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From: guy@rlgvax.UUCP (Guy Harris)
Newsgroups: net.physics
Subject: Re: quo vadis gravity?
Message-ID: <300@rlgvax.UUCP>
Date: Wed, 12-Dec-84 21:44:36 EST
Article-I.D.: rlgvax.300
Posted: Wed Dec 12 21:44:36 1984
Date-Received: Sun, 16-Dec-84 06:47:46 EST
References: <184@decwrl.UUCP> <181@talcott.UUCP>
Organization: CCI Office Systems Group, Reston, VA
Lines: 25

> > 
> > >The gravitational field does not "travel" at all.
> > 
> > If e=mc**2 and matter and mass are converted in an atomic bomb... shouldn't
> > something happen to the gravitational field of the "former" mass?
> 
> A common myth:  mass is not conserved, energy is not conserved, but the sum
> of the two is. This myth is in part propagated by Isaac Asimov and other
> popular science writers.  Now here's reality:  Mass *is* energy, and it is
> conserved. In fact, one more conservation law was thrown in:  momentum.
> Whereas B.E. (before Einstein), we had the conservation of mass, the
> conservation of energy, and the conservation of momentum, A.E. (after
> Einstein), we have the conservation of energy-momentum (energy being equal
> to mass).  Energy can be thought of as "momentum in the time direction".

And, since the source of the gravitational field is energy-momentum (well,
actually it's the stress-energy tensor, which more-or-less is to energy-momentum
as electric charge/current density is to charge, so energy-momentum is
actually the "charge" that generates a gravitational field), the "former mass"
is still around, because the energy-momentum of the bomb before the
explosion and after is the same (by conservation of E-M); therefore, the
gravitational field may be changed but it doesn't "go away".

	Guy Harris
	{seismo,ihnp4,allegra}!rlgvax!guy