better (mo' betta) to me. I love science, but I'm only a layman, and many papers and books are written above my level. Having peers, like here, who can explain things to my level without making me feel like a complete moron is quite lovely.
> 
> Bullgrit






Dungeoneer said:


> I'm in the same boat as you, man.
> 
> Have you ever read anything by Paul Davies? He's a physicist who write books that make arcane concepts fantastically accessible to laymen. I never really understood what relativity was about until I stumbled across his book "Other Worlds" a few years ago. Since then I've gobbled up anything I find by him. He has an amazing gift for explaining stuff: quantum mechanics, gravity waves, dark matter, unified field theory and even a bit of string theory (although even Davies admits that string theory is pretty much pure math). Also, he's not afraid to dabble in more speculative stuff like the possibility of alien life, whether we're all living in a computer simulation, and the ever-popular question of whether god exists (Davies is agnostic).
> 
> Personally, I wish I could understand a bit of the math, but mathematics really is its own language. When someone says "Here's the basic equation behind X..." and then there is a string of Greek characters, I feel a bit out of my depth.




And I'm really glad that you (and others here) can ask these questions; it's a real compliment to Morrus and the mods that there is a space for it.  I also don't want to say that I've really noticed anyone here claiming to have found a flaw --- those people wander into my office, send me emails, etc.  I've been fortunate never to have been threatened with a lawsuit for ignoring someone's pet "theory" (in quotes because of no mathematics), but I've known people who have.  Anyway, I'm just saying that these threads are an enjoyable form of public outreach for me, and I also find that a lot of you have a pretty strong appreciation both of the facts and subtleties involved.


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## freyar (Feb 6, 2014)

tomBitonti said:


> I'm (mostly) not suggesting this is a good way to understand black holes.  I just find it very curious that there is no way to observe the internal structure without performing the irreversible step of crossing the event horizon.  My understanding is that, according to known physics, there is no way to transmit information outside of an event horizon.  (With particle histories encoded in the topology of the horizon, such that there is no information loss.)  The reason to believe in an actual singularity is that even if the singularity does not exist, the details of the description put it back in (functionally) anyways.  There is no conceptual benefit to be realized.  Except, it does avoid extreme points in the mathematics.
> 
> Spinning black holes and ones with charge get a lot more complicated ... with multiple horizons and what not.  I understand that there are conditions which could conceivably produce a "naked" singularity, but I also thought that to reach this condition the physical conditions are even more extreme than those of a "normal" black hole.




As we've been saying, physics _is_ what the mathematics tell us.  It's not that there's a conceptual benefit or not to the singularity.  In Einstein's general relativity, it is just there (inside the black hole).  However, you can ask if that means there is a problem with relativity at the energy densities and curvatures you'd find at or near the singularity inside a black hole.  Most people believe that, yes, there is a problem, and general relativity is modified in some way so that the (modified) mathematics gets rid of the singularity.  This is a fairly big area of research among people who study gravity.



> The view for photon transmission is a thought idea.  The implication is that the receiver knows (somehow) exactly when to change state to balance the change in state of the transmitter, including the time and phase differences and possible paths, which feels implausible.  It's action at a distance taken to the extreme.  I also understand that accounting for the energy of the photon as it transits gets to be a mess if the transition is not made as a continuous transmission across space.
> 
> Still, the descriptions of particle motion which talk about "sum over possible paths" and what not seem to push us in this sort of direction.  One gets curiously close to a description that might arise if the whole system were being simulated using discrete techniques.




I'm not sure what you mean about the receiver changing state "to balance the change in state of the transmitter."  Quantum mechanics doesn't say this.  What it says is that we can only know the state of a system at a measurement event, and, if we know a particular measurement, we can predict the probabilities of the different outcomes of a subsequent measurement.  The calculation of these probabilities can be described as the sum over paths.  I'm also not sure I understand what you mean about accounting for the energy of the photon being difficult.


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## tomBitonti (Feb 6, 2014)

freyar said:


> I'm not sure what you mean about the receiver changing state "to balance the change in state of the transmitter."  Quantum mechanics doesn't say this.  What it says is that we can only know the state of a system at a measurement event, and, if we know a particular measurement, we can predict the probabilities of the different outcomes of a subsequent measurement.  The calculation of these probabilities can be described as the sum over paths.  I'm also not sure I understand what you mean about accounting for the energy of the photon being difficult.




It's looking at the physics by only interpreting the endpoint events: A photon is emitted; somewhat later, somewhere else, subject to spacetime, curvature, interference, and probabilities, a photon is absorbed.  That the photon crosses space is looked at as a bookkeeping detail: Figuring out where the photon will be absorbed.  The intrinsic event is the emission _and_ absorption, together as one unified event, which happens to occur at two different points in space-time.

That's what you might do if you were making a simulation, and only cared about the emit and absorb steps.  Computation of the path that the photon took (or could take) would be a way of figuring out where the absorb occurs.

Thx!

TomB


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## freyar (Feb 7, 2014)

tomBitonti said:


> It's looking at the physics by only interpreting the endpoint events: A photon is emitted; somewhat later, somewhere else, subject to spacetime, curvature, interference, and probabilities, a photon is absorbed.  That the photon crosses space is looked at as a bookkeeping detail: Figuring out where the photon will be absorbed.  The intrinsic event is the emission _and_ absorption, together as one unified event, which happens to occur at two different points in space-time.
> 
> That's what you might do if you were making a simulation, and only cared about the emit and absorb steps.  Computation of the path that the photon took (or could take) would be a way of figuring out where the absorb occurs.




That's a reasonable picture, but I don't think any physicist I know would consider emission and absorption as a single event.  In particular, "event" is usually defined to mean a point in space-time.  

I should mention, though, that is the _quantum mechanical_ picture.  That is how we describe physics at the microscopic level, but, in many contexts, it's much more useful to use the approximate picture of classical mechanics, in which the photon really does follow a definite path.


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## Scott DeWar (Feb 7, 2014)

So, A thought: Energy never really disappears, or is destroyed, right?It 'changes to a different form of energy' If I recall being told. Is there a way to follow the path using the Femto-cam technology?


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## Umbran (Feb 7, 2014)

freyar said:


> That's a reasonable picture, but I don't think any physicist I know would consider emission and absorption as a single event.  In particular, "event" is usually defined to mean a point in space-time.




Not as such.  You can have a single Feynmann diagram for a particle-antiparticle pair annihilating to a photon, and that then emitting to a particle-antiparticle pair, which is close....



> I should mention, though, that is the _quantum mechanical_ picture.  That is how we describe physics at the microscopic level, but, in many contexts, it's much more useful to use the approximate picture of classical mechanics, in which the photon really does follow a definite path.




Goodness, yes.  You'd only use such a treatment for very short-range interactions.  When you're talking about a photon that travels a light year, a mile, or even a few inches, you generally use the classical approach.


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## Janx (Feb 7, 2014)

Umbran said:


> Yep.  Massless particles are special.




I'm just extending my "what if the limit is because it's a simulation" idea here.

Is it possible that things like Photons exist because they artifacts of a rendering (aka display) proecess rather than actual matter?

consider in a video game (like minecraft), all the matter is modeled and effects are dealt with.  Then, the rendering step kicks in to compute what is displayed to the viewing portal (the player's screen based on it's relative position in the game universe).

As Minecraft actually has to project rays from light sources to computer light/dark levels as it renders the image, what if the proposed universal simulator has to project photons from pertinent objects, in order to compute what the individual observers (us) see?

Thus, these things are massless and have odd properties (like always traveling at c relative to the observer regardless of how fast the observer is traveling) because they serve functional purposes for the simulation?

I'm not actually wholly enamored of the recent "universe is a simulation" movement, but there are ideas that are interesting from it.


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## Bullgrit (Feb 7, 2014)

Something that twists my noodle:

Shoot a beam of light (like in the camera demonstration above), and you can see the beam from the side (90 degrees off its line of fire) -- that means photons are coming off the beam to the camera lens. I presume photons are being deflected off molecules toward the camera, and the beam is not *producing* more photons. Yes?

