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Non-Spooky Action at a Distance

  1. May 17, 2014 #1
    I think this is a relativity/frame of reference discussion and not QM but if I've got it wrong perhaps a mod can home it where it belongs, thanks.

    I've got an electrical engineer's background in physics from 30+ years ago and am totally rusted out on the math. My quest is driven by never being satisfied with the Copenhagen perspective on "spooky action" and other topics, so I've continued to read and think on this topic. I recently re-read the Carver Mead "Spectator" interview, which led me to John Gribbin's "Schrodinger's Kittens", and now my mind is totally blown! The following concept, quoted from Gribbin, has been staring me in the face and I never "got it" before, regarding Einstein's quest to understand how the Universe "looks" to a photon, and how does time flow for a photon?:

    "To answer the second question first - it doesn't. The Lorentz transformations tell us that time stands still for an object moving at the speed of light. ... And under such extreme conditions, the Lorentz-Fitzgerald contraction reduces the distance between all objects to zero. You can either say that time does not exist for an electromagnetic wave, so that it is everywhere along its path (everywhere in the Universe) at once; or you can say that distance does not exist for an electromagnetic wave; so that it touches everything in the Universe at once."

    BOOOSH! <--- sound of my head exploding

    So here's my thoughts/observations/conclusions on the above, and I'm hoping to find someone(s) with greater physics insights to talk through this with me:

    1. From a photon's perspective/frame of reference (FOR) a photon doesn't spend any travel time between emitting electron (emitter) and absorbing electron (absorber) - it's instantaneous. It also doesn't travel any distance.

    2. From #1 it must also be true that an emitted photon "knows" where it's going to be absorbed. It's not shot out of a cannon and waiting to see what it hits - the absorber is identified during the same instant the photon is emitted.

    3. In any two-slit etc. experiment likewise, from the photon's FOR, it "knows" which electron is going to absorb it the moment it is emitted, and the path isn't "through" any slit or anything else, since it "touches everything in the Universe at once" including the absorbing electron.

    4. All the spooky action / how can information travel superluminally etc paradoxes etc are FOR observation issues. Why it appears, in our FOR, that a photon takes 150 million years to travel to us from a distant star is interesting, but from the photon's FOR the journey was instantaneous. If two electromagnetic particles are able to be entangled and appear to communicate faster than C, it's because they touch everything in the Universe at once.

    5. In our FOR if I suddenly hold up a mirror to reflect a photon that travelled 150 million years to get to earth, it's still the case that from the photon's FOR that the journey was instantaneous, even though the mirror did not physically exist when (from our FOR) the photon appears to have been emitted. So the absorbing electron in the mirror might have been inside some ice in a comet 150 million years ago, found it's way to earth, found it's way into a mirror-making factory and into my hand. Did the photon maintain a "relationship" with the absorbing electron for 150 million years, knowing where it would be by the time it reached earth? (I'm mixing FORs here, not being as precise as I'd like)

    6. There are many possible explanations of Gribbin's "time does not exist for an electromagnetic wave." Here are some - for a photon:
    -time does not exist
    -it's every time all the time
    -it's any time all the time
    -time is stopped

    7. Maybe an easier physical way to think about entangled particles is that entanglement creates a spacetime warp between them - such that the particles are touching one another and in the same FOR.

    8. Since from a photon's FOR we are the ones moving at the speed of light, how would all these experiments look to an experimental physicist riding on a photon?

    Help me put my head back together again, please.
     
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  3. May 17, 2014 #2

    phinds

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  4. May 17, 2014 #3
    That's an interesting position, especially since it was Einstein's puzzling over what would happen if you could ride alongside a beam of light, at the same speed that light was moving, that led to his development of special relativity - and to have that kind of thinking dubbed "not meaningful" in an actual Special Relativity forum is just priceless irony.
     
  5. May 17, 2014 #4

    PeterDonis

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    You apparently haven't grasped the actual content of Einstein's thought experiment. What he actually found was that, when he asked himself what would happen if he could move at the same speed that a beam of light was moving, the result (a standing electromagnetic wave in space, unchanging with time) was *not a solution of Maxwell's Equations*. In other words, the thought experiment was based on a false premise: it's actually inconsistent with electrodynamics to assume that you can construct a frame of reference in which a photon is at rest. *That* understanding was what Einstein incorporated into special relativity, and is what the FAQ phinds referred to is based on.
     
  6. May 17, 2014 #5

    PeterDonis

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    You left out the correct explanation: for a photon, the concept of "time" is meaningless. "Time does not exist" comes close, but isn't quite right because it still tacitly assumes that there is something meaningful that "does not exist" for a photon. There isn't.

    Sure, glad to. :wink: All of the descriptions you gave for experiments involving entangled particles are non-relativistic (leaving out the issue with the concept of a "photon FOR" not being meaningful--I'm talking here about things like "spooky action at a distance", the wave function of the photon "being everywhere in the universe at once", etc.). None of them are relativistically correct. The correct relativistic description of these experiments (and indeed any relativistic quantum experiments) requires quantum field theory.

