NetNews Usenet Archive 1992 #27/NN_1992_27.iso/spool/sci/physics/19305

Text File | 1992-11-20 | 5.4 KB | 95 lines

Newsgroups: sci.physics
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From: [e-mail] (Daniel E. Platt)
Subject: Re: energy, mass, and all that
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Date: Fri, 20 Nov 1992 16:04:19 GMT
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In article <[e-mail]>, [e-mail] (SCOTT I CHASE) writes:
|> In article <[e-mail]>, [e-mail] (Daniel E. Platt) writes...
|>
|> >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|).
|>
|> What about after the photon is absorbed by some material, increasing
|> the kinetic energy of the material?  Are we going to say that there is
|> no such thing as heat energy, only mass increase of the material?  I
|> suppose you are free to do so, but I find this to be (a) a very big
|> stretch, and (b) a useless formulation for solving most problems.

You wouldn't catch me dead saying there's no such thing as heat
energy.  However, heat energy is only the movement of particles...
In the end, the partition function is computed on states of various
kinds of matter -- such as phonons being vibrations in a solid, or
black-body being the distribution of photons in thermal equilibrium
with matter.

As for the usefulness of such a formulation in solving problems,
how are you going to compute a partition function *without* a
Hamiltonian describing how a system manifests its energy?  In
other words, I don't know how you can talk about energy without
talking about the systems that can be 'excited' (have energy
transfered to them) by other systems, and what those interactions
look like.  Even in those situations where the total mass of
the final products is different from the total initial mass,
all that 'created energy' is being transmitted or mediated by
*something*.

|>
|> Martin Watts wants to say that mass and energy are non interconvertible.
|> That is, when Martin came into the world, there was so much mass and
|> so much energy.  From that point in time, he will say that all the
|> mass that became energy is really still mass, and all the energy that
|> became mass is really still energy.  Isn't this unphysical?  You have
|> to argue so hard to demonstrate that all mass continues to be mass that
|> you practically have to eliminate the possibility that energy exists in
|> the Universe (As you did above.)  Similarly, you have to argue so hard
|> that energy never becomes mass that you practically have to eliminate
|> the possibility that mass exists.
|>

If that is what Martin Watts is saying, then Martin is wrong.  On the
other hand, it opened the possibility for examining another question,
namely, what does it mean to say that 'matter has been converted to
energy?'  In the case of particle physics (not my strong suite), there
are restrictions on the number and kind of particles that can be
the product of a reaction -- such as iso-spin, charge, angular momentum,
etc.  In a way, those conserved quantities reflect internal symmetries
besides the one that describes the time-evolution of a system, and the
fact that a Hamiltonian commutes with itself (mass-energy conservation
in a Fock space); even mass-energy conservation has to be expressed in
terms of particles to carry the energy at that level.

In a sense, perhaps this is just a semantic argument.  The argument
I'm responding to is more like the old Star Trek's episode where
everyone is staring at a viewer at a glowing blob, and Spock says
that it is "...pure energy."  In response to that, I want to ask
"pure energy of what?"  A lot of people come out of their first
exposure to E=mc^2 thinking that you can completely convert the mass
in a chunk of desk into 'pure energy.'  Most often, it isn't made
clear that the 'conversion' occurs in reactions subject to strict
conservation laws.  Maybe I'm arguing about something pedantic.

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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