But the TED video, when showing the around the corner viewing potential, seems to show photons hitting a surface and "exploding" into more photons.

Bullgrit


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## tomBitonti (Feb 7, 2014)

Janx said:


> I'm just extending my "what if the limit is because it's a simulation" idea here.
> 
> Is it possible that things like Photons exist because they artifacts of a rendering (aka display) proecess rather than actual matter?
> 
> ...




(Text omitted.)

Yeah.  There turn out to a number of conceptual problems which clear up with a computational approach.  (Consider entanglement as the universe using lazy / deferred initialization.)

While not strongly advocating that approach, I do find it to be a curiously useful approach.  But maybe, that's because I do software, and the meaning of "computation" is pretty wide.

There _are_ problems: What about photons which vanish into space, never to encounter anything?


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## Scott DeWar (Feb 7, 2014)

Bullgrit said:


> Something that twists my noodle:
> 
> *Shoot a beam of light (like in the camera demonstration above), and you  can see the beam from the side (90 degrees off its line of fire) --* that  means photons are coming off the beam to the camera lens. I presume  photons are being deflected off molecules toward the camera, and the  beam is not *producing* more photons. Yes?
> 
> ...




I am guessing the sight of the 'photon bullet' in the coke bottle was diffused light from being scattered by the water. I am further guessing that a light beam in a vacuum would not have anything diffusing it and therefore no viewing from the side, as noticed when you look perpendicular to a line of sight between a star and earth, you can't see the beam or as long as it is not shining through an exploded star, 'dust motes'.


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## Janx (Feb 7, 2014)

tomBitonti said:


> (Text omitted.)
> 
> Yeah.  There turn out to a number of conceptual problems which clear up with a computational approach.  (Consider entanglement as the universe using lazy / deferred initialization.)
> 
> ...




As I figure, if Umbran said "yes, it could be that", it doesn't mean photons confirm the existance of a universal simulator.  Just as coal being useful for making torches, doesn't mean coal exists for the purpose of such (in minecraft anyway).

It would strike me as a wasteful process to compute the path of photons that don't go towards observers.  If it goes off to empty space, nobody's going to see it.  However, from an object oriented approach, you don't know that until the photon goes forever and doesn't actually hit anything (or travels max diameter of the universe in its last projection/deflection)

Though I also don't know that I would need photons to travel in a simulation.  As in a videogame, something is visible, the moment it is unobstructed.  Light travels effectively instantaneously.  Of course note, the video game is computing what's visible at 50-60 frames per second (at least on my PC for MC).  Assuming the Universal Simulator is just time slicing (at say c frames per second), what technical purpose is served to model photons, instead of just showing me whatever's in front of my eye in each frame?  I'm surmising that it might need to be some kind of performance optimization (so we don't have to check all 100 bajillion objects in the universe to see if I can see them to render an image for my eyes).


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## Bullgrit (Feb 7, 2014)

Wonder if anyone has studied images of the universe for duplicate galaxies on opposite sides of us, to determine if we're seeing wrap-around.

Bullgrit


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## Scott DeWar (Feb 7, 2014)

I took a peek the other day . . . . I will let you know what I saw when I catch up to myself. I had to borrow a bit of energy to do this so now I have to pay it back.


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## Morrus (Feb 8, 2014)

Bullgrit said:


> Wonder if anyone has studied images of the universe for duplicate galaxies on opposite sides of us, to determine if we're seeing wrap-around.




Of course! The Hubble telescope does that sort of thing all day every day. You should check out some of the Hubble Deep Field images.

And they didn't see that. However, we can't see all that far - we can only see about 14 billion light years (the age of the universe is 14 billion years, so at the speed of light which is one light year per year any light originating further than 14 billion light years from us hasn't had time to reach us yet).  That's what's called the "observable universe" - the small portion of it we can see.


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## Bullgrit (Feb 8, 2014)

Morrus said:
			
		

> Of course! The Hubble telescope does that sort of thing all day every day. You should check out some of the Hubble Deep Field images.
> 
> And they didn't see that. However, we can't see all that far - we can only see about 14 billion light years (the age of the universe is 14 billion years, so at the speed of light which is one light year per year any light originating further than 14 billion light years from us hasn't had time to reach us yet). That's what's called the "observable universe" - the small portion of it we can see.



I know. I've seen the Deep Field images -- in fact, my wall calendar over my desk is images of Hubble pics. They're mesmerizing!

But I haven't seen any references to anyone doing a wrap-around check.

Bullgrit


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## Morrus (Feb 8, 2014)

Bullgrit said:


> I know. I've seen the Deep Field images -- in fact, my wall calendar over my desk is images of Hubble pics. They're mesmerizing!
> 
> But I haven't seen any references to anyone doing a wrap-around check.




Well, you can only see 14b light years, so it's not possible to do that (at least - that isn't the case within 14b light years each way; if it is further out, we can't see it).

There's a bunch of theories on the shape of the universe and lots of research into it, but I'll leave an actual physicist to go into that; I get a bit lost when they start talking about "saddle shaped" universes and the like.


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## Umbran (Feb 8, 2014)

Janx said:


> Is it possible that things like Photons exist because they artifacts of a rendering (aka display) proecess rather than actual matter?




Is it possible?  Well, sure, it is possible.  But note that the simulation posit is what we'd call "non-falsifiable", meaning that there is no test we could do, no information would could ever have, even in theory, that would prove it *wasn't* a simulation.  Any evidence we had could be countered with, "Well, the simulation is written to give you that result."  So, the simulation idea is ultimately not a scientific question, but a philosophic one.

But, that something so central to how the Universe works is just an artifact?  In videogames, programmers go to great lengths to reduce the impact of rendering artifacts on the experience.  But photons are friggin' *everywhere*, and most definitely impact the simulation experience.  So, if they are artifacts, our programmers aren't very good at their job, now are they?


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## freyar (Feb 8, 2014)

Bullgrit said:


> Shoot a beam of light (like in the camera demonstration above), and you can see the beam from the side (90 degrees off its line of fire) -- that means photons are coming off the beam to the camera lens. I presume photons are being deflected off molecules toward the camera, and the beam is not *producing* more photons. Yes?
> 
> But the TED video, when showing the around the corner viewing potential, seems to show photons hitting a surface and "exploding" into more photons.






Scott DeWar said:


> I am guessing the sight of the 'photon bullet' in the coke bottle was diffused light from being scattered by the water. I am further guessing that a light beam in a vacuum would not have anything diffusing it and therefore no viewing from the side, as noticed when you look perpendicular to a line of sight between a star and earth, you can't see the beam or as long as it is not shining through an exploded star, 'dust motes'.




Right.  While there are processes that can create or destroy photons (like those in the laser!), the light propagation you see in that video is all just scattering.  Each pulse has many, many photons, and they are bouncing off the water (in the coke bottle), or, in daily experience, dust particles.  The "exploding" photons in the cartoon about looking around the corner is their way of illustrating that the single "light bullet" has tons of photons in it.  Each photon will hit a slightly different part of the wall and therefore bounce in different directions.  But anyway, you only see photons that go directly into your eye, so seeing a light beam "from the side" really means that some of the light is being scattered out of the beam toward you.

By the way, that video is really cool, but I have to point out a bit of "false advertising" or something in it that really bugs me.  Notice that the camera takes a frame every 1 trillionth of a second.  That's a _picosecond_, not a _femtosecond_, which is 1000 times smaller than a picosecond.  So calling it "femto-photography" is, well, misleading.  I mean, it's hard enough to explain science in a simple but reasonably correct way, so someone intentionally goofing up scientific notation is hard to take.  (And I'll eat my hat if no one in his lab knows the difference between "pico" and "femto.")

I'll make a separate post in a bit about the "wrap-around."


