The Twins Paradox
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.
Q: 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.
However, if something goes faster, time will slow down for it. It’s an effect called Time Dilation, which is part of Einstein’s Theory of Special Relativity. Long story short, if you’re put in a spaceship, the faster you travel, the slower time will be for you compared to time for people back on Earth.