How fast is gravity?

Astrophysicist Francesca Day took on this question, put to her by Chris Smith...

Francesca - When we say light can't escape a black hole that's absolutely true. And that's to do with the way that a black hole warps the space in which the light is travelling. Such that, whatever happens to it, it ends up going closer to the black hole. Since the observation of gravitational waves we know very precisely that gravity travels very, very close to the speed of light. We observed an event in which two neutron stars merged and we saw both gravity waves and light emitted from that event. And based on the arrival times of the gravity waves and the light we know that they travel very very similar speeds.

Chris - So basically it goes at the speed of light. But how is it travelling? Is it ripples in the fabric of space that are propagating along? If so, why should they travel at the speed of light? Is that a coincidence, or is there a physical reason why that should happen?

Francesca - There's a physics reason. So one of the great mysteries that we're trying to solve in particle physics at the moment is how quantum gravity works; how we can fit a picture of quantum mechanics and particle physics with a picture of gravity portrayed as spacetime. But we do know almost certainly that there is also a particle associated with a quantized theory of gravity and that's called the graviton. So, in the same way that the photon carries the electromagnetic force, the graviton would carry gravity; and because we think the graviton is massless like the photon, it travels at the speed of light.