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I Einstein's Train Example Seems to Exceed c...?

  1. Jun 27, 2016 #1
    Youtube vid of an Einstein's Train example:

    From the perspective of the man on platform, the rear of the train seems to be approaching the front lightning's light at greater than c (train's speed + c). Is this correct? I thought there was a rule against that...
  2. jcsd
  3. Jun 27, 2016 #2


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    Science Advisor

    That's fine. What is forbidden is that you ever measure speeds greater than c relative to you.

    Time dilation, length contraction, and the relativity of simultaneity conspire so that the guy at the back of the train measures the light coming towards him at c (you might want to Google for the relativistic velocity addition formula).

    Nothing is moving faster than c relative to you. Nothing is moving faster than c relative to the guy on the train. No rules broken.
  4. Jun 27, 2016 #3

    Doc Al

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    Staff: Mentor

    Yes, perfectly correct. The "closing speed" of the light and the rear of the train is greater that c.

    No. Note that no one measures anything to be moving faster than light with respect to themselves.
  5. Jun 27, 2016 #4
    Well, alright then. Thank you both
  6. Mar 20, 2017 #5
    I think not the rear of the train but front of the train approaches faster to the front strike and rear of the train "escapes" from the rear strike.
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