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Tethered to the Moon

  1. Feb 14, 2008 #1
    Imagine a solid chain from the Earth to the moon's surface (which I believe is 1.5 light seconds away).

    Now Imagine two scenarios.
    First scenario is that the chain is attached directly to the moon's surface.
    Second scenario is the chain is unattached.

    What happens if I tug on the chain in both scenarios? If information travels at the speed of light, how does the universe know whether I should be able or unable to pull the chain at that moment?
  2. jcsd
  3. Feb 14, 2008 #2


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    Science Advisor

    A shock wave creeps up the chain with a speed of ~5km/s. A highly sophisticated interferometer at the moon might have a chance (well, not really) to detect it one day later.
  4. Feb 14, 2008 #3
    The answer is that no material is completly rigid and both the chains will initially stretch (by the same amount) when tugged. For the time it takes the shock wave to get to the other end of the chain (A minimum of 1.5 seconds but in practice a lot longer because shock waves in a material travel at the speed of sound in that material) the chains will both behave the same, with the end you are tugging moving while the far end is stationary.

    In practice it would be difficult to find a material that will not stretch and break under its own weight even before tugging. Google for "space lift" or "space elevator" to see a related subject.
  5. Feb 14, 2008 #4
    ah so thats what they mean when they talk about rigid bodies

    thank you both :)
  6. Feb 14, 2008 #5


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    Staff: Mentor

    Btw, that 5km/s that Ich was talking about is the speed of sound in metal (didn't check the number, but it sounds reasonable).
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