The star Alpha Centauri is about 4 light-years away from us. Suppose that in our mutual reference frame an Earth comedian named Rebo tells a joke and a week later an alien at Alpha Centauri laughs. (a) How fast would a rocket have to be moving in order for its reference frame to conclude that the laugh happened at the same time as the joke? (b) Is it possible that the joke caused the alien to laugh? How do you know?

The star Alpha Centauri is about 4 light-years away from us. Suppose that in our mutual reference frame an Earth comedian named Rebo tells a joke and a week later an alien at Alpha Centauri laughs. (a) How fast would a rocket have to be moving in order for its reference frame to conclude that the laugh happened at the same time as the joke? (b) Is it possible that the joke caused the alien to laugh? How do you know?