system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: I seriously can’t wrap my head around the idea of “time is relative”.

I just don’t get how time is faster and slower on planets that aren’t Earth and how this affects how we age as well.
gpt
Time being relative is a byproduct of the fact that the speed of light in a vacuum is a constant regardless of your frame of reference.

That probably sounded science-jargony and didn't help, so let's take a step back and talk about velocity/speed and frames of reference. There's a classic physics thought experiment where you have a truck going down the highway at 55 miles per hour, and in the back of the truck is an athlete or robot or something that can throw an object out of the back of the truck at 55 miles per hour going the other direction. From the frame of reference of the truck, the ball will be going backwards at 55 miles per hour (because the robot/pitcher/whatever and ball were stationary from the reference point of the truck), but if you're looking at this from the side, the ball will seem to stay right where it was released, because the imparted force that accelerates the ball to 55 miles per hour backwards is exactly cancelling out the forward velocity (from earth's reference frame) that was bestowed onto it by the truck.

Now here's the next part of the trick, and where our intuition fails us: *Light in a vacuum moves at the same speed no matter what you're doing around it.* Let's take our truck and turn it into a rocket that can move at a significant percentage of the speed of light. And let's replace our ball-throwing person/robot and replace it with a flashlight. If this behaved the same way that the ball did, if we took our rocket and accelerated it to 0.5c (or half the speed of light) and from the reference frame of the earth looked at the light from the flashlight (it's a really bright flashlight), it would appear to be moving at 0.5 c as well (since it would be moving at 1c but would have to negate the 0.5c the ship was moving at), but we've tested this and despite it making no intuitive sense, *the light from the back of the rocket will be traveling at 1c (or just c)*. Similarly, if we take the flashlight on that same rocket and point it towards the front, and we take our intuition from the ball experiment, we might expect that light to be going at 1.5c, but it, too, will be going at 1c *no matter where we measure it from.* The ship sees it going at 1c, Earth sees it moving at 1c, Mars sees it moving at 1c... you get the picture.

This is a case of *special relativity* - where velocity will cause time dilation and an increase in mass, weirdly. This is true of all velocities, but it's only really significant once you get closer to the speed of light.

As for gravity, it's the same basic idea; the gravitational force should accelerate anything towards its center of mass, but since light's speed is constant, time in the local area gets distorted by the gravity.

And as weird and alien as this all seems, we've managed to prove this experimentally using nuclear clocks and fast planes. Perfectly synchronized nuclear clocks, one left stationary at sea level and one put into a plane and flown around at a high speed by our standards but low speeds by light's standards, desynchronized as much as Einstein's equations predicted they would in those conditions.