Re: Black Holes

Anders Sandberg (
14 Apr 1998 23:10:55 +0200

Reilly Jones <> writes:

> As a gedanken experiment, let's
> dribble itsy bitsy bits of matter towards this black hole and watch what
> happens at the event horizon Planck moment by Planck moment. We are
> carefully cooling the black hole until, at a given Planck moment,
> equilibrium is reached with the background temperature. At the very next
> Planck moment - Poof! - the black hole is now cooler than the background
> and begins to accrete even after we immediately turn off the dribble of
> matter. In one instant, the black hole turns from an everyday ordinary
> melting ice cube into a refrigerator powerful enough to suck in every bit
> of matter and energy that drifts by it, to suck in the whole universe if we
> could maneuver it into the black hole's vicinity.

Huh? I don't see any problem at all here. Of course, I'm deep down a
mathematician, so I might have lost my common sense long ago. Remember
that even an evaporating black hole is still absorbing energy that
happens to get to close to the horizon. There is nothing qualitatively
different between a hole just below the background radiation level or
just above, both absorb and radiate about the same amount of energy
with the only difference that the difference between them changes sign
at a certain outside temperature. It is an unstable fixed point, no

> The surface of the event horizon in such a scenario seems to me to be quite
> complex. Aside from tidal forces of rotation, which breaks the spherical
> geometry of the surface and creates complex mixing patterns, there should
> be peaks and valleys at the surface due to the underlying distribution of
> matter inside the black hole and due to quantum fluctuations.

The rotation doesn't seem to cause complex mixing in the models I have
seen (although chaotic orbits exist), it just tends to smear out
everything latitudally. The underlying distribution of matter inside
don't have any effect on the horizon shape if the no hair theorems
hold (it seems reasonable, since a horizon almost by definition means
no information can get out of it). However, when non-symmetric matter
drops into the hole there will indeed be quite a bit of wobble,
numerical simulations suggest that the horizon behaves like a drop of
liquid mercury as it gradually settles down by radiating away the
anisotropy as gravity waves. Quantum effects might create more fuzz, I
noticed some discussions among theoretical physicists about "crewcut
black holes", which might have some "hair" (somebody even mentioned
"hippie black holes"). But overall, macroscopic black holes likely
have rather smooth horizons.

> Evaporation would occur at the peaks from the tangential shear from
> incoming particles and accretion would occur in the valleys.

Interesting. I guess the evaporation might be anisotropic for a
excited black hole, further complicated by that the peaks and valleys
are radiating away themselves as gravity waves. I wonder if one could
get destabilizing effects by anisotropic evaporation that makes the
peaks accelerate or grow? Sounds like a wonderfully hard problem in
numerical black hole thermodynamics (if true, then black holes would
likely destabilize and radiate away very quickly and we would see
super-supernovas with total conversion of the black hole as the finale
of a stellar death. Gamma ray bursters, anyone? :-)

Anders Sandberg                                      Towards Ascension!                  
GCS/M/S/O d++ -p+ c++++ !l u+ e++ m++ s+/+ n--- h+/* f+ g+ w++ t+ r+ !y