🚀 go-pugleaf

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

You seem committed to using this set of terms to describe the phenomenon,
so we ought to at least be careful to note that this doesn't require
gravitational fields to behave in an acausal manner, or to consist of
tachyons, or any such thing, any more than the absence of aberration of
the electrostatic force due to a charge moving in a straight line (the
SR equivalent of gravitational free fall, after all!) implies such things
for the electromagnetic field. 
 
> A distant point in the system would feel slight
>variations in the strength of the gravitational force because the system has
>a small dipole component, since the two stars are separate bodies instead of
>concentrated at their center of mass.  But the dipole variations are felt
>without detectable delay.

Wouldn't the correction be from the *quadrupole* component?  The dipole
component of a system of masses is zero in the center of mass frame.

The same term produces the gravitational waves.  One way of saying all
of this is to point out that the gravitational waves emitted are much
smaller in magnitude than you'd expect from unaccelerated extrapolations 
of the retarded position. 

I don't really mind statements to the effect that static fields (of
any sort) don't propagate.  It helps to counter the excessively literalistic
image, fostered by cursory descriptions of quantum field theory, of
virtual particles spraying out from the source at the speed of propagation
of the field-- which leads in turn to some of the straw-man arguments against 
quantum gravity. 

-- 
Matt McIrvin, posting nonsense again