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Re: Entropy



I am not comfortable with what Leigh
wrote about "explaining" as opposed
to "making consistent." But I do not
know how to argue about this. In my
opinion both are subjective attempts
of "making sense" out of objective
things (according to accepted rules).
I do not know why one kind of logical
glue should be better than the other.
Ludwik Kowalski

----- Original Message -----
From: Leigh Palmer <palmer@SFU.CA>
Date: Thursday, April 8, 2004 5:11 pm
Subject: Re: Entropy

Joel is correct; I erred. I did mean to cast my aspersions toward
http://www.av8n.com/physics/thermo-laws.htm#sec-entropy

I really haven't the patience right now to discuss this in detail*,
which it surely deserves, since I now see that John Denker wrote
it. I
am also trepid about treading upon Michael Edmiston's endorsement of
the statistical definition of entropy as fundamental, a position that
certainly is popular now.

First let me set, briefly, a fundamental philosophical position fr
om
which I argue my points. It turns on the special meaning of a term we
physicists abuse. I claim that it is not the proper purpose of physics
to **explain** Natural phenomena. Instead, physics (through the
effortsof physicists) strives to **describe** Nature in a way that is
manifestly self-consistent. This is most easily accomplished by making
identifications of quantifiable attributes of physical systems
(physical quantities) with mathematical entities, since the
mathematical systems can be proved to be rigorously self-consistent.
This permits physics to make predictions, often with marvelous
success,based solely on extrapolation of a mathematical nature.

Physics, then, is a **consistent description** of Nature. If one
insists on **explanation** of nature then one should look to religion
instead of physics. Physicists should use the phrase "is consistent
with" in many places they employ "explains". Of course we physicists
all understand
that.

Let me, then, briefly outline my positions. The Denker excerpt
would be
greatly aided by a statement of definition, e.g. "The entropy of
... is
...". After all, the entropy is an attribute of something, most
usuallya physical system of some kind. If one wishes to define it,
thenspecification of the system is surely a required first step. In
thecited excerpt I have no clear idea of what system's entropy is
beingdefined. If it is meant to be the entropy of the deck of
cards, then I
have no doubt that this "definition" is incorrect, which is why I call
it science fiction.The entropy of a deck of cards cannot depend on
anything but the state of the deck, and it certainly cannot depend
uponwhat some arbitrary person knows about that state. That would
lead us
to infer that the entropy of a system has a value relative to some
observer based on the state of his knowledge of the system. It would
then conceivably have different valu
es for different observers
based on
their differing states of enlightenment.

Michael, the remarkable aspect of the statistical description of
entropy due to Boltzmann and Gibbs is that it is indistinguishable by
experiment from the classical description. Both descriptions are
exact,or, I should say, the physical theories within which they are
used are
equally valid descriptions of natural phenomena. I am prepared to
arguethis point at some length at a later date. I don't understand
yourpreference for the statistical description as an introduction,
and my
own is for the classical thermodynamic definition. The entropy, as
Rudolph Clausius noted, is very like the energy. Both are abstract
quantities, **functions** solely of the state of a system. Like
energy,it has no corporeal reality. I believe one important problem
encountered in introducing the entropy to students is that they have
been misinformed about the nature of the energy; t
hey believe energy
is, itself, a physically real substance. While energy is easy to
understand this way, if we call it caloric then the pejorative aspect
of that concept attaches, and no one today will defend caloric **per
se**.

There is material for hours of discussion here.

Leigh

*I'm leaving for a long holiday trip on Tuesday.

On 8-Apr-04, at 2:00 AM, Joel Rauber asks:

Leigh,

Did you mean http://www.av8n.com/physics/thermo-laws.htm#sec-entropy
instead
of http://www.av8n.com/physics/twelve-coins.htm

The first site contains the entropy explanation, the second 12-coin
page
merely uses entropy as part of an explanation of the solution to the
12-coin
puzzle.

Joel R

|
| > http://www.av8n.com/physics/twelve-coins.htm
| >
| > Once the customers understand what entropy is, you can
| > define temperature in terms of entropy.
|
| One does not *require* a definition of
entropy to define
| temperature. Many would argue that defining temperature is
| easier to do than that.
|
| Someone recommended the above site as a clear explanation of
| entropy. If you were confused before viewing this site, and
| you feel more confused after viewing it, I want assure you
| that you are reacting appropriately. The explanation given
| here is science fiction. See, instead, an appropriate
| textbook on the topic. My favorite accessible textbook is
| Reif's "Fundamentals of Statistical and Thermal Physics".
|
| The indiscriminate use of convenient sources on the web is to
| be discouraged, and classics should still be recognized for
| their great value. We should not reinforce the practice of
| uncritically passing on such convenient references to our
| students (customers!?) simply because they exist.
|
| Leigh