# Exotic Matter



## Joker (Mar 17, 2016)

I'm trying to wrap my head around exotic matter.  As far as I understand it, this hypothetical matter is partly used to make work the math that would make wormholes stable.

My question is could this be normal looking matter with strange properties?  If, instead of attracting like regular matter, it repels, would it have any noticeable effect on its surroundings?
Let's say you have a building made of the stuff that had as much mass as a building of regular matter;  Would it affect its environment in any way that was apparent to people going inside or walking around it?


----------



## Umbran (Mar 17, 2016)

"Exotic matter" is any theoretical matter that has properties we don't see in the normal stuff.  It is the more technical term for "unobtanium".

What it would look like would depend on exactly which properties the particular stuff in question has.

For example, let us consider a form of exotic matter that repels gravitationally, but has standard electromagnetic properties.

Your first question is - how do you find this stuff?  Your planet is brought together by gravitational attraction, but this stuff is repelled.  So, unless it was already bound in a molecule or other hunk of normal matter, it drifts *away* from the Sun as it and the planets form.  But, if it has normal electromagnetic properties, it can still form chemical bonds.  So, you can imagine finding it bound up with normal matter, and harvesting it, I suppose.

Assume, for the moment, that you did get a supply of this stuff.  Well, it has mass (so, it takes force to move it around), but is has negative weight.  What does that mean?  Well if you made a skyscraper out of it, people standing next to it wouldn't feel anything much - normal skyscrapers don't *pull* on you much, so this one wouldn't *push* on you much either.  However, the push between it an the Earth is another story....

This structure has negative weight.  Instead of resting on the ground with its full weight, it is *pulling away* from the ground with its full weight.  The Empire State Building weighs 365,000 tons or so.  That means the Exotic State Building will be pulling away from the Earth with a force of 365,000 tons.  What, pray tell, are you binding it to the ground with?  This sucker is going to rocket away from the ground, with a thrust equivalent to over 1700 Space Shuttle Main Engines.  The building has much more mass that the Shuttle, so it may not take off as fast, but it will be *inexorable*.  Good luck holding it down.


----------



## tomBitonti (Mar 17, 2016)

It would seem that if any such matter exists, it might as well as what seems to be normal matter, just with one of the properties changed.  Except for a very special case, an antimatter moon would be the same as our moon, to anti-matter people.

The exotic properties would cause a lot of problems, e.g., a repulsive force makes matter want move away from normal matter.  (What Umbran describes.)

I like the idea of matter which has a different coupling between inertial and gravitational mass.  A big hunk of matter which had a much smaller gravitational mass.  Say, if you attached normal matter to matter in a parallel space (that's magic), giving the ensemble the inertia of the combined mass but a gravitational interaction only base on the matter in our universe.  That would be an interesting MacGuffin for a Sci-Fi RPG.

Thx!
TomB


----------



## Janx (Mar 17, 2016)

Joker said:


> I'm trying to wrap my head around exotic matter.  As far as I understand it, this hypothetical matter is partly used to make work the math that would make wormholes stable.




Note: I know nothing about physics.  don't take any of my ideas seriously.

One problem I have with this usage of Exotic Matter, is having it be a place holder in math for mythical things like worm holes.  It's one thing to see that the math for the universe is off when we account for all visible matter, but it resolves if we insert "Dark Matter" to account for it.   The implication is that we need to find a way to confirm if Dark Matter exists to then finish solving the formula.  That's worlds different from "worm holes have never been observed and thus may not exist, but I can make up some math and magic matter that would allow them to exist."

The difference between the Universe and Dark Matter is that we know the Universe exists, and Dark Matter as a placeholder solves an equation.  We don't actually know that worm holes exist, so using exotic matter as a placeholder to solve worm hole math seems like a double-dependency on placeholders.


----------



## Umbran (Mar 17, 2016)

Janx said:


> One problem I have with this usage of Exotic Matter, is having it be a place holder in math for mythical things like worm holes.




Okay, well, here's the thing.  Physicists don't actually use it that way.  

In the case of wormholes, it goes like this:  Einsteinian General Relativity allows for wormholes to exist.  However, if we subject the result to perturbations (small changes - like say a star goes by in the vicinity, changing the local space by a small amount) we find that the wormhole should collapse.  Wormholes are found to be not stable to even very small perturbations, like "a butterfly flapping its wings in the China Sea" is big enough to make the thing collapse.

So, some folks tried to figure out what would be necessary to keep a wormhole open.  The answer seems to be that you need to create a region of negative energy density around the thing - the exact shape can vary somewhat, as I recall.  Now, we know how to make _very small_ regions with _very small_ amounts of negative energy density.  But this would be of a different order entirely.  As a practical matter, we posit that we'd need some material we currently don't have to manage it - not only dont we have this material, but we've no observatiosn that suggest it exists.  Thus, it is "exotic".

The argument that you need exotic matter to keep a wormhole open is typically an argument *against* stable wormholes existing.

There are a couple of things that we expect do actually exist, that we have next to no chance of getting our hands on, that are also lumped in with "exotic matter".  Neutronium - the stuff that neutron stars are made of, very likely exists (we have observed objects that have all the properties we'd expect a neutron star to have), but getting it?  How?  The stuff is down in a friggin' neutron star!  

There's also "degenerate matter", which you find in the cores of large stars, or in their white dwarf remnants, in which the main source of pressure is not the temperature of particles that bang against each other, but the quantum mechanical Pauli Exclusion Principle, is also considered exotic, but is not really possible to get.


----------



## Tonguez (Mar 17, 2016)

Umbran said:


> This structure has negative weight.  Instead of resting on the ground with its full weight, it is *pulling away* from the ground with its full weight.  The Empire State Building weighs 365,000 tons or so.  That means the Exotic State Building will be pulling away from the Earth with a force of 365,000 tons.  What, pray tell, are you binding it to the ground with?  This sucker is going to rocket away from the ground, with a thrust equivalent to over 1700 Space Shuttle Main Engines.  The building has much more mass that the Shuttle, so it may not take off as fast, but it will be *inexorable*.  Good luck holding it down.




ah so thats what happened to Sokovia!


----------



## Umbran (Mar 17, 2016)

Tonguez said:


> ah so thats what happened to Sokovia!




I suppose Ultron's machine that looked like a giant set of repulsor lift engines might instead have been infusing the land with exotic matter.  Sure, we can go with that.

Especially since Agent Carter has introduces "Zero Matter", which is the best implementation of Marvel Universe "Darkforce" I could ever have hoped for....


----------



## freyar (Mar 18, 2016)

Umbran said:


> Okay, well, here's the thing.  Physicists don't actually use it that way.
> 
> In the case of wormholes, it goes like this:  Einsteinian General Relativity allows for wormholes to exist.  However, if we subject the result to perturbations (small changes - like say a star goes by in the vicinity, changing the local space by a small amount) we find that the wormhole should collapse.  Wormholes are found to be not stable to even very small perturbations, like "a butterfly flapping its wings in the China Sea" is big enough to make the thing collapse.
> 
> ...




Yes, from looking at the original papers, I think Morris and Thorne kind of approached it as a game or challenge --- what does it take to keep a wormhole open, and does that make sense?  There is something of a school of thought in some areas of physics that you should do this kind of thing, that is, postulate a weird behavior that might seem counterintuitive, see what it would take to get it, and then argue whether that's actually possible.  It's somewhat popular in one branch of cosmology.  I don't personally find it terribly productive, since it seems like it spends a lot of time arguing about whether something that's probably impossible can exist or not.

More later...


----------



## Umbran (Mar 18, 2016)

freyar said:


> I don't personally find it terribly productive...




I expect it is as productive as any other form of play - which means the activity probably isn't *directly* productive, but has lots of good results that should not be discounted.


----------



## freyar (Mar 18, 2016)

Umbran said:


> I expect it is as productive as any other form of play - which means the activity probably isn't *directly* productive, but has lots of good results that should not be discounted.




It's possible, though, as a scientist, I have to make my best bet about what will actually tell us something interesting about the universe, whether directly or indirectly.  My bet is not on models that at least appear mathematically and physically inconsistent before you add lots of epicycles, that's all.


----------



## Umbran (Mar 18, 2016)

freyar said:


> It's possible, though, as a scientist, I have to make my best bet about what will actually tell us something interesting about the universe, whether directly or indirectly.  My bet is not on models that at least appear mathematically and physically inconsistent before you add lots of epicycles, that's all.




You know, Quantum Mechanics and the Standard Model have a number of problematic places that require renormalization techniques to help you deal with infinities.  Those things *looked* mathematically and physically inconsistent until someone figured out to employ those techniques - meaning someone had to fiddle and play around with them and then go, "Hey, I can do *this*, and it all works out!".  It turns out that in this case, you can make it work out... at the cost of having nonsensical materials.  Okay, it turned out not to be terribly useful - but they didn't *know* what the result would be when they started, and knowing one way or the other is kinda important, no?

I, personally, figure that the realms of scientific inquiry ought to be pretty wide open.  Do we really want to cast shade on some endeavors just because we don't expect they'll be fruitful?  I mean, that's the same logic used by those who claim that "pure research," without direct commercial relevance, should be curtailed.  And that would produce a hefty chilling effect, if we listened to it.

It is also important to remember that it isn't like this was some billion-dollar project to find out if a fringe-model is correct.  It was like, Kip Thorne and a blackboard, paper, and pencils, right?  Not a huge expenditure, or anything.


----------



## freyar (Mar 18, 2016)

Umbran said:


> There are a couple of things that we expect do actually exist, that we have next to no chance of getting our hands on, that are also lumped in with "exotic matter".  Neutronium - the stuff that neutron stars are made of, very likely exists (we have observed objects that have all the properties we'd expect a neutron star to have), but getting it?  How?  The stuff is down in a friggin' neutron star!
> 
> There's also "degenerate matter", which you find in the cores of large stars, or in their white dwarf remnants, in which the main source of pressure is not the temperature of particles that bang against each other, but the quantum mechanical Pauli Exclusion Principle, is also considered exotic, but is not really possible to get.




I want to clarify a little bit about the term "exotic matter."  It's used in a lot of different ways.  Just because neutronium is exotic in the English-language sense (you won't see any on earth), it's definitely not exotic in the same way as the stuff needed to make wormholes stable.  Yes, it's incredibly dense, but it doesn't have weird properties like negative energy density.  And we have very good evidence for the existence of neutron stars and therefore neutronium.  Degenerate matter isn't actually that hard to find --- the electrons in a piece of metal at room temperature are pretty degenerate, for example.  But the key thing I want to point out is that things like dark matter (at least usual theories of it), the Higgs boson, neutron stars, etc, aren't exotic in that they don't twist what's normally considered physically reasonable.  Negative energy densities often go beyond that.


