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
I’m going to ignore gravity because that stuff is difficult.
You’re moving at certain speed and direction. Everyone who is moving the same speed and direction as you is in the same “inertial reference frame” and you all experience time the same way. Your clocks all tick at the same rate. You age at the same rate. You all have the same concept of “now”.
If you and the other people watch someone in a different “inertial reference frame” you’ll all see that their clocks are moving slower and that they are aging more slowly.
Weirdly, those people in the other inertial reference frame will say the same about you. They’ll say it’s your clock that is moving slowly.
How to resolve this difference. How can you both be correct. It seems like there is a “twin paradox”. A pair of twins are born. One of them gets on a spaceship and travels very far very fast. Which twin is older? There isn’t an answer and there doesn’t have to be. The twins are far separated and nothing they do directly affects the other. The younger twin can’t use his smooth hand to feel the older twin’s wrinkled face. So long as that can’t happen, we don’t have to agree about which one is older. And even the twins don’t agree. They both think the other is younger. Both will disagree about who had their 40th birthday first. The order of events is different.
But what if one of them turns around and comes back? When they meet again don’t they need to agree which one of them has more wrinkles so the young hand can touch the old face? They do have to agree.
People in different inertial reference frames don’t have to agree about the order of events if the events don’t happen at the same location. They only have to agree when multiple events occur at a single location.
So how do we determine which twin is older? One of the twins changed his inertial reference frame. If they both stayed in the same inertial reference frame they could keep disagreeing about how much time had passed for each other. But in order to reunite, one of them changed inertial reference frames, and that moving between inertial reference frames changes the calculations.
Mathematics does not describe reality so it’s not really a mathematics question. It gives you tools to examine and predict reality, but does not tell you what reality is.
“Now” doesn’t really mean much aside of your own typically consistent’ish perception of time. You can’t, in any manner whatsoever, instantly tell anyone else that now is your now, not even theoretically, as the speed of light limits your ability to convey such information.
You also can not possibly know what’s happening somewhere else right now. You could make predictions, sometimes very good ones, but still, it’s just predictions.
But if you do apply some math, you can create a consistent understanding with others about when something happened to you, albeit how long ago it was in your perception might still be different than in someone else’s perception. E.g. to you something might have happened 5 hours ago and to them it was 10 hours ago.
The concept of “now” is not really something that physics defines, so.. No, you can’t say we all experience a now now.
Technically “now” isn’t really a thing anywhere but here.
The idea of a fixed “at the same time” doesn’t actually work in relativity, because “the same time” would differ based on the perspective.
Right now only exist at a single point at a time.
It is extremely counterintuitive and thankfully doesn’t really matter because at such small scales as different parts of the same planet, the reality is close enough to how we normally think of it that we might as well trat it like it is.
If you want to get really technical you have to deal with light cones and and different observers experiencing reality different based on where they are and how fast they are going.
I thought for a moment you meant the term now. Which is a rabbit hole especially where I come from.
Now – means soon
Now now – soon but later than now
Just Now – indeterminate time after now
Does “now” even exist? If so, how infinitesimaly small is it?
Ur time zone is different from other countries. So if it’s like around 1pm in the USA, in the UK it would be 6pm. It’s different times but you are experiencing things at the same “time”on the same globe.
“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.
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.
On Earth? Yes, there is an objective present. Things are happening across the world simultaneously as your existence. Someone across the word is scratching their nuts as I type this. Someone else is sneezing. But “now” is constantly moving. By the time I send this comment, the nut scratcher and sneezer are probably done doing their thing. But they’re still breathing, blinking, existing at the exact same moment as you and I.
Yes, though you will likely never notice the difference.
A scientist took his sons way up on a mountain, and after a weekend proved they had in fact traveled further in time than his wife had, by billionths of a second.
By being so far up, they literally traveled further through space that weekend than she did, and in turn, traveled further in time.
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