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From: gwyn@brl-tgr.ARPA (Doug Gwyn <gwyn>)
Newsgroups: net.physics
Subject: Re: A question about mass and energy
Message-ID: <11562@brl-tgr.ARPA>
Date: Sun, 14-Jul-85 06:30:41 EDT
Article-I.D.: brl-tgr.11562
Posted: Sun Jul 14 06:30:41 1985
Date-Received: Fri, 19-Jul-85 03:25:52 EDT
References: <378@sri-arpa.ARPA>
Organization: Ballistic Research Lab
Lines: 50

> 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?

The main problem here is that you are trying to take too literally
statements made (perhaps by more than one source) that:
	(1)  are qualitative, not quantitative
	(2)  oversimplify in order to convey a general feeling
		for an idea
	(3)  use terms loosely
	(4)  use terms in different ways

"Energy" is a concept that is almost defined by being conserved.
A certain amount of energy is in prinicple capable of doing a
certain well-defined amount of "work".  Work is measured as the
path integral of opposing "force".  Force is supposed to relate
fairly directly to one's common experience of muscular reaction.
There is nothing in this general idea of energy that requires it
to be associated with a material substance or to be in a well-
localized packet.  Indeed, a propagating plane electromagnetic
wave violates both but does carry energy as given by its Poynting
vector.

Special relativity shows that there is no essential difference
between mass and energy in that they are interconvertible
(E = m c^2), but certainly mass is characterized by being rather
localized.  If you look at the modern theories involving the
"elementary particles", the distinction between mass (particles)
and various forms of energy transport becomes blurred, since
these theories consider all interaction to be merely the exchange
of particles.  In particular, the electromagnetic field is just a
cloud of "photons", which are particles, and subsidiary particles.

Another view, less popular at present, is that "field" is the
fundamental entity and masses are either singularities in the
field or concentrations of the field.  (Just what field this is,
is a long story.)

To answer the last question, it is probable that any form of
energy contributes to producing the "gravitational field".  But
most forms of energy outside that which we normally call "matter"
are too unconcentrated to have much effect.  (It is even more
probable that a generalization of gravitation is fundamental and
energy is a derived notion, but that is part of the "long story".)