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A Question About Special Relativity's Basics

  1. Dec 25, 2007 #1

    I've read and learnt the fundamentals of SR and understand it to quite a nice extent. I'm talking about the time dilation's derivation using a light clock. I've understood the derivation's concepts, i.e. the proved principle of speed of light being 'c' in all frames of reference. So that part is clear.

    Now lets say the light beam is replaced by a soccer ball, which also oscillates the same way except the fact that it does so once in every one second (Just to keep things practical).

    Now, I wish to calculate the time dilation (though it would be extremely small). Now, I'm not sure if just like light, the balls speed would appear to be the same to the external observer. If yes, then can someone elaborate on this? In that case, what would have classical mechanics predicted?

    Thanks for your time,
  2. jcsd
  3. Dec 25, 2007 #2


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    that sounds familiar. didn't someone else (or the same, you're not Stellar1, are you?) ask this same thing about a soccer ball bouncing around in a "lightclock" instead of light?

    for some people SR is part of classical mechanics. for other's SR is "modern physics"; post-classical mechanics. anyway, in pre-SR times velocities added linearly. assuming a single direction of travel for all involved, if Observer A veiwed a soccer ball with a velocity of v1 (in, say the x direction) and another observer, who was also moving in the x direction with velocity v2 (relative to Obeserver A), would view that ball moving at velocity v1 - v2. but in SR they don't add (or subtract that way).
  4. Dec 27, 2007 #3
    No, the ball's speed would be different for different observers. Light is unique in that sense, and the "light clock" derivation indeed only works with "light."

    Personally, I never liked the light clock derivation very much. As the other poster said, it's really a very "classical" interpretation of SR, and leads some to believe that time dilation is just a consequence of the length of a light path being physically altered. In fact, the light clock derivation is more of a consequence of the mutability of space and time for different observers, rather than the other way around.
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