Ken said:
> All right, what cross-sections are possible for the
> accretion disc, depending upon the disc's angular
> velocity and the mass of the blackhole?
You've confused the accretion disc and the horizon here and in an
earlier message. The horizon is the boundary of the set of events from
which it's possible to escape to arbitrarily large distances from the
hole. If you pass through the horizon then you'll certainly collide
with the singularity. The so-called Cosmic Censorship Conjecture says
that all singularities are hidden from the outside world behind
horizons; this hasn't been proven. The accretion disc is just the stuff
outside the hole that's falling into it. This is often matter from a
companion star that's falling into the black hole.
A black hole without an angular momentum has a very simple structure
described by the Schwarzschild metric. A black hole with angular
momentum has a more complicated structure described by the Kerr metric.
In this case, the horizon is still spherical but is surrounded by a
spheroidal region called the "ergosphere". Bodies within the ergosphere
cannot remain at rest with respect to the distant stars because of an
effect called "frame dragging". The singularity within a Kerr black
hole is a ring, and it's possible to fall through the horizon and yet
miss the singularity. Charged black holes with angular momentum have a
more complicated metric again.
Rich, who used to know all about this stuff a long time ago.