----------



## freyar (Mar 18, 2016)

Umbran said:


> You know, Quantum Mechanics and the Standard Model have a number of problematic places that require renormalization techniques to help you deal with infinities.  Those things *looked* mathematically and physically inconsistent until someone figured out to employ those techniques - meaning someone had to fiddle and play around with them and then go, "Hey, I can do *this*, and it all works out!".  It turns out that in this case, you can make it work out... at the cost of having nonsensical materials.  Okay, it turned out not to be terribly useful - but they didn't *know* what the result would be when they started, and knowing one way or the other is kinda important, no?



I'm not sure precisely what you're saying, but renormalization techniques themselves are incredibly useful and have led to a really deep understanding of a number of subjects.  But the other key difference between early quantum mechanics and what I'm talking about is (1) there was a clear need for something new in developing quantum mechanics and particle physics and (2) the development of quantum mechanics, etc, didn't start from the position of self-inconsistency and then crossing fingers that you can dial it back.



> I, personally, figure that the realms of scientific inquiry ought to be pretty wide open.  Do we really want to cast shade on some endeavors just because we don't expect they'll be fruitful?  I mean, that's the same logic used by those who claim that "pure research," without direct commercial relevance, should be curtailed.  And that would produce a hefty chilling effect, if we listened to it.



We've had some variation of this conversation before, and, you know, I think we're just coming at it from different perspectives.  In a broad sense, I agree with you.  It's just that scientists have to prioritize at the personal, institutional, national, and even international levels.  Sometimes it just seems to me that there's too much of one direction vs another.  Another issue is that I read (and referee) a lot of papers, and it's easy to find poor quality work that springs from the attitude of "well, I can just make things up without much regard to physical or mathematical principles."  Let me put it this way: I don't have a problem with any specific direction of research as long as it could credibly be correct.  There are unfortunately many instances of large swaths of literature that don't easily pass that test.  That's all.  And I don't think it should all be cut out, but I think sometimes the balance is sometimes affected more by sales pitches than scientific principles.

I might also note that there's been a fair amount of shade cast on subjects like particle dark matter research or string theory in public and even on EN World (not in a disrespectful way, just in terms of disagreements).  Ironically, those types of research are based a lot more on growth of established principles.



> It is also important to remember that it isn't like this was some billion-dollar project to find out if a fringe-model is correct.  It was like, Kip Thorne and a blackboard, paper, and pencils, right?  Not a huge expenditure, or anything.




Oh, sure, the Morris-Thorne wormhole stuff was cheap and kind of a lark.  But, for example, the Large Synoptic Survey Telescope costs around $400 million and has a large goal to determine whether the universe's accelerated expansion is due to something like a cosmological constant or modified gravity.  The thing is, most of the modified gravity theories are mathematically nonsensical or close to it.  Now, the LSST is a great project with lots of other capabilities, so I like it a lot in general.  But it's too bad that's pushed as a big sales pitch for it.


----------



## freyar (Mar 18, 2016)

Umbran said:


> I suppose Ultron's machine that looked like a giant set of repulsor lift engines might instead have been infusing the land with exotic matter.  Sure, we can go with that.
> 
> Especially since Agent Carter has introduces "Zero Matter", which is the best implementation of Marvel Universe "Darkforce" I could ever have hoped for....




A colleague of mine was (is?) the scientific advisor for Agent Carter and gave them a lot of ideas for Zero Matter based on real ideas in physics (where it might come from in extra dimensions, behavior of things like superfluids, etc).  He put some reflections about it on his blog: Asymptotia (search for zero matter).


----------



## Janx (Mar 18, 2016)

Umbran said:


> I expect it is as productive as any other form of play - which means the activity probably isn't *directly* productive, but has lots of good results that should not be discounted.




I'm sorry to hit upon a sore subject.  I guess some physics people lean toward what I was saying, others don't.

Still, as what Umbran refuted my discomfort made sense, it sounds like playing "what if" games with placeholder concepts like Exotic Matter can sometimes lead to new ideas or sort of disproving ideas.

I'm inclined to recant my statement as badwrongfunism, and just let some set of scientists play their game of choice.


----------



## Umbran (Mar 18, 2016)

freyar said:


> I want to clarify a little bit about the term "exotic matter."  It's used in a lot of different ways.  Just because neutronium is exotic in the English-language sense (you won't see any on earth), it's definitely not exotic in the same way as the stuff needed to make wormholes stable.  Yes, it's incredibly dense, but it doesn't have weird properties like negative energy density.  And we have very good evidence for the existence of neutron stars and therefore neutronium.  Degenerate matter isn't actually that hard to find --- the electrons in a piece of metal at room temperature are pretty degenerate, for example.  But the key thing I want to point out is that things like dark matter (at least usual theories of it), the Higgs boson, neutron stars, etc, aren't exotic in that they don't twist what's normally considered physically reasonable.  Negative energy densities often go beyond that.




Entirely agreed.  That's why my first pass was "technical name for unobtanium", and I only added the things that don't raise eyebrows by physical laws and observation, but are just hard to get, on my second pass.


----------



## Umbran (Mar 18, 2016)

freyar said:


> A colleague of mine was (is?) the scientific advisor for Agent Carter and gave them a lot of ideas for Zero Matter based on real ideas in physics (where it might come from in extra dimensions, behavior of things like superfluids, etc).  He put some reflections about it on his blog: Asymptotia (search for zero matter).




Okay, that's awesome.  I want that job!  So often they do *sooo* badly with such stuff.


----------



## freyar (Mar 18, 2016)

Umbran said:


> Okay, that's awesome.  I want that job!  So often they do *sooo* badly with such stuff.




One of the best parts of my career has been getting to meet lots of cool people in science.  When I met Clifford, he was one of the many theoretical physicists in LA, but I've seen him start doing lots of public outreach, science documentaries, and now science advising.  Really neat.  I've also met the science advisor for the Big Bang Theory, but that show has of course drifted a lot from science as a central theme (and they've always been willing to make up some mumbo-jumbo if they thought it fit better than the real science).


----------



## Umbran (Mar 18, 2016)

freyar said:


> I'm not sure precisely what you're saying, but renormalization techniques themselves are incredibly useful and have led to a really deep understanding of a number of subjects.




Yes.  We say that now, in hindsight.  It turned out renormalization techniques, and quantum mechanics, were incredibly useful.  But, there was a time before these tools were used, when Einstein was still saying that God doesn't play dice with the Universe.  There was a time when QM was considered wacky, and many of the brightest felt it wouldn't go anywhere. My pofessors noted that there had been calls of the form, "Look at those infinities!  It's clearly non-physical!  Don't worry about it!"  Aren't you glad they didn't listen to the naysayers?

So, I'm not a big fan of pre-judging.  The results will speak for themselves, eventually.



> I might also note that there's been a fair amount of shade cast on subjects like particle dark matter research or string theory in public and even on EN World (not in a disrespectful way, just in terms of disagreements).  Ironically, those types of research are based a lot more on growth of established principles.




I've cast shade on string theory, in the form of "been at it for decades, still not much in terms of testable predictions," form*.  I have never said they shouldn't have bothered ever trying the theories out, though - I only gripe that after so long, perhaps we should have some of these brilliant people looking down other avenues.  I believe I am officially without preference on Dark Matter models, other than noting that certain forms seem to have been largely ruled out by observations.  Again, I don't believe I've ever said folks shouldn't have investigated the various models.  





*Modern Quantum Mechanics can be seen to have gotten its start in 1925.  Thirty five years later, in 1960, we had atomic bombs, solid state electronics, conventional superconductivity models, and nuclear power plants.  The "first superstring revolution" was around 1980.  And today?  Not much that we can even test to tell if we've got the right general direction.  Just sayin'.  Yes, yes, I know a great deal about the energy levels we are dealing with, and how difficult the math and everything else about it is.  Even knowing that, the point still remains in my mind.


----------



## Janx (Mar 21, 2016)

Umbran said:


> Yes.  We say that now, in hindsight.  It turned out renormalization techniques, and quantum mechanics, were incredibly useful.  But, there was a time before these tools were used, when Einstein was still saying that God doesn't play dice with the Universe.  There was a time when QM was considered wacky, and many of the brightest felt it wouldn't go anywhere. My pofessors noted that there had been calls of the form, "Look at those infinities!  It's clearly non-physical!  Don't worry about it!"  Aren't you glad they didn't listen to the naysayers?
> 
> So, I'm not a big fan of pre-judging.  The results will speak for themselves, eventually.
> 
> ...




I've seen the Einstein quote about dice and god.  Had he recanted his statement later in life?

Back to QM, in the past, you've said QM helped us do modern electronics.  Are you basically saying String Theory hasn't yielded anything (not just stuff, but tests that indicate the String ideas have merit)?


----------



## Umbran (Mar 21, 2016)

Janx said:


> I've seen the Einstein quote about dice and god.  Had he recanted his statement later in life?




Nope.  Though he got is Nobel Prize not for Relativity, but for his work on the Photoelectric Effect, a fundamentally quantum process, he had issues with the basic uncertainty of QM - he couldn't accept that the fundamental interactions between particles was not deterministic.  He stood by his expectation that eventually, it would be shown that QM was incomplete, and that another theory would be developed that would show that the uncertainty was not real, but an artifact of insufficient information and understanding.



> Back to QM, in the past, you've said QM helped us do modern electronics.  Are you basically saying String Theory hasn't yielded anything (not just stuff, but tests that indicate the String ideas have merit)?




Yeah.  A fundamental principle of science is that a hypothesis must be, in some way, testable.  For something like String Theory, this means it should make predictions that we can, at least in theory, test.  We may not have equipment of sufficient sufficient precision to do it yet, but it should be possible in principle.  General Relativity, for example, predicts that the orbit of Mercury should precess (and, it does).  GR predicts that gravity waves exist, and it seems we've finally detected them.  GR and QM both predict loads of physical effects we can look for, and observe.

String theories are a bit short in that department as yet.  There are darned few (if any) experiments we can imagine doing, where we'd look at the results and say, "This result is due to the string-nature of reality."  String theory has gotten to the point where it is consistent with many other theories for things we already observe, but that may just mean that String Theory is, mathematically, really just the same thing as, say, the Standard model.  We'd say one model "is equivalent to" or "reduces to" the other.  

