I’ve heard the term “space time” thrown around, and that gravity “bends space-time” but how does that work?
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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.
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