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From: guy@rlgvax.UUCP (Guy Harris)
Newsgroups: net.physics
Subject: Re: M = E/C^2 ??? How??? - (nf)
Message-ID: <1339@rlgvax.UUCP>
Date: Wed, 26-Oct-83 19:21:00 EDT
Article-I.D.: rlgvax.1339
Posted: Wed Oct 26 19:21:00 1983
Date-Received: Thu, 27-Oct-83 20:46:55 EDT
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Since, in relativistic terms, mass and energy are not separate things which
are interconvertible but are actually the same thing (ignoring the question
of whether mass as measured by the force required to accelerate something -
"inertial" mass - is the same as mass as measured by the force exerted on
the object by gravity - "gravitational" mass), the distinction between mass
and energy is more a distinction between the form of the mass/energy.

Generally, "mass" refers to the "rest mass" of something with significant
rest mass, where most of the energy that the object has is due to its rest
mass - if you make a ball go faster its mass increases because its energy
increases, but the difference is extremely small relative to the "rest energy"
of the ball.  Something having a mass of 10 grams has a rest energy of 10 grams
times C^2, which is (3E10 cm/sec)^2, giving 9E21 grams-cm^2/sec^2.  Such an
object moving at 4.5E3 cm/sec, which is 100 mi/hour, has a kinetic energy
of 1/2 m*v^2, which is 1.01E8 grams-cm^2/sec^2, which is about 18 orders of
magnitude less than the rest energy.  "Energy" in the case you mention
generally refers to the energy of something with no rest mass; i.e. electro-
magnetic radiation.  As such, a nuclear explosion (or a chemical explosion)
converts the rest mass/rest energy of the explosive materials into the
kinetic energy of what's left after the explosion (although all that kinetic
energy *does* remain as mass, because the flying pieces have greater mass due
to their greater kinetic energy) and into the kinetic energy of electromagnetic
radiation from the explosion (which, since it has no rest mass and can't
stop moving, has only kinetic energy).

One process which goes the other way would be that of breaking up the
products of a fusion or chemical reaction, or fusing the products of a
fission reaction, with the addition of electromagnetic or strong nuclear
energy.  In a chemical reaction which requires energy input, the final products
would have more mass than the inputs (although the same 18 orders of magnitude
would come in and you'd need the kinetic energy of 1E18 balls moving at 100MPH
to produce an increased mass equal to the mass of one of those balls).  One
other such reaction is "pair production".  A photon of electromagnetic energy
interacts with the electromagnetic field of a nucleus and becomes a pair of
a particle and its antiparticle, such as an electron and a positron.  This
is probably the purest form of the kind of reaction you're looking for, as
it converts a photon (which has no rest mass and acts more like what people
conventionally think of as "pure energy" than most anything else) into
particles of the type that make up everyday matter.

	Guy Harris
	{seismo,mcnc,brl-bmd,allegra}!rlgvax!guy