I was watching a video on YouTube called 'How Much Does A Shadow Weigh?' and towards the end, the following question was posed: Suppose you had a button that was one light year away from you and you built a rigid stick one light year long. If you pushed that stick, would the other end of the stick push the button instantaneously? The answer was no and the explanation was that when you push a rigid object, you're really putting a series of compression waves through it which travel at the speed of sound. So if we had a 12km long steel rod and we pushed it at what end, then the person on the other end would see it move about 2 seconds later (speed of sound in steel: http://www.engineeringtoolbox.com/sound-speed-solids-d_713.html). Would the steel rod then be momentarily contracted? Would the more we push, the more it becomes contracted?