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## freyar (Feb 8, 2014)

Bullgrit said:


> Wonder if anyone has studied images of the universe for duplicate galaxies on opposite sides of us, to determine if we're seeing wrap-around.
> 
> Bullgrit






Morrus said:


> Of course! The Hubble telescope does that sort of thing all day every day. You should check out some of the Hubble Deep Field images.
> 
> And they didn't see that. However, we can't see all that far - we can only see about 14 billion light years (the age of the universe is 14 billion years, so at the speed of light which is one light year per year any light originating further than 14 billion light years from us hasn't had time to reach us yet).  That's what's called the "observable universe" - the small portion of it we can see.






Bullgrit said:


> I know. I've seen the Deep Field images -- in fact, my wall calendar over my desk is images of Hubble pics. They're mesmerizing!
> 
> But I haven't seen any references to anyone doing a wrap-around check.
> 
> Bullgrit




Hubble isn't really the right instrument to do a search for a finite size universe/wrap-around effect.  The main problem is that Hubble only looks very far away in a very small area of the sky, and you've got to look in pretty much every direction if you want to see a wrap-around effect.  Think of it this way, if there is one galaxy we are seeing duplicated, we should see it both to the right and to the left.  You wouldn't see both duplicates in the same spot to the right, for example.

But you can look for wrap-around in the cosmic microwave background, which is the light left over from the end of the Big Bang and is the farthest away thing we can see (or ever will see, as the universe was opaque at earlier times).  This light is like a picture of the universe almost 14 billion years ago, and it tells us the universe was not quite the same temperature or density at every point.  So what people have done is look for evidence of repeating patterns in the hot and cold spots which could be explained by a wrap-around effect.  There's no credible evidence of any, which means the universe can't be smaller than 14 billion lightyears across (actually even a bit larger if I understand the results correctly).  I can't remember if there have been checks done for every possible shape (see below), so I suppose there might be some wiggle room, but I don't think so.  If you think about the usual theories of the early universe, it would be kind of weird for the universe to be just the size we see today anyway.





Morrus said:


> Well, you can only see 14b light years, so it's not possible to do that (at least - that isn't the case within 14b light years each way; if it is further out, we can't see it).
> 
> There's a bunch of theories on the shape of the universe and lots of research into it, but I'll leave an actual physicist to go into that; I get a bit lost when they start talking about "saddle shaped" universes and the like.




There are two sorts of shape issues here.  One has to do with whether space is curved.  In 2D, you can think of the three basic curvatures as being flat like a piece of paper, curved like a sphere, or curved like a Pringle chip.  Measurements (again from the cosmic microwave background, among others) tell us that our universe is flat (within error).  But the other question is if the universe is infinite or finite.  In 2D again, the old video game Asteroids is a good example of a flat but finite universe with wrap-around.  In fact, there aren't many other possibilities in 2D.  In 3D, there are a lot more choices.  I'm not a geometer, but my understanding is that the options more or less correspond to how you can take a regular polyhedron and glue the faces together.  What I'm not sure about in the cosmological searches if they've got a search algorithm that can handle all possibilities at once of if they've only been able to check a few.  I just don't remember, as it's not something I work on actively.


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## Ruzak (Feb 8, 2014)

PigKnight said:


> ...
> 
> But, for realsies, why is it called _c_? Latin? Kicks-n-giggles? Discovering scientist?




I believe c comes from the Latin celer, meaning fast.  This is also the origin of the words accelerate & celerity (like the spell).


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## Umbran (Feb 8, 2014)

Ruzak said:


> I believe c comes from the Latin celer, meaning fast.  This is also the origin of the words accelerate & celerity (like the spell).




The first known reference to c standing for "celeritas" is a 1959 essay by Isaac Asimov, and he cited no evidence to support the claim.  So, I'd take that with a grain of salt.

I believe it was c for "constant".  While physicists do get fanciful with naming sometimes, we do tend to keep it simple when it gets down to writing down the math.


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## Morrus (Feb 8, 2014)

Umbran said:


> The first known reference to c standing for "celeritas" is a 1959 essay by Isaac Asimov, and he cited no evidence to support the claim.  So, I'd take that with a grain of salt.
> 
> I believe it was c for "constant".  While physicists do get fanciful with naming sometimes, we do tend to keep it simple when it gets down to writing down the math.




You do NOT get fanciful about naming. You've spent half a century congratulating yourselves for "strange" quarks like that's some genius display of eccentric creativity. It's not. 

You all collectively need to read a comic book someday! 

(Which is weird, given that every physicist I know loves comic books. The - arguably - two most famous current popular physicists aside from Hawking - Neil deGrass Tyson and Prof Brian Cox - are both woefully ignorant of both).


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## Ruzak (Feb 8, 2014)

Umbran said:


> The first known reference to c standing for "celeritas" is a 1959 essay by Isaac Asimov, and he cited no evidence to support the claim.  So, I'd take that with a grain of salt.
> 
> I believe it was c for "constant".  While physicists do get fanciful with naming sometimes, we do tend to keep it simple when it gets down to writing down the math.




I stand corrected. That's what I get for taking information from a fluffy not-for-science-majors textbook. 
Thanks for the correction Umbran.  I've been telling it wrong for years.


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## Janx (Feb 9, 2014)

Umbran said:


> Is it possible?  Well, sure, it is possible.  But note that the simulation posit is what we'd call "non-falsifiable", meaning that there is no test we could do, no information would could ever have, even in theory, that would prove it *wasn't* a simulation.  Any evidence we had could be countered with, "Well, the simulation is written to give you that result."  So, the simulation idea is ultimately not a scientific question, but a philosophic one.
> 
> But, that something so central to how the Universe works is just an artifact?  In videogames, programmers go to great lengths to reduce the impact of rendering artifacts on the experience.  But photons are friggin' *everywhere*, and most definitely impact the simulation experience.  So, if they are artifacts, our programmers aren't very good at their job, now are they?




Note, you're using artifact like its a bad thing.  "artifact" in computing is only bad to graphics people as it means a defect in the image.  I may have chosen a less good word, as I really mean, "a component of the system"  Given that photons (in the universal simulator) ARE the rendering process, there aren't really any glitches.  Except now that people are studying them.. 

On the "It's Possible"  I was mainly seeing if "photon wierdness is Simulation Behavior" has more possibility of intent than "just happens to be there."  Kind of like walking into a strangers house a seeing a brick next to a door.  That brick might just happen to be there.  But when we notice that door doesn't tend to stay open, it becomes obvious that somebody intended the brick to be used to hold the door open.  That doesn't guarrantee that we're right on the presence of the brick versus the owner's intent.  But the proximity of a problem and a solution seems far from coincidental.


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## freyar (Feb 9, 2014)

Janx said:


> On the "It's Possible"  I was mainly seeing if "photon wierdness is Simulation Behavior" has more possibility of intent than "just happens to be there."  Kind of like walking into a strangers house a seeing a brick next to a door.  That brick might just happen to be there.  But when we notice that door doesn't tend to stay open, it becomes obvious that somebody intended the brick to be used to hold the door open.  That doesn't guarrantee that we're right on the presence of the brick versus the owner's intent.  But the proximity of a problem and a solution seems far from coincidental.




I feel like I should say that photons don't behave more "weirdly" than anything else.  I don't want anyone to get the impression that they do.


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## Umbran (Feb 9, 2014)

Morrus said:


> You do NOT get fanciful about naming. You've spent half a century congratulating yourselves for "strange" quarks like that's some genius display of eccentric creativity. It's not.




It isn't like we've reached energies such that we've seen tons of new things to name in those 50 years, though.  

And, well, penguin diagrams.



> You all collectively need to read a comic book someday!
> 
> (Which is weird, given that every physicist I know loves comic books. The - arguably - two most famous current popular physicists aside from Hawking - Neil deGrass Tyson and Prof Brian Cox - are both woefully ignorant of both).




The guys popular in the media are so busy, I don't expect them to have lots of time for popular entertainment.


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## Umbran (Feb 9, 2014)

Janx said:


> Note, you're using artifact like its a bad thing.  "artifact" in computing is only bad to graphics people as it means a defect in the image.




