From: metares@well.sf.ca.us (Tom Van Flandern)
Subject: Re: Spin
Organization: Whole Earth 'Lectronic Link
Date: Tue, 6 Oct 1992 04:31:21 GMT


ramsay@unixg.ubc.ca (Keith Ramsay) writes:

> How was it determined what kind of "aberration" would exist if gravitation
> propagated at the speed of light?

     "Aberration" is a technical astronomical term.  It usually refers to the
fact that astronomical bodies are not seen in their true positions because of
the motion of the observer and the finite speed of light.  It is analogous to
a vertically falling rain streaming across the window of a moving train at an
angle because of the motion of the train.  If the Earth's velocity vector is
<EV>, and the velocity of light toward a star is the vector <ES>, then we
actually see the star in the direction <EV> + <ES>.

     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.

> In particular, are you claiming anything which is known to be inconsistent
> with (a) the orbits are consistent with general relativity, and (b) gravity
> propagates at speed c in GR? If not, then please word the claim differently
> in the future.

     Your statement (b) is definitely false, as relativists will agree.  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.

     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.

     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.

     As a separate phenomenon, the presence of two large, moving masses
disturbs the "fabric of space-time" in their vicinity.  This sends out
ripples in space-time at the speed of light.  These ripples (not to be
confused with the gravitational force) can also affect masses they encounter,
although only in the slightest way.  The situation is loosely analogous to an
asteroid arriving from space and impacting nearby (analogous to gravity).
Shortly after its impact, a sonic boom arrives too (analogous to gravity
waves).  Only the sonic boom propagates at the speed of sound.

     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|-

-- 
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.