From: carlip@landau.ucdavis.edu (Steve Carlip)
Newsgroups: sci.physics
Subject: Re: Has the Hulse-Taylor Pulsar GR "Observation" Been Retracted? (was Re: Beckmann Physics)
Date: 14 Dec 92 22:03:45 GMT
Organization: Physics, UC Davis

In article <Byywu3.3r8@well.sf.ca.us> metares@well.sf.ca.us (Tom Van Flandern) writes:
>[...]
>The form of the equation for gravitational radiation contains a parameter
> whose value is not agreed upon.
>[...]
>I've heard it described as a sort of "permeability" of space-time.  A
>mathematical description of the parameter, together with the Einstein value
>and alternatives for it, may be found in: Ehlers, Rosenblum, Goldberg &
>Havas, Astrophys.J. 208, L77 (1976).  -|Tom|-
>
This is a very misleading description of the article by Ehlers et al.
The only thing in that paper even vaguely resembling such a parameter
is a coefficient they call A, which can be calculated in general 
relativity without ambiguity.  The main argument of this paper was
that _as of 1976_, approximations in general relativity had not been 
carried out to sufficiently high orders, or with careful enough
attention to potential problems of mathematical consistency, to be
completely confident of the calculated value of this coefficient.

Work on this question did not end in 1976; for a somewhat more
recent description, you might look at Damour's article in the book
_300 Years of Gravitation_ (Hawking and Israel, editors, Cambridge
University Press, 1987).  Damour stresses some remaining problems
in perturbation theory in general relativity, but also describes
three careful, independent calculations of gravitational radiation
damping, by three separate groups using different approximation
schemes, that all agree.  I am fairly sure that there have also
been numerical solutions of the Einstein field equations for a
pair of orbiting bodies that show the predicted radiation damping, 
but I can't quote references off hand.

It is certainly true that approximations to the two-body problem in
general relativity are difficult, and we have a way to go before
we can have complete confidence in computations.  But there is a
big difference between pointing out these problems and claiming
that there is a fundamental theoretical ambiguity.  In particular,
there is absolutely no doubt that general relativity predicts
gravitational radiation --- this can be shown through exact solutions,
numerically, by use of systematic approximation procedures, and by
general arguments about the asymptotic structure of spacetime.

Tom -- your real argument may be that general relativity itself is
not ``proven,'' and that other models may be found that also agree
with observation.  This is certainly true, although the binary
pulsar data does add a new and rather large constraint to the 
possible alternate theories.  Moreover, general relativity does
not have a bunch of adjustable parameters; any candidate replacement
theory would have to end up making a lot of very similar predictions
from a fundamentally different conceptual starting point.  (For
instance, there is most certainly no adjustable ``permeability of
spacetime'' in general relativity.)  For the sake of the nonexperts
reading this newgroup, I wish you would be clearer about the
distinction between saying ``there is an ambiguity within general
relativity'' and ``there is an ambiguity because there is more
than one possible theory.''  

Steve Carlip
carlip@dirac.ucdavis.edu
Newsgroups: sci.physics