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
It’s… weirder than that.
First off: it wouldn’t be people on different parts of the planet experiencing time differently. For relativity to be in play you have to be moving at different velocities through space and to oversimplify, everybody on Earth is moving at velocities so close to each others’ relativity won’t kick in. (We’ve done some stuff with jets where it came into play but let’s stick to easier examples.)
“Now”, as a moment, is a hard concept. If you imagine time as a line, “now” is like a point on that line. How wide is a point? They don’t have width. They indicate a VERY precise and infinitely accurate location on the line. So “now” as a moment will exist for everyone in every frame of reference, but it may feel like it happened at a different time.
That makes more sense if we talk about “a width” of time, or “a duration”. Let’s expand from “now” to “from 5 seconds ago to 5 seconds from now”, a 10-second interval. Without relativity, let’s say we draw that on a line representing time. It’s 1 inch long. It starts at a specific moment and ends at a specific moment.
What relativity does is says if we consider those start and stop moments between two people moving at significantly different velocities with respect to each other, someone is going to think that interval is longer than the other person.
Practically speaking, it means something like if you and a person on a spaceship started a stopwatch at exactly the moment they started moving at 90% of light speed, and you told them to count to 10 then exit light speed, something weird would happen. You’d start your watch and they’d start theirs. Then you’d see them come out of light speed. Then you’d stop your watch at 10 seconds. But if you asked them why they stopped early, you’d find their stopwatch still read 10 seconds. What happened? Relativity.
When they started going so fast, time got “wider”. (Or at least I think that’s the way it works.) So what feels like 10 seconds for them at that speed is not the same as 10 seconds for you at your much lower speed.
BUT.
If you said the point was you want them to come out of light speed at the same moment YOUR stopwatch hits “10 seconds”, they could do the math and pull it off. They would have to stop *later* than when their stopwatch says 10 seconds.
But since their slowdown and your stopwatch hitting 10s happen at the same time in your frame of reference, that’s absolutely “the same moment”.
So that’s the trick with relativity. You can’t just tell people moving at astronomical speeds “wait about an hour”. You have to say “Wait the amount of time one hour from now would be in my frame of reference” and they have to do math to figure out how much time they will perceive in their frame of reference to hit that moment.
Visually, it’s like you take a straight line from 0s to 10s, but they take a curved line. They took “more time” but did something at “the same moment”.
It’s weird for us because we don’t experience relativity in perceivable ways, so we’re used to just using time units and everyone staying in sync. If we involve relativity, everyone has to account for their frame of reference to stay in sync, because time moves differently for each of them even though it’s still one continuous thing and they will all experience the same moments.
# TL;DR:
The person in the spaceship would think you’re moving in slow motion because of relativity. They will reach the same “now” as you, but if you’re both counting seconds they will count more seconds than you to get there.
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