Respect to what time is calculated in space

Question

This may be the silly question but I have read that time appears to move slower near massive objects because the object's gravitational force bends space-time and the phenomenon is called gravitational time dilation i.e. time moves slower as gravity increases but I want to know how and concerning what time is calculated at a different position of space do time is calculated concerning earth time. Also, since times moves slower as gravity increases this means what

Does our clock hand move slower there if so how since it depends on the mechanical property or it means ageing of the body?
Does it depend on the ageing of the body if so how since body ageing depends on the environment and natural things?

I have searched this type of question on the internet but I did not get a proper answer majority of the answer are based on theories and formula but I am looking for an intuitive and rigorous answer.

Mauro Giliberti · Answer

What you're saying is correct: time slows down near a massive object (let's say you are near this massive object, and let's say the said object is the earth). The question "in respect to what time" is not stupid, indeed: it's this question that gives Einstein's theory its name. Time slown down with respect to somewhere else, in this case with respect to somewhere with a weaker gravitational field (let's say a person, Fred, is in a spaceship, far away from earth). Most importantly, time slows down in you frame of reference (where you stand) as seen from Fred's perspective, but not yours! This means that if you held a giant clock so that Fred could somehow see it, he'd say that it's running slower. For you, since everything in your frame of reference (the clock, your body, your mind, everything) is slowed down, everything would feel normal. But if Fred comes back to earth after a while, you could see that your clocks (that before Fred's departure were synchronized) are saying different times.

There are many YouTube videos about this: check out PBS SpaceTime and Veritasium for something more "visual".

Respect to what time is calculated in space

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