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If I were to stand at one end of a light-year long metal pole, and another person were to stand one light-year away at the other end, and then I were to push on my end of the pole. How long would it take for their side of the pole to move? I've been told that the matter in the pole would only move at the speed of sound. So it would take however long a sound-wave takes to travel a light-year for the other person to see the pole move. But if this is true, does that mean that if you were to take any still object, and hit it with another object moving at the speed of sound, the still object would compress into itself before expanding back out again?

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I would assume the answer to your second question will depend on the compressibility/rigidity of the body. For example, if you shoot a ping pong ball against a slab of metal at the speed of sound, nothing would happen to the slab, but I'm guessing the ping pong ball would experience something drastic. Solids can experience more than a certain amount of shear without tearing apart, so it'll depend on the momentum of impact between the two bodies. –  Kitchi Nov 25 '12 at 11:22
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You are quite correct, the movement of the pole would travel at the speed of sound in whatever the pole is made of, and this does apply to any object.

Although I suspect it's not the real point of your question, the speed of sound in steel is about 6100 m/s, so assuming your pole is made from steel it would take about 49,200 years for the other end of the pole to move.

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Hello John, Doesn't these waves suffer some energy loss as they travel to about 1 LY..? (it seems to be quite a large distance) They must lose energy as they travel through steel for such a long period. In fact, they're still pressure waves. Isn't it? –  Waffle's Crazy Peanut Nov 25 '12 at 7:46
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Well yes. I don't know what the attenuation of sound in steel is (though I'm sure it's on the Internet somewhere) but I imagine it's great enough that the pressure wave would fade away long before travelling a light year. But then it's only a thought experiment - at least I assume no-one is seriously intending to try it :-) –  John Rennie Nov 25 '12 at 7:51
    
Yeah. I definitely agree with that. Just a curiosity though... with a tiny bit of suspicion, "would this change my previous studies?" :-) –  Waffle's Crazy Peanut Nov 25 '12 at 7:56
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