gravity's speed

[Matthew B Hoyt]

On Mon, 26 May 1997 09:25:27 -0500 (CDT) amazing at mail.utexas.edu (dennis
grace) writes:

[Mr. Grace is responding to my presentation that during big gravitational
events, the gravity wave may propagate at speeds greater or smaller than
the speed 
of light.]

>Yeah, but the Special Theory of Relativity has an almost mystical, 
>almost
>canny capacity for enforcing that c speed limit. 

Unfortunately special relativity really only applies to flat (or
Minkowski) spacetime.  To back up this statement, let me turn to 
Misner, Thorne and Wheeler's _Gravitation_ again (which at least 
during my abortive tenure as physics grad student was the bible
of General Relativity (with a close second Played by Hawking and Ellis's 
_Large-Scale Structure of Spacetime_ (I hope that second reference is 
correct - it is close enough for someone to find in any case)).  Chapter
7 out of
42 is titled and concerns "Incompatibility of Gravity and Special
Relativity".
So you see special relativity has serious shortcomings during major
gravitation
events.

> Without even 
>looking, I'm
>guessing this gravity speed propogation thing mimics the action of
>microwaves in a waveguide.  Within the waveguide, the wave front 
>travels at
>velocities greater than c along a zigzag path, but overall end-to-end 
>speed
>in a microwave guide never exceeds c.  In other words, from point of 
>origin
>to point of effect, the observed propogation speed of the event never 
>exceeds c.

No, it is far deeper.  Spacetime's shape and size  are determined by 
gravitational effects.  Major gravitational events manipulate the medium 
against which you  determine the speed of the propagation.  Such events 
work against intuitions built up around special relativity.

Matt