In context, that seemed like the obvious meaning for you to have been aiming for.



> Given that photons (in the universal simulator) ARE the rendering process, there aren't really any glitches.




Well, to be honest, that's assuming there *is* a rendering process.  But, you can run a simulation without ever writing out the moment-to-moment state of the simulation.  Your experience does not need to be written to "screen" - everything you experience may be in memory/processor.



> On the "It's Possible"  I was mainly seeing if "photon weirdness is Simulation Behavior" has more possibility of intent than "just happens to be there."




Intent?  If you're looking to see "intent" in the structure of the Universe, you're definitely in the philosophical, rather than the scientific.


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## Janx (Feb 9, 2014)

Umbran said:


> In context, that seemed like the obvious meaning for you to have been aiming for.




Both yes and no.  In my line of work, artifacts are things we look for in analyzing a business process.  So it's not a negative so much as an attribute of their universe we are happy to find so we can handle it.



Umbran said:


> Well, to be honest, that's assuming there *is* a rendering process.  But, you can run a simulation without ever writing out the moment-to-moment state of the simulation.  Your experience does not need to be written to "screen" - everything you experience may be in memory/processor.




Again, yes and no.  If I was rendering to a central view screen, that's the more obvious use of a rendering process.  MOBs in a game don't actually need to see each other, as they are aware of each other's X,Y, and Z natively.

However, if I was writing an AI as its own object/entity in a FPS, I may indeed need to write a rendering process so it can "see" the universe and then it's object recognition sequence can execute and interpret what it "sees".  Game programmers skip that crap, because the AI and the game world are on the same plane of operation, so they can cut corners.

But if you were building an autonomous robot, you'd need a camera and an optical parsing engine to interpret what it sees into stuff it can respond to.

You could then disconnect the video camera, and connect it's brain to your Xbox's HDMI jack and make it play Halo (because it recieves a comparable visual image that it then inteprets into walls, people ,etc).

At this point, you have a universe (the game) that is going through a reaction/rendering cycle, and a separate process (the robot brain) that is unwittingly interacting with it.

Now when we go to super-technology, both the game world and the robot brain (AI) are running on the same super-duper computer, and in fact, the AI is part of the regular simulation.

So, it's like the rendering sequence (photons) is serving the AI's need to see and be self-contained from the simulation, while the AIs are actually made of the simulation.  Kind of like the guy who made an 8 bit computer inside of Minecraft.

Hypothetically, the photons exist because it's the only way any entity could "see" another entity in order to interact with it (or avoid falling into lava and dying).

Bear in mind, programmers often employ a concept of separation of church and state in our code.  One module (the AI) isn't allowed to know about or talk to other modules except through interfaces we devise for the purpose.  So while in a game, the bad guy knows you're in the room behind the door because the programmer didn't separate that, we might code it that the bad guy doesn't know anything, except that we can trace a line from his eye toward the direction he is looking and if it doesn't reach you, he doesn't respond to you.  That separation is what invokes more "real life" behavior.



Umbran said:


> Intent?  If you're looking to see "intent" in the structure of the Universe, you're definitely in the philosophical, rather than the scientific.




Again, yes and no.  Something that is an exception to the norm may indicate intent, rather than random coincidence.  One does not usually find a brick sitting in a house.  But when you see it next to a door, it is no longer random as somebody just left it there, now there's a strong probability that it was to prop the door open, because that's what one might do.

Just as we don't expect to find a brick in the middle of a cow pasture.  Just sitting there.  it's not likely to happen.  Which means somebody put it there.  Lacking any proximity to anything a brick might be useful, it's just sort of random and we cannot deduce a purpose.  Unlike the brick in a house next to a door that won't stay open.

It's possible the weirdness of photons is more like a brick in a field than a brick next to a door (or that they're very much less like a man-made brick given that they are as unnatural as quarks and protons).


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## Janx (Feb 9, 2014)

freyar said:


> I feel like I should say that photons don't behave more "weirdly" than anything else.  I don't want anyone to get the impression that they do.




They have no mass and they travel at the speed of c away from 2 observers who are also travelling at differing speeds.

I'd ask "what else does that" but there's likely a category of weird things that does it.

to us normal people who can't do that trick when we're on a train, that's wierd.


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## Umbran (Feb 9, 2014)

Janx said:


> But if you were building an autonomous robot, you'd need a camera and an optical parsing engine to interpret what it sees into stuff it can respond to.




Rendering is required to cross the simulation/non-simulation boundary.

We don't have a robot within a real space, taking in data and having some gaps in how we model the real world in his computer brain.  Nor do we have a computer world being displayed for a human being.   If we are positing the Universe is a simulation, then so far as we know, the space and everything within the space is also part of the model. If the Universe is a simulation, you, I, and everything in it are part of the simulation. 

As a part of the simulation, I don't need it to be rendered to interact with it.  My interactions are *part* of the situation.

Heck, if the Universe is a simulation, you'd kind of expect the bits of it that are supposed to be "sentient" to not recognize the fact - "Pay no attention to the man behind the curtain!" 



> You could then disconnect the video camera, and connect it's brain to your Xbox's HDMI jack and make it play Halo (because it recieves a comparable visual image that it then inteprets into walls, people ,etc).




You could, but... wouldn't that be kind of silly and pointless?  Let us take a game designed to entertain humans, and instead let it occupy a machine incapable of feeling entertained!   While any number of government grants get spent that way, I'm sure, isn't someone capable of consistently simulating an entire universe have something better to do with it's time?  What the heck kind of robot are you trying to train using an entire universe?  

The problem with Philosophical models of the universe is just this - you can go down incredibly deep ratholes of what *might* be true, with absolutely no real way to resolve questions.  Anything at all *might* be true, if you invoke such non-falsifiable forces to make things go.


----------



## freyar (Feb 10, 2014)

Janx said:


> They have no mass and they travel at the speed of c away from 2 observers who are also travelling at differing speeds.
> 
> I'd ask "what else does that" but there's likely a category of weird things that does it.
> 
> to us normal people who can't do that trick when we're on a train, that's wierd.




Gravitons, gluons, used to be people thought neutrinos... 

But that's not the issue with "we don't know where the photon is between emission and absorption."  That's quantum mechanics, which describes everything, massless or not.

And I'd put all kinds of other relativistic effects that massive particles experience just as high on the "weird" scale --- velocities don't add in the "usual" way at any significant fraction of c.  The case of light beams is just the limit.  Of course, the only reason we think of this as weird is because we don't interact much with really fast massive stuff.  And, in our daily lives, light moves so quickly as to be effectively infinitely fast.


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## Scott DeWar (Mar 2, 2014)

*need some humor here . . . . . good clean fun . . . . .*

A photon enters a hotel. The bellboy walks up and asks, can I help with  your luggage? 
The photon responds, no thanks, I'm traveling light. 

The bartender says, "I'm sorry, we don't serve faster-than-light particles here."
A tachyon walks into a bar.

Argon walks into a bar. The bartender says, I'm sorry, we don't serve noble gasses here. 
Argon doesn't react.

math jokes:

a group of mathematicians walk into a bar. The first asks for a liter  drink, the next half a liter, then a quarter liter, then an eighth  liter, etc. The bartender puts 2 liters on the table and says, you don't  know your limits.

A mathematician, a physicist, and an astronomer were traveling north by  train. They had just crossed the border into Scotland, when the  astronomer looked out of the window and saw a single black sheep in the  middle of a field. "All Scottish sheep are black," she remarked. "No, my  friend," replied the physicist, "Some Scottish sheep are black." At  which point the mathematician looked up from her paper and glanced out  the window. After a few seconds' thought she said blandly: "In Scotland,  there exists at least one field - in which there exists at least one  sheep - _at least one side of which_ is black."


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## Bullgrit (Mar 2, 2014)

Scott DeWar said:
			
		

> need some humor here . . . . . good clean fun . . . . .



Awesome jokes! Thanks. [Can't give you xp right now.]

