How does gravity actually work?

Tony sent this in via the webform, so particle physicist Chris Rogers took it on...

Chris R - So I have bad news, and the bad news is that no one really knows.

Phil - Oh no!

Chris R - I'm so sorry. But I can tell you what we do know. So in the 17th century, Newton and Hooke together figured out the laws of gravity, and they also figured out some things about force. So when we have a force, we push something which is really heavy, we accelerate it a little bit; and if we push something that's really light, we accelerate it a lot. That was one of Newton's famous laws of physics. And then the law of gravity says that a really heavy body will attract things more than a really light body. Now Einstein, he's a clever chap, right? So he came along...

Phil - I've heard that, yeah. He's got a good reputation.

Chris R - He does, doesn't he? So he came along at the beginning of the 20th century and what he said, he had a Eureka moment where he actually thought, well hang on, you've got these two different definitions of mass. One is that mass makes massive amounts of gravity, pulls things together really strongly. And the other one is that if you have a really massive object, it's really hard to push it. What happens if we say that those two definitions of mass are actually the same? And so this is something called the equivalence principle. And so together with another principle which he had, the covariance principle, the principle that the laws of physics are the same no matter how quickly you're moving, he tied those together to make a new theory of gravity as a sort of curvature of spacetime, because you can't tell whether it's just an acceleration or whether you're near to a gravitating body like a big planet or something like that.

Phil - I don't quite understand how you get to curved spacetime.

Chris R - Well it's an interesting question. So if you imagine for example, in a curved space time...

Phil - I can't imagine that. I have no idea what that looks like.

Chris R - Yeah, no, you do!

Phil - Nadia?

Nadia - Is it the same as getting close and closer to a black hole and everything is distorted? Is that something…

Chris R - Well so if you walk around the earth, that's like walking round a curved spacetime. If you start at the equator and walk North, you end up at the North pole of course. It doesn't matter where on the equate you start, you will always end up at the North pole. So curved spacetime because parallel lines are no longer parallel.

Chris S - People often use the analogy, Chris, of dropping a very heavy bowling ball on a trampoline, which distorts the trampoline down, bends the trampoline downwards. And if you were to roll a ball across the trampoline, it would roll down a hill. And they're saying, that is a two dimensional manifestation of the curvature of spacetime, if you imagine space being the fabric that's the surface of that trampoline. Is that a reasonable approximation?

Chris R - It's a lovely analogy actually. So it's exactly right. And it does show how spacetime can be curved.

Phil - So curving space time, is that what we're left with?

Chris R - So some theorists come along, they say that we have potentially a sort of subatomic particle called a graviton. Gravitons are really hard to see because gravity is a, what we think of as being a very weak force. But there's another, more fundamental problem. If we put these gravitons into our theories, turns out that the gravitons don't work in the theories, that you get infinite forces going around, which obviously isn't physical. So until we do observe a graviton it's only a theory.