Until we can demonstrate that string theory explains real physical effects _not explained by other theories_, it is really mostly a mathematical curiosity.


----------



## Scott DeWar (Mar 21, 2016)

Well, I have been silent thus far and I cannot remain son. My opinion on this, as limited as it is, is tus:

1. I am one of the bigger skeptics on dark matter/energy.

2. I am not privy to knowledgeable on issues of what any of this thread is talking on.

3. My answer is going to seem very simplistic, maybe overly so.

4. I believe string theory has made some headway since the last year or two

** perhaps we just haven't seen the effects on computer detection equipment yet. It may be as unobtrusive as neutrinos, just not able to see them in visible light the same as proving the existence of inferred or ultraviolet without the proper equipment or materials.

We may not have had the computing power as yet to detect them.**


----------



## Umbran (Mar 21, 2016)

Scott DeWar said:


> 1. I am one of the bigger skeptics on dark matter/energy.




Just to be clear, this has little to do with the string theory part of the discussion.  To my knowledge, string theory does not remove the need for "dark matter" or "dark energy" to explain the behavior of the universe on the large scale.



> 4. I believe string theory has made some headway since the last year or two
> 
> ** perhaps we just haven't seen the effects on computer detection equipment yet. It may be as unobtrusive as neutrinos, just not able to see them in visible light the same as proving the existence of inferred or ultraviolet without the proper equipment or materials.
> 
> We may not have had the computing power as yet to detect them.**




Not that I'm aware.  The problem isn't, "The theory makes predictions, but our machines aren't good enough yet."  The problem is that the predictions string theories make are the same as those made by conventional models.  So, there's nothing to say that string theory is the real thing we should be using.  String theory is, at best, as good as the simpler models we already have, and doesn't give us anything fundamentally different.

I've seen one argument that string theories suggest that the force of gravity should be material-dependent.  So, the pull of the Sun on a metallic asteroid should be different from the Sun's pull on an icy comet of the same mass.  However, I've never seen any prediction of *by how much* the pull should be different.  Just that they might be different.  If you can't make a *particular* prediction, it doesn't count.

We can put on top of that the fact that (again, to my knowledge) nobody has observed such variance.  It has to be pretty darned small to have eluded our notice, if it is there.


----------



## freyar (Mar 21, 2016)

Umbran said:


> Yes.  We say that now, in hindsight.  It turned out renormalization techniques, and quantum mechanics, were incredibly useful.  But, there was a time before these tools were used, when Einstein was still saying that God doesn't play dice with the Universe.  There was a time when QM was considered wacky, and many of the brightest felt it wouldn't go anywhere. My pofessors noted that there had been calls of the form, "Look at those infinities!  It's clearly non-physical!  Don't worry about it!"  Aren't you glad they didn't listen to the naysayers?
> 
> So, I'm not a big fan of pre-judging.  The results will speak for themselves, eventually.



Oh, of course.  I don't listen to naysayers either.  But I will just say again that the development of quantum mechanics is a very different situation than what I mentioned above.  And it's ok to have difficulties in the development of a theory.  However, there is always a need for scientists to make educated guesses at what will be productive areas of research.  My guess is that it is less productive to study theories that are just made up from whole cloth and then struggling to show that they are not nonsensical.  Of course there's a spectrum, and it's worth trying things, but I am not the only physicist that questions, for example, why certain papers get press releases and others don't.




> I've cast shade on string theory, in the form of "been at it for decades, still not much in terms of testable predictions," form*.  I have never said they shouldn't have bothered ever trying the theories out, though - I only gripe that after so long, perhaps we should have some of these brilliant people looking down other avenues.  I believe I am officially without preference on Dark Matter models, other than noting that certain forms seem to have been largely ruled out by observations.  Again, I don't believe I've ever said folks shouldn't have investigated the various models.



No, I don't think so, and I don't think you'll find me saying here that people shouldn't have investigated various models, either, but rather that I personally feel certain directions are perhaps over-represented.  It seems like you're saying the same thing about string theory just in this paragraph --- maybe it would be good to have smart people looking at other avenues.  I've noticed that the EN World community (at least in this Misc Forum) is fairly curious about science, so I try to offer my professional opinion for those interested in what someone who reads a lot of papers and follows what's going on in this branch of physics on a daily basis thinks.  I'm also not casting aspersions on Morris and Thorne.  I suspect they didn't realize the kind of interest they would generate, either, since AFAICT their initial article was in a publication that is mostly for the professional development of physics teachers, not a normal research journal.


----------



## freyar (Mar 21, 2016)

Scott DeWar said:


> Well, I have been silent thus far and I cannot remain son. My opinion on this, as limited as it is, is tus:
> 
> 1. I am one of the bigger skeptics on dark matter/energy.




There's extremely strong evidence for dark matter.  I made quite a long post about it last year in my AMA thread here. I'm happy to elaborate or answer questions, but that's a good place to start.  There's also quite good evidence that the expansion of the universe is accelerating, which is due to what we call "dark energy."


----------



## freyar (Mar 21, 2016)

Umbran said:


> Not that I'm aware.  The problem isn't, "The theory makes predictions, but our machines aren't good enough yet."  The problem is that the predictions string theories make are the same as those made by conventional models.  So, there's nothing to say that string theory is the real thing we should be using.  String theory is, at best, as good as the simpler models we already have, and doesn't give us anything fundamentally different.



I don't want to be too blunt, but that's just incorrect, actually.  If you build a big enough particle collider, you will see a very distinctive spectrum of resonances ("new particles") corresponding to the vibrational states of strings.  This will be on top of the other distinctive, but more model-dependent spectrum of resonances due to the presence of some kind of extra dimensions.  Now, is it possible to make some other model without strings that looks exactly the same way?  Yes, if you make up an infinite number of new particles with exactly the right properties.  It is also possible to describe the solar system with the earth at the center if you ignore Newtonian gravity and set up a bunch of epicycles, but we don't think of that as being the correct thing to do.  

It is of course not feasible to build a collider like this with current technology --- if you tried to scale up the LHC, you'd likely have to build something a significant fraction of the size of the solar system, given typical guesses about the scale of string theory.  But the problem is exactly "The theory makes predictions, but our machines aren't good enough yet" for this most basic prediction.

Here's another prediction of string theory: the existence of gravity.  That's a little flippant, but it's true --- no one expected gravity to turn up in string theory when it was first invented.  No other theory can explain why there is gravity, either.

More on this later, at least if people are interested.


----------



## Umbran (Mar 22, 2016)

freyar said:


> I don't want to be too blunt, but that's just incorrect, actually.




I don't mind blunt - I've been wrong before, and I am sure I'll be wrong again.  I hope I will not be like Einstein and not ever admit an error.  But, be blunt, not everyone agrees with you.  Or, in another way to put it, there are a few too many dodges to make me comfortable.



> If you build a big enough particle collider, you will see a very distinctive spectrum of resonances ("new particles") corresponding to the vibrational states of strings.  This will be on top of the other distinctive, but more model-dependent spectrum of resonances due to the presence of some kind of extra dimensions.




I will grant you that if you do an experiment at high energy, but well below the Planck energy, and see such resonances, then yes, you'd have evidence for strings.  Except... you might not see those things, and failing to see them doesn't invalidate the theory.  

The models that predict these are based in perturbative string theory, no?  That's explicitly an approximation, and to my understanding it is not at all clear that you expect that behavior in reality (which, as far as we can tell, is not perturbative).  Last I read, M-Theory, in general, does not require such resonances.  And there's a bazillion ways to collapse the multiple dimensions required by string theory down into 3d models that give particular predictions of what resonances you see - so, if you run an experiment, and it fails to show the resonances, you just say, "Well, I have the wrong perturbative series/compactification, but string theory hasn't been disproven!"

And, if you do somehow manage to run a test up a Planck energies, you still have an issue - more conventional theory also predicts that you will see some really strange things there (like black hole states), so you can't tell if what you are seeing is string theory, or something else.

Thus, the whole thing leans to the non-falsifiable.  If you can always dodge and say that your theory is still correct, there's an issue.

Meanwhile, the most recent stuff I saw on quark-gluon plasmas coming our of the LHC *failed* to match the string models used to describe them.  And no SUSY, though that should have been seen in Bs decay.  

Back in the 1980s, Feynman stated concerns that there was rather too much hype and groupthink surrounding string theory, and I don't see a lot of evidence that's gone away.  How many times does a model we've been trying to develop for 40 years have to fail to meet expectations before we collectively stop apologizing for it?  So, as you say above - yes, I think it fair to say that at this point string theory is over-represented, and we should start putting more legitimacy on other avenues of thought.  While there may be some people here or there working on other things, it seems to me that the community as a whole really has all the eggs in one basket.


----------



## freyar (Mar 22, 2016)

Umbran said:


> I will grant you that if you do an experiment at high energy, but well below the Planck energy, and see such resonances, then yes, you'd have evidence for strings.  Except... you might not see those things, and failing to see them doesn't invalidate the theory.






> The models that predict these are based in perturbative string theory, no?  That's explicitly an approximation, and to my understanding it is not at all clear that you expect that behavior in reality (which, as far as we can tell, is not perturbative).  Last I read, M-Theory, in general, does not require such resonances.  And there's a bazillion ways to collapse the multiple dimensions required by string theory down into 3d models that give particular predictions of what resonances you see - so, if you run an experiment, and it fails to show the resonances, you just say, "Well, I have the wrong perturbative series/compactification, but string theory hasn't been disproven!"




While I will grant that we don't know the final high-energy description of M-theory yet and can't give a quantitative prediction, the expectation would be to have membrane resonances there.  As for the perturbative/nonperturbative issue, one of the neat/amazing things about the framework of string theory is that almost every point in parameter space, even if it looks naively nonperturbative, can be mapped to another description where perturbation theory works pretty well.  (Remember that the coupling constant doesn't actually have to be that small for perturbation theory to be ok.)  So you would generically (in the technical sense) see stringy/membrany resonances with some width.  They'd only mush out and become indistinguishable (and get pushed right up to the Planck scale) in an unusual area of parameter space.  So, say you don't see the resonances.  Then, *just as in every other theory/model*, you can put limits on parameters.  This is worth emphasizing.  If I have *any* proposed theory, there is some kind of free parameter(s) to fit to experiment, and your prediction depends on those parameters.  So *the totally normal case of "falsifiability" means being able to put limits on free parameters.*  It's only when you have some way to *measure* parameters and then make an additional measurement that you can make a *quantitative* and possibly unique prediction.  String theory is no different than any other theory in that way.  Now, by the time we can build a Planck-scale (or even a bit below) accelerator, it's quite possible we could measure various parameters of string theory with cosmological measurements (or, more precisely, say what the parameters would be if string theory correctly describes cosmology), which would allow quantitative predictions for the collider results.  Again, falsifiable like anything else.