Bullgrit


----------



## tomBitonti (Mar 3, 2014)

Heh; pretty good jokes.  Although, the last is used often as a dig against mathematicians.

Thx!

TomB


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## Scott DeWar (Mar 3, 2014)

Actually I liked the last one the most as it gave the mathematician the most accurate statement.


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## freyar (Mar 4, 2014)

Here are a couple along the same lines:

A neutron walks into a bar and asks, "How much for a drink?"  The bartender replies, "For you, no charge."


An engineer, a physicist, and a mathematician all spend the night in a hotel, and each wakes up in the middle of the night with a fire in their trashcan (maybe they're still smokers?).  
The engineer gets up, fills the ice bucket with water, and puts out the fire.
The physicist gets up, estimates that the bucket is large enough to put out the fire, fills it with water, and puts out the fire.
The mathematician gets up, writes a proof that it's possible to put out the fire, and goes back to sleep.

Ba-dum-bum.


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## Umbran (Mar 4, 2014)

Scott DeWar said:


> Actually I liked the last one the most as it gave the mathematician the most accurate statement.




Well, perhaps this one will illustrate why:

Three men are in a hot-air balloon. Soon, they find themselves lost in a canyon somewhere. One of the three men says, "I've got an idea. We can call for help in this canyon and the echo will carry our voices far."
So he leans over the basket and yells out, "Helllloooooo! Where are we?" (They hear the echo several times).
15 minutes later, they hear this echoing voice: "Helllloooooo! You're lost!!"
One of the men says, "That must have been a mathematician."
Puzzled, one of the other men asks, "Why do you say that?"
The first says, "For three reasons. (1) he took a long time to answer, (2) he was absolutely correct, and (3) his answer was absolutely useless."

We can widen the scope a little, though...

A biologist, a statistician, a mathematician and a computer scientist are on a photo-safari in Africa. They drive out on the savannah in their jeep, stop and scout the horizon with their binoculars.
The biologist: "Look! There's a herd of zebras! And there, in the middle: A white zebra! It's fantastic! There are white zebras! We'll be famous!"
The statistician: "It's not significant. We only know there's one white zebra."
The mathematician: "Actually, we only know there exists a zebra, which is white on one side."
The computer scientist: "Oh, no! A special case!"


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## Scott DeWar (Mar 4, 2014)

Four engineers get into a new car, A Computer Engineer, a Electrical Engineer, A Mechanical Engineer, and a Chemical Engineer.

While they are on their way to Las Vegas for a symposium the car dies. The start discussing what is wrong, but egos get in the way and it breaks down to bickering and arguing.

Electrical Engineer blames the electrical wireing and charging system and says the problem is there.
Mechanical Engineer blames the engine and drive trane and insists that is where they need to look.
Chemical Engineer blames the fuels and lubricants and proclaims that is where they need to investigate.
Computer Engineer cries out and "SILENCE!" The others look at him with his smug look. "Try this: every one close their windows.
When they had all done that he said, "Now, re-open windows . . . "

How was that joke?


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## Scott DeWar (Mar 17, 2014)

*back on subject . . . . .*

I thought the idea of ftl was not possible?

http://www.nbcnews.com/science/space/smoking-gun-reveals-how-inflationary-big-bang-happened-n54686


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## Olgar Shiverstone (Mar 17, 2014)

How do you tell an extroverted engineer from an introverted engineer?
A: The extroverted engineer talks to _your_ shoes.

Or perhaps:

A mechanical engineer, an electrical engineer, and a civil engineer are sitting in a pub. Says the mechanical engineer: "I think God must be a mechanical engineer. Just look at all those bones, muscles, tendons, joints and how it all interacts so nicely." 
"Well, but nothing would be moving if it weren't for the brain and the nerves," throws in the electrical engineer. "No, I think God is an electrical engineer." 
The civil engineer takes a sip from his pint and then says: "You're both wrong. God is without any doubt a civil engineer." 
"Why's that then?" 
"Only a civil engineer would run a wastepipe straight through a recreational area."


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## Scott DeWar (Mar 17, 2014)

Olgar Shiverstone said:


> "Only a civil engineer would run a wastepipe straight through a recreational area."




heard this before, but wasn't sure how to keep it civil.


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## Mustrum_Ridcully (Mar 18, 2014)

Scott DeWar said:


> I thought the idea of ftl was not possible?
> 
> http://www.nbcnews.com/science/space/smoking-gun-reveals-how-inflationary-big-bang-happened-n54686




Not sure what you mean here with regards to FTL?


----------



## freyar (Mar 18, 2014)

Scott DeWar said:


> I thought the idea of ftl was not possible?
> 
> http://www.nbcnews.com/science/space/smoking-gun-reveals-how-inflationary-big-bang-happened-n54686




FTL motion is not possible in our best understanding of physics, and it would take a truly revolutionary and unexpected discovery to change that.

The phrase "a burst of inflation that was seemingly faster than the speed of light" is possibly one of the most misleading and damaging that people can use when talking about the expansion of the universe.  If a scientist said it, they either didn't know the subject, were being incredibly lazy in talking to the press, or weren't thinking.  And, in this case, the "FTL mistake" doesn't even require inflation.  Here's my explanation:

The expansion of the universe does not entail the motion of objects.  If I have two far-enough-apart galaxies, _they stay in the same places_ and _space appears between them_.  This does create the Doppler effect on light from one galaxy going to the other.  If you didn't know better, you'd think they were moving apart, but of course we at EN World know better now.   So saying that the expansion of the universe involves FTL motion involves the mistake of thinking things are moving.  It involves a simpler mistake, too: even if you think the galaxies are actually moving apart, you only see "FTL" motion if you use the simplified formula for the Doppler effect which _only_ works for speeds much smaller than the speed of light!  Using the correct relativistic formula never gives FTL speeds.

And there's one other problem with that phrase in this context: if you make _both_ mistakes and think there is FTL motion due to the expansion of the universe, you don't have to go all the way back to cosmic inflation to do that.  Some of the farther away galaxies we see would appear to be moving FTL.  No matter how fast the universe is expanding, you can always see "FTL motion" if you just look far enough away.  That's a feature of the expansion of space itself.  You don't need inflation.

BTW, there's another thread on this subject.


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## Umbran (Mar 18, 2014)

Mustrum_Ridcully said:


> Not sure what you mean here with regards to FTL?




That article has the followign passage:

"For decades, Guth, Linde and other theorists have advanced the view that the universe somehow inflated itself to huge size in as little as an undecillionth of a second (10 to the negative 36th power). If such an expansion were measured as a three-dimensional spatial phenomenon, the velocity would seem to exceed the speed of light. But in this case, the entire cosmos would have expanded into extradimensional space."

There are a couple of issues with that - they say, "the velocity would seem to exceed".  Note that velocities are attributed to objects.  They didn't say *what* would seem to have a velocity greater than light.  "The Expansion" is not an object with a velocity.

Though, come to think of it, "The Expansion" sounds like a Dr. Who antagonist....

Be that as it may, in inflationary scenarios, it isn't that objects move, it is that space gets added, which is not the same thing.  No object moves faster than light, but all objects not otherwise bound together wind up with more distance between them.  At that age of the universe, mind you, there aren't much of what we'd call "objects" in existence anyway.  We are talking about something that happened when the Universe was between 10^-35 to 10^-32 seconds old.  At the *end* of inflation, if I have my numbers right, what we now call the "observable universe" was still less than a millimeter across - still so dense that "things" aren't really a going concern.


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## Morrus (Mar 18, 2014)

Scott DeWar said:


> I thought the idea of ftl was not possible?
> 
> http://www.nbcnews.com/science/space/smoking-gun-reveals-how-inflationary-big-bang-happened-n54686




Objects cannot move through space faster than light. The expansion of space isn't motion through space.


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## tomBitonti (Mar 18, 2014)

Morrus said:


> Objects cannot move through space faster than light. The expansion of space isn't motion through space.




Is there no frame of reference into which to put co-expanding particles?  Say, of one or of the other particle?

