# Re: Black hole question

From: John Clark (jonkc@worldnet.att.net)
Date: Tue Feb 15 2000 - 13:52:51 MST

Jeff Davis <jdavis@socketscience.com> 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?

Gravity is nothing but Space -Time curvature. If you place a canon ball on
a rubber sheet the sheet will be distorted but that doesn't prove something
must come out of the ball to bend the rubber.

>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.

Perhaps not. According to General Relativity any accelerating mass generates
gravitational waves, as an object is being pulled into a Black Hole at ever increasing
speed most of the resulting gravitational waves will escape before the thing is inside
the hole. The waves might in theory contain enough information to reconstruct the
object. I'd hate like hell to try to do it in practice however.

>What if two black holes began ever so slowly to collide.

Unless the two holes were exactly lined up, and they never would be, they'd go into orbit
around each other. You'd get a spinning dumbbell shaped hole, but an unsymmetrical
very massive object rotating at nearly the speed of light would radiate huge quantities of
gravitational waves. When a gravitational wave leaves an object there is a recoil in the
opposite direction, this would push the two parts of the dumbbell together very very quickly
until you get a spherical hole. Stephen Hawking proved that the area of the event horizon of
the resulting hole is always greater than or equal to the area of the two holes that made it,
never less.

Hawking says this discovery gave him an important insight because he noticed that in this
regard the area of the event horizon is similar to entropy, it also never decreases and he
didn't think it was a coincidence. He wondered if this meant a Black Hole has entropy,
if so he wondered if it had a temperature too, if so he wondered if it gave off Black Body
radiation just like everything else that has a temperature, if so he wondered if the smaller
a black hole was the higher its temperature would be, if so he wondered if it would keep
getting smaller and thus hotter until it exploded. He found the answer to all these questions
was yes.

John K Clark jonkc@att.net

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