Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.3 4.3bsd-beta 6/6/85; site ucbvax.BERKELEY.EDU
Path: utzoo!watmath!clyde!burl!ulysses!ucbvax!brahms!weemba
From: weemba@brahms.BERKELEY.EDU (Matthew P. Wiener)
Newsgroups: net.space
Subject: Re: converting gravitational potential energy into useful energy
Message-ID: <12545@ucbvax.BERKELEY.EDU>
Date: Fri, 21-Mar-86 22:59:16 EST
Article-I.D.: ucbvax.12545
Posted: Fri Mar 21 22:59:16 1986
Date-Received: Sat, 22-Mar-86 23:07:07 EST
References: <8603220245.AA06331@s1-b.arpa>
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: weemba@brahms.UUCP (Matthew P. Wiener)
Organization: University of California, Berkeley
Lines: 23

In article <8603220245.AA06331@s1-b.arpa> REM%IMSSS@SU-AI.ARPA (Robert Elton Maas) writes:
>So it's obvious you can get energy by lowering something into a black
>hole, providing you don't just let it freefall, you have it's falling
>do useful work on some mechanical contraption. The hard part is
>computing how much energy you get for a given mass being lowered
>almost all the way to the event horizon before the rope breaks. I
>haven't done the calculations myself, but I read in some journal that
>the answer is you get exactly E=M*C**2 out of it, i.e. 100%
>mass-to-energy conversion as per Einstein's equation. (Would Gene
>Salamin or Hans Moravec or some other expert on physics & relativity &
>quantum mechanics who has actually done the calculation please confirm
>or correct the answer I quoted? (I wish Hawking were on this list,
>he's the real expert on black holes!!))

The calculation is rather simple.  Assuming a 100% conversion rate of
the infinitesimal gain in potential energy to photons emitted back,
and then accounting for the gravitational red shift on the way back
to the starting height, and then integrating from initial height to
the Schwarzchild radius, one gets that M*c**2 is the energy received
at the top.  Of course, there is an infinite delay at the end, but
never mind that.

ucbvax!brahms!weemba	Matthew P Wiener/UCB Math Dept/Berkeley CA 94720