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The Question

(Submitted June 01, 1998)

It is believed that gravity, like other forces, has a counterpart in the particle world, usually called the "graviton". How come the graviton can escape the inner side of a black hole ? Since the space-time fabric inside a black hole makes everything move toward the center, gravitons should never escape, therefore black holes should not have a gravity field outside the horizon. I'm an engineer in information technology, and I have a good knowledge of what a black hole is. Don't hesitate to show me formulas !

The Answer

The following website deals with issues such as you have raised:

In fact, it contains an answer to your question:

Subject: D.09 How can gravity escape from a black hole?
Author: Matthew P Wiener , Steve Carlip

In a classical point of view, this question is based on an incorrect picture of gravity. Gravity is just the manifestation of spacetime curvature, and a black hole is just a certain very steep puckering that captures anything that comes too closely. Ripples in the curvature travel along in small undulatory packs (radiation---see D.05), but these are an optional addition to the gravitation that is already around. In particular, black holes don't need to radiate to have the fields that they do. Once formed, they and their gravity just are.

In a quantum point of view, though, it's a good question. We don't yet have a good quantum theory of gravity, and it's risky to predict what such a theory will look like. But we do have a good theory of quantum electrodynamics, so let's ask the same question for a charged black hole: how can a such an object attract or repel other charged objects if photons can't escape from the event horizon?

The key point is that electromagnetic interactions (and gravity, if quantum gravity ends up looking like quantum electrodynamics) are mediated by the exchange of *virtual* particles. This allows a standard loophole: virtual particles can pretty much "do" whatever they like, including traveling faster than light, so long as they disappear before they violate the Heisenberg uncertainty principle.

The black hole event horizon is where normal matter (and forces) must exceed the speed of light in order to escape, and thus are trapped. The horizon is meaningless to a virtual particle with enough speed. In particular, a charged black hole is a source of virtual photons that can then do their usual virtual business with the rest of the universe. Once again, we don't know for sure that quantum gravity will have a description in terms of gravitons, but if it does, the same loophole will apply---gravitational attraction will be mediated by virtual gravitons, which are free to ignore a black hole event horizon.

J.K. Cannizzo
(for "Ask an Astrophysicist")

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