I’ve seen things of people aging slower in space, but any time I look it up it always comes with like 15+ words I have to look up to understand or concepts im not aware of. How does this work, and is it considered to be a form of time travel?
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We’re always travelling through time.
However, it is possible to travel forward through time at different rates; the passage of time is relative. One second for you might be two seconds for me, and so on.
There are two effects that mess with time (they also mess with space, or rather it is better to say they mess with spacetime, rotating and squishing time and space together in interesting ways):
* things that are going faster than you are slowed down in time; moving clocks run slow (Special Relativity),
* things deeper in gravity wells are slowed down in time; lower clocks run slow (General Relativity),
This gets fun when dealing with people in space, and specifically with stuff orbiting the Earth. The higher something’s orbit the faster its time runs compared with ours (because of the GR effect). The lower something’s orbit is the faster it has to be going (relative to the ground) to stay in orbit, so the slower its time runs compared with ours (because of the SR effect).
[We have to be a bit careful about language here, because “slower” and “faster” are terms that usually involve time as a fixed thing, we look at how much of something we get “per second” – by time running “faster” I mean that for every second you experience it will experience more than a second, and vice versa.]
This means that things in low orbits experience less time than us on the surface [overall – it gets a bit complicated because from their point of view *we* are moving fast, so *our* time is running slow], but things in higher orbits experience more time than us.
[Wikipedia has this handy graphic](https://en.wikipedia.org/wiki/File:Time_Dilation_vs_Orbital_Height.png), showing how this works out. The blue line shows how much time is sped up due to how high things are. The red line shows how much time is slowed down due to how fast it is going. The purple line is the net effect.
So the International Space Station (around 400km up) loses about 20μs per day compared to the Earth’s surface. Spend 6 months on the ISS and when you get back you will be ~0.004s younger than you would have been had you stayed on the ground (or if you had a really accurate clock, your clock would be 0.004s behind one left on the Earth).
GPS satellites are much higher – about 20,000km up. They are high enough that the GR effect is dominant, so their clocks run about 40μs per day fast. After 6 months a clock on a GPS satellite would be ~0.007s ahead of where it would be on Earth (and GPS has to account for this change).
The “break even” point is about 3000km up. That’s where the “slowed down due to being fast” and “sped up due to being high” effects cancel out.
Note that this only applies for *orbits.* Spacecraft or probes travelling will experience different effects depending on how fast they are going and how high up they are.
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