If a black hole starts rotating (say due to the effect of rapidly rotating matter falling into it), would we see the event horizon bulge because of the centrifugal force arising from the rotation? If yes, does that mean that at least some of the mass of the black hole has moved away from the singularity at its centre?
Someone will probably nitpick that you can’t “see” it, but yes, the black hole will bulge. See “Black holes and time warps”, by Kip Thorne, p. 51.
I don’t think this necessarily means any matter has moved away from the singularity, however.
All the matter in a black hole is located at the singularity. What “bulges” is the event horizon, an imaginary line surface. Light (or anything else) that crosses the event horizon cannot escape.
The event horizon is not so imaginary. It represents the distance from the “singularity” where the escape velocity is c. If all of the matter is at one point, this should always be spherical. I believe the bulging that is observed is a result of a singularity where all of the matter isn’t at a single point in space. I dimly recall a cosmology class where someone gave a presentation on the geometry of rotating black hole sigularities. The sigularities were not points, but circles where all of the matter is on a closed single dimensional line. I also recall that gravity in the center of the circle is repulsive. Strange stuff, indeed. Sorry I don’t have more information.
I found a site which describes ring singularities in rotating black holes. Near the bottom of the page it says:
What I meant by *imaginary * is that the event horizon isn’t made of matter–it’s a surface in space, not an object. The second sentence is equivalent to what I said.
Yes, a rotating black hole bulges. OK.
If it was originally not rotating and begins to rotate because of the infalling of rapidly spinning matter, does that make its radius (from singularity to event horizon) larger (at least along some axis)? If so, can the infalling stuff rotate fast enough to make this centrifugal force effectively cancel out some of the mass of the black hole? (I am thinking of infalling stuff that’s not very massive but has a very rapid rate of rotation).
We don’t know if all matter in a BH is compressed to a single point, we just use that point to make calculations easier. We also use this when we do ‘regular’ mass calculations and use the term center of gravity or center of mass. I think the latest thinking is that BH’s do not compress to a single point.