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Shining a laser out of a train - What does the stationary

  1. Feb 12, 2013 #1
    Lets say its the future your camera has an insanely fast shutter speed. You're standing on the ground watching a train travel past at just under the speed of light and the train driver is shining a laser pointer out the front window. With your insanely fast reflexes you take a take a picture of the train (or better yet you record a video). Would you see the laser light slowly moving ahead of the train, or would you see the laser beam traveling ahead of the train at full speed? If its the latter, I'm mighty confused because I can't see how you would record normal light speed for the laser beam, rather than light speed + the speed of the train.

    If its the former then I think I'm starting to get my head around special relativity.
  2. jcsd
  3. Feb 12, 2013 #2


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    Staff: Mentor

    You "see" the light as traveling at speed c. If the train is traveling at 0.99c for the sake of argument, then from your point of view, the separation between the train and the light pulse increases at the rate of 0.01c.

    From the point of view of someone on the train, the light pulse recedes at speed c.
  4. Feb 13, 2013 #3
    Ah right, so the stationary observer sees the light travel ahead of the train at 0.01c, thats what I needed to know, thanks a lot.
  5. Feb 13, 2013 #4


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    Gold Member

    Just keep in mind that that is a somewhat dangerous way of looking at things from the point of view of understanding physics. What the observer sees is light moving at c. The speed of the emitting object is irrelevant.
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