    In QFT, there are no such things as "wave functions" for photons or anything else. There are quantum fields and operators, and operators are tied to particular events in spacetime. The key idea you need to interpret all these experiments is that operators at spacelike separated events commute. In other words, the results of measurements done at spacelike separated events must be the same regardless of which order the measurements are made in. More precisely, there simply is no fact of the matter about which measurement "happens first", or whether both "happen at the same time". The only fact of the matter is that the results commute.

    As for the correlations between measurements of entangled particles, they can't be used to transmit information. The results of individual measurements satisfy all the criteria for being random; the information about the correlations can only be collected in the ordinary way, by communicating (using signals limited to the speed of light) results of measurements made at spacelike separated events to some common event in the future, where the results can be compared.

    That still leaves the question of what underlying mechanism makes the entanglement correlations possible. However, this is only a problem for certain interpretations of QM, the ones which include wave function collapse. (Gribbin is, IIRC, a fan of the "transactional interpretation" of QM, in which the ultimate underlying mechanism is signals propagating both forward *and* backward in time along lightlike paths between measurement events.) For no-collapse interpretations, e.g., the many-worlds interpretation, there is no issue at all; the correlations naturally emerge from the unitary evolution of the various Everett branches. (Of course, that may make your head explode again--sorry. :wink:)
     
  7. May 17, 2014 #6
    You're confirming that what I said WRT the thinking that led to special relativity was actually very meaningful, even if it exposed a faulty premise - thus ironic in this forum compared with the idea of it being "not meaningful." On the contrary it was very meaningful.

    But to the heart of the matter, my understanding from Gribbin is that the standing wave you reference led Einstein to observe that there is no need to invoke a preferred frame of reference. There does not have to be a standard of rest in the Universe against which all velocities are measured. All motion is relative - which means that anybody is entitled to say they are at rest, and to measure all motion relative to themselves.

    So given the above, I don't see how a photon is ruled out of being viewed as being at rest.
     
  8. May 17, 2014 #7

    PeterDonis

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    You're mis-stating what phinds said was "not meaningful". He didn't say Einstein's thought experiment was not meaningful. He said the concept of "the frame of reference of a photon" was not meaningful. It's quite true that Einstein's thought experiment was what *led* him to the conclusion that "the frame of reference of a photon" was not meaningful, so that the thought experiment itself certainly *was* meaningful; but phinds wasn't denying that.

    Can you give a specific reference to Gribbin here? It looks to me like either you're misinterpreting what he said, or he's mis-stating what Einstein's thought experiment said.

    The standing wave thought experiment wasn't what led Einstein to say that there is no preferred frame of reference; that was already a well-established concept in Newtonian physics. The problem was that the Newtonian equations for the behavior of mechanical objects, based on there not being a preferred frame, had *different* invariance properties than Maxwell's Equations; Newton's equations were Galilean invariant, whereas Maxwell's Equations were Lorentz invariant.

    So the real question was, which kind of invariance should "win"? Which set of equations should be adjusted? Many other physicists thought Maxwell's Equations were the ones that should be adjusted. But Einstein's standing wave thought experiment led him to conclude that it was *Newton's* equations that should be adjusted--that *mechanics* needed to be reformulated to be Lorentz invariant instead of Galilean invariant. *That* is what led him to special relativity.

    Because if it were at rest, it would be a purely spatial standing electromagnetic wave, with no time dependence, and that is not a solution of Maxwell's Equations. Every possible state for a photon must be a solution of Maxwell's Equations.
     
  9. May 18, 2014 #8
    Yes that's how I understood him. Einstein's thinking about the concept of "the frame of reference of a photon" is what led Einstein to special relativity. And if I understand the rule in this forum, that thinking would not be permitted here, thus it is ironic.


    Yes I'll give a few quotes here that address the points being discussed. All are from "Schrodinger's Kittens", page numbers are from the paperback version I have.

    p75 "Einstein's great gift was his genius for physical insight into what a problem was really all about. Mathematics was never his strongest point, although he was certainly stronger at maths than most people, but he had a great feel for physics. The insight that led him to the special theory was based on his sound physical intuition about what Maxwell's equations were really saying. He puzzled over what would happen if you could ride alongside a beam of light, at the same speed the light was moving. para Remember that the nub of Maxwell's equations is that a changing electric field produces the (changing) magnetic part of the wave, and the changing magnetic field produces the (changing) electric part of the wave. But if you were moving at the same speed as the wave, it would not be 'waving', from your point of view, at all. It would be stationary, like a wave on the sea frozen into ice before it could break. And Maxwell's equations quite clearly said that a stationary magnetic field would not make an electric field, and nor would a stationary electric field make a magnetic field. There would simply be no wave at all - not even a frozen one."

