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Instant Information Travel?

  1. May 18, 2007 #1
    Hello,

    This is my first ever post in an entirely science-dedicated forum. On another forum, another user and I theorised that the energy transfer in hydraulics from one point to another is instant. He initially posted about electricity being instant because electrons move at the same speed throughout a metal, and together we came up with this question:

    If I managed to make a pole one light-year long, and I pull on one end of this pole, would a friend standing at the other end of the pole experience this same movement instantly, or would there be a one-year delay?

    It's baffled me, and I was wondering if any of you wonderful people could shed some light on this question. :biggrin:

    Many thanks!
     
    Last edited: May 18, 2007
  2. jcsd
  3. May 18, 2007 #2

    Doc Al

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    Staff: Mentor

    Neither hydraulic impulse nor electricity travel "instantly".

    The effect of the pull would travel at the speed of sound in the pole, much slower than the speed of light--definitely not instantly!
     
  4. May 18, 2007 #3

    berkeman

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    For a mechanical pole yank, the force propagates at the speed of sound in the material of the pole. Similarly for hydraulics.

    For electricity propagating on wires, the impulse travels at a fraction of the speed of light -- the fraction depends on some of the characteristics of the insulators around the wire, etc.

    Nothing is instantaneous. You and your friend should stop speculating and do some reading. Welcome to the PF, BTW. Be sure that you've read and understood the PF Guidelines before getting too crazy with more posts:

    https://www.physicsforums.com/showthread.php?t=5374
     
  5. May 18, 2007 #4
    Thank you Doc! :)

    You seem very certain about it being the speed of sound. Does this not mean the the pole will be longer for part of the energy transfer?

    Berke, thank you for the link. I will be sure to follow the guidelines. I will do further reading about forces, and thanks for your welcome!
     
    Last edited: May 18, 2007
  6. May 18, 2007 #5

    Doc Al

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    Absolutely... it will stretch a bit.
     
  7. May 18, 2007 #6
    Ah, that makes this much clearer. It seems I need to research these theories more before I jump to conclusions.
     
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