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I have recently played with a thought experiment for which I could not find the answer: Assuming I have a very long stick of several million kilometers and I pull one end: how long does it take, that the impulse travels through the stick to the opposite end so that it moves? Intuitively, I would say that this happens immediately but obviously it must be slower than the speed of light...why is that do? How can this approach the speed of light? Infinitely tight packed atoms and zero mass of the whole stick?

MichaelScott
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The fastest you can have the pull on one end of the stick be transmitted to the other end is the speed of sound in the stick which will be of order ~1000 meters/sec. This is far, far slower than the speed of light.

niels nielsen
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  • Why is that? That's strange. –  Mar 19 '21 at 22:56
  • when you pull on one end of the stick, the molecules there transmit that pull to their nearest neighbors further down the stick, and then they pull on their nearest neighbors, and so on. the transmission speed of intermolecular pulls and pushes inside the stick material is the speed of sound in the stick. – niels nielsen Mar 19 '21 at 23:02
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    To add to Niels' answer, the speed of sound can approach the speed of light in extreme circumstances. Nothing like what you would find on Earth. Is the speed of sound almost as high as the speed of light in neutron stars? – mmesser314 Mar 19 '21 at 23:10
  • More specifically, when you are going faster than the speed of sound, you start having to deal with all of the shockwave mechanics. When this happens your "hockey stick" starts having very different material properties from what you thought they were. And, in many cases, the result is that the stick ceases to be one solid object. – Cort Ammon Mar 20 '21 at 00:02
  • Sounds absolutely plausible Niels! I should have thought about that...! Also thanks mmesser314 for bringing up the question with the neutron star! – MichaelScott Mar 20 '21 at 06:03