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
In modern relativity, *later* is considered a relative direction through spacetime. That means it’s relative to the way you’re pointing, like *ahead* is a relative direction through space.
You’re probably used to thinking of *later* as an absolute direction, like *north*: “I’ll see you later — in one hour”, you might say to your friend, agreeing that you’re both experiencing the same kind of time.
But in relativity, objects moving relative to one another are actually *experiencing time in different directions*. If you’ve ever drawn something in perspective, you know about foreshortening. Time dilation and all that weird stuff is just foreshortening, but applied to time itself rather than to a spatial direction.
One weird side effect of recognizing that the direction of *later* is different for different people, is that you also must recognize that *right now* is different for different people. *Right now* is the set of all directions that are perpendicular to *later*. So if you accelerate (change your direction of *later*) you also change your idea of *right now*.
People on the opposite side of the world from you are moving in the opposite direction because of Earth’s rotation. So *right now* points in slightly different directions through spacetime for them, than for you. Even if you synchronize your watches (and there are many ways to do that), you’ll disagree on what time it is at, say, Alpha Centauri.
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