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Photon Speed Same Always?

  1. Oct 6, 2007 #1
    Photon Speed Same Always??

    Hi there. I read the following in a wikipedia article (http://en.wikipedia.org/wiki/Cherenkov_radiation):
    Can anyone help me out to make sense out of this statement? The speed of a light wave [tex]v = c/n[/tex] in a medium is DIFFERENT that what it is in free space. Then how can the speed at which the photons travel be the same??
  2. jcsd
  3. Oct 6, 2007 #2
    Interesting. So if I were to see you through an aquarium, it would take longer for your image to reach me because the light is bouncing within the medium...? Wouldn't the image be lost? This doesn't make sense to me either.

    Hasn't it been proven that light can accelerate, that it can bend around the sun... why is everything suppose to be a constant? Isn't everything suppose to be relative? What if light, electrodynamically speaking is affected by the strength of the electromagnetic fields of its surroundings? What if air, and vacuum are one extreme, and water and such are another extreme. Couldn't the electromagnetic pressures between the molecules of water cause light to slow down...

    Anyways... I still have a lot to learn.
  4. Oct 6, 2007 #3
    Maxwell's equations imply the existence of electromagnetic waves (waves composed of varying magnetic and electric fields) which travel, in a vacuum, at a constant speed - related to the electric and magnetic properties of a vacuum (permeability and permattivity).

    This was initially thought to only hold in a certain reference frame (the reference frame of the ether - the medium in which light would propagate), but experimental evidence (e.g. Michelson-Morley experiment) showed this hypothesis isn't true.

    Einstein proposed that the speed of light (in a vacuum) is the same in every inertial reference frame, and this has held up against experiment. In General Relativity Einstein proposed that space-time is curved by (gravitational) mass.
    So while the light's path is affected by the sun, it can be thought of as the sun bending space-time rather than the light accelerating.

    This is the part where I'm not so confident, but I'll give it a shot. In a medium such as water the light is still traveling at the same constant speed, the speed of light, but the water molecules affect the light - the light interacts with electrons (and to a lesser extend the nuclei) of the molecules, being scattered and consequently is no longer traveling in a straight path, so appears to go at a speed less than c. In fact the difference is characteristic of the medium (at a particular wavelength) which is why we have a refractive index - the light always appears to travel at v=c/n.

    However in Quantum Electrodynamics the photons can travel in any path in space-time, so as I understand it there is a small, but non-zero, probability light really travels faster/slower than c. I'm not sure about this last part though, and would like it clarified if anyone knows more.
  5. Oct 6, 2007 #4


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    Staff: Mentor

    See post #4 in the Physics Forums FAQ.
  6. Oct 7, 2007 #5
    I read post 4, thanks, it is pretty good- I can almost buy it. The problem that i have is that if the medium is interferring with the continuous motion of light through it, and if it is doing so by absorbing and emitting, regardless of whether it is the atomic spectra or the phonon layer of the material- how the H does it re-emit a coherent beam with well defined boundaries.... For instance, I can still see your image through a meter of glass. And through that meter of glass, light takes longer to reach me than it does if there was no glass.

    The only solution that seems reasonable is that there is a pressure of electromagnetic field density that impedes the light as it traverses through the material= that is only my opinion...
  7. Oct 8, 2007 #6

    Claude Bile

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    The presence of a medium is not as intrusive as you might think to photon propagation.

    Provided the frequency of a given photon does not match a resonance of any sort (and thus WON'T be absorbed), the EM field acts to polarise the atoms that it is travelling through. It is this polarisation response that slows down photons in matter, and also causes nonlinear optical effects at very large field amplitudes.

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