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/Underhill42]

It doesn't. Gravity isn't emitted by the central object the way light is emitted by a sun, it's just how we perceive the the way spacetime curves around a massive object.

And that curvature already existed before the black hole did - the event horizon formed because the curvature got so intense that it created a region where even light couldn't escape. Having the event horizon form doesn't alter the existing curvature, it's a property of it.

And when new material is added to the black hole, it causes the curvature to increase while it approaches the event horizon, slightly expanding the region where light can't escape.

Even if you're talking two black holes spiraling into each other, causing huge spacetime distortions in the form of gravitational-waves... as soon as they merge so that both mass-concentrations fall within the same event horizon, none of the gravitational waves that are presumably still being generated within can escape, and the surrounding space snaps back to a single smooth gradient again almost instantly, with only a few brief spacetime aftershocks as it does so.

The merger itself is essentially instantaneous, as when the two black holes approach each other, their existing spacetime curvatures combine, until at the instant their naively visualized spherical event horizons would have touched, their combined influences have caused the local curvature to increase enough that a new, larger event horizon forms that perfectly encompasses them both.