From: Lee Corbin (lcorbin@tsoft.com)
Date: Tue Jul 22 2003 - 23:20:46 MDT
When I was a teen, I used to tell my friends that the
sun wasn't really where they thought it was. When they
evinced surprise, I proudly announced that it really was
two degrees to the west of where it looked like it was,
or about four sun-diameters.
I explained that this was (of course) because the light
took eight minutes to reach us, and so the sun's real
position was two degrees to the right (looking from the
Northern hemisphere).
None of them spotted the flaw in my logic :-) and it may
take a few on this list a moment or two to see where I was
going wrong.
Anyway, it seems that Laplace *did* correctly determine
that where the sun is as measured by a sensitive gravimeter
is not the same place that it looks like it is. There is a
very nice exercise in "Problem Book in Relativity and
Gravitation", Lightman, Press, Price, and Teukolsky:
"The position of the sun in the sky can in principle be
measured by a sensitive tidal gravimeter. What is the
angular difference between this position and its position
as measured optically? If the actual position of the sun
were at its optical position, there would be a force in
the direction of the Earth's motion. If this were the case
find the radius of the Earth's orbit as a function of time."
The answer has something to do with the aberration of light,
as I understand it. If the Earth is going 66,000 miles per
hour (30 km/sec) past the sun, then the sun appears slightly
shifted towards the forward direction. That makes sense.
So what I infer from this problem is that *gravitationally*
it isn't so! The gravity waves don't lie! There must not
be anything like "aberration of gravitons". (This thought
is also what lent me credence in my earlier post that perhaps
the frame of reference of the space-bending (i.e. gravitating)
object is the correct frame of reference to use. Mostly?
Lee
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