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Extended thought experiment

  1. Dec 31, 2009 #1
    Got this from extending the "faster than light" portion of the wiki page.

    Wiki notes that if a laser is swept quickly across a distant object, the point will move faster, but nothing else necessarily violates any rules so this experiment passes.

    Now what if you have an infinitely long rod of negligible weight opposing your holding of and swinging of the said rod, and you decide to spin it round and round at a 90 degree angle to your body.What are the SR effects at each end of the rod, particularly, the end that would otherwise move faster than c if it weren't for that speed limit compared to the end you are holding.

    Would it just be a connected twins paradox? And what of the apparent "ages" of the two ends of the rod. I know a rod doesn't "age", but if you were to compare the aging of the end you hold through time and a point very very far away, would it really be different or is there some linkage of information about their respective ages that prevents this (this last question may just be my over-thinking the situation)?
  2. jcsd
  3. Dec 31, 2009 #2
    There is no such rod of negligible weight though, which I believe is important, because I don't believe it would be impossible to rotate any physically existant rod to behave in such a manner where its outer points would be rotating above c.
  4. Dec 31, 2009 #3
    I'm definitely not one to argue but is your response a sound debate that would limit this experiment, not that I doubt what you say.

    In essence, I want a rod that would pose little effort to spin even if amazingly long but if your hunch is actually the limitation then I guess can we imagine some infinitely powerful machine can rotate it?

    Ignoring the particulars altogether, when the rod is good and going, what are the effects @ either end?
  5. Dec 31, 2009 #4

    Vanadium 50

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    The direction this is going - "would a rod that violates the laws of physics violate the laws of physics" - will not be helpful in understanding things.

    The motion of one end of a rod can only be affected by motion at the other end after this force has been propagated through the rod. The speed this is happens is the speed of sound in the rod, which is always less than the speed of light.
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