What do they mean when they say space time is curved? I keep hearing a lot of talk about Space Time being the 4th dimension, and no matter how many trampoline examples I see, I just don’t get it.
In: Physics
Okay, start with the concept of spatial dimensions.
Imagine a 3d game of Battleship played on a big wireframe mesh cube, with the coordinates given in terms of upness, rightness and forwardness from the bottom/left/front respectively.
Those three directions are all at right-angles to each other – the lines are all perpendicular, so changing your position in one coordinate doesn’t change your position in any other.
Every object, every atom, every subatomic particle takes up a little chunk of that massive, universe-engulfing grid.
So that grid is what we call ‘space’.
Now we extend that concept: we add *another* dimension to that grid, *at right-angles to all the others*.
This is of course impossible to visualise, but the concept is straightforward enough.
This new dimension is called ‘time’, and its coordinate is given in terms of laterness, from the big bang.
Every object on the grid *also* takes up room in the ‘time’ direction – a dinosaur wouldn’t just be dinosaur-shaped; if you could see in 4d, it would be like a long extruded length of [dinosaur pasta](https://nuts.com/images/rackcdn/ed910ae2d60f0d25bcb8-80550f96b5feb12604f4f720bfefb46d.ssl.cf1.rackcdn.com/1e6c14229d87184e-Y1Shzv_q-large.jpg) that changes shape down its length (or as time went on on, from its own perspective)
That extra-confusing grid is called ‘spacetime’.
Here’s the fun part: you put a big massive object in the grid, and it *bends the actual fucking lattice* that all your objects are stuck to.
Just a little bit, not enough that you’d see it – but it’s there. The whole grid was poorly designed and is made of cheap shitty plastic, so objects don’t *quite* fit properly, and if you wedge a whole bunch of atoms in next to each other, you get some noticeable distortion start to build up.
TFW you order your universe from wish.com…
Anyway, now you’ve got this trippy concept where the strands of the lattice *are no longer completely perpendicular along their length*, so travelling along an X-coordinate strand can actually move you a little bit in the Y direction as well.
It’s like haring down the freeway on your motorbike, when the road starts to take a very slight curve – you keep going in an absolutely straight line, but somehow this means you drift out of your lane, wtf.
And this is how gravity works.
Mass bends the time-strands into spacey directions around it, so when you’re standing still near a planet-sized chunk of mass, just minding your own business going forwards in time, somehow this means you drift out of your position in space, wtf.
(actually, it bends *all* the strands, but we – and planets – are in fact very, *very* long pasta shapes, so the drift from laterness into downness is a hell of a lot more noticeable from our perspective)
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