How does gravity get out of a black hole?

1. Oct 2, 2015

nburns

Forgive me if this is a dumb question. If nothing gets out of a black hole, how does the gravity get out?

2. Oct 2, 2015

phinds

This is a very common question and the answer is not obvious. Remember that gravity is a field.

Basically, gravitational fields don't "propagate" after they are formed, but the CHANGE in the field does propagate as the field is forming. Let's consider two ways of a black hole forming.

First, A star explodes in a supernova and leaves a black hole. The gravity field of the star, at a point some distance removed from it is actually decreased because some of the star is blown away in the explosion of the supernova. The remaining field, at that same distance, is actually weaker. BUT, the gravity field still exists and in close to what is now a black hole, the field is quite strong. Had the entire mass of the star compacted down to a tiny size, the field strength at the farther distance would be the same, but now you can get in closer to the center of mass and the field is strong. How did the gravitational field of the star get as strong as it is in the first place? Same was as in the following case:

Second, a mass accumulates, enough to finally compress into a black hole. I don't know if this actually happens except in the rare case of more mass being added to a neutron star but it is a VERY informative case to look at. AS the mass coalesces, the gravitational field continues to increase at a given distance from the center of mass because more mass is now closer to the center than that point is. So the gravitational field continues to increase as the mass increases, and even after it becomes a black hole, at some distance out from the event horizon, the field is just the size that it would be if the same amount of mass were there but not a black hole.

So gravitational fields surround a black hole for exactly the same reason that they surround the Earth or the sun or any other celestial object.

3. Oct 2, 2015

nburns

Thanks for your answer. So you are saying that, basically, the gravitational field surrounding the black hole doesn't need to "communicate" (for lack of a better word) with the mass at its center as long as the field isn't changing?

4. Oct 2, 2015

Drakkith

Staff Emeritus
Remember that in GR the gravitational field is described by a metric tensor. It is the curvature of this metric that gives rise to gravity. There's no need for gravity to 'get out' of the black hole because it is this metric that's curving to create the black hole in the first place!

5. Oct 3, 2015

nburns

Does the same logic apply to magnetic fields of black holes?