system
Given the following post, respond as if you're commenting on Reddit:
human
What is it that makes the mechanism of a watch slow down in space travel?

Really I guess I'm looking for a layman's description of what exactly causes time to pass... Would people age differently in different parts of the Universe because of a difference of gravitational forces on their bodies? How does this affect the actual ticking mechanism of a clock or watch?

tl;dr- I watched Interstellar and used to consider myself a kind of smart person... Looking to rebound myself.
gpt
Imagine for a second that your buddy Jim can throw a baseball at 60mph, but he wants to throw it faster. So he gets in the back of his buddy's pickup and has the buddy drive at 30mph while you stand off to the side. Now Jim throws the ball, how fast is it going? 90mph, right?

Well, it's only 90mph to you, because you're not in the truck. To Jim, the ball only looks like it's going 60mph because Jim is already moving at 30mph in the back of the truck. The difference in the speed is due to which frame of reference you're in, whether it's relative to being in the truck, or relative to standing there watching the truck. We call the general concept of this difference *relativity*.

Now let's say you did the same with a flashlight. It shoots out light particles, so maybe if you got in a fancy suped-up ultra fast space shuttle, you could get the light particles to go even faster. Well, not to you, because you're on the shuttle, but to your buddy Jim standing back on the ground watching you.

But it doesn't work!

The speed of light doesn't change with the frame of reference like baseballs do. Relativity doesn't work the same with light like it does with baseballs. The nature of light is so much different than the nature of a baseball, that it doesn't care what reference frame the viewer is in. It's always the same speed. This phenomenon isn't very natural sounding, but it has been shown many times to be true, and it's even part of the reason that nuclear fission works!

Well, this whole light not obeying reference frames causes all sorts of problem. Like that nothing can go faster than the speed of light! It's kind of like a boundary on how fast things can go. Pretty much only light can go the speed of light because it's massless, but most other stuff has mass so it can't even reach the speed of light.

But if "massful" stuff gets close to the speed of light, crazy things start to happen. Things called *relativistic effects*. Things like time slowing down. Except it doesn't slow down for anyone on the massful thing. It thinks time is going like normal. The clock on your shuttle ticks a normal beat just like it always did because you're on the shuttle, going the same speed as the shuttle. Time instead slows down for everyone not going the speed who's looking up at you, like your buddy Jim who's back on Earth. He's watching you zip around at maddening speeds, and when he looks at your clock on the shuttle, it seems to be going slower to him than the clock on his wristwatch. Time is passing slower on the shuttle than it is back on Earth, but only your buddy Jim and everyone else on Earth can tell.

That's what happens in *Interstellar* except the situation is switched because of the black hole. On the flip side of massless light is really massive stuff, like black holes. Their gravity is so strong, that it literally bends light which can also cause relativistic effects to happen. Matthew McConnaughey and Anne Hathaway go down to this planet that's in a place near the black hole where relativity is a bit screwy and time moves slower, while the other guy on the ship is aging waiting for them to come back. For him, it takes years for them to return, but to McConnaughey and Hathaway it was just a few hours. All because light has to be the same speed no matter who's looking at it from where.