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Information transmission

  1. Nov 24, 2004 #1
    Here's a scenario I thought of a few days ago. I know there must be an easy explanation, but it hasn't come to me ...

    Bob and Alice (no Eve this time) are separated from each other by about a million miles, and they would like to communicate with each other without the five second delay of radio waves. Fortunately, there lies between them a million-mile-long pole. After talking back and forth over radio, they agree to communicate by poking each other in Morse code with the long pole. Are they communicating with no five-second delay? Is this a problem?

    I wouldn't be surprised if this is a well-known relativity-style "puzzle". If someone could explain what's wrong with the reasoning, I'd appreciate it.

    JP :biggrin:
  2. jcsd
  3. Nov 24, 2004 #2


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    The signal will travel down the bar at the speed of sound in the material, which will be far less than the speed of light.

    It's a standard puzzle, the resolution is that there are no rigid bodies in relativity, a rigid body would have an infinite speed of sound, while the actual speed of sound is less than 'c'.
  4. Nov 24, 2004 #3
    That makes sense. Thank you!

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