Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Staff Emeritus
    Science Advisor

    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!

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook