Why does Space-Time curve and more importantly, why and how does Space and Time come together to form a “fabric”?
In: Physics
[We don’t know.](https://xkcd.com/1867/) It just… seems to do that when heavy things are around. Maybe, when you grow up, you can figure it out.
That’s not a satisfying answer? Ok, then. Erm… well, technically Space-Time isn’t a real thing. [It’s just something Einstein made up.](https://xkcd.com/895/) A story, if you will. We tell ourselves stories about how the universe works, like “a person lets go of an apple, and it falls to the ground”, and then we look at the universe and, if we see those stories in the universe, we remember them for later. This is a story with a lot of maths in it, which makes some people think it’s real, but it’s actually just a story. We’ve already noticed places where the story doesn’t tell us what actually happens, and we’re trying to find a better story. This story’s good enough for most of the things we need it for, though, so we’re keeping it in the meantime.
We used to have a story written by Isaac Newton, that told us that things just fell down, but we got rid of the story when we noticed that the story said that the planets move in a certain way, but they were actually moving in a different way. It’s really interesting, actually. You know that the planet is a big ball, right? Well, when things fall “down”, they’re falling towards Earth. So if you’re in [country on the “bottom” of the globe] and you throw an apple into the air, it’ll still fall down to Earth, even though “down” is _that_ way instead of _that_ way. So, Newton’s story goes that if you put a cannon on top of a mountain and fire it sideways, the cannonball would cuuurve and hit the ground. Like this. But if you fire it further, it’ll curve around like this… and hit the ground, but it’s sort of curved around the Earth because “down” is always towards Earth, no matter where you are. But if you fire it hard enough… it’s just going round and round! Because it’s going so fast that it’s rushing past and, even though it’s being pulled towards Earth, it’s still curving. In fact, if you fire it fast enough, like this, it’ll shoot off into space and _never_ fall down again!
And Newton’s story is accurate enough for almost everything you’ll ever need. But it’s not quite right. It says that things move sliiightly differently to the way they actually move. So Einstein came up with a slightly better theory – one that’s a lot more complicated, though still quite simple if you’re really good at maths – (I’m not good enough at maths to understand it) – and Einstein’s theory predicted that, among other things, light was bent around the sun. We knew that light was bent around the sun before that, because we saw it, but because it had a lot of maths Einstein’s theory predicted _exactly how much_ the light was bent around the sun. (Actually his first theory was wrong about this, but he made a second theory that was better, and predicted it right.) Now, the only light that we can see that could’ve been bent around the sun was light from stars, which would make the stars look like they’d moved _very_ slightly when they were near the sun, but you’ll probably have spotted the problem with trying to spot stars moving near the sun.
Yes, you can’t see the stars in daytime. So they had to wait until a solar eclipse, and Eddington and his friends got a telescope with a special filter to stop them from going blind and took photographs of the stars near the sun, and found that the prediction made by Einstein’s second theory was right and Newton’s theory’s prediction was wrong.
We’ve got two main stories about the universe at the moment. One of them is Einstein’s theory, called “General Relativity”, which is the one about gravity, and the other one is called the “Standard Model”, and talks about really tiny things. These two stories predict different things, and we’ve measured that the Standard Model is wrong about gravity, and that General Relativity predicts contradictory things to the Standard Model. But that doesn’t make these stories useless. In fact, Newtonian Physics is still useful, and it’s what you’ll get taught in school until you’re an adult, and most adults don’t even use General Relativity when they’re working things out.
You do need General Relativity if you need to be really exact, or need to deal with clocks moving at different speeds to you, because Relativity says that time passes at different speeds depending on how fast you’re travelling. (Yes, it seems confusing, but that’s because your brain is designed for Newtonian Physics with time that passes at the same speed everywhere, and that’s just a story.) For example, [the GPS satellites that orbit Earth](https://physics.stackexchange.com/q/1061/105169) (like Newton’s cannonball) have clocks on them, and those clocks need to have the right time on them, so they need to use relativity to make their clock go at a different speed so it still matches up with the clocks on Earth.
General Relativity is an incredibly useful story. But it’s just a story. We don’t really understand the universe; we’re not even close.
Not yet.
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