From: lip@s1.gov (Loren I. Petrich)
Subject: Re: That Great Pulsar Timing Flame War
Organization: LLNL
Date: Mon, 11 Jan 1993 05:04:28 GMT

In article <1993Jan9.203021.10884@Princeton.EDU> steve@puppsr.princeton.edu writes:
>In article <C0IoBJ.4x8@well.sf.ca.us>, metares@well.sf.ca.us (Tom Van Flandern) writes:
>
: lip@s1.gov (Loren I. Petrich) writes:

: > 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.

>As Loren has already pointed out, this is not correct. [detailed
	explanation followed...]

	I'm glad to see that someone else agrees with me on this subject :-)

	[About consistency with GR, Brans-Dicke (scalar-tensor)
theory, and Damour's biscalar-tensor theorie...]

	...No
>matter how good the test becomes, as the data continues to come in, you
>can always sneak through this alternative theory by making the small
>parameters smaller.

	Certainly correct, and it does not support van Flandern's
contention of serious departures from GR being in evidence.

>Note this is not the end of the story with binary pulsars.  As Taylor and
>others pointed out in a Nature article recently, PSR~B1534+12 offers
>a new "test" which uses Shapiro time delay measurements rather than
>P_b_dot and is thus "nonradiative", and useful in squeezing on a
>different set of alternative theories.

	[Exactly what is the article?]
	
	The Shapiro time delay in the isotropic PPN gauge is

	2*(1+gamma)*mc*log(r0/(r + n.r))

	where gamma is a PPN parameter, mc is the companion mass, r is
the separation from the companion to the pulsar, and n is the
direction to the observer. Note: gamma = 1 from GR and is constrained
to within 10^-3 by Solar System experiments.

	There are additional effects that manifest themselves as
eccentricity offsets; these have sizes of

	[2*(1+gamma)*mc - mp*mc*(mp+2*mc)/m^2]*e*sin(i)

	and

	[2*(1+gamma)*mc - mp*mc^2/m^2]*e*sin(i)

	The sizes of these effects are essentially independent of the
orbit's size, and are, for mp = mc = 1.4 solar masses,

	4*mc = 28 microseconds

	mp*mc*(mp+2*mc)/(2*m^2) = 2.7 microseconds

	mp*mc^2/(2*m^2) = 0.9 microseconds

	How good is the timing accuracy for PSR~B1534+12? These
estimates suggest that one needs accuracies of at least a few
microseconds to get good results on these effects.

>As a scientist working in the field, let me restate my own opinion,
>offered some weeks ago.  Taylor and Weisberg, in their observations of
>the binary pulsar PSR~B1913+16, have demonstrated the existence of
>gravity waves beyond any real doubt.  The fact is that a prediction of
>an orbital decay rate due to the loss of energy and angular momentum
>into gravitational radiation was made, and the measurement was done and
>the results matched to better than 1%.  Any serious claim against GR
>or gravitational waves must now reproduce this result in a natural
>way.  I am all in favor of the continued exploration of alternative
>models, done so well recently by Damour.  My PhD thesis included much
>discussion of alternative models.  Someday the real limits of GR will
>be reached, and it will be quantized and perhaps folded into a GUT, and
>I believe this explorations of the underpinning of GR may show the
>way.  But in the meantime, there is no reason to believe that the
>"standard model" is incorrect.

	One interesting question about GUT's that include gravity is
whether they predict any Brans-Dicke-like effects, any field
dependencies of the gravitational constant. I don't recall any
discussion of this question in any of the GUT literature I have read.

-- 
/Loren Petrich, the Master Blaster
/lip@s1.gov