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Non moving photon

  1. May 27, 2009 #1
    I am new and so is my interest in quantum mechanics. I am curious to know if all photons have motion? and if a photon is not in motion is it detectable?
  2. jcsd
  3. May 27, 2009 #2


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    Photons always move at the speed of light (being as they ARE light), approximately 300,000,000 meters per second.
  4. May 27, 2009 #3
    I dunno. Is it there if you don't measure it, or is it there when you measure it?
  5. May 28, 2009 #4
    A photon is always in motion relative to any observer.

    Under most other circumstances, one can choose a frame of reference and "coast along" with an object, such as a fast moving space ship; not so at the speed of light where only massless objects (such as photons) can go.
  6. May 28, 2009 #5
    If a photon can only be observed because it is moving, then how would it be possible to know if it were in a non moving state? also, is it assumed that it is always in a moving state or is it a proven fact?
  7. May 28, 2009 #6
    does the uncertainty principle somehow tie into that?
  8. May 29, 2009 #7
    another of those intractable basics - from our perspective, photons "travel" at C, but from the photon's perspective, there is no travel, because at C, distance and time do not exist - once a photon is emitted, it is essentially everywhere in the universe simultaneously - hence the "all possible paths" description.
  9. May 29, 2009 #8
    I think it is simply a property of the object we call a photon. It just so happens that experiments agree with this 'definition' of the photon, so there is no need (at this time) to assume that it would be false.
  10. May 29, 2009 #9
    Don't want to quibble, but wouldn't make more sense to say that they move at the speed of light because they are massless -- not because they are light.
  11. May 29, 2009 #10
    A photon or an electromagnetic wave in a transparent medium may travel with v < c. Any massive particle can travel also with v < c. So they can co-travel together. In a moving reference frame such a wave is a just standing wave. If it is sufficiently intense, it can ionize a co-travelling atom, for example.

    Particles in medium can travel faster than light (Cherenkov's effect for charges, nothing for neutral particles).

    If photon travels in a free space with c, a fast (v→c) accompanying particle will see it moving with c anyway but with a very low frequency.

    Last edited: May 29, 2009
  12. May 29, 2009 #11


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    It is a basic concept of relativity (not quantum physics) that a photon moves at 300,000,000 m/s relative to any frame of reference. And relativity has an enormous number of confirming experiments.
  13. May 31, 2009 #12
    The only thing i've heard of in the sense of slowing down photons is that a bunch of scientists manged to slow light down to less than 1mph using some sort frozen phosphorus or something like that, but incase you mean photons in empty space then my anwser is no light always moves at (c)
  14. May 31, 2009 #13
    You can slow light down, but not the individual photons. They still move at c. Even in a medium where the speed of light is less than c. Only in a vacuum is the speed of a lightwave determined by the speed of photons. In a medium it is the properties of that medium that largely determine the speed of lightwaves. The photons are absorbed and re-emitted many times, which causes the 'delay' and slower net speed.
  15. May 31, 2009 #14
    What a rubbish!!! A transparent medium works as an effective vacuum for photons - it does not change the photon direction. Absorption and re-emission lead to diffusion and losses like in fog, which is not case for a transparent medium.

    Other material particles are also de Broglie waves. They can travel even faster than photon in a medium. Are they absorbed and re-emitted too?

  16. May 31, 2009 #15
    So you are saying that photons travel less than c in a medium, where c is the speed of light in a vacuum?
    I have always heard otherwise...?

    If a particle moves faster than a photon in a certain medium, isn't that a violation of special relativity?
  17. May 31, 2009 #16
    1) Yes, v=c/n, where n > 1 is the medium refraction index.

    2) No, it is not since particle velocity is always smaller that c but may be higher than c/n.

  18. May 31, 2009 #17
    I know that. I am in doubt however if the v in your equations refers to the speed of a light-wave, or to the speed of individual photons! There's a big difference!

    I thought the main postulate of relativity was that a photon always moves at the speed of light, no matter in what frame of reference you look at it. If you are moving faster than a photon, you can look at the photon but there would be no way you saw it moving at the speed of light... Unless it goes back in time or something...
  19. May 31, 2009 #18
    From our very own Physics FAQ (post #4):
    So, maybe I was wrong that photons are absorbed/emitted by atoms. Rather they are absorbed by the material as a whole, eg the lattice.

    So I still believe that photons travel at c, always.
  20. May 31, 2009 #19
    Radio waves are much longer than the lattice step. The photon slowing down in the medium transparency window is of course a collective effect but the light wave is not different from one photon if the wave is not so intense (no non-linear medium polarization effects). The medium serves as an effective vacuum.

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