system
Given the following post, respond as if you're commenting on Reddit:
human
Time dilation: relative to what?

This question may be badly phrased, so please bear with me.

I know that time slows down for moving objects.  But I've also been told that there's no such thing as an absolute frame of reference.  So, if a spaceship is moving very quickly relative to a planet, who's to say that it's the spaceship moving instead of the planet?  Would an observer on the spaceship see clocks moving more slowly on the planet, since the planet is moving quickly relative to the ship?  And would an observer on the planet see the exact opposite?  So, how does one determine which object is "moving", and whose clock moves more slowly?
gpt
You don't tell which one is moving and which one's clock is running slow. You just say that the people on the ship observes that the clocks on the planet are running slower than on the ship and that the people on the planet observe that the clocks on the ship are running slower than on the planet.