Say you (somehow) constructed a very long, sturdy rod, about a light year in length. Say it was made of unobtanium, so it has the same mass as a relatively short steel rod. Now say there are two astronauts, one at each end, and one of the astronauts gives the rod a firm push in the direction of the other astronaut. Does the other astronaut instantaneously see the rod move in his direction? If not, how exactly does the rod behave? (Does it move at all when the astronaut pushes on it? Or does it take a really long time to start moving?) Thanks in advance for your input on this thought problem.