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Homework Help: Violation of Special Relativity

  1. Dec 8, 2015 #1
    1. The problem statement, all variables and given/known data
    Consider a searchlight on the ground that casts a spot on a cloud 1500 m overhead. If the searchlight is rotated rapidly−say, 40 ∘ in 1 μs−—how fast does the spot move in the Earth reference frame when the searchlight is directed vertically upward?

    I have solved this part and found the velocity to by 3.5c.

    Is this a violation of special relativity?
    1. No, this is not a violation of the special relativity, light spot is not a physical object and can move with any speed.
    2. Yes, this is a violation of the special relativity, the searchlight cannot really rotate this fast, data given in the problem statement is incorrect.
    3. Yes, this is a violation of the special relativity, this situation should be studied using general relativity.

    2. Relevant equations

    3. The attempt at a solution
    I think it is a violation, but i'm not sure exactly the reasoning behind it. My educated guess would be number 2 is the correct answer.
  2. jcsd
  3. Dec 8, 2015 #2


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    No, there isn't anything that is actually moving except the rotating search light. This canard is regularly debunked here on PF. No 1 is correct.
  4. Dec 8, 2015 #3

    Ray Vickson

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    Suppose you replaced the searchlight by a laser and the distance 1500m by the distance to the moon. You could rotate a real laser at a not-excessively fast rate entirely within practical bounds, and make the light-spot on the moon's surface move much faster than c. So, (2) is not really relevant, although it might be for a big, heavy searchlight---that's why I suggested a laser. That is, moving an actual searchlight through 40° in 1 μsec might be impractical, but you can easily wave a laser by hand with no trouble.

    That leaves (1) or (3).
    Last edited: Dec 8, 2015
  5. Dec 8, 2015 #4


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    [post deleted]

    EDIT: OOPS. I thought I was responding to the OP. Sorry Ray.
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