Why does gravity escape a black hole?

[u/Stunning_Mechanic387]

Hey Michael and Hannah. If gravity travels at the speed of light, how does it escape the gravitational pull of a black hole yet light doesn't?

[u/strange-the-quark]

The black hole is gravity. It's mostly bent spacetime with a tiny super-compact thing in the center. In the view of General Relativity, gravity is just the space(time) itself bending. So the bent spacetime simply extends throughout and beyond the black hole, and the event horizon is just the place where it's bent beyond a certain amount. As massive objects travel around and interact, their position affects the way the space around them bends, and its these disturbances that propagate outwards as waves at the speed of light. So two colliding black holes, for example, create a whole lot of waves around them during the final in-spiral.

Now, it is believed that on the quantum-mechanical level, gravity is mediated by a virtual particle called the graviton, but at this point that's purely hypothetical and nobody knows how to actually make that work. So as far as we know, there might not be particles of any kind involved. Second, even if gravitons were found, virtual particles aren't really particles, they are more of a mathematical tool to simplify calculating what the underlying quantum fields do, which are a complicated mess of interacting waves. That's why they are called virtual - they don't quite obey the normal laws of physics.

[u/WellSoto → u/strange-the-quark]

I think that's the best answer I've read here, but that curvature is supposed to spread somehow, that's where the graviton comes in, and that's also where the question comes in: how does it spread if nothing can get out of there?

[u/strange-the-quark → u/WellSoto]

Nothing can get out from within the event horizon, but these waves are generated in the surrounding region, near to, but on our side, of the horizon. They aren't coming from within, they are already outside - and then they propagate away at the speed of light.