Black holes, gravity, and c

[Enola Straight]

Black holes have the strongest gravitic fields in the universe.

The escape velocityof a black hole is greater than the speed of light, therefore nothing, not even light, can escape from a black hole.

Gravity propagates at the speed of light.



How can gravity escape from a black hole when light can't?

[Kuroneko]

Black holes have the strongest gravitic fields in the universe.

In the sense that the static force on the surface of a black hole is infinite, yes.

The escape velocityof a black hole is greater than the speed of light, therefore nothing, not even light, can escape from a black hole.

Well, that's one way to look at it, anyway.

Gravity propagates at the speed of light.

Gravitational waves propagate at the speed of light. If there is a pertubation in the stress-energy configuration of a system, it will be gravitationally detected at a delay dictated by the speed of light.

How can gravity escape from a black hole when light can't?

If an observer inside the horizon wiggles a bit, the gravitational pertubation will not be detected outside the horizon, as expected by your reasoning. What's the problem? Gravity is geometry; it doesn't need to 'escape'--if the spacetime manifold is curved in a certain way, then gravity behaves in a certain way. If this really bothers you, then you can take some consolation in the fact in regions external to the black hole, spacetime gets this curvature before the black hole finishes collapsing (in fact, it never finishes collapsing relative to any external observer). On the other hand, if this is a question in regards to some hypothetical gravitational force carrier (gravition), then the answer there even simpler--virtual particles are not limited by lightspeed.

[Surlethe]

Another way of thinking about it is to note that gravity isn't something which moves with respect to spacetime, because it is spacetime. If I have my terminology correct, the reason a black hole has an escape velocity of c is because no timelike geodesic leads from inside the black hole to outside. Gravity, on the other hand, doesn't follow any particular path in spacetime, because it's described as the curvature of spacetime -- asking whether gravity can escape a black hole is like asking whether or not grass soil can escape an especially steep hill.