How does gravity slow down or speed up time?
In: Physics
I’ll take a crack at this but it is hard to ELI5, probably because I am still learning it myself. Space and time are essentially one thing, spacetime. When something has mass, which matter does, it causes the spacetime to warp. Now imagine that I want to measure spacetime itself. I need a coordinate system of some kind.
So let’s imagine this in two dimensions first. I am standing on a sheet that is really really big, like infinitely big, and i want to make a grid with lines that are one meter apart. Now I cover my whole sheet with this grid and wherever the lines cross I call a ‘point’ and now I can know the distance from where I am to any point on my grid. Well what about time? I would need a clock, then I would know what time it was where I am. If I want to know what time it is at another point though I need to put a clock at that point. It does me no good to to have a bunch of random times on these clocks so I must synchronize them. I get a bunch of friends and tell them to all start their clocks when I flash a light (because they are too far for me to yell) so we can all start our clocks at once.
When my friends see the flash of light they start their clocks, but while light is fast, it is not infinitely fast so the further away my friends are from me, the further their clocks will be behind mine. Well, that’s okay if I need to know the time somewhere I can call my friend and they will read off their time to me. Now we have a grid of times and distances across the whole sheet.
Now suppose I have a friend who is really fat, I mean enormous. So fat that he actually bends the sheet he is standing on. When I look at where he is standing the lines will look curved because he is stretching them, even though I drew them straight. But we said that space and time are connected so when he bends the lines he is also ‘bending’ his clock. And it looks like it is moving slower that all the clocks around him.
The theory of relativity says that no reference point is special, meaning it doesn’t matter where I stand on my sheet, I should be able to measure the same spacetime. When my fat friend curved the sheet I should see the same curve of the lines from anywhere.
This is a sorta bad analogy for what is happening when gravity bends spacetime. The lower the gravitational potential, the slower the clock at that point must run. This is because gravity warps spacetime so the closer you are to the source of the warp, the slower your local clock will run.
Now this is all a very convoluted way of saying that because there is no “special” observation point that all reference frames must show us the same thing and the only way the math tells us this can happen is if there is a time “warp” or what we call time dilation.
If someone can do a better job, or if I got something wrong please let me know.
Okay. Not a physics major, just engineering but have had a physics teacher briefly explain. Sorry if I get this wrong, if I do I would love to be corrected!
The faster you move through space, the slower you move through time. So if you have a high acceleration (in this case gravity) you move through time slower. But remember u still will only live your same time length. Say you will die at 80, you will still feel like you lived 80 years, it’s just that earth time will move quicker relative to you.
This is an exaggerated example but you should get the point. If we were able to withstand an infinite amount of gravity on an infinitely large planet with infinite mass (yes I know it would be a black hole and realistically we would be crushed) we would cease to go through time, relative to everyone else at least. If we ever left the planet during our 80 year life span and try to get to Earth, Earth and our solar system would probably have been gone LONG ago.
Or on a planet with still insanely large gravity but not infinite (we still would be crushed), one day if we were able to teleport to Earth from this planet or view it somehow, it might be year 3000, the next day would be 4000 and so on.
I can’t answer the in depth physics question of WHY “The faster you move through space, the slower you move through time” exists. That would be a question for Einstein, or someone that’s certainly not myself. I hope this somewhat helped…
“time- dilation” effect is a consequence of Einstein’s theory of general relativity, which posits that the gravity of a massive body – such as the Earth – warps the space-time around it, causing the flow of time to speed up or slow down depending on its distance from the mass.
Gravity only slows time down. A clock floating in an empty universe would run the fastest and as the gravity gets stronger, the clock would begin to slow down. This is because bending space to cause gravity requires time to slow down to keep the invariant interval the same. I don’t want to get bogged down in math, but the invariant interval is something that all inertial observers will agree on regardless of how they’re moving.
Let’s imagine two cars each going at 100mph (units not important use. km if you’d rather). One is going directly east and one is going northeast. After one hour they have each covered the same distance–analogous to the invariant interval–but the northeast car has traveled less in the east direction than the first one–like time passes slower even though it was just as much space-time.
The car example is a little extreme since the normal time dilation is tiny inless travelling very far or close to the speed of light. A clock here on Earth will measure a year as 21 milliseconds shorter than the same clock on the Moon.
There’s a lot to unpack here. This ELI5 won’t be correct, but something this complex can’t really *be* correct at ELI5 level. Don’t take this explanation at face value. Take it more as a mere introduction to some of the general concepts, from one armchair Reddit user to another.
The universe happens to have a speed limit. No *thing* that is *in* space can go faster than this speed *through* space. We call it *c*. It stands for *causality*. It is the maximum speed by which anything can *cause* anything else to happen in the universe. Things that do not have mass will always travel at this speed in a vacuum, all the time, always, no exceptions. Photons (light) happen to be one of these things, which is why you almost always hear *c* referred to as “the speed of light”. *c* actually has nothing to do with light in particular, but the name is stuck.
