From: Anders Sandberg (asa@nada.kth.se)
Date: Wed Oct 23 2002 - 03:54:48 MDT
On Tue, Oct 22, 2002 at 11:43:56AM -0400, Alexander Sheppard wrote:
> There seems to be a discussion going here regarding the possibilities for
> harvesting energy from stars. Well, someone mentioned more potent energy
> sources. There would seem to be a very potent source of energy that I don't
> think anyone has yet mentioned (while I've been on the list anyway). That
> would be rotating black holes.
Well, there is a lot of energy in a rotating black hole, but it is not
close to the mass-energy of the hole. Extracting energy from the
rotating Kerr metric is a popular textbook example, but it is limited by
the total angular momentum the hole formed with.
According to http://cxc.harvard.edu/fellows/viewgraphs/li/li.ps one
could simply put a disk or coil around a charged rotating hole and get a
transfer of angular momentum through electromotive forces. This way you
can get 0.15 of the mass-energy if the original hole was extremal
(http://arxiv.org/pdf/astro-ph/0002004). The Blandford-Znajek mechanism
only gives 0.09
(http://scienceworld.wolfram.com/physics/Blandford-ZnajekProcess.html),
and I think the Penrose process even less.
While this is nice, non-rotating holes would be useless. But they can be
used as energy sources if they are small enough to radiate significant
Hawking radiation. If you top off the mass loss with new matter to keep
the hole stable, you get a very efficient mass-energy converter. You
could also improve the efficiency by attaching cosmic strings to the
hole; according to http://xxx.lanl.gov/abs/hep-th/0012260 you can get
significant improvements in efficiency this way.
Big black holes are useful for cooling instead.
> Have you noticed that nobody EVER portrays what a black hole would really
> look like cast against the stellar background? In fact you ought to be able
> to see your ship, too, assuming it is large enough..
I think most people have a hard time even thinking in terms of bent
light; renderings like http://www.photon.at/~werner/bh/ usually just
confuse people.
-- ----------------------------------------------------------------------- Anders Sandberg Towards Ascension! asa@nada.kth.se http://www.nada.kth.se/~asa/ GCS/M/S/O d++ -p+ c++++ !l u+ e++ m++ s+/+ n--- h+/* f+ g+ w++ t+ r+ !y
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