I’ve heard the term “space time” thrown around, and that gravity “bends space-time” but how does that work?
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Imagine a large bedsheet, being held up at all four corners. If you drop a marble in the middle, it’s going to make the bedsheet sag, or “bend”, in the middle.
If you drop a baseball, or a bowling ball in the middle, it’ll make the bedsheet sag even more. If you drop the marble in with the bowling ball, it’ll roll right towards the bowling ball, along the fabric of the sheet.
It works similarly with spacetime. Mass bends spacetime in such a way that it attracts things with mass towards each other. The more mass, the more bending, and the more attraction.
Edit: [here’s a literal children’s book about it.](https://old.reddit.com/r/nextfuckinglevel/comments/w4x3ds/books_for_babies_that_are_on_another_fucking_level/)
spacetime: suppose you are floating in the middle of space, and i pass you at near the speed of light. we will disagree on long a second is, and we’ll disagree on how long a meter is. however, we will always agree on how fast light travels (in meters per second). it seems strange to those of us who crawl around the universe at infitesimal fractions of the speed of light, but it’s a fact of the universe that’s been borne out time and time again by experiment. distances in time and distances in space dilate in an equal an opposite way that leaves distances in space-time constant regardless of inertial frame (ie how fast we’re travelling relative to each other). it’s natural for physicists to consider space and time together like this so they can describe the laws of nature in a way that don’t privilege any inertial frame. from my perspective it is I that am floating in space and you are passing me at the speed of light.
curvature of spacetime: imagine you and I were two-dimensional beings living on a sphere. locally, space looks flat: if I draw a triangle, its angles add up to 180 just like Euclid said they would. however if I draw a REALLY BIG triangle, like one that goes from Panama to the North Pole to Africa, I find that in fact I make a triangle that has three right angles in it! this clues me in to the real nature of reality, that I live on a curved surface, despite the fact that I, as a 2D being, am not able to physically point into the third dimension. curved spacetime is basically the same concept, except with one more dimension. locally, space looks flat and I can refer to points as orthogonal x, y, z coordinates, but at large scales, particularly around massive objects, I can see that space is not flat, but it is bending in a way that can at least be mathematically described as “3D space being bent around another dimension which we are unable to point into”.
Spacetime is exactly what is sounds like. You probably have an intuitive grasp of “space”. You also probably have an intuitive grasp of “time”.
When you combine the two and look at *where* and object is at any given *moment*, you’re looking at the object’s path through spacetime. It’s pretty much just that. Spacetime is just the “position” (both in space and time) of events.
So why is it important, and why do we combine them together? Well, it turns out that your movement through *space* affects time, and your movement through *time* affects space.
For instance, when you stand still, you’re travelling directly through time, but not through space at all.
When you start moving, you’re essentially “rotating” in spacetime, so part of your motion is through space now.
This “rotation” so that part of your movement is through space actually means you move through time more slowly! (It’s not noticeable to you because you’re moving through space reallllly slowly, relative to light)
Things tend to follow straight paths in spacetime. You either keep moving forward through time, or forward through space, but you don’t usually turn unless you put effort into it.
We’ve noticed that those straight paths “bend” around massive objects though. And every object “bends” the exact same way. It’s almost like they’re not bending at all, and instead they’re all following a straight path, just like you’d expect, and for some reason we see that straight path bending.
It turns out the “bend” around massive objects can be perfectly described if you think of *spacetime itself* bending around massive objects, and everything just following a straight path in that bent spacetime.
Why does spacetime bend around massive objects? We… don’t know. We’ve observed some effects that can be perfectly described and predicted by the math that you get if you assume spacetime bends a certain way. Black holes, gravitational lensing, gravitational waves, all stuff that we can predict perfectly by assuming spacetime bends. We know it *does* bend because of these perfect predictions. Why it bends? No clue.
