Lightspeed

Arthur C. Clarke on near-lightspeed travel, from Childhood's End:

    "The important fact was that I knew how far they had to travel, and therefore how long the journey took. NGS 549672 is forty light-years from Earth. The Overlords ships reach more than ninety-nine per cent of the speed of light, so the trip must last forty years of our dine. Our time; that's the crux of the matter.
    "Now as you may have beard, strange things happen as one approaches the speed of light. Time itself begins to flow at a different rate-to pass more slowly, so that what would be months on Earth would be no more than days on the ships of the Overlords. The effect is quite fundamental; it was discovered by the great Einstein more than a hundred years ago.
    "I have made calculations based on what we know about the Stardrive, and using the firmly-established results of Relativity theory. From the viewpoint of the passengers on one of the Overlord ships, the journey to NGS 549672 will last not more than two months-even though by Earth's reckoning forty years will have passed. I know this seems a paradox, and if it's any consolation it's puzzled the world's best brains ever since Einstein announced it.