From: pg@sanitas.stortek.com (Paul Gilmartin)
Newsgroups: sci.astro
Subject: Re: The Hole Story
Date: 20 Dec 92 03:34:09 GMT
Organization: Storage Technology Corp.
X-Newsreader: TIN [version 1.1 PL7]

Chris Flatters (cflatter@nrao.edu) wrote:
: 
: Disregard most of what Tom is saying.  Tom is trying is confusing the effects
: of a field with those of a propagating wave.
: 
: If you want to calculate the gravitational potential at the Earth (say) using GR
: you would use retarded potentials to do the necessary integrations in the most
: general way.  You would find that the gravitational force on the Earth is
: directed towards the point in space where the Sun was about 8 minutes ago
: (the light travel time between the Sun and the Earth).  In the reference frame
: of the solar system this is a near as makes no difference to the point in space
: where the Sun is now so we can ignore the retardation.
: 
: The correct analogy in terms of electromagnetism is not to consider the
: direction of the force acting on the Earth due to light from the Sun
: (the Poynting-Robertson effect is due to the fact that light has a
: non-zero momentum) but to consider the what would happen if the Sun
: had a net positive charge and the Earth had a net positive charge.  In this
: case the force on the Earth would again be directly towards the Sun.
: 
: Now: going back to the beginning.  Light from the Sun does not appear
: to come directly from the Sun but appears to come from a direction that
: is shifted slightly forward of the Earth (defining forward by the
: instantaneous motion of the Earth) in the reference frame of an
: observer moving with the Earth.  This is simple aberation and has
: nothing to do with retarded potentials (in the Sun's reference frame
: you could say that the Earth is running into the side of the light);
: the apparent direction from which the light comes is not the direction
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
: of the retarded position of the Sun.  Since light has momentum and the
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
: light comes from slightly in front of the Earth there is a slight
: slowing of the Earth.  This effect is significant for small particles
: and dust does indeed spiral inwards towards the Sun because of it.  The
: effect is real: Tom's explanation is garbled.

I disagree with you.  I believe the apparent direction from which the light
comes is _exactly_ the direction of the direction of the retarded position
of the Sun.  I'll defend my position with a simple thought-experiment
using Special Relativity.  Suppose I measure the apparent position of the
Sun exactly at noon with a sensitive meridian telescope.  I'll see the
Sun slightly west of its true position because of "simple aberration".
I fully agree with your explanation that "in the Sun's reference frame
you could say that the Earth is running into the side of the light."
But how can I explain the same apparent displacement in the reference
frame of the Earth?  In that reference frame, the Earth is at rest, and
can't be regarded as "running into the side of the light."  Instead,
the explanation is that photons leave the Sun and travel in straight
lines reaching the Earth in 8 minutes.  When I measure the apparent 
position of the Sun at noon, I'm actually observing the direction of
motion of photons that left the Sun 8 minutes previously, so the Sun
appears to be at its position at that time.

So the effect of retardation is exactly the same as the effect of aberration,
and it is impossible for the descriptions "as near as makes no difference"
and "significant" both to be true.

-- gil