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Question about photons and relative velocity.

  1. Dec 25, 2003 #1
    From the reference point of a photon, at what velocity are the photons around it traveling at?

    I'm asking it because doesn't light ALWAYS travel at light speed, no matter what the reference point? Does that principle only apply to things that are not photons or gravitational disturbances, or does it apply to absolutely everything?

    I guess this is a rather dumb question, but confusing to me nonetheless.
  2. jcsd
  3. Dec 26, 2003 #2


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    It's not clear to me what you mean by "photons around it", but the general answer to your question is that, yes, all photons have velocity c relative to any (inertial) coordinate systeml. However, since the time coordinate is singular at light speed, one photon does not "see" other photons moving away from it, even though they have velocity c relative to it.
  4. Dec 27, 2003 #3
    Suppose that an atom at rest emits two photons simultaneously, but in opposite directions, and then one second later, the same atom emits two more photons in the same direction. Thus, looking at things from either side, we have two photons which are 299792458 meters apart, and the distance they are apart isn't varying in the atom's frame. Now switch to the photon's frame. In that frame the photon's do not move relative to each other either, rather they stay at some fixed distance apart, but if the theory of relativity is correct, then this photon frame which you have switched to is not an inertial reference frame, hence the relativity postulate that the speed of light is equal to c in any inertial reference frame is not violated.

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