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Hypothetical question about speed and distance across space

  1. Sep 26, 2011 #1
    I have two hypothetical questions about breaking the speed of light that has been bugging me for quite a while.

    Question 1:

    Assume an impossibly sturdy and light pole is attached to the earth. Its length spans a distance of 200 million light years into space. Since it is anchored on earth, it rotates with the earth. Unbelievably, as the earth spins, the end of this pole (or any part of it) does not hit any bodies in space.

    Wouldn't the end of this pole be moving across the universe in a full 360 rotation in 24 hours? Moving at speeds way over the speed of light?

    [PLAIN]http://img823.imageshack.us/img823/8533/question1k.jpg [Broken]
    (ignore the "tap like morse code, that is for question2)
    http://img823.imageshack.us/img823/8533/question1k.jpg [Broken]

    Question 2:

    This same pole, is not attached on earth and does not rotate with the earth. However, it can be pushed (remember, it is extremely sturdy and light). At the opposite end of this pole is a morse code machine. Couldn't we effectively communicate across 200 million light years instantaneously?

    [PLAIN]http://img23.imageshack.us/img23/6382/question2c.jpg [Broken]
    http://img23.imageshack.us/img23/6382/question2c.jpg [Broken]

    Does that make any sense?
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Sep 26, 2011 #2


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    Staff: Mentor

    Welcome to PF. This is a common question. The answer is pretty simple: An impossibly stiff rod is impossible. In reality, Any motion will propagate along the rod at the speed of sound in the rod.
  4. Sep 26, 2011 #3
    As stated above, welcome!!!

    For the second question, remember that the electrons used to send the message are only traveling at the speed of light, so it will just take 200 years for the Morse code to reach the other end of the pole (assuming no dissipation in the signal).

    Good thinking, though. These types of mind experiments were how the Theory of Relativity was developed in the first place. Glad you asked. :smile:
  5. Sep 26, 2011 #4
    Thanks for the responses!

    Did you mean speed of sound or light in regards to the answer to question 1? So you're saying that the rod at 200 million light years across is being whipped around the earth, slowly creating a spiral?

    In regards to question 2, so by pushing on the rod, it would move along the rod at only the speed of light? So the end wouldn't move for 200 million years, but it's moving from the beginning like a ripple at the speed of light?

  6. Sep 26, 2011 #5
    Oh, sorry, I misread that it was being used as a morse code machine. The movement would still only propogate at the speed of sound of the material.
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