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Speed of light question

  1. Sep 12, 2009 #1
    Hi all,
    I know there is a postulate saying "The speed of light is the same to all observers in uniform motion", but I have a question about this:
    What if the observer is moving at the speed of light and measuring its own speed. The result should be zero, right? I can't imagine how something could be moving faster than itself.
    Any replies appreciated.
  2. jcsd
  3. Sep 12, 2009 #2

    Doc Al

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    Staff: Mentor

    Any observer measures his own speed as zero with respect to himself. But no observer can move at light speed with respect to something else.
  4. Sep 12, 2009 #3
    The general Concept is:

    "If a man chases an escaping light beam at speed very very close to that of light, He still finds the light beam escape from him with the same velocity as it would have escaped if he didn't chase it but simply stayed at the same place. The same velocity is the universal constant c"

    Read the above paragraph again if you don't get it, I have worded it carefully.
  5. Sep 12, 2009 #4
    I understand that, but what is the speed of a particle of light with respect to itself?
  6. Sep 12, 2009 #5


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    We would have to define it to be 0. Any other definition would be absurd. Note however that it is something we would have to define, since there's no obvious way to associate a coordinate system with the motion of a massless particle. See my posts in this thread for more on that.
  7. Sep 12, 2009 #6
    Thank you very much. It's as clear as it will ever get, I suppose.
  8. Sep 12, 2009 #7
    These statements can be misleading to a beginner in SR. When you make a statement about speed or velocity, you must say what it is relative to. Also, the statement "stayed at the same place" is meaningless. It implies somehow that someting is not moving. Whether or not something is moving is meaningless. You can only say that something is moving or not moving relative to something else.

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