Image: Mooc Summers / Wikimedia Commons

# Science explained: Gravitational waves

Over the past couple of weeks you’ve no doubt heard a lot about gravitational waves. They’ve been everywhere: in the papers, on Facebook and, of course, here in the Boar. Everyone has been ranting about how amazing it is that they’ve been discovered, but there’s a simple question that a lot of people have been left with – what the hell are they?

To understand that, we need to go back to the basics and answer a fundamental question: what is gravity? Now you’ll probably remember that story about Isaac Newton and an apple falling from a tree – the big mass (Earth) pulling the small mass (the apple) down because it has a greater mass. Both of the masses exert a force upon the other mass, but because the Earth is pretty big in comparison to the apple, the Earth always wins the tug-of-war. Albert Einstein (a name you’ve probably heard before) came up with a different way of thinking about things, however, and it’s a bit… weird.

Imagine that the universe is all made out of a fabric a bit like a trampoline, and that it’s called spacetime

Imagine that the universe is all made out of a fabric a bit like a trampoline, and that it’s called spacetime. Now if you put a mass on it (like a tennis ball), the fabric sinks around where the mass rests. A bigger mass (like a bowling ball) sinks the fabric more than a smaller one, and that’s kind of the basics to Einstein’s ‘theory of general relativity’. As objects move around in the universe (and in spacetime), they get attracted into orbits around bigger objects because of the dimple in spacetime the bigger objects cause. When huge objects like black holes start orbiting, though, things get extreme.

Black holes have enormous masses – anything from ten to several billion times as massive as our sun – so when they move they disturb spacetime just like you do when you roll on a trampoline. This disturbance can stretch and compress the fabric that makes up the universe, and these stretches and compressions are known as gravitational waves. When you get two black holes orbiting each other, the gravitational waves produced are very energetic, meaning that when they reach Earth the entirety of space and time – including everything on our planet – momentarily stretches and compresses as the wave goes by.