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Speed of light and Relativity

  1. Jun 4, 2006 #1
    "Speed of light and Relativity- Do they hold in any media?"

    If I believe correctly, one of major considerations in relativity is 'c', being constant speed of light in vacuum. But, velocity in any media is different than vacuum for light. Speed in water is almost 75% of c.(Jean Foucault in 1850) now, v/c is refrective index of medium and is less than unity. Also the medium turns the light's direction while change in media.

    One more fact, Cherenkov effect, shows electrons in heavy water pool of pool type nuclear reactor travelling faster than speed of light. This tends to emission of photons.

    Now, this makes a series of questions in my mind:
    1. Does laws of relativity hold for vacuum only?
    2. How can law be established if vacuum is never measured to be perfact, u can just decide it to be almost zero probability of finding any atom in media.
    3. What changes velocity of light in a media?
    4. What do u predict for photons in above case coming out of even faster electrons? Does the inertia laws apply to them? read the effect in detail to know about emission's nature and suggest me the answer.
    Last edited: Jun 4, 2006
  2. jcsd
  3. Jun 4, 2006 #2
    The speed of a photon never varies, it always travels at exactly C whether it is propagating in a vacuum or in a medium. Actually the photon always travels in a vacuum else it is absorbed (annihilated) and then re-emitted. It is this absorption/re-emission process that is responsible for the apparent slowness of light when passing through a medium. A light beam consists of many photons. As the beam propagates through a medium, each individual photon generates an electromagnetic field. As the group of photons in a light beam propagates, each photon’s EM field combines algebraically with the others depending on the phase relationship of the each contributor. This produces a group waveform and an apparent group velocity. The group velocity usually appears to be slower than C even though the contributing photons are traveling at exactly C!

    Always hold.

    There cannot be a perfect vacuum.

    The absorption/re-emission process.

    The speed of a photon is always C, even if the emitting particle has velocity of almost the speed of light. As a massless particle, a photon has no inertia. Of course no one knows what causes inertia.
  4. Jun 5, 2006 #3
    @ Geniere

    Thanks for reply.

    Does that mean electromagentic field of all photons in one light beam can be cancelling each other at a time to become stationary and still all are moving at same speed of C?

    I am sorry, but I can not control my curiosity when it comes to such interesting things.


  5. Jun 5, 2006 #4
    one more thing

    One more thing, does the same thing make a light beam bend while change in media? Your reply concludes the air to absorb more or less photons than water. Does that mean absorbing photons is a tendency of media which depends upon density of media? And how the bending direction is decided?

    I am sorry, but I can not control my curiosity when it comes to such interesting things. Sorry if I am going too much off the topic for relativity but I feel that my ignorance to understanding fundamentals of time and light always keeps me away from understanding it completely.

    Thanks again.
  6. Jun 5, 2006 #5
    If two people are whistling the same note 180 out of phase, you cannot hear anything, but they are, never the less, still whistling. Just because the effect is cancelled doesn’t mean the sources are annihilated or in any way altered.
  7. Jun 5, 2006 #6


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    I've seen people use this explanation before, but I've seen others say it doesn't really resemble the explanation for why light slows down in a medium that would be given by quantum electrodynamics. Is there anyone familiar with QED who can comment?
  8. Jun 5, 2006 #7
    In the visible spectrum, the human body is opaque but it is transparent in the x-ray spectrum. The energy of the photon is a factor also. Remember a photon does not bend when passing through a medium, the bending is a combined effect of a multitude of photons.

    You need only remember that a photon always travels at exactly C. The effect of a medium on a group of photons (light beam) has nothing to do with relativity. These effects were not even considered by Einstein.
    Last edited by a moderator: Jun 5, 2006
  9. Jun 5, 2006 #8

    Meir Achuz

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    1. I would say yes. In a material medium, there is a preferred frame, the rest frame of the medium.
    2. All that says is that perfect vacuum is unattainable by man (or woman), but space (and even air) is a good enough vacuum that SR works fine.
    3. Maxwell's equations in the rest frame of matter give v=c/n.
    Max's eqs. don't work in a moving frame of matter. Some textbooks write LTs for D and H, but they would give the wrong answer if put into M's eqs for moving matter.
    4. Photons from the Cerenkov effect are normal photons.

    The model of photons being absorbed and reimitted is not consistent with the long wavelength of the photons. The motion is a wave phenomena.
    That model works for conduction electrons because their wave length is very short, so they can be considered to propagate classically between collisions.
  10. Jul 3, 2006 #9
    The reason a photon must travel at c is that it is massless.
    Every massless object must travel at c and every object that has a mass cannot possibly travel at c.
  11. Jul 3, 2006 #10


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    Why not? I know it's pretty much impossible to create a perfect vacuum on Earth, but I see no reason why it would not be possible to enclose an arbitrarily small volume of space such that there are no atoms in it.
  12. Jul 3, 2006 #11
    There could be areas without atoms but atoms are not the only kids on the block. And then, if I am not mistaken, there is the uncertainty principle which means that particles are created and destroyed using "borrowed" energy on the fly as it were, and there is no way to prevent or control that.
  13. Jul 4, 2006 #12


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    I woudl think that vacuum applies only to matter, though I suppose I could be wrong.

    I suspect you're thinking of virtual particles, which have nothing to do with HUP. I don't think they apply in a definition of vacuum.
  14. Jul 4, 2006 #13
    So only atoms are matter and other particles are not? :confused:

    So virtual particles cannot interact with photons? :confused:
  15. Jul 4, 2006 #14


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    They can, but presumably not in a way that slows the photons down like a real medium would. After all, virtual particles figure in any quantum field theory including quantum electrodynamics, but I'm pretty sure QED still says that light in a vacuum travels at c.
  16. Jul 4, 2006 #15


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    Yes Indeed! The physicists who developed QED were very proud that it was "manifestly covariant", obeying relativity completely from the git-go.
  17. Jul 4, 2006 #16
    Sorry but that is complete rubbish, in fact quite the opposite is true.

    For QED violates the principle of locality, which would invalidate GR completely.
    QED and GR cannot both be correct.

    The only thing that works in QED is SR, in fact it requires it.
    Last edited: Jul 4, 2006
  18. Jul 6, 2006 #17


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    At low (i.e. ordinary) energies, it's quite possible to quantize gravity as an "effective field theory". (I forget who pointed this out to me originally). See for instance


    Thus there is not nearly as much conflict between GR and QED as you assume, in fact one can even derive general relativity from a spin 2 quantum model (except possibly for issues of global topology).

    See for instance


    and Chris Hilman's comments on the later

    http://math.ucr.edu/home/baez/RelWWW/grad.html [Broken]

    Last edited by a moderator: May 2, 2017
  19. Jul 6, 2006 #18


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    According to the way "locality" is defined in quantum theory, I believe QED is considered a local theory, it obeys "causality" in the SR sense. See this thread. Entanglement can never be used to transmit information faster than light, and in terms of your interpretation of what's "really" going on, I think it's possible to explain entanglement without any nonlocal effects in the many-worlds interpretation of QM.
    Last edited: Jul 6, 2006
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