NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/sci/astro/13399

Internet Message Format | 1992-12-24 | 2.6 KB

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From: [e-mail] (Steve Carlip)
Newsgroups: sci.astro
Subject: Re: The Hole Story
Message-ID: <[e-mail]>
Date: 24 Dec 92 06:20:07 GMT
References: <[e-mail]> <[e-mail]>
Sender: [e-mail]
Organization: Physics, UC Davis
Lines: 39

In article <[e-mail]> [e-mail] (Leigh Palmer) writes:
>[...]
>That being the case, one cannot infer from the existence of plane wave
>solutions that waves generated by, for example, a binary compact sytem,
>will exist which are solutions of GR.[...]
>Thus we return to the question of whether a binary compact system can
>radiate. It is not immediately obvious to me that it should radiate plane
>waves (indeed that possibility is ruled out because plane waves are
>infinite in extent). Does a radiative solution to GR exist which has a
>symmetry more appropriate to the case of the binary pulsar?
>
There are no known exact solutions to GR describing gravitational radiation
of a physically realistic source, unless there's been amazing progress in
the past year.  But
(1) There are rigorous _existence theorems_ for radiative solutions with
isolated sources (see Cutler and Wald, Class. Quantum Gravity 6 (1989) 453);
and
(2) there are systematic approximation schemes with reasonably rigorous
justifications.  I have one reference that says that Damour and Schmidt
have a proof that one such approach (the "post-Minkowskian expansion")
gives an asymptotic series, but I don't know where this is published.

To be fair, the solutions Cutler and Wald prove to exist aren't binary
pulsars --- they don't have the appropriate stress-energy tensor as a source,
and they radiate away _all_ of their mass.  From my quick look at the
literature (hey, it's vacation), one of the main problems seems to be
realistically modelling the details of the matter configuration, which
is not surprising (try this with Newtonian gravity).  And there is work
to be done to establish the connection between short-distance approximation
schemes and exact results at large distances more rigorously.  However, I
haven't seen any hint of evidence that there's something fundamentally
flawed with the approximation methods now used, and the existence theorems
certainly establish that at least some compact systems radiate.

Nonstandard disclaimer: I work in quantum gravity, not classical GR, and it's
certainly possible that I'm missing or misunderstanding something.

Steve Carlip
[e-mail]