Theory Relativitas

Why is it called a theory of RELATIVITY?
Because time and length are no longer absolutes. You’ve got your digital watch on your wrist and a metre ruler on your desk. These seem like absolutes: a second and a centrimetre for you must be the same as they are for me, and the same as they are on Alpha Centauri. But they’re not.

If I stay on my balcony while you start a career as an astronaut flying round the galaxy at an incredible speed (and it would have to be pretty close to the speed of light: 300,000km/sec), and if you could later whiz past my balcony so that we could somehow compare watches and rulers, your metre ruler would be smaller and your watch would be going slower than mine. (Actually that wouldn't be possible because the human eye can't spot things moving at that kind of speed, and spaceship rockets do nasty things to balconies that are only a few metres away. But if it were practically possible, it would be fun.)

While you’re out in space travelling at some unbelievable speed nothing seems to you to have changed. It’s only if you have a chance to compare measurements of time and length with those back home that you see that something odd has happened.

All the introductions to Einstein talk about the twin paradox. What's that?
One 25 year old twin stays on earth while the other, fresh out of astronaut school, sets off on a space voyage travelling at 90% of the speed of light. After 10 years in space, with her mission accomplished, she turns round and heads back to earth. By the time she lands she knows from her on-board clock that 20 years have passed. She is now 45 years old. Fortunately, her study of relativity has prepared her for the shock when she sees her twin sister, who is now 71 years old.

Conclusion: Space travel, when it is really, really fast, is also time travel: you travel into the future without getting that much older yourself.