space travel and time

In the classic scifi movie Planet of the Apes, the shocking plot twist is(wait... you haven't seen the movie?! But I'm about to spoil the ending! Quick! Look away!) that the strange planet they are exploring is Earth, millions of years in the future. Humans have lost the power of speech and are nothing more than animals. Apes rule the world. They shouldn't have been so surprised to find themselves there, because this happens all the time in science fiction. Hapless astronauts in Twilight Zone episodes, space travellers in many scifi books, and even the comedy series Red Dwarf, are based on the premise that the characters wake up millions of years in the future. Typically, this is done through the magic of cryogenic freezing- not yet perfected, sadly - in combination with Einstein's Law of Special Relativity, which says, if I may paraphrase the great man somewhat, that the faster you go, the more time slows down.
Therefore, if you get in a spaceship and cruise around for a couple of years at speeds approaching the speed of light, when you parachute back into home, you'll find that everyone you know is dead, and monkeys rule the earth. Or something like that.
That's the theory that launched a thousand paperbacks.
And in principle, it's perfectly valid. Time slows down the faster you are moving. We don't notice it ourselves because everything around us is moving so slowly. But clocks run slightly faster at the equator than at the north or south pole because of the rapid spinning of the Earth. So an astronaut travelling at, say, half the speed of light would be very well preserved.
What this neat theory of space travel doesn't take into account is that the atmosphere shields us from radiation. Travelling through space means being exposed to harmful cosmic rays, which may age you just as fast, or faster, than if you had stayed on Earth..
So a jaunt to Alpha Centauri and back might only seem like a stroll down the shop, but it will age you by several lifetimes. So much for the science fiction of space travel. But that doesn't detract from Planet of The Apes, or any other story based on the premise of the returning astronaut.
To quote my father, "Never let the facts get in the way of a good story."