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From: leon@hhb.UUCP
Newsgroups: net.physics
Subject: Re:  Is gravity instantaneous?
Message-ID: <123@hhb.UUCP>
Date: Wed, 25-Apr-84 15:49:00 EST
Article-I.D.: hhb.123
Posted: Wed Apr 25 15:49:00 1984
Date-Received: Fri, 27-Apr-84 04:31:39 EST
References: <12552@sri-arpa.UUCP>
Organization: HHB-Softron, Mahwah, NJ
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	Geneerally, Gauge theories are local quantum field theories.
This does not mean that distant locations are not affected, but rather
that they are affected only through a signal propagation mechanism.
In terms of the formulation of field theory through lagrangian densities
this means that only a single space-time location appears in the
lagrangian.  Thus:

		    df(x)    df(x)          4 
                    -----  . -----   + f(x)
                    dxi      dxi

is local(xi is x - sub i, summation convention in effect), but a 
lagrangian like:

                   df(x)     df(x-y)        2         2
                   ----   .  ------  + f(x)   . f(y)
                   dxi       dxi

would be non-local.  For a historical perspective on this issue,
one could study the history of field-theoretic descriptions of
weak interactions(fermi lagrangian vs. the weinberg-salaam model).

By the way, classical maxwell electrodynamics is also a gauge theory
(although it is an abelian one unlike gravity).  Needless to say,
electrodynamics is local.


Although the quantization of gravity is by no means a finished program,
much work has been done on it - most recently in the context of path
integral quantization of gauge theories.


					leon gordon
					decvax!philabs!hhb!leon