I'm not saying that it isn't different, but how is that frame different than other frames of reference?  Why aren't the relative particle velocities meaningful in that frame?

Note that we don't need inflation to create pseudo-velocities, of the sort caused by inflation, which are greater than the speed of light.  The ongoing, much slower, expansion of the universe is doing so for particles at extreme distances.  Inflation shrinks the radius at which the speed of light is reached as a pseudo-velocity.

Thx!

TomB


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## Morrus (Mar 18, 2014)

tomBitonti said:


> Is there no frame of reference into which to put co-expanding particles?  Say, of one or of the other particle?
> 
> I'm not saying that it isn't different, but how is that frame different than other frames of reference?  Why aren't the relative particle velocities meaningful in that frame?




I'll leave it to the actual physicists to answer that.  You just went above my pay grade.  Usually that means the answer = equations, which is where I stop following along!


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## Umbran (Mar 18, 2014)

tomBitonti said:


> Is there no frame of reference into which to put co-expanding particles?  Say, of one or of the other particle?




Well, there's the trick - pick two particles close enough together that there are no other particles in the space, then the relative speeds will be under that of light.  Pick two distant particles... then you have a whole lot of other particles int eh field of view, and you can see the issue.



> I'm not saying that it isn't different, but how is that frame different than other frames of reference?  Why aren't the relative particle velocities meaningful in that frame?




Ah, there's the phrase we can use.  The particle velocities aren't meaningful in that frame, because the frame isn't constant!  The frame is expanding!

One way to think of it is thus:  when we pick a frame of reference, we don't actually pick just a central point.  We pick that central point, and some distant points - usually implicitly we pick "relative to the distant stars" which are fixed for most human purposes.  The reference points of the frame, Einstein tells us, are arbitrary, so for most cases they don't matter.  But we are now talking about a case where it does matter.

When we are talking about cosmological expansion, we are no longer talking about small distances or times.  We are talking about distances and times which span... all.  There is no larger, fixed outer edge of the frame of reference to which we can refer, and we have to worry about the fact that the frame itself is no longer constant!

There are other ways to explain the difference.. I remember a really cute video I saw recently.  Let me see if I can find it again...


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## tomBitonti (Mar 18, 2014)

Umbran said:


> Well, there's the trick - pick two particles close enough together that there are no other particles in the space, then the relative speeds will be under that of light.  Pick two distant particles... then you have a whole lot of other particles int eh field of view, and you can see the issue.
> 
> Ah, there's the phrase we can use.  The particle velocities aren't meaningful in that frame, because the frame isn't constant!  The frame is expanding!




Not sure what other particles do to change the situation, except to make the observations a lot messier.  (Some bad astronomy pun deserves to be attached to "messier", but, being humor impaired, I'll leave that to others.)

Is a non-constant frame of reference a problem?    There are all sorts of non-constant frames which are used in physics.  There are distinctions made between inertial and non-inertial frames of reference (giving rise to pseudo-forces; for example, a frame of reference defined from a point on a spinning disk is non-inertial, and gives rise to centrifugal forces).  But, I thought that a frame of reference was just a coordinate system, with more or less utility based on how well the frame exposes particular physical properties of the system which is being studied.

Is the problem perhaps that frames of reference (which is how we are used to thinking) don't work very well when applied to an expanding (or contracting) region of space-time, because of the non-uniform scale factor which results?

Note, I'm really not trying to say that we shouldn't treat recession velocities as special.  I'm trying to understand exactly how we know they are special.  At least, how do we know without doing a global analysis.  For a pair of particles, how is a recession velocity distinguishable from simple motion?

Thx!

TomB


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## Umbran (Mar 18, 2014)

tomBitonti said:


> Not sure what other particles do to change the situation, except to make the observations a lot messier.  (Some bad astronomy pun deserves to be attached to "messier", but, being humor impaired, I'll leave that to others.)




They make the situation obvious.  Like Edwin Hubble noted, *everything* is moving away from us?  Really?  *Everything*?  How does that make sense? 



> Is a non-constant frame of reference a problem?




It isn't a problem, insofar as we can work with it, once we identify it.  It is a problem if we assume that the thing is constant, and interpret our observations as if it were constant, but it isn't!



> There are distinctions made between inertial and non-inertial frames of reference (giving rise to pseudo-forces; for example, a frame of reference defined from a point on a spinning disk is non-inertial, and gives rise to centrifugal forces).




Okay, so if you understand that -  special relativity, in which the whole "limit to the speed of light" is initially given to us, is about inertial frames of reference. Choosing the rest frame of one of the two particles is an attempt to choose an inertial frame of reference.  And, locally, that works.  But, on the cosmological scale, that rest frame is not actually an inertial frame, and special relativity does not hold.  



> Note, I'm really not trying to say that we shouldn't treat recession velocities as special.  I'm trying to understand exactly how we know they are special.  At least, how do we know without doing a global analysis.  For a pair of particles, how is a recession velocity distinguishable from simple motion?




Here's how:

Take the expanding universe.  The apparent speed of an object is related to the object's distance away from you. This is Hubble's Law:  v=H*D, where v is the apparent velocity, and D is the distance.  The farther a thing is away from you, the faster it is moving away from you.

So, at a given time, an object is some distance away, and moving at some speed away.  At a later time, it will then be farther away, and thus moving *faster*.  The object is accelerating! 

But now, we can go back to Newton.  Objects at rest stay at rest, unless a force acts upon them.  That thing is accelerating, but there's no force acting on it!

Moreover, we note that while it is moving quickly relative to us, we can also note that it is *not* moving quickly relative to it's neighbors or its own local space.  

Voila!  We know something is hinkey!


----------



## tomBitonti (Mar 18, 2014)

Right right.  But, what we think of as forces is a little trickier than that: Note that the force of gravity can be viewed as a fictitious force.  From:

http://en.wikipedia.org/wiki/General_relativity



> Definition and basic properties
> 
> General relativity is a metric theory of gravitation. At its core are Einstein's equations, which describe the relation between the geometry of a four-dimensional, pseudo-Riemannian manifold representing spacetime, and the energy–momentum contained in that spacetime.[33] *Phenomena that in classical mechanics are ascribed to the action of the force of gravity (such as free-fall, orbital motion, and spacecraft trajectories), correspond to inertial motion within a curved geometry of spacetime in general relativity; there is no gravitational force deflecting objects from their natural, straight paths. Instead, gravity corresponds to changes in the properties of space and time, which in turn changes the straightest-possible paths that objects will naturally follow.[34] The curvature is, in turn, caused by the energy–momentum of matter. Paraphrasing the relativist John Archibald Wheeler, spacetime tells matter how to move; matter tells spacetime how to curve.[35]
> *



*

Bold added by me.



Umbran said:



			Here's how:

Take the expanding universe.  The apparent speed of an object is related to the object's distance away from you. This is Hubble's Law:  v=H*D, where v is the apparent velocity, and D is the distance.  The farther a thing is away from you, the faster it is moving away from you.

So, at a given time, an object is some distance away, and moving at some speed away.  At a later time, it will then be farther away, and thus moving *faster*.  The object is accelerating! 

But now, we can go back to Newton.  Objects at rest stay at rest, unless a force acts upon them.  That thing is accelerating, but there's no force acting on it!

Moreover, we note that while it is moving quickly relative to us, we can also note that it is *not* moving quickly relative to it's neighbors or its own local space.  

Voila!  We know something is hinkey!
		
Click to expand...



I'm thinking we need to be a big more careful to define what we mean by velocity to understand recessional velocity.

Velocities already don't add in a simple linear fashion -- v_ab + v_bc != v_ac (v_xy == velocity of y measured by x) -- because of special relativity.  That there are other non-linear phenomena should not be too much of a surprise.

Thx!

