Edward Green says... > >On Jul 8, 6:28=A0am, stevendaryl3...@yahoo.com (Daryl McCullough) wrote: > ><...> > >> Physicists *DO* reject the generalized principle of relativity expressed >> as the requirement of general covariance because it is physically vacuous > >Aha. That's exactly what I was trying to express when you asked me to >elaborate. Just how long have "physicists" felt this way, and when did >they start reading my Usenet posts? :-) :-) :-)
The position I'm quoting was in Misner, Thorne and Wheeler's "Gravitation", which is at least 30 years old.
>(I had an argument along these lines with John Baez years ago.
I think it must have been subtly different, because John certainly knows that every theory can be made generally covariant. There is a related property that *isn't* vacuous, that I mentioned, which is the lack of non-dynamic scalar, vector, or tensor fields. In GR, all fields are dynamic.
Roughly speaking, the way to make an arbitrary theory generally covariant is to stick in extra fields to correct for whatever changes in the theory occur when you do a coordinate transformation. But in general, those extra fields are non-dynamic.
So I would like to know exactly what the argument was with John.
>I noticed another argument that I had with him -- to the effect >that force is momentum flux -- later surfaced as an outstanding >revalation in his home pages).