Although we can't technically do this currently, what if we could? What would be a reason to send a person on a mission into the future? If Bob travels at near the speed of light from earth for lets say 5 years and then returns to see Alice. Alice has, and in fact the entire population of earth and everything on it has aged much more than Bob. Bob is now the only one who hasn't aged more than 5 years. Seems like there is no valid reason to ever do this, even if we could.