They’re produced by pretty much anything – you, me, cars, planes. But we’re tiny, and space is very ‘stiff’ – it really doesn’t like to deform, and so while we do produce gravitational waves, they’re minuscule in terms of amplitude. Basically, we’re looking for violent things. To get a signal that’s strong enough to measure on Earth, or in orbit near Earth, it really has to be a very big, violent event in the universe.
Gravitational waves are created by an acceleration of mass. If you think of two objects orbiting each other, you can have an angular acceleration – if you have two stars orbiting and you’re viewing them from a long way away, then at some times you see the two stars separated, with one gravity field, and…