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Understanding the speed of light

  1. May 4, 2010 #1
    1. is it appropriate to infer that, for a photon, time and distance do not exist?

    2. if so, is it therefore appropriate to infer that once a photon is emitted, it's wave function permeates the entire universe immediately?

    3. and if so, does our measurement of the "speed of light" at a fixed rate of C imply something peculiar about our own reference frame, ie that our measurement of C may be more reflective of the time required for the wave function to collapse, or some odd aspect of our ability to "measure", rather than the time required for a photon to travel a given distance?
  2. jcsd
  3. May 4, 2010 #2


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    Yes, I guess you could say that.

    No, for many different reasons. The main reason being that there is no such as a photon wavefunction (at least not in the usual sense); photons are not "particles" as such. It is perfectly possible to confine a generated photon using e.g. a cavity.
  4. May 4, 2010 #3
    Well… I would moderate that given the beautiful experiments of S. Haroche where he does just that : trap a photon in a cavity :smile:
  5. May 4, 2010 #4
    I think it might be useful to point out that time dilation and length contraction only apply to things which have mass. since a photon is massless, it doesn't experience either time dilation or length contraction.
  6. May 4, 2010 #5

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    jnorman: Do note that f95toli begrudgingly said "Yes". A better answer is that your question is nonsensical. That is not meant to belittle you; you are trying to come to grip with some weird concepts.

    By way of analogy, I suspect you have seen various "proofs" that 0=1, 1=2, etc. There is almost always a division by zero hidden somewhere in these proofs. Dividing by zero is a nonsensical concept in the sense that division by zero leads to nonsense results. Because of this dividing by zero is not allowed. All that it takes to shoot down a proof as invalid is to show that some step involves a division by zero.

    Back to the problem at hand: Asking about questions about how things look from the perspective of a photon is nonsensical is precisely because of division by zero.
  7. May 4, 2010 #6
    Photon has a wavefunction. It also is a particle, no less than anything else. It carries momentum, it interferes, it can hit an electron and can do virtually everything wavefunction or particle can do.
  8. May 4, 2010 #7
    Not exactly… It is rather similar to a quasiparticle, like a phonon… :smile:
  9. May 4, 2010 #8


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    No it doesn't. The properties you list have nothing to do with whether or not you can write down a wavefunction for a photon.
  10. May 4, 2010 #9
    What is necessary to have a wavefunction then? For me if it quacks like a duck, then it is a duck.

    I don't see a strict line between quasiparticles and "real" particles. On the other hand: all particles currently known are just quasiparticles if Higghs mechanism is correct. Except photon :), it's unaffected by Higgs.
  11. May 4, 2010 #10


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    The "point of view of a photon" is an ever-popular topic next door in the relativity forum. There's a thread about it going on right now, in fact:

    If I was light...

    You might like to check out that thread and similar ones that we have had in the past.
  12. May 4, 2010 #11
    Last edited by a moderator: Apr 25, 2017
  13. May 4, 2010 #12
    It depends what you mean by “wavefunction”.

    If you mean “something which obeys a Schrödinger equation”, you can define such a thing for a photon.
    If you mean “something which gives the probability density of finding the photon at a certain point in space”, you cannot define such a thing for the photon.

  14. May 4, 2010 #13
    Photon wavefunction in a Schrödinger sense gives me interference pattern in double-slit experiment. So I can just claim that the intensity of light is the probability of finding photon particle at some point.

    I don't see much difference to any other particle's wavefunction, neither mathematical nor physical.
  15. May 5, 2010 #14
    Nope, that's not correct :smile:
    Have a look at this reference : Iwo and Zofia Bialynicki-Birula “Why photons cannot be sharly localized”, PRA 79, pp. 032112 (2009).
    As one say : the title says it all :biggrin: They tried, and succeed in a sense, to construct a wavefunction for the photon which can be used to spatially localize it. Two problems : 1) you can either localize the electric or magnetic character of the photon, not both. 2) upon time evolution, this localization is lost at the speed of light… Talk about localization… :biggrin:
    Last edited: May 5, 2010
  16. May 5, 2010 #15
    i think the original question was, 'does a photon experience the passage of time - since it travels at the speed of light.' i think the answer is yes, a photon experiences the entire year of a 1 lightyear trip. the reason being, that relativistic time dilation doesn't apply to a massless object.
  17. May 5, 2010 #16


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    That's not the correct conclusion. See the thread that jtbell linked to.
  18. May 5, 2010 #17
    A photon can't have an Anti-Symmetric wave function. It requires an Anti-Symmetric wave function to have point particle like nature from what I understand.
  19. May 6, 2010 #18
    This is available here on arXiv as well:

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