Quote:
Originally Posted by hung
If gravity travels at the speed of light, how do black holes have gravity, since their escape velocities are faster than the speed of light?
|
Surely Hung, a man of your intellect must know the answer to this apparent conumdrum.
The General Theory of Relativity allows for the gravity of a black hole to be felt outside of it.
The gravity doesn't have to get out of the black hole. General relativity is a local theory, which means that the field at a certain point in spacetime is determined
entirely by things going on at places that can communicate with it at speeds less than or equal to c. If a star collapses into a black hole, the gravitational field outside the black hole may be calculated entirely from the properties of the star and its external gravitational field
before it becomes a black hole. In this sense the black hole is a kind of "frozen star": the gravitational field is a fossil field. The same is true of the electromagnetic field that a black hole may possess.
Often your question is phrased in terms of gravitons, the
hypothetical quanta of spacetime distortion. If things like gravity correspond to the exchange of "particles" like gravitons, how can they get out of the event horizon to do their job?
Gravitons don't exist in general relativity, because GR is not a quantum theory. Gravitons might be part of a theory of quantum gravity
when it is completely developed, but even then it might not be best to describe gravitational attraction as produced by virtual gravitons.
Nevertheless, the question in this form is still worth asking, because black holes can have static electric fields, and we know that these may be described in terms of virtual photons. So how do the virtual photons get out of the event horizon? Well, for one thing, they can come from the charged matter prior to collapse, just like classical effects. In addition,
virtual particles can go faster than light! Consequently the event horizon, which is really just a surface that moves at the speed of light, presents no barrier to virtual particles.