It’s possible we’ll come up with a deeper theory of reality that explains spacetime bending. As of now though, the best we can really do is say that it does.
spacetime is a unified model of space and time. In day to day life, we think of there being 3 dimensions in space, and time as something unrelated. However, in certain circumstances, it becomes clear that the 3 spatial dimensions and the 1 time dimension are closely enough related that it makes sense to think about not just space and time separately, but as one big four-dimensional “spacetime.”
Objects trace out paths through spacetime. If you see an object standing still, you can think of it as tracing out a path that’s always at the same place in space, but moving straight ahead through time. A moving object, on the other hand, has a path that’s tilted a bit. It stretches through time, but varies where it is in space depending on where it is in time, since the object’s moving.
Gravity doesn’t bend spacetime, gravity is the *result* of bent spacetime.
Objects will tend to move in straight lines through spacetime unless something else acts upon them. For instance, if two objects bounce off eachother, the paths they take would look like lines that got bent at the place they touched. But “Straight lines” behave strangely on curved surfaces. For instance, if you and a friend stood on the equator and walked in straight lines due north, at first it would look like you were walking perfectly side by side. But if you kept going, the curvature of the earth would mean that your paths would eventually cross at the north pole. If you wanted to keep apart, one of you would need to stop walking in a straight line and turn away. Gravity works in a similar way. Mass bends spacetime, and it bends it in such a way that a straight line through spacetime will tend to aim closer to that mass. Even though the orbit of the earth around the sun seems curved, it’s actually the straightest path the earth can take through spacetime, and when you stand on the ground, the weight you feel on your feet is actually the feeling of the ground forcing you to “turn” through spacetime instead of letting you continue in a straight path to the center of the earth.
Once you get down to that level, ‘how’ and ‘why’ don’t have a lot of meaning. You can put names on a bunch of magical unicorn shit, but that just moves the problem.
The ‘what’ is a more approachable question, though.
Think of the axes on a graph, or the coordinates in a game of battleship – that describes a 2d space, with the left-right and up-down lines at right angles to each other.
However, the space we walk around in is 3d; it has a back-forward axis as well, at right-angles to the other two. The universe is divided up not into a grid of squares, but a lattice of cubes, like minecraft, with its x/y/z coordinates.
As it turns out, time is just more of the same – past-future is just another axis *at right-angles to all the others*.
The universe has X, Y, Z **and T** coordinates to define a point.
It’s hard to imagine directly, obvs – but imagine all the pages of a flipbook stacked up on top of each other. Cut out all the blank bits of page, and you can see the characters as these kind of tall extruded shapes that curve as the character moves through the action of the scene.
The characters can’t perceive that shape as they’re only 2D, and only see things on the same page, but in our 3D space, we can see their shape-through-time. We can see the whole block, the whole 2d+T lattice that they’re embedded in, at once.
Well, real 3d objects in real space are the same – long curvy extruded shapes-through-time, but we can’t see the long-curviness directly, we can only see the same page.
That 4D lattice that 3d objects-in-time are embedded in, that’s spacetime.
Now as it happens, when you get a bunch of mass in the same place…. *the lattice itself bends inwards*.
Why or how? Magical unicorn shit. The question doesn’t mean a lot, right at the bottom of things.
What it *means* though, is that just a little bit of your movement through time becomes movement through space. As objects just sit there getting older, they end up closer to the mass, futurewards from now. Think of taking a dead-straight path on a motorbike, but the road curves a little beneath you – you end up in the gutter, even though you’re facing dead-ahead.
And that’s gravity. Be near a bunch of mass, you get closer to it over time. It’s not a force pushing you off the road, it’s just the road bending beneath you.
And by the same token – if you’re drifting sideways across the road, you’re making a little less progress down it – which is why time slows down a little in a very strong gravitational field. You’re spending a little of your futureward progress on downward progress instead.
All the responses in here are far too complicated so here’s a super simple version.
Spacetime is 4 dimensions. 3 dimensions you know XYZ, but time is also a dimension, and you are always traveling along that at a set speed.
Gravity is a warping of that spacetime, sort of like if you had a bowling ball held up by a bunch of elastic ropes, the mass of objects can’t pass through them, but will instead stretch the ropes. Those stretched ropes are gravity, they are trying to push to a state of equal energy.
