[Fis] Re: Only One Entropy

To: <fis@listas.unizar.es>
Subject: [Fis] Re: Only One Entropy

Dear Michael,

About symmetry and entropy, I wrote: <<For these latter, logical symmetries
(degeneracy of states), and spatial symmetry (chambers) lead to decrease
the combinatorial enumeration of the microstates, and thus there is an
effect on the calculation of thermodynamical entropy.>>
Here, I was myself writing what seems to be an usual concept of
the impact of symmetry, although I admit that not all authors agree.

In fact, there are various way to introduce symmetry in the model
of a thermodynamical system, at least because there are several
concepts of symmetry (and see the very interesting work of Gyuri in
his post <<Re: [Fis] Re: miscellanea / temperature / symmetry>>
of the 22 April).
My own conception about the relation between symmetry and entropy
is in the email <<Re: [Fis] Re: miscellanea / symmetry>> posted the
26 April: there is little connection between symmetry and entropy
(not all authors agree).

About the "unique" entropy":
It is a philosphical discussion: when we have several models
(i.e. sets of mathematical equations) competiting for some physical measure
in a real system (think also to economy and social sciences), nobody
would confuse the system with one of its mathematical models.
But sometimes, when there is only one model currently available,
or when models are just of increasing accuracy due to a formal
complexification (e.g. newtonian approximation of gravity in
the relativistic model). I do not claim that there are several
statistical mechanics theories which offer no compatibility
within themselves. Now, I assume that our knowledge of some physical
system is so deep, that we are able to produce a mathematical
model working perfectly at any level and scale. Here again,
I consider that the system and its model are two different entities.
It is why, in some practical situations dealing with entropy, I consider
that the informational entropy is able to modelize the physical entropy,
but IS NOT the physical entropy. If the two entropies are the same,
informational entropy has at least to work in all situations where
thermodynamical entropy is measured. This is difficult to believe.
And what about the converse ? I cannot see thermodynamics
in all fields in which informational entropy is used.

This is just a philosophical discussion, which could be of interest
in any area of science.
So, here comes my question to philosophers (and scientists of course):
what is "modelling" ?

Michel Petitjean Email: petitjean@itodys.jussieu.fr
Editor-in-Chief of Entropy entropy@mdpi.org
ITODYS (CNRS, UMR 7086) ptitjean@ccr.jussieu.fr
1 rue Guy de la Brosse Phone: +33 (0)1 44 27 48 57
75005 Paris, France. FAX : +33 (0)1 44 27 68 14
http://www.mdpi.net http://www.mdpi.org
http://petitjeanmichel.free.fr/itoweb.petitjean.html
http://petitjeanmichel.free.fr/itoweb.petitjean.freeware.html
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Received on Mon May 3 14:12:06 2004