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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.

I disagree with this.  It is possible for gravity to propagate at finite
speed and yet produce no aberration.  If the force is non-central by exactly
the amount corresponding to the propagation delay, then the force will
still point towards the instantaneous position.  Given that the direction
of the gravitational force is not Lorentz invariant, and given that the
force points towards the instantaneous position regardless of which frame
is used to determine the direction of the force and the instantaneous
position, and given that the direction of the force is not even a well
defined concept except when space-time is almost flat globally, I would
prefer to think of gravitation as propagating at finite speed but with
a non-Lorentz-invariant non-central component.  Even better, I like the
statement made earlier in this thread that static fields by definition
don't propagate and the non-static component of gravity does propagate
at the speed of light.

>     Although I'm trying out the following wording for the first time, I
>believe it to be accurate.  Consider two large masses in close orbit; for
>example, a binary pulsar.  Each body responds gravitationally to the true,
>instantaneous position of the other (not its position retarded by lighttime,
>nor its extrapolated retarded position; but its exact, true position, as
>required by observations).  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.

This is good, but I would make just a few minor amendments.  Most
important, you need the requirement that each source follows a geodesic
from its retarded position until it reaches its instantaneous position.
Also, since the direction of the force is not well defined except when
motions can be compared with a flat background space-time, I would add
the restriction that space-time must be approximately flat (small M/R
for all sources).  Finally, since gravitational radiation is predicted
to carry angular momentum, and since conservation of angular momentum
then implies an aberration in the direction of the force (if you can
define the direction of the force), I might want to require that we
only apply your statement to cases where gravitational radiation is
insignificant (is that implied by the previous condition?).

     Richard M. Mathews			Lietuva laisva = Free Lithuania
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