From: matt@physics16.berkeley.edu (Matt Austern)
Newsgroups: sci.physics
Subject: Speed of propagation of gravity (was Re: Spin)
Date: 29 Sep 92 03:40:52 GMT
Reply-To: matt@physics.berkeley.edu
Organization: Lawrence Berkeley Laboratory (Theoretical Physics Group)
NNTP-Posting-Host: physics16.berkeley.edu
In-reply-to: b5249-23@mathcs.sjsu.edu's message of 28 Sep 92 22:17:35 GMT

In article <1992Sep28.221735.35@sjsumcs.sjsu.edu> b5249-23@mathcs.sjsu.edu (Kurt Stocklmeir) writes:

> Physics teachers give hours of homework problems to physics students and
> because of that physics students do not ever have time to learn
> anything about physics.  That explains why John Baez and others
> do not know and can not figure out P.177.
> 
> The only thing that was meant on P.177 was that gravity travels
> faster than the speed of light.  But in relativity, nothing
> travels faster than the speed of light.  That means any gravity
> moving faster than the speed of light does not fit into relativity.

I think it is possible that we're getting confused here by
terminology.  First of all, Misner, Thorne, and Wheeler are not
claiming that gravity violates relativity---not on page 177 or any
other page.  Second: we have to be very careful defining exactly what
it means to talk about the speed of a force.  In the case of gravity,
in particular, you have to be more careful than usual.

Let's look at a simpler example first: electromagnetism.  Now, it's
well known that when you write Maxwell's equations in terms of the
vector potential, you can change the form around quite a bit by making
a gauge transformation, right?  Well, there is one gauge, the Coulomb
gauge, in which it looks as if the electromagnetic force is
instantaneous.  (Read Jackson, he works out all the details.  As
usual.)  So what's going on here: does this mean that light travels
faster than light?

What's going on is that in the Coulomb gauge, you're dividing the
electromagnetic potential into two pieces, the "static" piece and the
"radiation" piece.  The 1/r potential looks like it's instantaneous,
but that's only because you've put all of the physics of propagation
delays (i.e., electromagnetic radiation) into the other piece of the
potential.  In other words: you can say that the electromagnetic force
is instantaneous, but only in those cases where propagation delays are
irrelevant anyway.

The math is more complicated in General Relativity, but I believe that
the physical principles are the same: the "force" is instantaneous in
static configurations---that is, in configurations where gravitational
radiation is negligible.  Since gravitational radiation is negligible
in the dynamics of the Solar system, it shouldn't surprise us that
people don't put in propagation delays when they calculate the orbits
of planets.

Again: we really shouldn't be surprised by this.  Waves are the only
things that propagate, and we're talking about situations where there
are no waves.  Why should we find it surprising to learn that there
aren't propgation delays, if there isn't any propagation?



--
Matthew Austern                   Just keep yelling until you attract a
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Newsgroups: sci.physics