From: metares@well.sf.ca.us (Tom Van Flandern)
Subject: Re: That Great Pulsar Timing Flame War
Organization: Whole Earth 'Lectronic Link
Date: Fri, 8 Jan 1993 03:57:19 GMT


lip@s1.gov (Loren I. Petrich) writes:

> I don't know if Tom Van Flandern has returned from vacation yet, but for
> the benefit of all you net.people, I have decided to give the model used to
> fit binary-pulsar timing.

     Welcome back, Loren, and I hope the holidays were relaxing and
refreshing for you.  I was on travel in South America to find suitable sites
for the 1994 total solar eclipse.  (I'll report on that later.)

> So, we have the following parameters all from observation:
> - The projected semimajor axis a1*sin(i)
> - The period P
> - Its first derivative dP/dt [*]
> - The eccentricity e
> - The periapsis angle omega
> - Its rate of change d(omega)/dt [*]
> - The redshift parameter gamma = sqrt(a/m)*(mc*(mp+2*mc)/m^2)*e [*]

     The three parameters I have marked with [*] actually form a single
solution parameter, according to Taylor and Damour.  There is insufficient
observational power to separate them.  The masses and orbit must be
determined from observations along with this single relativistic parameter.
So the relationship between the three components of the relativistic
parameter must be set by theory, since it cannot be measured by observations.

> If one assumes the GR result for G-wave emission, then one predicts an
> unambiguous result for dP/dt, which is very close to what is observed.

     A part of the difference of viewpoint here centers on exactly what is
"the GR result for G-wave emission."  The Yu paper [Yu Xin (A. Yu), "Does
Einstein's G.R. theory predict gravitational radiation of PSR 1913+16?",
Astrophys. & SpaceSci. 194, 159-163 (1992)], for example, argues that G-wave
emission in GR must actually be zero.

     Even if one derives the amount of gravitational radiation in the way you
prefer, that still makes assumptions that are reasonable, but not confirmed
by observation or experiment.  These assumptions fall into two categories:
(1) G-waves will differentially affect two independent masses in a detectable
way, as opposed to setting up oscillations within a single mass.  That point
is distinctly arguable.  But note that the whole design of LIGO assumes it is
true.  That is one reason some of us feel that the LIGO design is premature.

     (2) There is still considerable vagueness about exactly what comprises
the medium through which G-waves propagate.  Gravity waves are supposed to be
"ripples in spacetime."  But is this the same spacetime through which
electromagmetic signals propagate?  Then what distinguishes a G-wave from an
electromagnetic signal of very long wavelength?  It seems more reasonable to
assume that the carrying media for electromagnetism and gravity are
distinguishable, and that we will eventually develop the observational
capability of distinguishing them.  Then we are back to the original
situation that we can't be sure we know the key properties of the propagating
medium for G-waves.  We are presently just making unverified assumptions
based on a highly questionable analogy with electromagnetism.

> It _might_ be possible that there is some alternate theory of gravity which
> predicts (say) a value of d(omega)/dt different from GR and a value of
> G-wave emission different from GR, with the fitted parameters resulting
> from a1*sin(i), P, gamma, and d(omega)/dt giving _exactly_ the observed
> value of dP/dt, but it would have to be a very contrived theory.

     Whether or not you agree with Yu's theoretical argument that GR should
actually predict zero gravitational radiation, you can see that the pulsar
timing observations do not allow us to disprove Yu's argument.  GR with zero
gravitational radiation can be made to fit the data equally well.  It was
common to see the amount of gravitational radiation predicted by GR disputed
up until the announcement that the binary pulsar had "confirmed" a particular
value.  We now see that the observations did not confirm a particular value;
rather, a particular value was assumed to be "the GR value," and that value
was shown to be in agreement with observations.  But other values make
equally good fits.

> I presume that Damour has risen to the challenge of coming up with such a
> theory (where's his paper, again?).

     His latest paper is: Damour, T., "General relativity and experiment: a
brief overview," Class.Quantum Grav. 9, 55-59 (1992).  -|Tom|-

-- 
Tom Van Flandern / Washington, DC / metares@well.sf.ca.us
Meta Research was founded to foster research into ideas not otherwise
supported because they conflict with mainstream theories in Astronomy.