In Einstein’s general relativity, it is said that space and time are actually two pieces of the same thing, called *spacetime*. Spacetime is four-dimensional, with three “space-like” dimensions and one “time-like” dimension. …perhaps I should back up for a moment.
Think of a piece of paper. It’s two-dimensional, right? Draw a doodle on it. Now lay another sheet of paper on top of it. Draw another doodle, similar to the first, but slightly shifted. Now another sheet, draw another doodle. And another. And another. And another, lots of times. What do you have? A lot of sheets of paper with static, 2D drawings on them. But if you bind them to one side, and flip through them very quickly… the doodle starts to look like it’s a single illustration that’s animating! You could say each page of the flip book is a snapshot of time, and flipping through the doodle is like moving ahead through time, one snapshot at a time. The sheet still only has two “space-like” dimensions, but by stacking them up you’ve created a third, “time-like” dimension. Congratulations, you’ve invented spacetime! A 3D spacetime, anyway. You could think of our 4D spacetime as something vaguely similar — the universe is a stack of 3D frames, stretching into the fourth dimension of time. To move through time is to advance frames up the stack.
Now, let’s look at *c* again. As it turns out, *c* isn’t just the speed that massless things travel through space. It’s the speed that *everything*, even *you*, travels, through space *AND* time. If we think back to our flipbook, and we ask which directions our doodle could move, it would appear to make sense that it could only move left, right, up, or down. But you could also think of pointing that arrow *out* of the page, through time! Such it is with spacetime in four dimensions — your speed can point purely through space, and you can use the full speed to go through it, or you can divert some of that speed to start moving through time. So, the faster you move through space, the slower time ticks for you, and the slower you move through space, the faster time ticks for you.
(Sidenote: remember I said that massless things always travel at *c* through space? So, they aren’t diverting any of their speed to move through time, so… does that mean they don’t experience time? Yes! If you were a photon, you would have no concept of time. You would be instantly created and destroyed in the same instant from your perspective. In fact, it doesn’t even make *sense* to say “from a photon’s perspective”. The mathematics completely break down when you pretend to observe things as a photon. Physicists would phrase this as “lightspeed reference frames are invalid”. By the same principle, anything that has mass CANNOT move through space at *c*. So a tiny amount of speed will *always* be pointing through time. Thus, anything with mass will ALWAYS experience time, no matter how fast they go.)
Another thing to note about spacetime is that, unless acted on by a force, anything traveling through it is ALWAYS traveling in a straight line. This should be a pretty simple concept to grasp. Newton’s first law, right? An object in motion stays in motion. If I shoot a marble across a table, it will keep shooting in a straight line until it hits something. That should be pretty intuitive. Put a pin in this idea, we’ll come back to it in a second.
So what does gravity have to do with all of this? Well, funny thing about general relativity… in general relativity, gravity as a “force” doesn’t exist! It’s an *illusion*. What’s really going on, is that things with mass *distort spacetime itself* just by existing in it. The classical way to demonstrate this is with a 2D bedsheet, or a trampoline surface. If you put a heavy marble on the sheet, it will sink a bit, and deform the sheet around it into a funnel. The closer you get to the marble, and the heaver the marble is, the steeper the funnel gets. Now, let’s take our marble shoot example from before and shoot it so it just barely misses the heavy marble. What happens? The shot marble goes in a straight line… until it hits the funnel. And then it appears to deflect and bend around the massive marble. What just happened there? Did the heavy marble “attract” the small marble through some invisible force? Well… no. The heavy marble was bending the sheet itself. All that flat space around it has now been stretched out into a funnel with more surface area. The light marble did what it was always doing, traveling in a straight line, but since the sheet got bent into a funnel, the path that is a “straight line” now follows the contour of the funnel for a bit, creating an arched path. From the light marble’s perspective, it was traveling in a straight line the whole time, but from our distant perspective, it looked like it was pulled in to the heavy marble by some invisible force. That is Einstein’s explanation for gravity. It is the effect you see when something moves through deformed, or *curved*, spacetime.
So, okay, gravity can deflect things moving through space, but what does this have to do with the *time* part, specifically? Well, mass’s curvature effect on spacetime affects the time dimension, too. There’s really no super intuitive way to explain this, and any real physicist will probably weep at this gross and wrong simplification, but here we go: the way mass is “stretching” 4D spacetime changes the way you advance through time, compared to how you would if you weren’t near something very massive. Just like how the heavy marble stretched the space of the sheet, effectively cramming more space into an area than flat space would normally contain, and the light marble had to traverse that extra space and got deflected, mass stretches the time dimension, effectively “cramming” more time there than normal. And you, being close to that mass, have to traverse that “extra” time, but at the same speed of *c*. To an observer far, far away, it looks like your clock is ticking slower as you are bogged down with this stretched, curved time dimension.
And that is why gravity “slows down” time.
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