> And, if you do somehow manage to run a test up a Planck energies, you still have an issue - more conventional theory also predicts that you will see some really strange things there (like black hole states), so you can't tell if what you are seeing is string theory, or something else.




Again, generically, the string resonances should be lower-energy than black holes.  And, of course, to understand quantum black holes in a particle collider, you need some kind of quantum gravity theory, none of which are conventional field theories.  In more general situations, yes, it is generally possible to cook up a quantum field theory that does basically anything that string theory or any other CPT-preserving unitary theory can do.  But that may require a lot of epicycles. At what point do you allow Occam's razor?



> Thus, the whole thing leans to the non-falsifiable.  If you can always dodge and say that your theory is still correct, there's an issue.



Let me ask a rhetorical question.  Do you think quantum field theory is falsifiable?  I can certainly falsify specific quantum field theories, but within the framework of QFT, it's possible to get many many different results and/or avoid all kinds of limits.  It's a similar situation with string theory --- there are many possibilities overall, but a given set-up can be ruled out or limited as normal.



> Meanwhile, the most recent stuff I saw on quark-gluon plasmas coming our of the LHC *failed* to match the string models used to describe them.  And no SUSY, though that should have been seen in Bs decay.



We need to be careful here.  We've so far been talking about string theory as a "theory of everything" (or ultimate description of the universe).  With the quark-gluon plasma issue, you're now talking about string theory as a dual description of nuclear physics, meaning it's a way to calculate in a theory (quantum chromodynamics) where calculations by usual means are prohibitively difficult.  In that case, it would have been a big surprise to see quantitative agreement between the string models and the experiment, because the precise string models used are not supposed to describe real-world QCD closely but rather similar theories which are a bit easier to work with.  The lesson here is that QCD is hard, and we have a way to go with both traditional methods and dual string theories.  It's worth noting that the first qualitative understanding of quark-gluon plasma results from RHIC was due to a dual string theory model.

As for supersymmetry, you're again talking about limits on parameter space.  Yes, there are now strong limits on the minimal version of SUSY, though it's not nearly closed off yet (I'd warn against articles in the popular press about that, though, since the "SUSY is ruled out" soundbite sounds so good despite being inaccurate according to community consensus).  On the other hand, the Standard Model is a far-from-minimal extension of the subatomic physics we knew 100 years ago, so it's maybe not so surprising if low-energy supersymmetry turns out to be non-minimal.




> Back in the 1980s, Feynman stated concerns that there was rather too much hype and groupthink surrounding string theory, and I don't see a lot of evidence that's gone away.  How many times does a model we've been trying to develop for 40 years have to fail to meet expectations before we collectively stop apologizing for it?  So, as you say above - yes, I think it fair to say that at this point string theory is over-represented, and we should start putting more legitimacy on other avenues of thought.  While there may be some people here or there working on other things, it seems to me that the community as a whole really has all the eggs in one basket.




Interestingly, I used to work at CalTech, where of course Feynman spent a large part of career, and so has John Schwarz, known as "the father of string theory" (and, at least until his recent retirement, the resident of Feynman's old office).  From what Schwarz and others who were there with Feynman have told me, Feynman was not so negative about string theory in general as his famous quote would lead you to believe and was in fact supportive of Schwarz's work, at least.

Anyway, I'm starting to feel like we're just getting into a back-and-forth (and rather away from the OP's topic) between just the two of us, so I'll leave with a last couple of thoughts unless other posters ask for more information:
1) It's certainly fair to have an opinion that string theory is over-represented.  It's an opinion.  I'd note that there are actually quite a few people working in different areas of quantum gravity.  Though they are not as unified in what they're working on, I wouldn't classify it as "all the eggs in one basket."  Anyway, it's also fair to note that there is sometimes overlap between different approaches (including string theory) and that all approaches to quantum gravity face the same fundamental obstacle in making predictions --- they have to extrapolate over about 16 orders of magnitude in energy.  If you think quantum gravity is worth understanding, it's going to take time, no matter what approach you favor.
2) If it seems like string theorists have been "apologizing" a lot, it's because a relatively small handful of physicists decided to attack it in the popular press.  There isn't a need to apologize --- string theory has actually been remarkably productive whether or not it is an ultimate theory of everything.  I don't have time to type it all out, but it has led to numerous important discoveries in mathematics (related to more than one Fields medal), new ways to evaluate scattering amplitudes of relevance to the LHC, dual formulations of many field theories, phenomenological models of extra dimensions and cosmology, and recently improved understanding of complex systems in condensed matter physics also through dualities.


----------



## Joker (Apr 3, 2016)

So from what I gather, when we talk about exotic matter, we're more in the realm of fiction than science.  It's strictly a placeholder value to make a formula work?  Is that a fair assessment? 

If we're in the realm of fiction, let's make the assumption that matter with such properties exists.  
To overcome the problem of a particle that repels instead of attracts (at the same strength, but negative, of normal attraction), could it be possible that said particle has magnetic properties?  That during a planet'ss formation it could have attached itself to a ferrimagnetic material?  Thus allowing deposits to form on the planet.  And I don't possible in the sense of, it's fiction so anything is possible.  I mean, is there any rule that says an material with a repelling force can't also be magnetic?

I wonder if anyone, as an intellectual exercise, has tried to figure out what such matter would look like.  I mean, if you know all the properties of a certain particle, shouldn't it be possible to know what color it has, and how it feels?


----------



## Umbran (Apr 3, 2016)

Joker said:


> It's strictly a placeholder value to make a formula work?  Is that a fair assessment?




It is not *strictly* a placeholder.  There are examples of things that we are pretty sure exist, but just aren't like the normal matter that makes up your dinner table.  And it isn't usually so much that you put a placeholder value in to make an equation work - that's just making crap up. 

In some instances, the exotic matter has never directly observed the material, but *everything* we know and observe says it should exist (neutronium, degenerate matter).  In others, it is the most reasonable physical interpretation that best describes what we see in the universe (Dark Matter and Dark Energy, f'rex).   And in yet other instances, it is a result of asking, "Assume this effect occurs.  What does that imply?"  If the effect doesn't actually happen, then the material probably doesn't exist.  This is the wormhole-exotic matter case.  This last is the more speculative.



> And I don't possible in the sense of, it's fiction so anything is possible.  I mean, is there any rule that says an material with a repelling force can't also be magnetic?




Well, in most cases, you are positing the existence of a material to fit a very particular problem.  In such cases, you are fairly restricted in terms of what properties the stuff can have and still be consistent with what we do know.  Dark Matter, for example - if it interacts by means other than gravity and perhaps the weak nuclear force, it won't fit what we observe in the universe.

If you want ot create complete fiction, and posit a material with arbitrary properties, that's fine, but this solves no problems.



> I wonder if anyone, as an intellectual exercise, has tried to figure out what such matter would look like.  I mean, if you know all the properties of a certain particle, shouldn't it be possible to know what color it has, and how it feels?




Well, individual particles do not have "color" in the visual sense, nor texture.  These are properties of matter in bulk, often dependent on how the material comes to be - carbon can be opaque grey graphite powder, or clear solid diamond, for example.


----------



## Dannyalcatraz (Apr 3, 2016)

Umbran said:


> Neutronium - the stuff that neutron stars are made of, very likely exists (we have observed objects that have all the properties we'd expect a neutron star to have), but getting it?  How?  The stuff is down in a friggin' neutron star!




Well, first we launch an unobtanium skyscraper...


----------



## Umbran (Apr 3, 2016)

Dannyalcatraz said:


> Well, first we launch an unobtanium skyscraper...




Basically.  The thing is, neutronium should only exist in the situation in which normal matter can't - where gravity is so strong that it compacts matter *so* much that electrons get crushed down into the atom's protons, and become neutrons.  So, if you reach out to it with any normal matter, you're reaching into a place where that normal matter, by definition, cannot exist.


----------



## Dannyalcatraz (Apr 3, 2016)

FWIW, Larry Niven gave me my intro to the exotic critters called neutron stars when i was a wee laddie.

https://en.m.wikipedia.org/wiki/Neutron_Star_(short_story)


----------



## freyar (Apr 4, 2016)

Umbran said:


> It is not *strictly* a placeholder.  There are examples of things that we are pretty sure exist, but just aren't like the normal matter that makes up your dinner table.  And it isn't usually so much that you put a placeholder value in to make an equation work - that's just making crap up.
> 
> In some instances, the exotic matter has never directly observed the material, but *everything* we know and observe says it should exist (neutronium, degenerate matter).  In others, it is the most reasonable physical interpretation that best describes what we see in the universe (Dark Matter and Dark Energy, f'rex).   And in yet other instances, it is a result of asking, "Assume this effect occurs.  What does that imply?"  If the effect doesn't actually happen, then the material probably doesn't exist.  This is the wormhole-exotic matter case.  This last is the more speculative.
> 
> ...




Yes, all this.  I'd also add that magnetic fields have normal gravity, so there's no reason your fictional exotic matter can't both have antigravity and magnetic properties, but the magnetic properties can't be the cause of the antigravity.


----------



## Scott DeWar (Apr 4, 2016)

Have they discovered if there is a particular particle that creates magnetism? or is  it the movement of the electron around the nucleus and being stronger with certain combinations of electron/proton/neutron combinations?


----------



## freyar (Apr 4, 2016)

Scott DeWar said:


> Have they discovered if there is a particular particle that creates magnetism? or is  it the movement of the electron around the nucleus and being stronger with certain combinations of electron/proton/neutron combinations?




Photons --- particles of light --- are what carry the electromagnetic force.  So, like electric fields, we can say magnetic fields are composed of many photons (that's a really loose way of putting things).  Now, if you're asking why certain materials are magnetic, it mostly does have to do with the electrons, namely their spins and/or orbits, which cause each atom or ion to act like a miniature magnet.  Certain materials, like iron, have magnetic properties because there are small interactions between the atoms/ions that make those miniature magnets preferably align, so they add up into a big magnet.  There are also materials where the miniature magnets like to anti-align, and those materials tend to suppress, rather than create, magnetic fields.


