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Trying to understand special relativity.

  1. Mar 11, 2013 #1
    Many explanations of time dilation use light sent from mirror to mirror on moving spaceships to explain that since the distance the light travels increases then time must increase in order to keep the speed of light the same . . . however, in order for the light to leave one spaceship and hit the other one, the light must have a horizontal velocity equal to the speed of the spaceship . . . if it has c as its speed toward the other ship and it has a horizontal velocity . . . then, in fact, its speed is greater than c . . which is not allowed. How does this work?
  2. jcsd
  3. Mar 11, 2013 #2

    Doc Al

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    Staff: Mentor

    Yes, the standard "light clock" thought experiment.
    The light just bounces back and forth between two mirrors inside the one spaceship.
    When the moving light clock is viewed from another frame it is certainly true that the light pulses must have a horizontal component of velocity equal to that of the spaceship. But from that frame, the light moves at an angle with speed c.

    In the moving spaceship's own frame, the light just moves vertically. Also at speed c.
    Last edited: Mar 11, 2013
  4. Mar 11, 2013 #3


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    Science Advisor
    Gold Member

    Here's an animation I made that might help you:


    If you want detailed explanation, see this thread.
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