How much time will it take to move an object whose length is equal to one light year? Suppose there's a stick whose length is one light year and I push it from one side by one centimeter. How much time would it take for its other side to move by one centimeter and why?
 A: It depends on the material. When you push one end of the stick, you move the atoms at the very end of the stick. Those atoms push the atoms next to them, those atoms push the next atoms, and so on down the stick. This is a sound wave that travels down the stick, so the time you have to wait for the other end to move is the length of the stick divided by the speed of sound in the material of the stick. If the stick is wooden, the speed of sound is about 4000 m/s (compared with 330 m/s in air). It would take $\frac{9.5\cdot10^{15}\,m}{4000\,m/s} = 2.4\cdot10^{12}$ seconds (74 000 years) for the other end of the stick that is a light-year away to move.
A: The answer to this depends on the material properties of the stick.  If it were a mythical "rigid body," the entire stick would move all at once, but that "rigid body" is an approximation that isn't valid in situations like this.  The rigid body assumptions assume that you can transmit information about movement instantaneously.  In many cases, that's close enough to correct, but in situations like this, the speed at which you can propagate the force through the stick is not infinite, and matters.
Instead, you would need to set up a compression wave along the stick which transmits the information about you pushing it down the length of the stick.  The speed of that compression wave depends on the material you use.  Thus, in the real world, the "best case scenario" is dependent on the speed of sound in the medium you build the stick out of.
