Replies

Paradoxes.
If a spaceship were accelerated to the speed of light – taking 50 years in your example – we first have to decide “whose” 50 years. That is, as measured by someone on Earth, or by someone in the spaceship.

Let’s assume that we mean 50 years for the ship to go out and return to Earth, as measured by those on the spaceship.

One paradox is that when the spaceship returned, the people on Earth would have measured a much longer time elapsed (perhaps 100 years or longer). "Time" would run more slowly on the spaceship than on Earth. The reason for this is that (as mentioned in another post), clocks, experiences, events and sensations would "run" more slowly on the fast-moving spaceship than on Earth. (Relativity means that the speed between two objects is “relative”; so on which do clocks and events slow? The one that was accelerated slows.)

A second paradox is that neither the people on Earth nor those on the spaceship could tell which is “clocking” slower, if they could see one another. That is because of a phenomenon called Time Desynchronization, where the “clocks” at the rear-end of a fast-moving object are “ahead” of the “clocks” at the front-end of the object.