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Question on relative time.

  1. Jun 11, 2008 #1
    Imagine the following scenario.

    There is a long train moving at very fast speeds (approaching light speeds), one man stands at the back of the train with a watch and another at the front. A third man stands exactly halfway between them with some gunpowder and a match. The men at the back agree to set their watches to 1 o clock when the light from the gunpowder reaches them (I said fast and long train so that it will actually take a reasonable time to reach them). A forth man watches from outside the ground.

    The gunpowder goes off, and both men set their watches. The third man views this and says it was a fair timing and both had set their watches to the same time. However the man from the ground sees the light reach the man at the back of the train 2 seconds earlier (since relative to him the man at the back is moving towards the light and man at the front away). So he thinks the clocks are set to 1:00 and 1:02.

    The train slows down and the man from the ground goes and sees the watches. What will they say?

    I'm having a bit of trouble coming up with an answer for this because I know both views are both equally valid views, but the clocks can only say one thing when they all look at it togeather.
  2. jcsd
  3. Jun 11, 2008 #2


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    Hi Alex! :smile:

    It's easier if you consider what happens if the back man (after they've set their watches) walks slowly towards the front man.

    The observer on the ground will gradually see the times on the watches get closer.

    Similarly, if the train gradually slows down, but the two men stay apart, the observer on the ground will gradually see the times on the watches get closer. :smile:
  4. Jun 11, 2008 #3
    Ah ok I see thanks.
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