----------



## Scott DeWar (Apr 4, 2016)

I am sorry for hijacking the thread but I have another question:

So, if you are 100 feet below the magnetic southpole ground level, and the compass you are testing spins because you are at the magnetic south pole, You are looking at the result of photons from the sun creating that field, even though you are 100 feet below ground?


----------



## freyar (Apr 4, 2016)

Scott DeWar said:


> I am sorry for hijacking the thread but I have another question:
> 
> So, if you are 100 feet below the magnetic southpole ground level, and the compass you are testing spins because you are at the magnetic south pole, You are looking at the result of photons from the sun creating that field, even though you are 100 feet below ground?



Actually, if you're looking at the effect on a compass, that's due to photons from the earth since it's the earth's magnetic field.  Also, that magnetic field isn't a freely-traveling wave, so, if you want to think of it as made of photons, you might want to think of it as "long-range virtual photons."  Basically, these aren't photons that can just fly off and go somewhere on their own but make up a large scale field that particles like electrons feel.  (The problematic issue in translating the math is that the magnetic field isn't really made up of a fixed number of photons at all; this is one of those weird quantum things.)


----------



## Scott DeWar (Apr 5, 2016)

freyar said:


> Actually, if you're looking at the effect on a compass, that's due to photons from the earth since it's the earth's magnetic field.  Also, that magnetic field isn't a freely-traveling wave, so, if you want to think of it as made of photons, you might want to think of it as "long-range virtual photons."  Basically, these aren't photons that can just fly off and go somewhere on their own but make up a large scale field that particles like electrons feel.  (The problematic issue in translating the math is that the magnetic field isn't really made up of a fixed number of photons at all; this is one of those weird quantum things.)



I am going to have to sit here and dwell on this for a bit. Thank you for the information, I will, In most likelihood, ask more on this.

I asked about magnetic field particle theory as I wondered about the possibility that something like anti photons, or some other heretofore unknown magnetic field producing matter, are what is needed to be seen as the exotic material.


And you thought I was just going down a rabbit trail! HA!


----------



## Umbran (Apr 5, 2016)

Scott DeWar said:


> I asked about magnetic field particle theory as I wondered about the possibility that something like anti photons




Photons are their own anti-particle. 

When an electron and a positron (aka an anti-electron) collide, they annihilate and release energy.

When a photon and another photon of the same energy collide, they annihilate, and release energy.


----------



## Dannyalcatraz (Apr 5, 2016)

Umbran said:


> When an electron and a positron (aka an anti-electron) collide, they annihilate and release energy.
> 
> When a photon and another photon of the same energy collide, they annihilate, and release energy.



I can almost hear Michael Bolton crooning that one out.


----------



## Scott DeWar (Apr 5, 2016)

Umbran said:


> Photons are their own anti-particle.
> 
> When an electron and a positron (aka an anti-electron) collide, they annihilate and release energy.
> 
> When a photon and another photon of the same energy collide, they annihilate, and release energy.



oh. so no anti photons. got it.


----------



## tomBitonti (Apr 5, 2016)

Wait, I thought photons didn't interact with each other, although, they like to be in the same state.

TomB


----------



## Umbran (Apr 5, 2016)

tomBitonti said:


> Wait, I thought photons didn't interact with each other, although, they like to be in the same state.




They don't.

Well, to first-order approximation they don't - they are electromagnetic waves, and so they like to interact with electric charges, even when they themselves are neutral.  There are some more rare effects that make for a non-zero probability of it happening.

But either way - when a particle-antiparticle pair interact, they turn to energy, which likely comes out as photons that had energy that totaled up what went into the particle-antiparticle pair.

When the photon-antiphoton pair interact, they turn to energy, which likely comes out as... photons.  Photons with the same total energy as what went in.  So... they'd look *exactly the same*.  How would you really know if they'd interacted or not?  

Freyar will likely grump at me now for being a tad inexact.  I'll deal


----------



## Scott DeWar (Apr 5, 2016)

well, In your defense, explaining fizziks to simple 'lektrishuns like my self is an inexact science.


----------



## Jester David (Apr 5, 2016)

Exotic matter is odd. It's rule lawyering physics. "This isn't strictly against the universe's RAW so it's possible."
If the right exotic matter exists we could make wormholes or warp drives. And if we find pixie dust we can fly.


----------



## Mustrum_Ridcully (Apr 5, 2016)

Jester Canuck said:


> Exotic matter is odd. It's rule lawyering physics. "This isn't strictly against the universe's RAW so it's possible."
> If the right exotic matter exists we could make wormholes or warp drives. And if we find pixie dust we can fly.




I liked when some scientist said to new discoveries that the Alcubierre drive might not need exotic matter equivalent to the Jupiter mass, but only that of, say, the Voyager probe: "So basically, we don't need a million unicorns to go faster than light, we just need one?"

But, that comparison might not be quite fair. Or at least not if we replace unicorns with pixie dust - pixie dust can have any property we want, there are no restrictions or descriptions that would allow us to test for its existence. For exotic matter of the "negative mass" type, we could at least postulate properties that we could test for. We're much closer to explaining how something happens - how the pixie dust does make us fly is undefined (does it remove gravity? Invert our mass? does it create an electro-magnetic field? Alter the laws of physics.)
Negative Mass Exotic Matter would be in contrast well-defined.


----------



## Jester David (Apr 5, 2016)

Mustrum_Ridcully said:


> I liked when some scientist said to new discoveries that the Alcubierre drive might not need exotic matter equivalent to the Jupiter mass, but only that of, say, the Voyager probe: "So basically, we don't need a million unicorns to go faster than light, we just need one?"
> 
> But, that comparison might not be quite fair. Or at least not if we replace unicorns with pixie dust - pixie dust can have any property we want, there are no restrictions or descriptions that would allow us to test for its existence. For exotic matter of the "negative mass" type, we could at least postulate properties that we could test for. We're much closer to explaining how something happens - how the pixie dust does make us fly is undefined (does it remove gravity? Invert our mass? does it create an electro-magnetic field? Alter the laws of physics.)
> Negative Mass Exotic Matter would be in contrast well-defined.



90% of the mass of the galaxy already can't be explained, and now we're adding in something that would reduce the effective mass? 

It's cool and we can look for it, and maybe it exists. But it's not worth assuming it's a thing. 
Especially since the Alcubierre drive has other problems, such as the inability to stop, steer, or have anything inside NOT be flashfried by Hawkings radiation.
An Alcubierre drive is only slightly less ridiculous than an infinite improbability drive.


----------



## Umbran (Apr 5, 2016)

Mustrum_Ridcully said:


> Negative Mass Exotic Matter would be in contrast well-defined.




The two things that have been mentioned are matter with negative energy density (which is not necessarily negative mass), and matter which exhibits gravitational repulsion (which again, is not necessarily negative mass).



			
				Jester Canuck said:
			
		

> Especially since the Alcubierre drive has other problems, such as the inability to stop, steer, or have anything inside NOT be flashfried by Hawkings radiation.




With respect, this is kind of like trying to use the lack of a bit and bridle to dismiss the overall concept of a horse.

Those issues are things that we in the field would call "engineering details".  If you can get it to work at all, these are not apt to be massive problems. Steering, for example, is almost trivial.  Assume, for a moment, that you have the exotic matter required to form a warp bubble.  Steering comes from small alterations in the distribution of that exotic matter, to slightly change the shape of the bubble.  Compared to the immensity of the base assumption, mere steering is child's play.   

As for Hawking radiation - dude, you're talking about having a warp drive.  A main deflector dish isn't far behind


----------



## Jester David (Apr 5, 2016)

Umbran said:


> With respect, this is kind of like trying to use the lack of a bit and bridle to dismiss the overall concept of a horse.
> 
> Those issues are things that we in the field would call "engineering details".  If you can get it to work at all, these are not apt to be massive problems. Steering, for example, is almost trivial.  Assume, for a moment, that you have the exotic matter required to form a warp bubble.  Steering comes from small alterations in the distribution of that exotic matter, to slightly change the shape of the bubble.  Compared to the immensity of the base assumption, mere steering is child's play.
> 
> As for Hawking radiation - dude, you're talking about having a warp drive.  A main deflector dish isn't far behind




Steering is tricky because you're cut-off from regular space and unable to respond to new signals or changes. Which isn't much of an issue, since if you see something you have to steer around, by the time you can see it you've already hit it. Stopping is the bigger issue. And not releasing a death wave of energy that has built up in front of you. 

All these could be worked around given time and energy. But figuring out a reliable way of stopping and not unleashing a planet destroying radiation way isn't very useful science until you can build such a device. And building such a device isn't useful until you have the unicorn to power it. 
To me it feels like making a budget for spending your lottery winnings. Sure, if you win the jackpot and have a detailed financial plan it's easier and you're less likely to make rash & foolish financial decisions, but you might be better off working towards an independent financial future. 
In this case, working on rotating ships that can withstand the radiation of space or perfecting suspended animation/cryosleep. Or downloading brains into robot bodies so we can live as infomorphs before uploading into robots upon arrival.


----------



## freyar (Apr 5, 2016)

Umbran said:


> They don't.
> 
> Well, to first-order approximation they don't - they are electromagnetic waves, and so they like to interact with electric charges, even when they themselves are neutral.  There are some more rare effects that make for a non-zero probability of it happening.
> 
> ...




Nah, this is all good.   As usual, I can elaborate, though.  There are a couple of rare types of events Umbran is talking about.

One is that two photons can interact and turn into an electron/anti-electron pair _if they have enough energy to do so_.  If you had an easy way to make very energetic photons, this would happen as much as electron/anti-electron annihilation.  People usually call it pair creation from the electron's point of view.  These events are rare not because they are unlikely to happen given the right circumstances but because we don't really have too many ways to make lots of high energy photons, at least not without using some kind of matter/anti-matter annihilation to start with (so the right circumstances are rare).

The other type of rare event is photons actually bouncing off of each other.  At a microscopic level, the photons don't interact with each other but with quantum "virtual" electrons.  This is a rare event just because, even when photons get close to each other, they don't generally both interact with virtual electrons at the same time.


----------



## tomBitonti (Apr 5, 2016)

freyar said:


> The other type of rare event is photons actually bouncing off of each other.  At a microscopic level, the photons don't interact with each other but with quantum "virtual" electrons.  This is a rare event just because, even when photons get close to each other, they don't generally both interact with virtual electrons at the same time.




