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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Due to very complicated reasons called General Relativity, time “elongates” the more gravity there is.
Now, at first it would seem it means someone on earth would age slower than someone on the ISS.
But also time elongates *with acceleration and speed*.
And the ISS is going wayyyyyy faster than all of us ground-locked humans (it revolves around the Earth in 90 minutes !). As such, its astronautes experience a slower time than we do.
In a sense… If we assume this change is noticeable (it’s not by human standards), they would still *feel it* normally, from their point of view, but would see someone on the surface as moving faster than that someone would see themselves.
TL;DR: From an outside observer, someone that’s going real fast would seem to age slower than someone immobile. Because gravity, speed and acceleration dilate time.
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.
Time is affected by both gravity, and how fast you’re moving. Both of those things are different for the people up in orbit on the International Space Station. Not by much, but by a tiny bit. A very tiny bit. Like, a smaller amount than you could ever notice.
> is it considered to be a form of time travel?
Only really by clickbait articles. Theoretically if you could go fast enough, you could “travel into the future”. As in, you could experience, say, 1 hour on your space ship, while the Earth experiences 1 year. That’s what’s happening when astronauts go into space, except it’s such a small difference as to be essentially meaningless to humans. And of course, if you did travel to “the future” like this, you’d be stuck there.
One area where it DOES matter is in GPS systems. If you’re driving down a motorway in your car, your sat nav knows where you are because of communications with satellites. In this situation, the time difference is big enough that it does actually matter, so scientists have to be very clever and take this into account when designing them.
People age slower in space *compared to people on the ground*. People in space age exactly the same – no slower, no faster – compared to *other people in space with them*.
Real ELI5:
Time depends on 2 things – speed and gravity. Faster the speed- slower the time and vice versa. Stronger the gravity- slower the time and vice versa.
So when you see movies where people are aging slower in space, they are either traveling at a very high speed or they are under the impact of a lot of gravity(bigger plants and blackholes).
Time is relative, so just because you are traveling very fast or are near strong gravity field, doesnt mean you will experience the time in slow motion. You will experience it as you are doing it right now. Its only when you compare your clock to someone who was in opposite condition, is when you know your time passed differently.
It’s not being in space per se but the speed at which you’re moving. The faster you move in space, the slower you move in time relative to someone travelling more slowly (e.g. on Earth).
If you took a trip to Alpha Centauri and back, travelling at close to the speed of light, you’d find on your return that you had aged significantly less than people left behind on Earth had, though you wouldn’t personally have any perception of time having slowed down.
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