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JoshuaFox comments on A sense of logic - Less Wrong
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Right. You might answer that the dot is not actually reaching the stars, and so is not traveling faster than the speed of light.
A similar problem, though, is a thought-experiment with a rigid rod which is one light-year long. If you rotate it with yourself as the axis, at even a small angular velocity, explain why the tip doesn't go faster than the speed of light.
I'm guessing that "rigidity" is actually a complicated engineering sort of thing when you really look at it, so that the motion takes time to propagate down the rod.
Yep. If you tried to rotate a giant rod, it would look like a spiral.
Galaxies have much looser internal connections than a rod has.
However, this suggests that light speed puts an upper limit on the rigidity of materials.
The galaxies were just there as a visualization- I don't think they started out as rods (but I Am Not An Astronomer).
Yep- the fundamental mechanism underlying rigidity is the electromagnetic potentials between atoms, and those can't propagate faster than the speed of light. Typical speeds of actual propagation are significantly slower- vibrations travel at the speed of sound in that material, and so on.