Can you expound on that?  I've read (if memory serves me) of interaction of photons with virtual particles as a cause for "tired light" which is an alternate (but dis-proven) mechanism for photon red-shift.  I've wondered why such interactions don't cause a lessening of the speed of light in a vacuum, based on the proportion of time the photon spends interacting with the virtual electron, or extra dispersal of a collection of photons which are initial in phase.

Thx!
TomB


----------



## megamania (Apr 5, 2016)

Feel like this is an episode of Big Bang Theory and I'm Penny


----------



## Scott DeWar (Apr 5, 2016)

freyar said:


> Nah, this is all good.   As usual, I can elaborate, though.  There are a couple of rare types of events Umbran is talking about.
> 
> One is that two photons can interact and turn into an electron/anti-electron pair _if they have enough energy to do so_.  If you had an easy way to make very energetic photons, this would happen as much as electron/anti-electron annihilation.  People usually call it pair creation from the electron's point of view.  These events are rare not because they are unlikely to happen given the right circumstances but because we don't really have too many ways to make lots of high energy photons, at least not without using some kind of matter/anti-matter annihilation to start with (so the right circumstances are rare).
> 
> The other type of rare event is photons actually bouncing off of each other.  At a microscopic level, the photons don't interact with each other but with quantum "virtual" electrons.  This is a rare event just because, even when photons get close to each other, they don't generally both interact with virtual electrons at the same time.






tomBitonti said:


> Can you expound on that?  I've read (if memory serves me) of interaction of photons with virtual particles as a cause for "tired light" which is an alternate (but dis-proven) mechanism for photon red-shift.  I've wondered why such interactions don't cause a lessening of the speed of light in a vacuum, based on the proportion of time the photon spends interacting with the virtual electron, or extra dispersal of a collection of photons which are initial in phase.
> 
> Thx!
> TomB




is all of this still in theory or has it been actually observed and reproduced in a lab?


----------



## Scott DeWar (Apr 5, 2016)

megamania said:


> Feel like this is an episode of Big Bang Theory and I'm Penny



believe me, I know what you mean!


----------



## Scott DeWar (Apr 5, 2016)

Umbran said:


> As for Hawking radiation - dude, you're talking about having a warp drive._*  A main deflector dish*_ isn't far behind



and that device is handy for so many other uses!


----------



## Umbran (Apr 6, 2016)

Scott DeWar said:


> and that device is handy for so many other uses!




It slices!  It dices!  It vaporizes!  Whitens sheets, freshens breath, and is guaranteed to be able to channel more energy than any other emitter on your starship!

If you order now, you get a free set of phaser banks!

Plus, as a special added bonus, a matched bat'leth and dk'tagh set, for all your kitchen needs!


----------



## tomBitonti (Apr 6, 2016)

Scott DeWar said:


> is all of this still in theory or has it been actually observed and reproduced in a lab?




The bit about "weak light" is a well disproven idea.  Virtual particles are demonstrated by the Casimir effect (putting two plates very close to each other).  That's based on a very thin understanding and reading.  I defer to others for anything in more depth.

Thx!
TomB


----------



## Scott DeWar (Apr 6, 2016)

Umbran said:


> It slices!  It dices!  It vaporizes!  Whitens sheets, freshens breath, and is guaranteed to be able to channel more energy than any other emitter on your starship!
> 
> If you order now, you get a free set of phaser banks!
> 
> Plus, as a special added bonus, a matched bat'leth and dk'tagh set, for all your kitchen needs!



shut up and take my money!


----------



## freyar (Apr 6, 2016)

tomBitonti said:


> Can you expound on that?  I've read (if memory serves me) of interaction of photons with virtual particles as a cause for "tired light" which is an alternate (but dis-proven) mechanism for photon red-shift.  I've wondered why such interactions don't cause a lessening of the speed of light in a vacuum, based on the proportion of time the photon spends interacting with the virtual electron, or extra dispersal of a collection of photons which are initial in phase.




You're right, this wouldn't lead to "tired light" or slow down the speed of light.  Basically, as a single photon travels along, it constantly excites (and de-excites) virtual electrons but in such a way that the wave speed of light is unaffected (and specifically is still independent of frequency).  The reason for this is that the interaction between the photon and the electrons still respects special relativity.  What it does effect, however, is how strongly photons and electrons interact, in other words, the value of the electric charge.  You might have heard something to the effect that an electron is surrounded by a cloud of virtual electrons & anti-electrons that collectively "screen" the electron's charge.  A photon hitting an electron with more energy gets closer to the electron, so it is screened less and effectively sees a larger electron charge.

This same kind of effect means that two photons can also "hit" each other.  Most of the time, two photons will just pass each other by, but sometimes they will excite the same virtual electrons and can both bounce off that virtual electron.



Scott DeWar said:


> is all of this still in theory or has it been actually observed and reproduced in a lab?



I'm not an experimentalist, so I don't keep track of every result like this terribly carefully.  The following is my understanding, but I might have missed something: 

Two photons coming together and creating an electron/anti-electron pair has been observed in a lab starting in the late 1990s AFAIK.  The caveat is that the only way we have to produce energetic enough photons on earth is in high energy collisions of other things, so the photons involved are themselves fairly virtual (it's worth mentioning that virtual is not a binary descriptor but a continuous thing --- every particle is in reality a little virtual).  On the other hand, this process appears to happen fairly commonly in astrophysics since there are lots of things in space that create high energy photons.

I don't believe two photons bouncing off each other in the vacuum has been observed yet, and in fact the predicted rate for this to happen is below current experimental sensitivity --- if we'd seen it by now, it would mean there's something we don't understand going on.

But it's worth mentioning that both of these types of events are embedded and inevitable in the Standard Model of particle physics AND any way we might know how to modify particle physics consistent with things we've already measured.  If they didn't happen, it would mean we'd literally have to scrap almost a century of particle physics theory which has been otherwise remarkably successful.  What I'm trying to say is that there are different degrees of "still just in theory."  There's stuff like this, which pretty much has to happen given what we know already; there is a concept like dark matter, which we don't know a lot about but can be extremely confident exists from what we know; there is something like string theory or another approach to quantum gravity, which we can't directly access experimentally for the time being but is a logical extension of what we know and have observed (and can be tested mathematically); and there is something like the exotic matter needed for wormholes and warp drives, which doesn't really naturally come up for any other reason and can easily violate basic physical principles if you're not really careful about it.


----------



## Istbor (Apr 6, 2016)

Please continue, just don't mind the grey matter trickling from my ears.


----------



## Umbran (Apr 6, 2016)

Istbor said:


> Please continue, just don't mind the grey matter trickling from my ears.




S'okay, we won't mind it.

Unless it is *exotic* grey matter.  Then, we're interested.


----------



## Scott DeWar (Apr 7, 2016)

I am aware of the electron / anti electron [or positron IIRC] production from previous discussion, more notably PET scanners used in cancer cell observation on the molecular level. I was once told I do not want a pet scan. In the last year I have had it explained WHY I  don't want one. More specifically, the gamma ray produced when the mater and anti matter introduce them selves.


----------



## Scott DeWar (Apr 7, 2016)

Istbor said:


> Please continue, just don't mind the grey matter trickling from my ears.



Yeah, I understand. Mine has been coming out bubbling and steaming.


----------



## tomBitonti (Apr 7, 2016)

freyar said:


> This same kind of effect means that two photons can also "hit" each other.  Most of the time, two photons will just pass each other by, but sometimes they will excite the same virtual electrons and can both bounce off that virtual electron.




An idea, musing about this: If I setup a double slit experiment and put a reflector on the outward side that causes the washes from the slits to cross, then send through individual photons or electrons, will these sometimes reflect off themselves?

Since a single interaction can't conserve momentum, does that mean this particular reflection can't happen?

Edit: Trying to answer this myself on my drive in to work: One line of reasoning: Particles arriving from the left or the right are distinguishable events, meaning, normal probabilities should be used, not amplitudes.  A particle crossing from the left is exclusive with a particle crossing from the right.  No self collisions are possible. Second line of reasoning: If particles always reflect exactly, then a reflection would be indistinguishable from a non-reflection.  There is no noticeable result (except the reflection is indistinguishable from a non-reflection, so some considerations of amplitudes with this). Third line of reasoning: If a scattering interaction can occur (not just a reflection) and amplitudes must considered, then a bizarre result seems possible, where the electron or photon can (very rarely) scoot off in a random direction.

Thx!
TomB


----------



## freyar (Apr 7, 2016)

Scott DeWar said:


> I am aware of the electron / anti electron [or positron IIRC] production from previous discussion, more notably PET scanners used in cancer cell observation on the molecular level. I was once told I do not want a pet scan. In the last year I have had it explained WHY I  don't want one. More specifically, the gamma ray produced when the mater and anti matter introduce them selves.




As in PET scans, the positrons (anti-electrons) we'd normally encounter are produced as a product of nuclear decay; large particle accelerator experiments also produce them a lot, though not necessarily from photon-photon interactions.  The electron/positron annihilation into 2 gamma rays is precisely the reverse process of two photons hitting each other to form an electron/positron pair.

I agree with you and your medical professional that you don't want a PET scan if you don't medically need one.  Compared to an X-ray, you get a high dose of radiation from them. But I do think it's worth noting that the scary estimates of how likely that dose of radiation is to cause a future medical problem, like cancer, are based on extrapolation from extremely high doses of radiation (like people who were exposed to nuclear accidents, etc).  There's no direct evidence that PET or CT scans cause cancers --- in fact, one study I saw on the BBC news page a while back concluded that CT scans don't increase the risk of cancer at all. So, while I think best medical practice is to avoid unnecessary testing and unnecessary risks, I wouldn't be scared of getting a PET scan if the doctors think it would be helpful for treatment.


----------



## freyar (Apr 7, 2016)

tomBitonti said:


> An idea, musing about this: If I setup a double slit experiment and put a reflector on the outward side that causes the washes from the slits to cross, then send through individual photons or electrons, will these sometimes reflect off themselves?
> 
> Since a single interaction can't conserve momentum, does that mean this particular reflection can't happen?




If I understand what you want to set up, the mirror will change the interference pattern from the double slit experiment, but that's normal quantum mechanical interference, not due to the photon "hitting" itself in the sense I was talking about.  Scattering really requires two different particles.


