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Observers at light speed

  1. Aug 28, 2009 #1
    I am curious as to how a photon travelling at 'c' is described by an observer also travelling at 'c'.

    Elementary education tells us that photons, always travel at the same average speed in the same medium, regardless of the motion of the observer.
    However, if both were travelling at 'c', from point A to B, could the observed photon cannot arrive at B before the observer? I don't think it should, because then it would indicate that information could be transmitted to B faster than light.

    My only 'solution' to this comes from the lorenz contractions of time seem to suggest that in a photon's lifetime, from its own 'experience' the entire distance travelled on its journey (including A and B) are contracted as though it travels no distance at all. The observer in the example above, would also be subect to these contractions. Therefore travelling 0 distance at ANY speed would always take the same time (also 0 )

    How accurate is this hypothesis? Am I missing something or confused somewhere?
  2. jcsd
  3. Aug 28, 2009 #2


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    An observer cannot travel at c. More specifically, we cannot define an inertial frame travelling at c relative to another inertia frame. Therefore, your question is moot.
  4. Aug 28, 2009 #3
    Thanks for the response. The impracticalities of massive ojects moving at 'c' aside, I was merely curious as to the 'what if' scenario.
    Your statement regarding inertial frames, though, I think is more crucial and certainly would nullify the possibility of being able to provide a real answer. This helps a lot, because it ironically does answer te problem by highlighting its impossibility!
  5. Aug 28, 2009 #4
    Yes, it can travel at c or faster if c means the light velocity v in a transparent medium. In a transparent medium the light velocity v is smaller than c but particle velocity V can be equal or exceed v. In the latter case the particle arrives earlier than the light. Remember the Cherenkov's effect for charged particles.
    Last edited: Aug 28, 2009
  6. Aug 28, 2009 #5


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    What do you mean when you say "it"? Are you saying that you can define an inertial reference frame with a velocity v > c as measured from the lab frame?

    You should note that the OP specifically mentions observers travelling at c, rather than observers travelling faster than light. If the OP would have stated the latter, then of course it would be appropriate to discuss Chenkerov radiation. However, since the question was related to the former, Chenkerov is irrelevant here.
  7. Aug 28, 2009 #6


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    Basically, you are saying "If relativity were NOT true, what would it say about this situation"! And the answer, of course, it that if it were NOT true, it would NOT say anything meaningful.

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