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From: desj@brahms.BERKELEY.EDU (David desJardins)
Newsgroups: net.physics,net.philosophy
Subject: Re: Thermodynamics / Statistical Physics
Message-ID: <12772@ucbvax.BERKELEY.EDU>
Date: Sat, 29-Mar-86 07:09:30 EST
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Posted: Sat Mar 29 07:09:30 1986
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Reply-To: desj@brahms.UUCP (David desJardins)
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In article <541@umich.UUCP> torek@zippy.UUCP (Paul V. Torek ) writes:
>Heat is too the same thing as its microphysical realization, and it was
>even 200 years ago.  What are different are our *ideas* of temperature and
>mean kinetic energy.  Pardon my naive realism, but the idea of a thing
>and the thing itself are different.

   We make models of the universe.  These models have properties which
we give names.  One very effective model is called thermodynamics and
it contains concepts called "heat" and "entropy" (among others).  The
"laws" of thermodynamics make statements about the behavior of large-
scale systems based on these macroscopic variables (e.g. heat flows in
a certain direction).  
   This model has three main weaknesses.  It describes only macroscopic
and not microscopic behavior (e.g. energy distribution among individual
atoms/molecules).  Its correspondence with the real world is not precise
(it is impossible to come up with a consistent definition of heat that
applies in all cases; for example is vibration in a normal mode of a
crystal lattice "heat"?).  And its predictions are only *statistical*;
that is, even given an appropriate correspondence with the real world
its predictions can (with small probability) be incorrect.

   What we are doing here (Platonists and conceptualists agree) is
defining an *abstract* concept called "heat" which satisfies certain
ideal properties (always well-defined and always obeys the laws of
thermodynamics).  It is this *concept* that is useful in actual
scientific practice (e.g. we pretend that a thermometer actually
measures this ideal quantity).
   "Td-heat" (which Matt and I are using to describe the actual heat
computed from the actual velocities etc. of the constituent particles)
does not satisfy any of these properties (even if you could define it
in general, which you can't).  In particular the "laws" of thermodynamics
are *not* laws when applied to this actual quantity, but only statistical
approximations.
   I guess I am saying that I agree with you that "heat" and "the concept
of heat" are not the same thing--the actual heat in a physical body is
what I am calling "td-heat," and our idea of what that heat represents is
what I am calling "heat."  Maybe I should give the latter its own special
name ("id-heat," for ideal) to avoid confusion.

   -- David desJardins