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Faster than light speed Comunications?

  1. Nov 20, 2008 #1
    Hello, I'm newbie to physics and relativity and I wish to know more.
    I've had a question for 5 years and nobody has been able to give me a clear answer.
    Here it goes!

    Suppose there were 2 astronauts in space one in front of the other. There is a distance of one light year between them. Both are grabbing a pipe, or whatever solid object, which is also one light year large.(assuming that were technically possible to create an object that large...).What woud happen if one of them pushed this "pipe"? (and of course were strong enough to move it...). When would the other astronaut feel this push? a year later, instantaneously?

    Regards and thank you for your answers!
  2. jcsd
  3. Nov 20, 2008 #2


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    speed of sound in metal

    in steel it is given variously as 4500 to 5000 m/s
    for convenience suppose the pipe is some material is 3000 m/s, that is slower than light by a factor of 100,000.

    You ask how soon the signal would get there. The answer is it would get there in 100,000 years.


    explanation: mechanical disturbances in a material travel at the speed of sound in the material.
    no material is infinitely stiff.
    when Mr A pushes on the pipe, the pipe is mushy-springy so it compresses at his end. And then the compression re-expands and compresses the pipe further on. And so a kind of compression wave travels along the pipe (at the speed of sound in the material) towards Mr B.

    The issues of what a realistic long metal pipe would actually do, is something else. Even if it were out in intergalactic space thousands of lightyears from any other massive body, it would still be feeling its own gravity. It might tend to collapse or crush down under its own gravity. Also it would probably take a superstrong astronaut to give it a shove that could be felt, because it would be very massive. But I think the realistic issues do not matter, because it would be such a slow form of communication even if it could be realized in fantasy.
    Last edited: Nov 20, 2008
  4. Nov 20, 2008 #3
    no, it's not that marcus, the astronaut is not knocking the pipe, it's pushing or "moving" the pipe towards the other one. when would the other one notice that movement?
  5. Nov 20, 2008 #4


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    marcus is correct, if you push one end of a solid object, it will create a compression wave which moves through the object at the speed of sound in its material, the other end won't accelerate until this wave reaches it. See here or here for a more detailed discussion.
  6. Nov 20, 2008 #5
    Like some have said, the person on the other end would finally feel or sense that movement in the pipe for one light year divided by the velocity of the speed of sound in that specific material.
  7. Nov 20, 2008 #6
    thank you for your replies!

    One final question. So no matter how fast that push may be, the compression would never exceed the speed of sound through that material right?
  8. Nov 20, 2008 #7
    Basically, since the speed of sound in that material dictates the maximum speed at which the particles can transmit information.
  9. Nov 20, 2008 #8
    just to sum up.

    Suppose that I had a huge steal beam, let's say 10KM. If I twisted it by its near end really fast I could see how a wave is turning it as if it were made of licorice until it got its far end.
    It's that correct?
  10. Nov 20, 2008 #9


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    Yes, assuming you twist it at a normal speed (if some force twisted it extremely fast it could just snap).
  11. Nov 20, 2008 #10


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    You could, of course, push on it fast enough to destroy the pipe, but in that case there would be no compression wave moving forward from your finger, you'd just have your finger destroying the pipe as it moves through it.
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