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B Thought experiment re speed of light / information

  1. Dec 2, 2015 #1
    I've built a rod 186K miles long, of ultra-light, ultra-rigid material. I am on one end of the rod, and an observer is at the other end (we're both in the near vacuum of space). I want to send a signal to the observer on the other end of the rod signaling whether the Packers have won the superbowl - signal received, they won, no signal, they lost.

    The packers win, so I bang my end of the rod with a fast and massive hammer (moving the rod quickly) and instantaneously / simultaneously on my end, there is a great flash of light sent towards the observer.

    The light signal reaches the observer in about a second. What prevents the observer from seeing his / her end of the rod move in say 1/2 second? (No fair blaming compression of the rod or mass of the rod).
  2. jcsd
  3. Dec 2, 2015 #2


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    Staff: Mentor

    The signal will propagate in the rod at the speed of sound. There is no way to make this speed faster than the speed of light: on an atomic level, it is the electromagnetic force that propagates the momentum from atom to atom.

    This is the 35323. thread about a perfectly rigid rod, and you can find the same answer (and more details) in all of them, there is no point in repeating the discussion again.

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