Imagine spacetime as a big elastic blanket. Celestial objects like stars & planets interact with spacetime much like how a bowling ball would react with our blanket. If you set a bowling ball on the elastic blanket, it will stretch the space around the ball thus stretching time as well.
Spacetime is a 4D geometric construct on which general relativity is performed. This object bends and curves and twists in such a way that we can predict how things in the Real World behave. How it bends it dependent on the [Stress Energy Tensor](https://en.m.wikipedia.org/wiki/Stress–energy_tensor) which accounts for mass (mostly mass), pressure, velocity, etc.
Whether it actually bends depends on your model. For instance in theories of quantum gravity (note that such a theory does not *currently* exist in a complete form but it is assumed that this is the next big step in quantum physics) **gravity is not described as a bending of spacetime** however the actual effects would be the same.
People say “space-time” because space and time are not two entirely different things, but two aspects of the same thing.
“Bending” space-time is all to do with geometry. So, the rules of geometry you’re probably aware of are called “Euclidean” after the ancient Greek Euclid. They include rules like: parallel straight lines will never cross. It turns out that you can imagine what would happen if that rule didn’t hold. When you’re letting rules change, you sometimes have to make sure that you’ve got a rigorous definition of what before was pretty obvious, like: what is a straight line, anyway? A good general definition is that a straight line segment is the shortest distance between two points.
The easiest non-Euclidean geometry to imagine is the surface of a sphere, like the Earth. On a sphere, the shortest distance between two points is always a segment of a ‘great circle’, where the equator and lines of longitude are examples of great circles. Airplanes will travel along great circles when they’re trying to minimise the distance they travel between two airports. (Although in practice, they will often not travel on the shortest path for various reasons, but that’s not important here).
Great circles are the analogue of straight lines, but they’re weird. They loop back on themselves. The lines of longitude all start at right-angles to the equator, which you’d assume means they’re parallel to each other, but then they all cross each other at the north and south poles.
The surface of a sphere is an example of a non-Euclidean geometry. Another example is hyperbolic geometry, where parallel lines don’t cross, but they diverge away from each other. This is a lot harder to visualise. People have made video games set in hyperbolic geometries, like [HyperRogue](https://store.steampowered.com/app/342610/HyperRogue/) and [Hyperbolica](https://store.steampowered.com/app/1256230/Hyperbolica/).
What people mean by “bending space-time” is that instead of space-time being Euclidean (strictly speaking it’s ‘Minkowski’ rather than ‘Euclidean’, but that distinction isn’t important right now), space-time actually has a non-Euclidean geometry, and the amount of non-Euclideanness is dependent on how much mass and energy there is.
As previously mentioned, a key aspect of non-Euclidean geometries is what happens with straight lines. Consider this: if an object isn’t accelerating, i.e. moving at a constant speed, then it’s on a straight line through space-time. You can draw that on a graph with time on one axis and position of the object on another to satisfy yourself of that!
So, we expect that if we have two objects sitting alone in space, travelling at the same speed, not accelerating, their paths in space-time are parallel lines. They should stay the same distance from each other. We can tell if they’re accelerating by affixing an accelerometer to each object. Except, we do the experiment, and the two objects will come together, neither recording anything other than 0 on their accelerometers. Because of gravity!
Because they didn’t accelerate, they must both still be going on parallel straight lines in space-time. But their parallel straight lines ended up intersecting! Sort of like what happens with parallel straight lines on the surface of a sphere! What happened is the mass of each object caused space-time to deviate from being Euclidean. That’s what’s meant by “bending space-time”.
If we are playing baseball, and I hit a line drive at you, it will take a very flat path and get there quickly, right? But if I hit a high pop fly, it will take a big arc path and a long time to get to you.
The path we take through space and the path we take through time are related. If you want to track both at once, you call it space-time.
And just like the fly ball path seems “bent” compared to the line drive path, things in the universe can “bend” the path we take through space and time.
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