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From: tan@ihlpg.UUCP (Bill Tanenbaum - AT&T Bell Labs - Naperville IL)
Newsgroups: net.physics
Subject: Re: A question about mass and energy
Message-ID: <835@ihlpg.UUCP>
Date: Fri, 12-Jul-85 13:47:30 EDT
Article-I.D.: ihlpg.835
Posted: Fri Jul 12 13:47:30 1985
Date-Received: Sat, 13-Jul-85 15:06:14 EDT
References: <378@sri-arpa.ARPA>
Organization: AT&T Bell Laboratories
Lines: 25

> Here's a question I've had in my mind that I haven't resolved yet.
> 
> In physics I was taught that energy is not a substance and does not have
> a definite location.
> 
> However relativity says that energy is/has mass.  Mass does have a
> definite location.
> 
> If energy is mass, how can it be and not be in a definite location?  If
> energy has mass, where is the gravitational field caused by that mass if
> it has no definite location?
> 
> Dan

An excellent question.  The answer is simply that energy DOES have a
definite location (at least if we ignore quantum effects).  In
elementary physics, concepts such as potential energy prove useful.
The potential energy is viewed as a property of the entire configuration
of the system, rather than as a local quantity.  In a more sophisticated
formulation, we see that the potential energy can be explained as the
energy stored in the gravitational or electromagnetic field.  The energy
density of the electromagnetic or gravitational field is well defined
at every point, i.e. has a definite location.
-- 
Bill Tanenbaum - AT&T Bell Labs - Naperville IL  ihnp4!ihlpg!tan