From: dwr2560@zeus.tamu.edu (RING, DAVID WAYNE)
Subject: Re: Spin
News-Software: VAX/VMS VNEWS 1.41    
Organization: Texas A&M University, Academic Computing Services
Date: Tue, 6 Oct 1992 22:55:00 GMT

metares@well.sf.ca.us (Tom Van Flandern) writes...
>     If gravitation propagated at the speed of light or anything comparable,
>it too would be subject to aberration.

No. The retarded potentials of GR predict no abberation just like your theory.

>                                        If an attractive force eminates from
>the Sun with some speed g, then we can represent the force by a velocity
>vector from the Earth toward the true Sun with magnitude g.  Call this vector
><EG>.  Then on a moving Earth with velocity <EV>, the apparent direction of
>the force will be <EV> + <EG>.

This is a straw man. Noone important thinks gravity works this way.

>                                Observations flatly contradict this, so we
>can be sure that gravity does not propagate at the speed of light or anything
>comparable.

Or we can be sure that the straw man is wrong.

>> (b) gravity propagates at speed c in GR
> 
>     Your statement (b) is definitely false, as relativists will agree.

I'm not a relativist, but I most definitely disagree. Gravitational
disturbances propagate at c.

>                                                                          In
>Newtonian gravity, the propagation speed is infinite.  This is required by
>both observations and calculations, as my previous post mentioned.  In GR, to
>avoid this ftl problem, it is postulated that nothing about the force of
>gravity *propagates*.  Instead, curved space-time awaits the Earth's arrival
>and affects the Earth in place, without any dynamic component which would
>depend on the Earth's velocity.

Not quite. First GR does not 'try' to do anything. It is a complete formalism.
There is no room for interpretation. It predicts that in situations where the
sources are moving along geodesics and a 'direction' toward these sources can
be defined with minimal ambiguity, the forces will point toward the current
positions. Second, there is no such postulate. Period. In fact it is false
anyway. If the masses do not follow geodesics the force won't point toward
the current positions (until the retarded time :)

>     You are probably confusing the speed of gravity with that of gravity
>waves.  Gravity waves are a different phenomenon from ordinary gravity.  When
>the Earth (assumed to have negligible mass in comparison to the Sun) orbits
>the Sun in a circular orbit, no gravity waves are generated.

They are produced but they are not strong enough to be concievably measured.

>     GR has adapted its descriptions to agree with observations, so there can
>be no contradiction.  All that remains to be argued is whether or not the GR
>descriptions are the correct ones; or whether perhaps a slightly different
>description of the same phenomena might make more sense and provide a deeper
>understanding of nature.  I do lean toward the latter view.  -|Tom|-

GR has adapted nothing. The formalism is the same as it was in 1916. And
the predictions agree with all data we have. So (ignoring binary pulsars)
the only reasons for preferring one theory over the other is Okham's
razor: Which theory is simpler? And GR is by far the most parsimonious
theory of all time.

Dave Ring
dwr2560@zeus.tamu.edu
Newsgroups: sci.physics