How quickly can force propagate?

08 January 2012

Question

If you had a really long tube and filled it with rigid balls and then put another one in the end, would a ball come out the other end instantly or would there be a speed of light delay?

Answer

Chris - Okay, so you've a tube full of balls and you're going to apply force to the ball at one end and you want to see how quickly the force can propagate through the assemblage of balls, yes?

Dominic - Now what's going to happen when you push on the end of the tube is that they're going to compress one of the balls ever so slightly elastically and that is going to then exert a force on the next ball on the tube that's going to compress that force on the next ball on the tube. And that compression can only propagate at the speed of light because no influences can propagate faster than the speed of light. The force you're actually applying here is electromagnetic, so the protons and electrons in your hands will be exerting electromagnetic force against the photons and neutrons in the ball and causing it to compress.

Dave - In fact, it's probably going to be a lot slower than the speed of light because effectively what you're doing, by pushing on something very quickly like that, is sending a sound wave down it and the speed of sound is actually the fastest information can pass through a material elastically. With something like a steel ball, I think that's 5 or 6 kilometres a second, much faster than the speed of sound in air but still, very slow compared to the speed of light.

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