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How does a very long object in space act with time

  1. Oct 1, 2011 #1
    If i put a 100 light year long string in space in a straight line and i put a astronaut at each end. Then the astronaut at one end took hold of the string and pulled it, would the person who is 100 light years away at the other end of the string see immediately the string move away from him?

    Just i find it difficult to favom that it can possibly be instantaneous , would it be like every molecule inside the string sends a flow of energy through it continuously to the next nearest one, stretching it and pulling it closer to that initial position. And these tiny forces between the molecules are tiny forces which do not occur instantaneously?

    Thank you for any input, its all much appreciated

  2. jcsd
  3. Oct 1, 2011 #2


    User Avatar
    Science Advisor

    This is probably in the "FAQ" since we get it every few weeks. No the other end of the string (or 'rigid' bar) does not move instantaneously. For one thing the speed of light restriction would prevent that. But in fact it would move much slower than that- the string will stretch (or bar compress) as the force moves through it. You are talking about a stress wave moving through the material and such waves move at the speed of sound in the material.
  4. Oct 1, 2011 #3
    sorry about that.
  5. Oct 1, 2011 #4


    Staff: Mentor

    Actually, it is not in the FAQ. This is a pretty big omission. I will propose a FAQ and get some feedback.
  6. Oct 1, 2011 #5


    Staff: Mentor

    Here is my proposed FAQ:

    One common proposal for achieving faster than light communication is to use a long rigid object and mechanically send signals to the other end by pushing, pulling, or tapping it. The fundamental idea is that when one end is moved the other end is disturbed instantaneously, however, a closer analysis of the motion of an extended object shows that any mechanical disturbance will propagate at a finite speed.

    This speed is called the speed of sound in that material, high stiffness materials like metal have a very high speed of sound and low stiffness materials like jello or air have a very low speed of sound. When you push on something made of jello, you can easily see that the disturbance propagates at a finite speed. When you push on something like metal, it is not so easy to see visually, but the disturbance still propagates at the finite speed of sound in the metal.

    The speed of sound in diamond is about 12000 m/s which is about 25 thousand times slower than the speed of light (299792458 m/s). But what about some hypothetical "unobtainium"? Why couldn't unobtainium's speed of sound be faster than the speed of light? The answer is that all materials, even unobtainium, are held together by electromagnetic forces at the molecular level. When one molecule moves then the change in its electromagnetic field propagates to its neighboring molecule at the speed of light. So even in principle it is not possible for any material to have a speed of sound faster than the speed of light.

    d3nd3, can you give me some feedback? Do you feel that is a full and complete answer?
  7. Oct 1, 2011 #6


    User Avatar
    Staff Emeritus
    Science Advisor

    This should be posted for the other SAs as well however I think it's very good. The one thing I would say it needs is a simple examples (bolded are my additions)

  8. Oct 1, 2011 #7


    Staff: Mentor

    Thanks, I have posted it to the SA's with your suggested edit.
  9. Oct 3, 2011 #8


    Staff: Mentor

    Here is the final version of the FAQ published:
    https://www.physicsforums.com/showthread.php?p=3537287#post3537287 [Broken]
    Last edited by a moderator: May 5, 2017
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