    "Einstein saw that there is no need to invoke a preferred frame of reference at all. There does not have to be a standard of rest in the Universe against which all velocities are measured. Instead , he said that all motion is relative - which means that anybody is entitled to say that they are at rest, and to measure all motion relative to themselves."

    p.79 "So, what happens when we push this time-dilation business to the limit? Getting back to the original question that Einstein asked about light, how does the Universe 'look' to a beam of light (or photon, if you prefer), or to a person riding on a light beam? And how does time flow for a photon? para To answer the second question first - it doesn't. The Lorentz transformations tell us that time stands still for an object moving at the speed of light. From the point of view of a photon, of course, it is everything else that is rushing past at the speed of light. And under such extreme conditions, the Lorentz-Fitzgerald contraction reduces the distances between all objects to zero. You can either say that time does not exist for an electromagnetic wave, so that it is everywhere along its path (everywhere in the Universe) at once; or you can say that distance does not exist for an electromagnetic wave, so that it 'touches' everything in the Universe at once. para This is an enormously important idea, which I have never seen given due attention. From the point of view of a photon, it takes no time to cross the 150 million km from the Sun to the Earth (or to cross the entire Universe), for the simple reason that this space interval does not exist for the photon."

    Gribbin (see above quotes) says above that there would be no wave at all - not even a frozen one. My guess is that zero EM wave is a solution of Maxwell's Equations.
     
  10. May 18, 2014 #9

    PeterDonis

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    It's certainly permitted to talk about Einstein's thought experiment. It's not permitted to talk as though the concept of "frame of reference of a photon" is actually valid (as opposed to a hypothesis that gets refuted), when Einstein's thought experiment showed that it isn't.

    But he says there *would* be a stationary electric field and a stationary magnetic field. That would require charge and current density to be present, but by hypothesis they're not--we're talking about a region of space that is vacuum except for the EM wave.

    A field that is identically zero is certainly a solution--but that solution equates to nothing at all being present, not even a wave, in *any* frame.

    A nonzero stationary field is a solution, but it's not a vacuum (source-free) solution--it requires charge and current density to be present, as above, which violates the original conditions of the thought experiment (that we are talking about an EM wave in vacuum).

    Some other comments on the Gribbin quotes:

    As I noted in a previous post, Einstein was by no means the first person to say this. Galileo said it more than two centuries earlier. So did many others. By Einstein's time, the principle of relativity was a well-established part of Newtonian physics. The problem, as I said before, was that the Newtonian principle led to Galilean invariance, whereas the principle in electrodynamics led to Lorentz invariance.

    No, they don't. The Lorentz transformations are mathematically undefined for ##v = c##. You can't conclude anything from a mathematically undefined operation. Gribbin's whole discussion here is wrong for this reason.
     
  11. May 18, 2014 #10
    Yes and is there any evidence that Einstein changed his view away from the idea that "all motion is relative - which means that anybody is entitled to say that they are at rest" (including a photon). I've not been able to find it. And Gribbin explicitly expresses the idea of a photon at rest while being well aware of the invariance issues.

    Well, please don't take any insult, but faced with conflicting assertions in an area where I don't have the depth to sort it out on my own, I'm going to stick with Gribbin.
     
  12. May 18, 2014 #11

    PeterDonis

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    No, of course not. But you mistakenly included an additional qualifier:

    That was never Einstein's view to begin with. He used his standing wave thought experiment to *test* whether the standard view in Newtonian mechanics that all motion is relative could be applied to a photon. He found that it couldn't. But that wasn't a "change" in view because the idea of applying the principle of relativity to photons had never been part of the principle before.

    The invariance issue isn't what makes the idea of a photon being at rest invalid. What makes it invalid is the fact that the Lorentz transformation is undefined for ##v = c##.

    Of course I don't take any insult; only you can decide what you want to believe. But you should be aware that this is not just a conflict between my assertions and Gribbin's.

    First of all, the fact that the LT is undefined for ##v = c## is easy to check for yourself. Have you done so? If you don't feel comfortable doing that, then, with all due respect, you should focus on fixing that before even trying to interpret what *any* book about relativity physics is saying.

    Second, I'm certainly not the only one asserting that the idea of a photon being at rest isn't valid in relativity physics. You've already been pointed to the PhysicsForum FAQ that says the same thing, so it's not just me, it's all of the experts here on PF (many of whom have a great deal more experience in relativity physics than I do). Check out any of the standard textbooks in the field and see what they say; you'll find that they say the same thing I'm saying. (Gribbin's book, btw, is not a textbook or a scientific paper; it's a pop science book. Pop science books are *not* good sources to use if you actually want to learn about physics.) Or check out other online sources like these:

    http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/headlights.html
     
  13. May 18, 2014 #12

    Dale

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    It seems rather pointless for you to have started this thread if you never intended to listen to the answers.

    Thread closed.
     
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