TomB*


----------



## tomBitonti (Mar 18, 2014)

Ok, to get more funky:

http://en.wikipedia.org/wiki/Comoving_distance

See the section "Uses of the proper distance":



> Uses of the proper distance
> 
> If one divides a change in proper distance by the interval of cosmological time where the change was measured (or takes the derivative of proper distance with respect to cosmological time) and calls this a "velocity", then the resulting "velocities" of galaxies or quasars can be above the speed of light, c. This apparent superluminal expansion is not in conflict with special or general relativity, and is a consequence of the particular definitions used in cosmology. Even light itself does not have a "velocity" of c in this sense; the total velocity of any object can be expressed as ...




The tail has this statement, which links to:

http://arxiv.org/abs/astro-ph/0310808



> The issue of how best to describe and popularize the apparent superluminal expansion of the universe has caused a minor amount of controversy. One viewpoint is presented in (Davis and Lineweaver, 2003).




Thx!

TomB


----------



## tomBitonti (Mar 19, 2014)

Ok, got some time to scan over that last referenced paper.



> In the ΛCDM concordance model all objects with redshift greater than z ∼ 1.46
> are receding faster than the speed of light. This does not contradict SR because the
> motion is not in any observer’s inertial frame. No observer ever overtakes a light beam
> and all observers measure light locally to be travelling at c. Hubble’s law is derived
> ...




pp 3-4

Thx!

TomB


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## freyar (Mar 19, 2014)

tomBitonti said:


> Is there no frame of reference into which to put co-expanding particles?  Say, of one or of the other particle?
> 
> I'm not saying that it isn't different, but how is that frame different than other frames of reference?  Why aren't the relative particle velocities meaningful in that frame?




Umbran already covered this, but I'll add to it.  You need to be careful about the difference between coordinates and frames of reference in general relativity.  A coordinate system can stretch throughout the universe (subtleties do exist, of course), but a reference frame is something that is attached to a single observer and only exists *locally*.  An observer can only observe things at the same location, essentially.  So you can't say, "I see that galaxy 50 megaparsecs away moving so fast."  All you can say is that "That galaxy 50 megaparsecs away has a redshift of X," meaning we see the light from the galaxy at a longer wavelength than what it was when it was emitted (which we can tell by measuring "signposts" in the light from atomic transitions).  Because we're used to the Doppler effect here on earth, people sometimes translate that redshift into a velocity, but it's not the same thing.



tomBitonti said:


> Right right.  But, what we think of as forces is a little trickier than that: Note that the force of gravity can be viewed as a fictitious force.  From:
> 
> http://en.wikipedia.org/wiki/General_relativity
> <snip>
> I'm thinking we need to be a big more careful to define what we mean by velocity to understand recessional velocity.




What we usually think of as the "force of gravity" that, for example, makes the moon's path circle the earth, is not a force in the theory of general relativity.  It is the path that the moon takes _in the absence of forces_ but in the curved spacetime caused by the presence of the earth (and sun).



> Velocities already don't add in a simple linear fashion -- v_ab + v_bc != v_ac (v_xy == velocity of y measured by x) -- because of special relativity.  That there are other non-linear phenomena should not be too much of a surprise.



That was one the points of my previous post --- even if you make the mistake of thinking of redshift as a velocity, the conversion isn't a linear one in relativity.



tomBitonti said:


> Ok, to get more funky:
> 
> http://en.wikipedia.org/wiki/Comoving_distance
> 
> See the section "Uses of the proper distance":




All I can say to this interpretation of "expansion velocity" is ugh.  This isn't a useful way of thinking about things in any practical sense.  Here is one:

Imagine there is a grid extending through space.  Galaxies are at fixed points on the grid and, to a good first approximation, *don't move* with respect to the grid.  What happens is that the grid itself gets bigger.  We get a redshift because the light waves also expand as the grid expands --- basically the peaks and troughs of the wave are at fixed grid points.  

Back to velocities: the galaxies can of course move around on the grid even as the grid expands.  This is called "proper motion" and is usually only noticeable for closer galaxies.

Last point: that arXiv paper is really not a good source, I think.  It makes some ok points, but absolutely no one talks about recessional velocities like that.  Basically, they have made up a new way to define "recessional velocity" just to try to make a mathematical point.  I don't doubt their math, but they aren't making a lot of sense in terms of physics.  The key point is that "the motion is not in any observer's inertial frame," which says that they're definition has no observational meaning.


----------



## Scott DeWar (Mar 20, 2014)

*head spins - world tilts* Ok, looks like I kicked an ants nest of information, there.


----------



## Scott DeWar (Mar 20, 2014)

Mustrum_Ridcully said:


> Not sure what you mean here with regards to FTL?




someone mentioned what I was thinking - about the phrase " - - - - - -" aw crumbs, I can't find it now. Basically it had something to do with an unintentional misunderstanding on my part about the actual velocity exceeding "M"  instead it appeared to exceed "M" as the gravitational waves caused matter to expand in x^-nth seconds (can't remember the actual time)


----------



## Scott DeWar (Mar 20, 2014)

freyar said:


> FTL motion is not possible in our best understanding of physics, and it would take a truly revolutionary and unexpected discovery to change that.
> 
> *The phrase "a burst of inflation that was seemingly faster than the speed of light*" is possibly one of the most misleading and damaging that people can use when talking about the expansion of the universe.  If a scientist said it, they either didn't know the subject, were being incredibly lazy in talking to the press, or weren't thinking.  And, in this case, the "FTL mistake" doesn't even require inflation.  Here's my explanation:
> 
> ...




yup. That is the phrase that screwed me up. And I can see by your commentary that this phrase should never have been "Put to print"

I think I am getting the feeling tha fro a fixed point to any one direction expansion is moving at "M", but to two points in the universe that are moving from the starting point at the same velocity, they see the opposite point looking like the other is moving at "M + x" velocity  or even (2M) velocity. Is that the general idea?


----------



## freyar (Mar 20, 2014)

Scott DeWar said:


> yup. That is the phrase that screwed me up. And I can see by your commentary that this phrase should never have been "Put to print"
> 
> I think I am getting the feeling tha fro a fixed point to any one direction expansion is moving at "M", but to two points in the universe that are moving from the starting point at the same velocity, they see the opposite point looking like the other is moving at "M + x" velocity  or even (2M) velocity. Is that the general idea?




That's basically right. Suppose we have two galaxies that are relatively "nearby" us (but still millions of light-years away) that happen to be the same distance away but in opposite directions.  We would translate the redshift of each galaxy into a speed v moving away from us (v stands for the size of velocity, ie speed).  Either of those galaxies would see us moving at v away from them and the other galaxy moving at 2v away.  (Remember, I don't really mean a velocity in the normal sense here.)  If we look at a 3rd galaxy which is twice as far away at the first two, it would have an apparent speed of 2v away from us.  

That's the behavior expected if the universe is expanding at a fixed rate, which roughly holds for the universe that's nearby us (on cosmological scales).  However, if you look really far a way, you find deviations from that linear distance-speed relation (which is called the Hubble law).  That deviation is due to the fact that the expansion of the universe is changing rates.  For example, gravity from normal matter acts to slow down the expansion.  We also discovered in 1998 that there is some other source of energy (often called "dark energy") that acts to increase the expansion rate of the universe, and that effect is winning.  The expansion of the universe is actually getting faster.


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## tomBitonti (Mar 20, 2014)

Scott DeWar said:


> yup. That is the phrase that screwed me up. And I can see by your commentary that this phrase should never have been "Put to print"
> 
> I think I am getting the feeling tha fro a fixed point to any one direction expansion is moving at "M", but to two points in the universe that are moving from the starting point at the same velocity, they see the opposite point looking like the other is moving at "M + x" velocity  or even (2M) velocity. Is that the general idea?




That phrase shows a misconception: _Any_ expansion will lead to apparently faster-than-light motion.  The current expansion does so with a horizon measured in billions of light years.  The very fast expansion during inflation did so with a much much smaller horizon.

Hmm, this puts a figure of about 10^-35 cm for the particle horizon during inflation.  (I'm not sure if I'm reading that correctly.)

http://adsabs.harvard.edu/full/1991ApJ...383...60H

Anyways, the amount of expansion (between two points) is proportional to the distance between those points.  The rate of expansion is a proportional amount over time, not a fixed velocity.

