Question: Why do people age slower in space?

Duncan McNicholl answered on 13 Nov 2017:

You mean the twin paradox? That’s some crazy complicated stuff, but basically it’s about the fact that they’re moving really fast, not that they’re in space. Also, it’s not just that they’re aging slower: time is moving slower, so they think that less time has passed when they get back than we do. The time-slowing-down thing is a bit of head-scratcher, but it’s because of something that Einstein discovered called relativity: if the speed of light is just the speed of light, and it can’t be any faster or slower, then when you’re moving really fast, time has to go slower to stop the universe from breaking. The same thing actually happens if you’re next to a really big thing, like a black hole (this happens in Interstellar) or the sun, or even the earth. In fact, GPS satellites are far enough away from the earth that from their perspective time down here is moving more slowly, and they have to correct for that to be able to do their jobs properly.