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## Main Question or Discussion Point

A common thought after one learns about the limit of the speed of light is that a long rod when pushed would seem to cause the other end to move instantly, thus carrying information faster than light. The reason this isn't the case is because the movement will only travel through the rod at the speed of sound in that material, quite a bit slower than light.

I was thinking about this the other day and I realized that it must also be true that the amount of force required to move the rod at a certain speed must also be less than the amount for the full rod, at least initially. If pushing the rod with x force caused it to move at y speed you could easily work out it's mass z, and if the rod was a known density and width you'd then know the length. That could be used to send info faster than light.

My question is then how would the local end of the rod behave when receiving a single large push? It would seem as though it would have to move initially fast, and then slow down as the wave of the push moves farther and farther down the rod, and thus more and more mass is moving.

However, I can't resolve the following. Imagine a rod of known density and dimensions, this rod is a light hour long and the end points are at two space stations, with an additional space station at the midway point. At a predetermined time the local end is given a push. For the sake of simplicity let's assume the speed of sound in this rod is 0.01c, so the far end should begin to move at 100 hours, the midpoint at 50 hours. At 49:59 hours the midpoint station decides if it wants to send a 1 or 0 bit, to send a 0 it does nothing. For 1 it cuts the rod in half and moves the far half out of the way. When the wave of movement reaches the midpoint (which is now the endpoint of the half rod) the wave stops, and the local end of the rod stops slowing down. The local station observes if the rod continues slowing down or not and from this knows if the midpoint station intended to send a 1 or 0.

Because of that example I feel my thought for how the motion would travel through the rod must be wrong. Can someone please explain how the end would act?

I was thinking about this the other day and I realized that it must also be true that the amount of force required to move the rod at a certain speed must also be less than the amount for the full rod, at least initially. If pushing the rod with x force caused it to move at y speed you could easily work out it's mass z, and if the rod was a known density and width you'd then know the length. That could be used to send info faster than light.

My question is then how would the local end of the rod behave when receiving a single large push? It would seem as though it would have to move initially fast, and then slow down as the wave of the push moves farther and farther down the rod, and thus more and more mass is moving.

However, I can't resolve the following. Imagine a rod of known density and dimensions, this rod is a light hour long and the end points are at two space stations, with an additional space station at the midway point. At a predetermined time the local end is given a push. For the sake of simplicity let's assume the speed of sound in this rod is 0.01c, so the far end should begin to move at 100 hours, the midpoint at 50 hours. At 49:59 hours the midpoint station decides if it wants to send a 1 or 0 bit, to send a 0 it does nothing. For 1 it cuts the rod in half and moves the far half out of the way. When the wave of movement reaches the midpoint (which is now the endpoint of the half rod) the wave stops, and the local end of the rod stops slowing down. The local station observes if the rod continues slowing down or not and from this knows if the midpoint station intended to send a 1 or 0.

Because of that example I feel my thought for how the motion would travel through the rod must be wrong. Can someone please explain how the end would act?