Thx!

TomB


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## Scott DeWar (Mar 20, 2014)

dang, wonky internet just ate my previous post.



freyar said:


> The expansion of the universe is actually getting faster.




is it getting faster at a constant increase? has it been doing this since the beginning? what kind of implications does this have for the age of the universe?



tomBitonti said:


> That phrase shows a misconception: _Any_ expansion will lead to apparently faster-than-light motion.  The current expansion does so with a horizon measured in billions of light years.  The very fast expansion during inflation did so with a much much smaller horizon.
> 
> Hmm, this puts a figure of about 10^-35 cm for the particle horizon during inflation.  (I'm not sure if I'm reading that correctly.)
> 
> ...




you know, it kind of is scary to say, but I think it is starting to make sense to me.


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## tomBitonti (Mar 20, 2014)

Scott DeWar said:


> is it getting faster at a constant increase? has it been doing this since the beginning? what kind of implications does this have for the age of the universe




The rate of expansion (often, the _Hubble Parameter_) is only recently known to any decent accuracy.  (To about 2% is what I'm seeing, say, around 71-73.  Previously, values between 50-80 seemed usual.)

This seems to make the basic case for acceleration:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC314128/figure/fig6/

That seems to show acceleration starting some time in the past, and increasing since then.

Given the uncertainties in the measurements, telling the rate of acceleration seems unlikely.

The basic implication is that a computation of the age of the universe cannot rely on a constant Hubble parameter values.

One measure of the age of the universe is telling the amount the background radiation is red-shifted: That shift basically integrates the effect of expansion over time.  Then, knowing the rate of expansion, and inverting the integration, the age can be obtained.

An increasing Hubble parameter will make for the same shift over a smaller amount of time.  Not sure quantitatively how much that would be, but I'm guessing on the order of 10%, or a small multiple thereof, for the measured acceleration, and not of the order of 100% or more.  But, I could be very off on that estimate, and the estimate is very probably thrown off anyways by the basic uncertainty of the current parameter value.

Just to say, I find just a bit disturbing the image of an increasingly sparse universe, with its parts flying apart close to the speed of light, as an end-state of the universe.  Hopping 100 trillion years forward would leave you in a very empty universe.

---

So many interesting links found while looking up various questions in this space:

http://www.google.com/url?sa=t&rct=...03RWYtg0GHSq8Zw&bvm=bv.62922401,d.dmQ&cad=rja

This one is amusing:

http://www.answersingenesis.org/articles/tj/v9/n1/hubble

Thx!

TomB


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## Umbran (Mar 20, 2014)

tomBitonti said:


> Hopping 100 trillion years forward would leave you in a very empty universe.




It would look empty even if it weren't expanding.  In 100 trillion years, the entire Universe will have completely used up all its hydrogen and so on - all stars will have burned out, and their corpses cooled down to the ambient temperature of vacuum.  The only light to be seen will be the occasional flash as a stellar corpse encounters a black hole or neutron star.


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## tomBitonti (Mar 20, 2014)

Umbran said:


> It would look empty even if it weren't expanding.  In 100 trillion years, the entire Universe will have completely used up all its hydrogen and so on - all stars will have burned out, and their corpses cooled down to the ambient temperature of vacuum.  The only light to be seen will be the occasional flash as a stellar corpse encounters a black hole or neutron star.




Over so much time*, all the baryonic matter will decay to nothing.

There would just be the occasional flash of black holes evaporating.

Such a dark and gloomy place.

Thx!

*100 trillion years might not be long enough.  At least for black hole evaporation, I think we are up in the 10^100 years range


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## Scott DeWar (Mar 20, 2014)

lets see, 100 trillion.
100,000,000,000,000

still 10 ^4 to go.


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## Umbran (Mar 20, 2014)

tomBitonti said:


> Over so much time*, all the baryonic matter will decay to nothing.




The end of normal matter is,  I think, currently estimated at 10^30 years.  The end of black holes at 10^100 years.


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## Scott DeWar (Mar 20, 2014)

*I am normally not this anal, but . . . . .*

10,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000

ok, 100 trill. uses 12 '0's so 10 trillion^ 8.5 power, i think
or maybe
[1 bill ^ 11] * 10 = 100 billion billion times 10? this one might be wrong.


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## freyar (Mar 21, 2014)

tomBitonti said:


> The rate of expansion (often, the _Hubble Parameter_) is only recently known to any decent accuracy.  (To about 2% is what I'm seeing, say, around 71-73.  Previously, values between 50-80 seemed usual.)



I'm always kind of amazed we can make this kind of measurement at all.  The various methods used are really ingenious.



> That seems to show acceleration starting some time in the past, and increasing since then.
> 
> Given the uncertainties in the measurements, telling the rate of acceleration seems unlikely.




Yes, for most of the life of the universe, the expansion was slowing down, as you'd expect from gravity pulling things together.  The positive acceleration (speeding up) has only 
kicked in recently (the last billion years or so, if I recall correctly).  It's basically an issue of scales, meaning that the "dark energy" has just recently become of the same density as normal matter.



> The basic implication is that a computation of the age of the universe cannot rely on a constant Hubble parameter values.
> 
> One measure of the age of the universe is telling the amount the background radiation is red-shifted: That shift basically integrates the effect of expansion over time.  Then, knowing the rate of expansion, and inverting the integration, the age can be obtained.
> 
> An increasing Hubble parameter will make for the same shift over a smaller amount of time.  Not sure quantitatively how much that would be, but I'm guessing on the order of 10%, or a small multiple thereof, for the measured acceleration, and not of the order of 100% or more.  But, I could be very off on that estimate, and the estimate is very probably thrown off anyways by the basic uncertainty of the current parameter value.




That's right.  With a constant Hubble parameter, you can just extrapolate back linearly: age of universe = 1/H_0 in appropriate units.  Acceleration makes H_0 (the value of the Hubble parameter now) bigger than it "should" be, so the simple estimate of the age of the universe isn't old enough.  And the correction is not quite 10%.


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## Mustrum_Ridcully (Mar 21, 2014)

tomBitonti said:


> Over so much time*, all the baryonic matter will decay to nothing.
> 
> There would just be the occasional flash of black holes evaporating.
> 
> ...




Of course, that may be nothing compared to the scenario of the Big Rip, where the acceleration of expansion is so fast that at some point, even atoms get torn apart... But it seems with our current understanding, the expansion won't get that bad.



I wonder if there could be some new types of patterns emerging on that scale?


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## Umbran (Mar 21, 2014)

Mustrum_Ridcully said:


> Of course, that may be nothing compared to the scenario of the Big Rip, where the acceleration of expansion is so fast that at some point, even atoms get torn apart... But it seems with our current understanding, the expansion won't get that bad.




It isn't quite about "how bad the expansion gets".  It is about how much pressure "dark energy" gives for its density (via the ratio of those things).  Essentially, it is about the strength of the "force" that causes expansion.  If it is strong enough, it'll eventually overcome gravity, the electromagnetic, and nuclear forces, and matter will just sort of dissolve away.

At the moment, the experimental errors are large enough that we can't tell for sure which state we are in - looking at a Big Rip or not.





> I wonder if there could be some new types of patterns emerging on that scale?




Patterns... of what?  And what is "that scale"?


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## Mustrum_Ridcully (Mar 21, 2014)

Umbran said:


> Patterns... of what?  And what is "that scale"?



Imagine those left-over black holes that slowly evaporate forming a pattern. I figure that most of these would be galactic black holes, so the scale would be grand. Theoretically, a group of black holes clusted around another black hole might evaporate so much energy in the direction of the center black hole that it grows instead of shrinks for a while. I wonder if there could be something creating a pattern, some form of replication possible that we haven't considered yet... I don't currently know anything particular that could come, mostly it seems that, if such black hole "clusters" that can feed a central black hole where to exist, it would just mean that the center black hole last a bit longer, nothing more fancy.


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