NetNews Usenet Archive 1992 #27/NN_1992_27.iso/spool/sci/physics/19249

Text File | 1992-11-19 | 5.3 KB | 103 lines

Newsgroups: sci.physics
Path: sparky!uunet!newsgate.watson.ibm.com!yktnews!admin!platt
From: [e-mail] (Daniel E. Platt)
Subject: Re: energy, mass, and all that
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Date: Thu, 19 Nov 1992 19:57:24 GMT
Disclaimer: This posting represents the poster's views, not necessarily those of IBM
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In article <[e-mail]>, [e-mail] (SCOTT I CHASE) writes:
|> I am more convinced than I was before that this argument is more than a
|> matter of different definitions of mass.  No matter which definition you
|> adhere to, you can't get the right physics by assuming that matter and
|> energy are not interconvertible in the way that Bondi seems to suggest.
|>
|> In article <[e-mail]>, [e-mail] (Martin S T Watts) writes...
|> >
|> >    "It has come to our notice (for example Warren, 1976) that there is
|> >    quite widespread misunderstanding about the interpretation of
|> >    Einstein's famous equation E=mc^2. It seems that this is often
|> >    regarded as something rather like a monetary rate of exchange, such
|> >    as L1.00=$1.45, and that mass and energy are thought to be
|> >    interconvertible, each to the other. This is not so. Mass and
|> >    energy are not interconvertible.
|>
|> No matter which definition of mass ("old" or "modern") you use, energy
|> and mass are still interconvertible.  When an electron and positron
|> annihilate into a pair of photons, which are absorbed by a piece of lead,
|> you end up with hot lead.  You can extract work from the temperature
|> difference thus created.  The mass of the original pair (whether you
|> count just the rest mass, or the relativistic mass) has been converted
|> into work.  If this is not mass being converted to energy, what is it?

Perhaps part of the problem has to do with the vagueness of the language?

When 'mass' is destroyed, and 'energy' is created, actually, what's happening
is some kinds of particles are anihilated, and some are created.  For instance,
in electron-positron anihilation, 2 photons are created; there's Feynmann
diagrams for the processes, and cross-sections may be computed, to yield
spectra, etc.  There's no energy without some particular kind of particle
to carry the energy.  However, there's different kinds of particles that
CAN carry the energy, and the total mass of the particles after the
reaction may be different than the mass before the reaction; the total
mass-energy (E^2 = p^2 + m^2) must be conserved (reflected in a delta-function
in the S-matrix).  Even here, saying that the mass was 'converted' to
energy is a misnomer; photons were created that carry the energy in a
way characteristic of the photon dispersion relationship: E=pc (omega = c|k|).
Is this energy the kinetic energy of a photon?  Somehow, that's not how
I've ever thought of it...

|>
|> >    They are entirely different
|> >    quantities and are no more interconvertible than are mass and
|> >    volume, which also happen to be related by an equation, V=m*rho^-1.
|> >    Mass and volume are different quantities and have different
|> >    dimensions. So have mass and energy. They feature differently in
|> >    equations.
|>
|> That's just gibberish.  The equation you quote is not a physical relationship
|> between two physical quantitites.  It is the definition of density.  It
|> has no physics content.  E^2 = m^2 + p^2 tells you how much total energy
|> a system contains, i.e., how much work you can extract from it.  It includes
|> the physically necessary contribution from the mass, which, when converted
|> into energy, allows more work to be done than can be accounted for by
|> just the kinetic energy of the original system components.
|>

If Martin meant what it looks like he meant, then he's indeed spouting
gibberish; however, I will still point out that out of the gibberish
does emerge a distinction worth noting; for instance:

|> A question for Martin:
|>
|> If energy and matter are not interconvertible, where does all the energy
|> of an atomic bomb blast come from?
|>

The energy released in an atomic bomb is carried in photons (gamma rays),
alpha particles, beta particles, various different fission products, as
well as kinetic energy ultimately transmitted to the ground and atmosphere.
The total energy at the end is carried by mass and other particles (neutrinos,
photons, etc).

|> -Scott
|> --------------------
|> Scott I. Chase            "It is not a simple life to be a single cell,
|> [e-mail]         although I have no right to say so, having
|>                  been a single cell so long ago myself that I
|>                  have no memory at all of that stage of my
|>                  life." - Lewis Thomas

Dan

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Daniel E. Platt                                      [e-mail]
The views expressed here do not necessarily reflect those of my employer.
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