From: Richard.Mathews@West.Sun.COM (Richard M. Mathews)
Newsgroups: sci.physics
Subject: Re: Spin
Date: 28 Sep 1992 22:19:24 GMT
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Originator: richard@astro.West.Sun.COM

b5249-23@mathcs.sjsu.edu (Kurt Stocklmeir) writes:

>Gravity has an infinite velocity.
>Gravitation   Minser, Thorne, Wheeler   P. 177

Others have already commented on how this out-of-context quote does not
really represent what MTW were saying.  I will concentrate on...

>Inertia can be explained if gravity has an infinite velocity.
>Inertia could be caused by a particle interacting with many other particles
>in the universe using gravity that has an infinite velocity.
>Superforce   Paul Davies   P. 215

This does not appear to be an exact quote at all, but Stocklmeir's
interpretation.  Pages 213 through 216 do discuss Mach's principle and
possible origins of inertia, but at no point does Davies say "gravity
has an infinite velocity."  Rather, he says,
	The trouble with the simple [Machian] picture given
	above is that, according to the theory of relativity,
	a gravitational disturbance cannot travel faster than
	light....  Einstein believed that he had found a way to
	overcome the time delay problem by formulating Mach's
	principle as part of his cosmological investigations....
	There followed a protracted and confused debate that
	continues to this day, as to what extent Einstein's
	general theory of relativity does or does not incorporate
	Mach's principle.   [page 214]

>Electric fields and magnetic fields have an infinite velocity.
>Most books on electromagnetic theory.

There is a bit of "magic" in E&M that the electric field "vector" points
towards the instantaneous position of the source no matter what reference
frame is used to make the observation provided that the source is moving
inertially.  This is sort of like the "spooky" pictures in Disneyland's
haunted house which follow you around the room.  Those eyes aren't really
moving; just your perception of them is.  Similarly, the electromagnetic
antisymmetric rank two tensor is not changing when you change reference
frames.  The part of the tensor which happens to act like a vector under
rotations and reflections, the part we call the "electric field vector,"
appears to be changing because we are looking at it from different
perspectives.  That doesn't mean that the field moved with infinite
velocity.  I would rather think of it as a field which propagates at
the speed of light but which appears to be a non-central force in most
frames.  The force is non-central by exactly the amount necessary to
make it point to where the particle currently is in that frame.

The situation with gravitation is similar.  In a situation where space-time
is almost flat, you can supply a global Lorentz frame.  With respect to
that frame, you can determine a Newtonian-like gravitational acceleration
vector.  No matter which such global frame you choose, the gravitational
acceleration vector points towards the instantaneous position of the
source provided that the source is moving on a geodesic.  Since different
frames see the "gravitational acceleration vector" pointing in different
directions, and since such a vector and such an instantaneous position
can't even be defined in a more general case with strong fields, I can't
see it as more than a coincidence that the vector points towards the
instantaneous position.  The field propagates at finite speed, but in
a way which makes the "eyes follow you around the room."

Actually, in neither case is the situation really a coincidence.  It
is a requirement of conservation of angular momentum.  That makes the
result much less surprising.

     Richard M. Mathews			Lietuva laisva = Free Lithuania
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