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 relatively, whether things happen at the same time (simultaneity) is also relative to an observer.
That is to say, it’s possible for event A to happen before event B by one observer, but for B to happen before A by another observer.
The answer of your question lies within the definition.
is it “now” as the exact defined moment in time you are experiencing right now? If so, yes, the moment is identical to them because time may be relevant for the object. “Now” is always now, if you’re at a space station or on earth because it all has to be relative to that object (For example you in this case).
But of course, the relativity of time makes it possible that your “now” in relation to another object (for example a space traveller) is different.
Long story short:
Your now is always now. Now is one exact point in time.
Imagine you have 2 exact watches.
You are wearing one and your twin going to the end of our solar system and come back your watches may have a small difference.
This is time dilation. The faster an object moves through space, the slower it moves through time. People on the Earth are in fact moving at different speeds based on the rotation of the Earth (faster at the equator or at higher elevations, for example), but the difference is so small that the relativistic effects are negligible.
Once you get into space, things change. The ISS is traveling at 17,500 MPH relative to Earth, and that is measurable. It’s not significant, though – in a year on the ISS, you will have aged 0.01 seconds less than someone on Earth.
So, yes, as you are reading this, someone in space or on the other side of the planet is experiencing time differently, but not in any way that affects the human experience, and whatever they are doing is the same “now” as yours.
Simultaneity – the idea of things that happen “at the same time” is relative. For all practical purposes their experience is simultaneous to yours but in terms of actual physics the order of events doesn’t have to be the same from all perspectives. This is bizarre and unintuitive, but such is physics sometimes.
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.
Now is now everywhere. But due to relativity, you could wait 10 seconds, and someone else might wait 10.001 seconds, and you’d still be at the same point in time together. Because time moves faster/slower due to relativity.
It’s not even possible to define ‘now’ mathematically for one person. What you see, what your brain is making sense of and what your body is subject to all happen at different times.
Time is an artificial construct we imposed on the observable universe. Technically, everything you sense is in the past because it takes time for light to reach your eyes, then for the sight/sound/touch type signal to be translated into electrical signals, then more time for it to travel to your brain, and for the brain to interpret those signals. I want to say our sight “delay” is 80 milliseconds if I remember correctly. Something like that.
Having said all that, the concept of “now” becomes hard to determine. Let alone compare two events to see if they happened at the same time
I believe there’s actually no correct answer for this because there’s no definitive, universal point of reference that everyone can use
> 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?
Yes, because time is relative there is no such thing as one single “right now” in our universe anymore then there is one single “right here”. If we paused the universe “right now” then there are events that have happened from my perspective that haven’t yet happened from yours, and vice versa. We call this [Relativity of Simultaneity](https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity).
It gets complicated (because this is legit Einstein shit we’re talking about) but the effect is tied to spatial distance. In other words the closer our clocks are synchronized to each other the further out we have to go spatially before “right now” begins to look different for us, and the further out we go the more our “right now” becomes separated. So if the difference in the passage of time between the both of us is really small then we have to look really far before “right now” begins looking different.
Here on Earth the time dilation we experience in our everyday life between one another is so infinitesimally small (but very real nonetheless) that it’s pretty much indistinguishable… But if we hit the pause button and look out far enough in the universe there is different “right now” for every single person reading this.
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