Re: Black hole question

From: Robert J. Bradbury (
Date: Sun Feb 13 2000 - 19:07:41 MST

On Sun, 13 Feb 2000, Jeff Davis wrote:

> If a black hole has an event horizon from within which nothing can escape,
> then how can the mass within the black hole exert a gravitational
> attraction on anything outside the event horizon? How can anything *orbit*
> a black hole?

Though John or Hal can probably give much nicer answers, I'll take a shot.

Mass creates a distortion in space (a black hole being the ultimate
distortion). Its typically represented as a "depression" in space.
When you "orbit" something your orbatal velocity "out" of the depression
is balanced by the net acceleration into the depression (towards
the mass). Speed up and you move out, slow down and you move in.

The effect of gravity isn't the effect of one body on another,
its the effect of two spatial distortions interacting. Gravity
waves presumably result from great distortions in space (probably
from colliding black holes, neutron stars, etc.). Smaller masses
presumably generate them as well, but they are way to small to measure.
An interesting question would be -- how small the masses have to be
before they start getting down to the extra dimensions postulated
in string theory.

So, I'm guessing (since I haven't taken astrophysics), that the
reason the event horizon exists is because your orbital velocity
would have to be faster than light-speed to overcome the acceleration
pulling you into the singularity.

> Doesn't the indisputable fact of gavitational influence by the foremost
> candidates for black holedom suggest that either they are not black holes,
> or that the theory of black holes is flawed?
Nope, everything seems consistent.

The only problem I've seen pointed out with black holes is that they
violate a "conservation of information" principle. The basic problem
is that once matter goes into the black hole, its impossible to get
back any information that was contained in that matter. Now, I'm not
sure how "strict" that principle is, unlike say "conservation of momentum",
so it isn't clear (to me) whether that creates big problems or only
little ones.


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