I have only a very vague awareness of the idea of relativity but I’m aware that there’s a concept that people in orbit experience less time than those on the planet due to gravity, in some way.
Does this mean that the idea of “now”, as in a moment that is right now, is marginally different for people in other places? Are they experiencing a moment that is in my objective future/past, in a mathematical sense?
In: Physics
“Now” is a single point. To compare things you need two points.
You have a “now,” that is your “now.” But your “now” isn’t even the same for you, your current “now” is about to become your “then.”
So I think your question is more “if two things happen at the same time for you, do they happen at the same time for everyone else?”
To which the answer is no.
Two things can happen at one time for one person and at different times for another person.
In some cases the order can change as well; two events can be in one order for one person, but a different person for another person.
But there is a limit on that. Events that are “causally connected” (i.e. something can get from one to the other) always happen in the same order. If light can get from event A to event B, event B always happens after (or at the same time as) event A. There is no point of view in which their order is reversed.
> people in orbit experience less time than those on the planet due to gravity
To be pedantic, people in orbit experience *more* time than those on the planet *due to gravity*. Gravity squishes time (and space) up, so time runs slower the lower you are – the more gravity there is. However, people in orbit are travelling really quickly (compared with us on the surface of the Earth), and that means their time runs slower than ours. And in low orbits that effect beats the gravity one, so things in low orbit experience less time than on Earth. About 12,000km above sea level this balances out, and above that things in orbit experience more time than we do on the surface.
[Wikipedia has a handy diagram](https://en.wikipedia.org/wiki/File:Time_Dilation_vs_Orbital_Height.png) to show this. The red line is how much time is slowed down due to zooming around the Earth in orbit. The blue line shows how much extra time people get due to being higher up (away from the Earth’s gravity). The purple line is the overall effect.
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