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From: cjh@petsd.UUCP (Chris Henrich)
Newsgroups: net.misc,net.physics
Subject: Re: perpetual motion
Message-ID: <480@petsd.UUCP>
Date: Sat, 23-Mar-85 21:12:57 EST
Article-I.D.: petsd.480
Posted: Sat Mar 23 21:12:57 1985
Date-Received: Sun, 24-Mar-85 06:49:08 EST
References: <5079@elsie.UUCP>
Organization: Perkin-Elmer DSG, Tinton Falls, N.J.
Lines: 42
Xref: watmath net.misc:7627 net.physics:2295

[]
> > Also, conservation of momentum is a direct consequence of spatial
> > translation invariance of the fundamental laws of physics, and
> > conservation of energy is a result of time translation invariance.
> > 
> > The general relativist would point out that the corresponding exact
> > physical law (vanishing divergence of stress-energy-momentum tensor)
> > shows that the exactly-conserved quantity is not purely physical, and
> > the purely physical quantity is not exactly conserved.
> >
> 
> 	Could someone kindly translate the above into English
> 	for those of us who don't often see phrases like
> 	"time translation invariance"
> 		 and
> 	"vanishing divergence of stress-energy-momentum tensor"?

The basic message of the first statement is that conservation
of momentum is deeply imbedded in modern physical theories,
and cannot be removed without totally wrecking the theory.

The second sentence depends on the distinction between "purely
physical" quantities and something else (in the name of
Murphy, what??).  If we ignore this distinction, then it also
says that you cannot get away from conservation of momentum
without wrecking general relativity.

Perhaps the writer's meaning is that in general relativity, no
region of space-time can be truly isolated, and so any
measurement of momentum is slightly affected by all the matter
in the universe.  And this effect is non-constant, because we
can't get the universe to hold still while we do our
experiment.

Regards,
Chris

--
Full-Name:  Christopher J. Henrich
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