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Problem of sumultaneity

  1. Jul 9, 2009 #1
    i don`t mean to be beating a dead horse but if every observer measures the speed of light to be C, doesn`t that make for the possibility of a given photon being in more than one place at once? I realize there can be a problem of sumultaneity which i don`t understand.
  2. jcsd
  3. Jul 9, 2009 #2


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    Re: c

    How are you getting the conclusion that a photon could be in more than one place at once? It can't, but I'm not sure how best to explain why unless you present your reasoning.
  4. Jul 10, 2009 #3
    Re: c

    I think what the OP is trying to say is if you're a photon going at the speed of light then light will travel away from you at the speed of light and so maybe the photon will travel faster than c?

    Anyways, this isn't true due to relativity. If you're in the photon's reference, light will travel away from you at c. However, if you are an observer, you'll just see the photon going at c. The photon isn't in multiple places, it's just dependent on reference.
  5. Jul 10, 2009 #4


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    Re: c

    There is no absolute "at once" for all observers. Simultaneity is relative. I guess that is your problem, but you have to describe your reasoning more accurately.
  6. Jul 10, 2009 #5


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    Re: c

    This concept doesn't really make sense. See my posts in this thread for an explanation.

    I think you're right about what the OP is thinking.
  7. Jul 10, 2009 #6
    Re: c

    i know i`m on the simple side but here goes - if a moving emitter sends a photon out and the photon moves at c away from the moving emitter such that the photon is always at c x time distance from the moving emitter, then that photon has a position at any instant. If another observer is moving in the opposite direction from the moving emitter and he also measures that same photon, then his c x time from his position to the photon would yield a different position for the photon. They`re both using c as the speed and using the same time so that should yield different positions for the photon. I must be haywire here. Sorry. Your help is appreciated.
  8. Jul 10, 2009 #7
    Re: c

    Well, since the emitter and the observer are going at different speeds relative to each other then they would measure different positions due to the time delay of relativity. If the emitter and the observer waited 1 second, then that one second would be different in either reference frame since they are moving at different speeds relative to one another.

    Though this difference may be extremely small, they are using this time to find a position of a photon going at c. If one was to factor in the time delay with respect to the other's movements, then both would measure the same position of the photon.

    I should note that this is without regard to the application of the Heisenberg Uncertainty Principle and the ability to measure a photon going away from you.
  9. Jul 10, 2009 #8


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    Re: c

    Measuring the speed of light means measuring speed.
    Speed is defined as distance divided by time.
    Time is passing at different rates in the two frames of reference experienced by the two moving observers. A second in one frame is longer than a second in the other, thus, the photon is measured as being farther from the emitter.
  10. Jul 10, 2009 #9

    Doc Al

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    Re: c

    That's true in the frame of the emitter, using that frame's distance and time measurements.
    No, the same photon has a single position. What's different are the coordinates of that position. That second observer will measure the photon's distance from him using his own distance and time measurements.
    No. Same position, just differently described.
  11. Jul 10, 2009 #10
    Re: c

    thank you much for your help.
    Another question is - how does cosmic time and cosmic position figure in. It seems to me that relativity emphasizes each point of view as being valid but isn`t there a cosmic point of view and time?
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