On one episode of QI they asked the question, "How fast do electrons move travelling around an electric current."
The answer is (more or less) "very slowly", with the explanation that it's not the electrons that move fast, it's the force (I think). They likened it to pushing on one end of a long tube of touching marbles and observing how quickly one fell out the other end.
This made me think about the tube. I realise that this tube cannot allow a force to propagate through the tube instantaneously because that would be faster than the speed of light, so my question is, why not?
TL;DR; Why doesn't a marble exit a long tube filled with touching marbles immediately when a force is applied to the marble at the other end?