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
In maths lessons you’ve probably worked on paper with grids; lines equally spaced, making a bunch of different squares of the same size.
You can use this easily to make a set of axes (usually x-y) and a grid. This is how we normally think of (2D) space; we could describe points by a grid – any point we can identify by how far it is in the x-direction (or how many lines along), and how far in the y-direction. The two directions are at right angles so any motion in one is independent of the other – we can move freely left-right and/or up-down.
Now imagine instead of that graph being drawn on paper it was drawn on something stretchy, like the elastic you’d get from a balloon if you cut it open and flattened it into a giant square (but still kept flat on a table). Now you could twist or squish the elastic, which would twist or curve the lines. You could get the lines to bunch up in some places, and be more spread out in others, you could get them to curve – although you couldn’t get them to cross over each other – you’d still have the same number of squares overall, some just wouldn’t be square any more, and they’d be of different sizes. You might be travelling left (along an y-direction line), but due to curving find yourself travelling up a bit instead, or vice versa.
It turns out that real space is stretchy/squishy, and mass (or energy) squishes it around. It bunches up space (like the elastic sheet), so what should be straight lines start being a bit curved. Also you might find that two points that should be 5 squares apart (so if you went up 10, along 5 and down 10 you’d get from one to the other) are actually six if you try to cross them directly because the lines are bunched together more there.
And it turns out this doesn’t just affect the three dimensions of space (it’s a bit harder to visualise this in 3D, but don’t worry about that too much), but also the 4th (or 0th) dimension of time. Not only do you find there are places where space is all bunched up, time also gets stretched out as well (so there is more time between two events than there ‘should’ be). And to make it even weirder, space and time aren’t these separate things, but part of a combined space-time thing; so you might find that while you thought you were just travelling in time, your time-direction has been twisted or curved a bit into a space-direction, so rather than just moving through time you seem to move in space as well (when viewed from the outside); i.e. you fall.
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