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Rocket-man measuring speed of light

  1. Aug 19, 2011 #1
    Suppose a rocket is travelling almost at the speed of light.....but relativity says he will still measure the speed of light 3*10^8.....if a beam of light is sent at the time the rocket started.....if the beam of light and the rocket both travel the same distance, let 2 light minutes, then what will the observer in the rocket see?will he see that both the rocket-he is in-and the beam of light covered the distance almost in same time? And what will an observer who is relatively standing still in another reference frame, see? Sorry for this childish question...!! Thanks...
  2. jcsd
  3. Aug 19, 2011 #2


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    Staff: Mentor

    Suppose the ship is going at 0.99c (99% of the speed of light). The ship and a pulse of light start out from the Earth at the same time. They can't go the same distance in the same time because they're going at different speeds, so let's make the time the same, 2 minutes.

    According to an observer on the Earth, after 2 minutes, the light is 2 light-minutes away from the Eartth, and the ship is 1.98 light-minutes away. The light is 0.02 light-minutes ahead of the ship.

    According to an observer on the ship, the ship is stationary. After 2 minutes, the Earth is 1.98 light-minutes behind him, and the light is 2 light-minutes in front of him.
  4. Aug 19, 2011 #3


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    You need to realize that no one can directly see light in transit. We can only "see" it after it reflects off of something and returns to us. So a stationary observer located at the start of the scenario won't see the light hit a sign at the finish line until four minutes later. He will also see the rocket arrive there a short time later. The rocket man will also see the light hit the sign just before he gets there but to him, it all happens in a matter of seconds instead of minutes and the distance is only light seconds instead of two light minutes.
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