----------



## Umbran (Apr 7, 2016)

freyar said:


> If I understand what you want to set up, the mirror will change the interference pattern from the double slit experiment, but that's normal quantum mechanical interference




Yah.  Consider that, in the standard double-slit experiment, the particle goes through *both* slits, which is what sets up the interference pattern in the first place.  It is, in effect, already interacting with itself.


----------



## tomBitonti (Apr 7, 2016)

freyar said:


> If I understand what you want to set up, the mirror will change the interference pattern from the double slit experiment, but that's normal quantum mechanical interference, not due to the photon "hitting" itself in the sense I was talking about.  Scattering really requires two different particles.




I was thinking about a device like the Stern-Gerlach (?sp) device used by Feynman in his lecture notes.  If that can be used to split a particle stream then to recombine the streams to test quantum effects, then in a similar fashion the device could be used, not to split and merge the stream, but instead to split and then cause the branches to cross.

Thx!
TomB


----------



## Umbran (Apr 7, 2016)

tomBitonti said:


> I was thinking about a device like the Stern-Gerlach (?sp) device used by Feynman in his lecture notes.  If that can be used to split a particle stream then to recombine the streams to test quantum effects, then in a similar fashion the device could be used, not to split and merge the stream, but instead to split and then cause the branches to cross.




In a quantum mechanical sense, that's what the double-slit experiment already does.


----------



## tomBitonti (Apr 8, 2016)

Umbran said:


> In a quantum mechanical sense, that's what the double-slit experiment already does.




Ok, but the objection was that a mirror would cause problems.  If it is unnecessary, then remove it from the setup.  The goal was to create places where particles from either slit had a probability of reaching with the same travel time.

I think what is throwing me off is that having an amplitude to reach a coordinate along two paths is not to say that the particle traverses both paths simultaneously, at least not in an everyday sense.

Thx!
TomB

Thx!
TomB


----------



## freyar (Apr 8, 2016)

tomBitonti said:


> Ok, but the objection was that a mirror would cause problems.  If it is unnecessary, then remove it from the setup.  The goal was to create places where particles from either slit had a probability of reaching with the same travel time.



That also happens in the regular double-slit experiment.  As Umbran says, the interference pattern is a type of basic quantum mechanical "interaction" but not the type of scattering that I was talking about above.



> I think what is throwing me off is that having an amplitude to reach a coordinate along two paths is not to say that the particle traverses both paths simultaneously, at least not in an everyday sense.




Right, the "everyday sense" is the issue.  There's a mathematical formulation due to Feynman that looks like the particle takes every possible path simultaneously, but that's far from the whole picture.


----------



## Umbran (Apr 8, 2016)

tomBitonti said:


> I think what is throwing me off is that having an amplitude to reach a coordinate along two paths is not to say that the particle traverses both paths simultaneously, at least not in an everyday sense.




Yes, but as soon as you start talking about quantum mechanics, you have to remember that "the everyday sense" probably does not apply.  Your intuition is formed and trained using your observation of the slow-moving, macro-world - the world of baseballs and pool tables.  And quantum objects just *don't* follow the same behaviors.

For example, you say above: "having an amplitude to reach a coordinate along two paths is not to say that the particle traverses both paths simultaneously".

But, the double-slit experiment exists to demonstrate and maximize the *WAVE* nature of light or matter.  In this setup, discussing the *particle* traversing anything is going to confuse you, because it isn't behaving at all like a particle.


----------



## tomBitonti (Apr 8, 2016)

Umbran said:


> Yes, but as soon as you start talking about quantum mechanics, you have to remember that "the everyday sense" probably does not apply.  Your intuition is formed and trained using your observation of the slow-moving, macro-world - the world of baseballs and pool tables.  And quantum objects just *don't* follow the same behaviors.
> 
> For example, you say above: "having an amplitude to reach a coordinate along two paths is not to say that the particle traverses both paths simultaneously".
> 
> But, the double-slit experiment exists to demonstrate and maximize the *WAVE* nature of light or matter.  In this setup, discussing the *particle* traversing anything is going to confuse you, because it isn't behaving at all like a particle.




Yeah.

What kindof still throws me is that if we put two double slits facing each other, and sent different particles through each at the same time, there is a probability that the particles will scatter, at locations as determined by the diffraction pattern across the line of intersection of the streams.  And if you split a stream of particles and sent through through the slits, if there were many particles at once, there would be scattering, but for just one particle at a time there won't.  Then, there is a bit of care needed when representing what is happening, in that an approximate description of what is happening, which works for multiple particles, doesn't work for sending one particle at a time.  I think this ends up being a matter of how to interpret an amplitude and what that means for just one particle vs. how it is used for multiple particles.

Thx!
TomB


----------



## freyar (Apr 8, 2016)

tomBitonti said:


> What kindof still throws me is that if we put two double slits facing each other, and sent different particles through each at the same time, there is a probability that the particles will scatter, at locations as determined by the diffraction pattern across the line of intersection of the streams.  And if you split a stream of particles and sent through through the slits, if there were many particles at once, there would be scattering, but for just one particle at a time there won't.  Then, there is a bit of care needed when representing what is happening, in that an approximate description of what is happening, which works for multiple particles, doesn't work for sending one particle at a time.  I think this ends up being a matter of how to interpret an amplitude and what that means for just one particle vs. how it is used for multiple particles.




We should be careful by what we mean by the word "interacting."  When I, as a physicist, talk about interference in a double-slit experiment, which as Umbran says is a manifestation of the wave nature of particles, I do not mean an interaction.  The idea is that a wave passing through the slits naturally has some points where it will be zero.  On the other hand, an "interaction" is what it sounds like --- two different things interacting with each other (ie, doing things together). Mathematically, the universe can tell the difference between one and two particles.


----------



## Umbran (Apr 11, 2016)

tomBitonti said:


> What kindof still throws me is that if we put two double slits facing each other, and sent different particles through each at the same time, there is a probability that the particles will scatter, at locations as determined by the diffraction pattern across the line of intersection of the streams.






> And if you split a stream of particles and sent through through the slits, if there were many particles at once, there would be scattering, but for just one particle at a time there won't.




I think I may now have a more clear picture of what you mean... but that leaves me going, wha?  I'm not sure why this is a question.

If you send a bazillion particles through the slits, you have roughly a bazillion chances for scattering.  If you reduce it down to one particle at a time, you reduce your chances of interaction to nigh zero, simply because the number of particles to interact with is near zero.  

The classical model may help us here:

Take two nerf guns, and point them at each other.  Nerf guns aren't terribly accurate, so there's a lot of scatter from shot to shot.  Turn them on full-automatic fire, and let them run.  You'll see some darts bouncing off each other, because there's darts all over the place.  Compare this to having each gun shoot only one dart.  What's the chance that those two darts will just happen to hit each other?  Very small.  

Now, for your reflective surface case - 

Take one of the guns, and fire it at a wall.  Darts will bounce back from the wall.  If you have this on full-auto, some bouncing darts may collide with new incoming darts, again, because there's a bazillion darts flying around to run into.  But, if you go to single shot, a dart that bounces *has no incoming dart to collide with*.  It *can't* scatter off itself in that sense.  If you have it on slow-auto, the chance of the bouncing dart colliding with a new incoming dart is still extremely small, simply because the chance that those two will be in the right place is very small.


----------



## tomBitonti (Apr 12, 2016)

Umbran said:


> I think I may now have a more clear picture of what you mean... but that leaves me going, wha?  I'm not sure why this is a question.
> 
> If you send a bazillion particles through the slits, you have roughly a bazillion chances for scattering.  If you reduce it down to one particle at a time, you reduce your chances of interaction to nigh zero, simply because the number of particles to interact with is near zero.
> 
> ...




The issue is how to reason about the motion of the particle after it passes through the double slits.  One chooses a mode of reasoning, then uses it to determine expected outcomes, then compares those outcomes with experimental results as a test of the mode of reasoning.

Then, there is an equation which describes the probability of a particle being at a particular location (for a certain class of measurements), and an amplitude (for a different class).

For a self intersection, how do we decide to use probabilities instead of amplitudes? We use amplitudes for the particle arriving at a detector.  We use simple probabilities for the particle interacting with itself.  Why?  What is different about these two types of events?

Edit1: Let me ask in this way: When a particle moves through a double slit, can we describe the motion as the particle moving simultaneously through both slits?  Or, must we say that a field describing the probable location of the particle moves through the slits simultaneously?

Edit2: The line of reasoning that led to the question of self-interaction was the possible interaction of two photons in free space, where the two EM fields have a chance to interact with a virtual electron.  The local description of that interaction: Two EM fields meeting in free space, seems exactly the same -- locally -- as the interaction of two segments of the EM fields of a single photon.

Thx!
TomB


----------



## Umbran (Apr 12, 2016)

tomBitonti said:


> For a self intersection, how do we decide to use probabilities instead of amplitudes? We use amplitudes for the particle arriving at a detector.  We use simple probilities for the particle interacting with itself.  Why?  What is different about these two types of events?




Well, note something - the amplitude is a *probability* amplitude.  Square the modulus of the amplitude, and you get a probability (well, probability density, at least).  

In fact, if we are going to talk about actual measurements, we eventually have to talk about probabilities - the probability of a thing arriving in and being seen by a detector.  The amplitude is a non-physical thing, and outright imaginary number.  In the basic double-slit, we eventually talk about the *probability* that the particle will strike a particular point on the screen.  In a scattering interaction, we discuss the "cross section" (basically, again, the probability) for the scattering to occur.  And, in both cases, before we can arrive at the probability, we work with the amplitude that we will have to square.


----------



## tomBitonti (Apr 12, 2016)

Umbran said:


> Well, note something - the amplitude is a *probability* amplitude.  Square the modulus of the amplitude, and you get a probability (well, probability density, at least).
> 
> In fact, if we are going to talk about actual measurements, we eventually have to talk about probabilities - the probability of a thing arriving in and being seen by a detector.  The amplitude is a non-physical thing, and outright imaginary number.  In the basic double-slit, we eventually talk about the *probability* that the particle will strike a particular point on the screen.  In a scattering interaction, we discuss the "cross section" (basically, again, the probability) for the scattering to occur.  And, in both cases, before we can arrive at the probability, we work with the amplitude that we will have to square.




Yes.  There is a careful consideration, though, of whether to add the amplitudes then square (interference) or if to square the amplitudes then add (no-interference).  For indistinguishable contributions, we add the amplitudes first.  For distinguishable contributions, we square first.  I'm fitting this to the virtual particle photon-photon interaction, and am using the double slit case to test my understanding.

