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From: rimey@ucbmiro.ARPA (Ken Rimey)
Newsgroups: net.physics
Subject: Re: YAQ about BLACK HOLES
Message-ID: <10085@ucbvax.ARPA>
Date: Thu, 22-Aug-85 04:55:17 EDT
Article-I.D.: ucbvax.10085
Posted: Thu Aug 22 04:55:17 1985
Date-Received: Sat, 24-Aug-85 14:46:10 EDT
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Reply-To: rimey@ucbmiro.UUCP (Ken rimey)
Organization: U.C. Berkeley
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Todd Aven asks,
>Can a black hole collide with another black hole in a finite
>length of time in
>	a) its reference frame or
>	b) in an inertial reference frame outside the pair?

The easy case to consider is that where one of the black holes is
much smaller than the other.  Then the smaller one will not perturb
the larger one, and it will follow the same trajectory any small
particle would follow.  It will cross the Schwarzschild radius and
hit the central singularity in a finite time as measured by someone
riding on the particle.  Of course, in speaking of a rider on the
particle I am presuming that it is not a black hole.  It is not clear
what would be meant by the "reference frame" of a black hole.

Question (b) makes sense in the general case.  Everyone is taught the
answer for the case of a black hole and a small test particle (which
might as well be a black hole):  The small particle never reaches the
Schwarzschild radius.  But what happens if instead of a small test
particle we use a black hole comparable in size to the other black
hole?  I don't know.  Do any of you people?

					Ken Rimey
					rimey@dali.berkeley.edu