Or, why do black holes have accretion disks?
The obvious answer is gravity.
But wait! We have experimental knowledge that gravity propagates at the speed of light. And a black hole has so high a gravity that nothing, not even light, can escape. So how can gravity itself escape the black hole? Being a singularity, shouldn’t it just pinch itself off? Could it be that the accretion disk itself is the gravitational source?
Gravity is more mysterious than light, in a lot of ways. It is better visualized as an alteration of the shape of spacetime rather than as a form of radiation.
Well yes, but that distortion must be propagated by something, right?
Must it?
I presume when the black hole forms the hole itself forms at the speed of light. A light ray just ahead of the growing event horizon will escape, one just behind is stuck.
Gravity doesn’t need to “escape” to manifest itself. A static gravitational field is just fine. Or, for that matter, a static electromagnetic field, if the hole has a net charge.
Or, consider that the gravitational field of the stuff falling into the hole is the same before and after it falls, from the point of view of things further out. You don’t need the gravitational field to originate inside the hole. The stuff that made the hole in the first place was already causing a field around it while it was on the way in.
Yes.