Why doesn't pushing balls in a tube propagate the movement faster than the SoL?

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Is it possible for information to be transmitted faster than light by using a rigid pole?

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?

Duplicate of Is it possible for information to be transmitted faster than light by using a rigid pole? — 410 gone, Dec 11 '12 at 10:21
Pressure in marbles propagates approximately by the speed of sound which is approximately 1 million times slower than the speed of light. — Luboš Motl, Dec 11 '12 at 10:21
@EnergyNumbers: Sorry, I did try searching used things like tube and (mar)balls. — George Duckett, Dec 11 '12 at 10:30

score 0 · Answer 1 · answered Dec 11 '12 at 10:12

The marbles are not perfectly rigid, so when you push on the first one, it will deform. This leads to a compression wave passing through the marbles from one end to the other; only when it reaches the other end will the last marble begin to exit the tube.

One of the lessons of special relativity is that rigid bodies are impossible, even in principle.

Why doesn't pushing balls in a tube propagate the movement faster than the SoL?

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