Thx!


----------



## freyar (Apr 12, 2016)

tomBitonti said:


> The issue is how to reason about the motion of the particle after it passes through the double slits.  One chooses a mode of reasoning, then uses it to determine expected outcomes, then compares those outcomes with experimental results as a test of the mode of reasoning.
> 
> Then, there is an equation which describes the probability of a particle being at a particular location (for a certain class of measurements), and an amplitude (for a different class).
> 
> For a self intersection, how do we decide to use probabilities instead of amplitudes? We use amplitudes for the particle arriving at a detector.  We use simple probabilities for the particle interacting with itself.  Why?  What is different about these two types of events?



Actually, no, we also use amplitudes to describe the interactions of particles.  It's all quantum mechanics, and the calculation always proceeds by finding the amplitude and then squaring for the probability.  If you're familiar with Feynman diagrams, those are all amplitudes.  In fact, not all the calculations would be self-consistent if it weren't for specific cancellations and additions between the diagrams.



> Edit1: Let me ask in this way: When a particle moves through a double slit, can we describe the motion as the particle moving simultaneously through both slits?  Or, must we say that a field describing the probable location of the particle moves through the slits simultaneously?



Both, if I understand what you mean.  There are two equivalent mathematical ways to describe a particle in a double slit experiment.  One is to solve the Schrodinger equation (or appropriate relativistic generalization) describing the wavefunction in the experiment.  That automatically shows you the constructive and destructive interference.  The other method is to consider every possible path through the experiment (including weird discontinuous paths) with an amplitude assigned to each path, which you add up to find the interference patterns.  In a usual double slit experiment, the straight-line paths through the slits are the main contributions.



> Edit2: The line of reasoning that led to the question of self-interaction was the possible interaction of two photons in free space, where the two EM fields have a chance to interact with a virtual electron.  The local description of that interaction: Two EM fields meeting in free space, seems exactly the same -- locally -- as the interaction of two segments of the EM fields of a single photon.



I think the reason it seems that way is that you don't have the full mathematical description.  The wavefunction of two photons is not mathematically the same as the wavefunction of a single photon.  In fact, they don't even depend on the same number of variables.  Locally, it's still different


----------



## tomBitonti (Apr 12, 2016)

To followup:

A consequence seems to be that looking at an EM field as the superposition of many fields from individual photons is not a complete description of the EM field.

For example, if the EM field was from a laser (coherent; all of the photons are in the same state), then there should be no scattering interactions where the laser crosses itself, where-as, if the EM field is no-coherent (say, a narrow beam from the sun), there will be a scattering interaction.  Simply knowing the field strength and direction of travel doesn't seem to be sufficient.

Thx!
TomB


----------



## freyar (Apr 12, 2016)

tomBitonti said:


> Yes.  There is a careful consideration, though, of whether to add the amplitudes then square (interference) or if to square the amplitudes then add (no-interference).  For indistinguishable contributions, we add the amplitudes first.  For distinguishable contributions, we square first.  I'm fitting this to the virtual particle photon-photon interaction, and am using the double slit case to test my understanding.



I think maybe you're getting a couple of different ideas mixed up.  But the point is that two photons make a very different system than a single photon, as in my last response.  The coherent/incoherent addition of amplitudes here is something of a red herring if you want to know if a single photon passing through a double slit experiment (with or without a mirror) can bounce off of itself.


----------



## freyar (Apr 12, 2016)

tomBitonti said:


> To followup:
> 
> A consequence seems to be that looking at an EM field as the superposition of many fields from individual photons is not a complete description of the EM field.
> 
> For example, if the EM field was from a laser (coherent; all of the photons are in the same state), then there should be no scattering interactions where the laser crosses itself, where-as, if the EM field is no-coherent (say, a narrow beam from the sun), there will be a scattering interaction.  Simply knowing the field strength and direction of travel doesn't seem to be sufficient.




If you take a laser beam, leave it on continuously, and send it though mirrors so that it crosses its own path, one laser photon that's gone through the mirrors absolutely can scatter from a photon that has not gone through the mirror.  It would be an extremely rare event, but it can happen.


----------



## Scott DeWar (Apr 12, 2016)

freyar said:


> If you take a laser beam, leave it on continuously, and send it though mirrors so that it crosses its own path, one laser photon that's gone through the mirrors absolutely can scatter from a photon that has not gone through the mirror.  It would be an extremely rare event, but it can happen.




Would there be a significant event when the one "Old" photon (post mirror reflect) collides and scatters from th3e "new" (pre-mirror) Photon? Would this be the event of Photon meeting with the electron - positron shindig and the gamma ray hangover? (Pardon the humorous bent, please)

also, can someone tell me why pre-mirror was not in my spell check library, but pee-mirror was?? on a side note, I do not want to know what a pee mirror is, so don't tell me especially it is used to detect exotic matter.


----------



## Umbran (Apr 12, 2016)

tomBitonti said:


> For example, if the EM field was from a laser (coherent; all of the photons are in the same state), then there should be no scattering interactions where the laser crosses itself




As freyar has noted, there *can* be scattering interactions in this scenario.  They are rare, but can happen.


----------



## freyar (Apr 12, 2016)

Scott DeWar said:


> Would there be a significant event when the one "Old" photon (post mirror reflect) collides and scatters from th3e "new" (pre-mirror) Photon? Would this be the event of Photon meeting with the electron - positron shindig and the gamma ray hangover? (Pardon the humorous bent, please)



  Well, I'm not sure it would be significant enough to notice unless we looked quite carefully, but I think you have the right idea.  Two photons could "bounce" off each other or even create an electron-positron pair (if they have enough energy).



> also, can someone tell me why pre-mirror was not in my spell check library, but pee-mirror was?? on a side note, I do not want to know what a pee mirror is, so don't tell me especially it is used to detect exotic matter.



You have a pervy spell-checker?


----------



## tomBitonti (Apr 12, 2016)

freyar said:


> If you take a laser beam, leave it on continuously, and send it though mirrors so that it crosses its own path, one laser photon that's gone through the mirrors absolutely can scatter from a photon that has not gone through the mirror.  It would be an extremely rare event, but it can happen.




Didn't get back to that point in time.  A continuously firing laser will have a lot of different photon bundles (the wave front for a single photon and its coherent partners).  Then, when different bundles interact they can scatter.  But that is the same as scattering of non-coherent photons.  The case of interest is a bundle induced by a single photon.  Can that scatter with itself?  (Will different photons in the bundle take different paths on the outgoing side of the double slit?  I'm thinking yes, but it's something to think about more.)

Since a mirror is a problem, would an electron through an electric field also create a problem?  The goal is to get the streams from the two slits to cross with a large scattering cross section.  The streams overlap already, so this isn't necessary, just convenient to increase the cross section.

Thx!
TomB


----------



## freyar (Apr 12, 2016)

tomBitonti said:


> Didn't get back to that point in time.  A continuously firing laser will have a lot of different photon bundles (the wave front for a single photon and its coherent partners).  Then, when different bundles interact they can scatter.  But that is the same as scattering of non-coherent photons.  The case of interest is a bundle induced by a single photon.  Can that scatter with itself?  (Will different photons in the bundle take different paths on the outgoing side of the double slit?  I'm thinking yes, but it's something to think about more.)




I feel like I'm kind of talking across you or somehow I'm not getting my point over.  The point is this: a single photon cannot interact ("bounce") off itself.  In the case of the double-slit experiment, you seem to be worried that the photon can take multiple different paths and that the "photons" in the "different paths" hit each other, even though there is only one photon.  That's not the case.  In fact, if you have an experiment set up with a double-slit or mirrors or whatever, the way to _define_ a photon _includes_ all the interference patters from those "different paths."  It's not a photon scattering off itself. I'll put it another way: scattering requires that two different particles trade energy and/or momentum between each other.  That doesn't happen if there's only a single particle.


----------



## Umbran (Apr 13, 2016)

Or, to put what freyar is saying a different way:  The self-interference of the probability amplitude after it goes through the slits is *not equivalent* to an interaction between particles.  The self-interference is *not* akin to having the particle go through both slits and scatter "off itself".    

These are entirely different effects, and should not be conflated.


----------



## tomBitonti (Apr 13, 2016)

freyar said:


> I feel like I'm kind of talking across you or somehow I'm not getting my point over.  The point is this: a single photon cannot interact ("bounce") off itself.  In the case of the double-slit experiment, you seem to be worried that the photon can take multiple different paths and that the "photons" in the "different paths" hit each other, even though there is only one photon.  That's not the case.  In fact, if you have an experiment set up with a double-slit or mirrors or whatever, the way to _define_ a photon _includes_ all the interference patters from those "different paths."  It's not a photon scattering off itself. I'll put it another way: scattering requires that two different particles trade energy and/or momentum between each other.  That doesn't happen if there's only a single particle.




This: I fully believe that a photon doesn't interact with itself.  But I'm not particularly interested in that specific result, other that it arising from a false but interesting line of reasoning.  I'm much more interested in the reasoning used to reach the result.  (My background is pure math, with some depth in formal systems.  I rather like proofs.  To me, the form of an argument is as much an thing to be understood as the topic of the argument itself.)

From the interaction of two photons, what seems to happen is two EM fields interacting.  Looking only locally, the two EM fields don't look different than two parts of the field of a single particle.  Or don't seem to look different.  I'm missing something in my view that the fields look the same.

That is an "aha" moment for me.  Looking just at EM fields doesn't seem to capture enough information.

For example, the field of two photons which are in the same state seemingly might look the same as a field from one, looking at just a portion of the field.  This seems possible by confining the single photon to radiate in half of the angle of the two photons. This *seems* possible, but I might be missing something.

Then the difference between the two fields doesn't seem to arise until the entire fields are viewed, with the two photon field summing to twice the sum of the single photon field.

Thx!
TomB


----------



## freyar (Apr 13, 2016)

OK, I see the confusion.  Here is the short answer in which I grossly oversimplify.  The field of two photons is different than the field of a single photon: it is *bigger* (at any given point).  That's why we say classical fields are made of lots of photons.

The more involved and more correct answer is that the amplitude of the field does not have a definite value for a state with a definite number of photons.  This is similar to how you can't know both the position and momentum of a particle at the same time in quantum mechanics.  However, if you average the strength of the field (specifically the root-mean-square average), you will find that it gets bigger in states with more photons.


----------

