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Photon's frame of refernce

  1. Oct 15, 2009 #1
    Sorry if this has been asked, but I did a search and didn't find a relating topic.

    This thought came to me a few days ago when a friend asked me about time dilation. Time beats slower and distance is shorter for a moving reference frame with respect to a non-moving reference frame, and the amount of each is dependent upon the speed.

    So, for a photon that travels at c, would time not pass and would all space be a single point in the photon's moving reference frame with respect to the non-moving reference frame?
     
    Last edited: Oct 15, 2009
  2. jcsd
  3. Oct 15, 2009 #2
    Photons don't have reference frames. By postulate, photons travel at c in all reference frames. Thus there are no reference frames in which photons are at rest.
     
  4. Oct 15, 2009 #3

    JesseM

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  5. Oct 15, 2009 #4
    I've seen a few arguments on PF to the effect that a photon doesn't have a frame of reference where it is at rest. But the inability to provide a map, as these arguments go, from an inertial frame to a hypothetical frame of reference, is only weak evidence of nonexistance.
     
    Last edited: Oct 15, 2009
  6. Oct 15, 2009 #5

    pervect

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    We've really had this discussion several times. See for instance https://www.physicsforums.com/showpost.php?p=1254103&postcount=8

    The highlights, since I don't think you'll read the thread otherwise:

    But while a photon doesn't "experince time", it is possible to mark out regular intervals along its worldline.....

     
  7. Oct 15, 2009 #6
    How does that answer anything, pervect?

    The OP asks about the proper time of a photon,
    and asks how the measure of space in an inertial frame maps to the measure of space in the comoving frame of a photon,
     
  8. Oct 16, 2009 #7
    When I claimed above that the photon has no reference frame, I did not argue anything about maps. I pointed out that such a frame would directly contradict the second postulate. That should clearly end any debate, unless it was something other than Relativity you were wanting to talk about.
     
  9. Oct 16, 2009 #8

    Fredrik

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    He's also talking about the "photon's frame of reference" as if there's a standard definition of "this object's rest frame" that works for all objects including photons. This is certainly not the case. A good answer should explain the standard definition for massive particles and explain why it doesn't work for photons. I included such an explanation in the thread I mentioned in this quote:
     
  10. Oct 18, 2009 #9
    Fredrik. We've gone over this before, of course. thank you, I've reread your posts. You might also feel free to read my posts at the end of the same thread.

    One can spend a great deal of time showing how one cannot make sense of the question using some particular reasoning. But the best we can do, in support of the negative, is say that we cannot make sense of it, rather than sense cannot be made of it.

    If I become sufficiently motivated, I might go over my notes to see if I found a sensible answer to this.
     
  11. Oct 19, 2009 #10

    A.T.

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    I think It is OK to say that proper time doesn't pass for photons. I'm not convinced by the arguments why the concept of proper time is completely inapplicable to photons:
    Here you a linking the concept of proper time which is a measurable physical quantity with one particular abstract mathematical concept: the Minkowski spacetime. There are other geometrical interpretations than Minkowski's, like the space-popertime diagram, where the proper time of photons is simply zero.
    We also cannot place an observer at infinity, but will still use his proper time to define gravitational time dilation. And how do you know that photons are not clocks?
     
  12. Oct 19, 2009 #11

    Fredrik

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    I took a quick look at them, but didn't examine every detail. When you define your new variables a and b, you're really just saying that we take the photon's world line to be the time axis of a new coordinate system, and then you pick another straight line to be the spatial axis. This is of course fine, but you need to be aware of two things: a) the result isn't an inertial frame, and b) after you have chosen the time axis that way, the photon will be stationary in your new coordinate system, no matter what other choices you make in your definition (e.g. what curve to use as the spatial axis). So if you're going to say that a particular coordinate system is "the photon's rest frame" (rather than one of infinitely many rest frames for the photon), you're going to have to come up with a very good reason.

    I think that what's interesting is that neither the theory we're talking about, nor the standard definitions used by people working with it, has made sense of it. The possibility that some weird definitions might give this concept meaning isn't very interesting to me.
     
  13. Oct 19, 2009 #12
    Let us take a plane wave Acos(ωt-kx) in a transparent medium with n. The light velocity is c/n. What is the wave solution in a reference frame moving with V=c/n? Isn't it A'cos(k'x') ? And photon energy is not ћω'=0 but ћck'>0?
     
    Last edited: Oct 19, 2009
  14. Oct 19, 2009 #13

    Fredrik

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    I can tell that we have pretty different ways of thinking about these things. To me a photon is a mathematical concept defined by a theory, and the definition implies that a photon doesn't have the internal structure it would need to change with time. You're probably talking about "actual photons" in the real world, but I consider that an ill-defined concept, and therefore less interesting to talk about. Also, to me "proper time" is a mathematical property of a curve in Minkowski space, and special relativity is the theory that tells us how to interpret the mathematics of Minkowski space as predictions about results of experiments. If you use another "geometrical interpretation", I would say that you're talking about a different theory. It's an equivalent theory (I hope), but not the same theory, and I tend to interpret questions in the relativity forum as questions about the standard formulations of SR and GR, not as questions about other formulations, or even as questions about the real world (since the concepts we're talking about are defined by the theories we're using).
     
    Last edited: Oct 19, 2009
  15. Oct 19, 2009 #14
    Is it in theory impossible to construct an observer out of zero mass particles?
     
  16. Oct 19, 2009 #15

    Ich

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    It is possible in principle, as long as they go in different directions. That's equivalent to the system having rest mass.
     
  17. Oct 19, 2009 #16

    Dale

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    Woohoo, around and around in circles we go over and over and around and around again and again woohoo!
     
  18. Oct 19, 2009 #17
    Yes, instead of answering my questions.
     
  19. Oct 19, 2009 #18
    Maybe somebody mentioned this, but another reason why it's not meaningful to talk about a photon's rest frame is that if it were actually at rest, its momentum would be zero in that frame, so its rest mass would be zero. Then its energy content must be zero, and arguably it wouldn't exist at all. A photon is like a shark, it exists only in motion.

    There also is a Heisenburg Uncertainty problem, because if the photon's momentum is exactly fixed (at zero in its rest frame), its location must be entirely unbounded.
     
  20. Oct 19, 2009 #19
    And how about my question in post #12?
     
  21. Oct 19, 2009 #20
    Agreed.
     
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