Re: Poynting-Robertson effect

[David Bowman <dbowman@tiger.gtc.georgetown.ky.us>]

Alex Burr writes:
> Palmer raises an interesting point (as he is wont to do) in his last
> post on P-R effect. He states that the gravitational force points directly
> to the center of the sun because it is a feature of space and does not
> propagate as does light.
> How does one reconcile this statement with the concepts of gravitons
> and gravity waves?

When I first read what Leigh had wrote about this in his post, I too had an
alarm bell go off in my head.  But I re-read what he had actually written,
and concluded that the bell was a false alarm.  What Leigh said was:

>    .... That gravitational force is strictly central; it appears to point
> directly to the center of the Sun. That is because the gravitational force
> does not in any sense have to propagate; it is a property of the space
> itself. Thus the speed of light does not limit it. ....

Leigh doesn't say that gravitational effects cannot propagate as retarded
waves.  He said that the "gravitational force does not in any sense HAVE TO
propagate".  I take this to mean that the sun's gravitational field is
*static*.  The field doesn't have to propagate because it long ago was 
established and has since settled down to a static configuration.  The field
configuration would indeed have to propagate with speed c as a retarded wave
IF it were dynamically changing -- but it is not; it is static.  The earth
merely enters different regions of different static solar gravitational field
as the earth orbits.  As an E&M analogy we know that accelerated charges
produce propagating EM waves/radiation, but a stationary proton sitting at 
the origin of an inertial coordinate system has a static field that doesn't
have to propagate for another passing particle to interact with the local
value of that stationary field.  (Here the proton's electrostatic field is
treated classically and not as a quantum artifact of virtual photon
exchange.)

If I misstated anything regarding Leigh's position on the matter, I'm sure
that he will be glad to correct me.

David Bowman
dbowman@gtc.georgetown.ky.us