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The problem of simultaneity

  1. Jul 17, 2008 #1
    recently i read about the embankement-train thought experiement, however i find it diffcult to understand
    supposer the right end of the platform is A and left B, a train of same length as platform travels from B to A direction.
    In the reference frame of the platfrom, a observer in the middle M sees two light arrive simultaneously from A and B, so he measeure his distance from A and B and conculde he is in the middle, therefore the lightning is simultaneous.
    Also from his frame(platform), and the two light pulse travels at same speed toward observer 2 on the mid point of hte train M' as the trains front and back are aligned with the platform. Since he sees obervers 2 moving towards A, he will conclude that observer 2 will not see the events as simultaneous.

    However the problem is, since the notion of simultaneous does no take into account the time taken to travel from the event to the observation point(ie that time lights' travel is subtracted from time seen).
    in the statement Since he sees obervers 2 moving towards A, he will conclude that observer 2 will not see the events as simultaneous. the first oberver has not taken into account that oberver 2 has to subtract the time taken for light to travel to him(observer 2)

    What actually does the observer 2 see on the train?
    When light reach him from point A and B, the train is not aligned with the platform, so when he subtracts the time it taken light to travel from A to him and B to him, what does he observe, and what is the space time coordinate (x,y,z,t) for the two events????
  2. jcsd
  3. Jul 17, 2008 #2


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    This is accurate. It does not mean that that the observer cannot calculate the whole scenario and express it from different frames. 'Seeing' in this context just means that the light reaches the observers eyes. If the two light beams are seen by the platform observer to arrive at the same time, he will correctly conclude that they will arrive at different times for the observer on the train.
  4. Jul 17, 2008 #3
    he indeed conclude they arrive diff time for observer, however
    what does the observer on train see, and what time will he conclude for the lightning strikes at A and B when he subtractes the time taken to travel from the time the light reaches him respectively
  5. Jul 17, 2008 #4


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    If the light from each flash reaches the observer at the center of the train at different times, and yet he knows he's an equal distance from each flash, then if he assumes the light moved at the same speed in both directions, he must conclude the flashes happened at different times in his frame.
  6. Jul 17, 2008 #5
    why is it that he is equal distance from flash?
  7. Jul 18, 2008 #6


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    That's just an assumption of the thought-experiment, that each flash strikes at the position of either end of the train car, and that the observer on the train car is right at the midpoint between both ends. In his frame the train is at rest, so both ends of the train car always stay at the same position in this frame.
  8. Jul 18, 2008 #7
    Note that things are only simultaneous by convention not in an absolute sense!
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