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Physical Interactions over Great Distances

  1. Feb 28, 2007 #1
    Say you (somehow) constructed a very long, sturdy rod, about a light year in length. Say it was made of unobtanium, so it has the same mass as a relatively short steel rod. Now say there are two astronauts, one at each end, and one of the astronauts gives the rod a firm push in the direction of the other astronaut.

    Does the other astronaut instantaneously see the rod move in his direction? If not, how exactly does the rod behave? (Does it move at all when the astronaut pushes on it? Or does it take a really long time to start moving?)

    Thanks in advance for your input on this thought problem.
  2. jcsd
  3. Feb 28, 2007 #2

    Doc Al

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    Staff: Mentor

    The "push" moves along at the speed of sound in the rod--nowhere near the speed of light, much less instantaneously.

    See this thread: https://www.physicsforums.com/showthread.php?p=1245509
  4. Mar 1, 2007 #3
    Can you explain the significance of the mass of the rod in your question?
  5. Mar 2, 2007 #4
    Thanks for the quick re-direct, Doc Al. I had been feeling so proud of myself for thinking up the idea all on my own, but am now humbled to see that the exact same idea was brought up a few short weeks earlier.

    country boy -- my thinking was that if it didn't have a small mass, it would have a huge inertia and would be virtually impossible to push by an astronaut anyway.
  6. Mar 2, 2007 #5
    I see. You might be interested in this thread also, which uses rockets to propel a rod:

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