Entropy is in Flux
Abstract
The science of thermodynamics was put together in the Nineteenth Century to describe large systems in equilibrium. One part of thermodynamics defines entropy for equilibrium systems and demands an everincreasing entropy for nonequilibrium ones. However, starting with the work of Ludwig Boltzmann in 1872, and continuing to the present day, various models of nonequilibrium behavior have been put together with the specific aim of generalizing the concept of entropy to nonequilibrium situations. This kind of entropy has been termed {\em kinetic entropy} to distinguish it from the thermodynamic variety. Knowledge of kinetic entropy started from Boltzmann's insight about his equation for the time dependence of gaseous systems. In this paper, his result is stated as a definition of kinetic entropy in terms of a local equation for the entropy density. This definition is then applied to Landau's theory of the Fermi liquid thereby giving the kinetic entropy within that theory. Entropy has been defined and used for a wide variety of situations in which a condensed matter system has been allowed to relax for a sufficient period so that the very most rapid fluctuations have been ironed out. One of the broadest applications of nonequilibrium analysis considers quantum degenerate systems using MartinSchwinger Green's functions\cite{MS} as generalized of Wigner functions, $g^<$ and $g^>$. This paper describes once again these how the quantum kinetic equations for these functions give locally defined conservation laws for mass momentum and energy. In local thermodynamic equilibrium, this kinetic theory enables a reasonable local definition of entropy density. However, when the system is outside of local equilibrium, this definition fails. It is speculated that quantum entanglement is the source of this failure.
 Publication:

arXiv eprints
 Pub Date:
 March 2014
 DOI:
 10.48550/arXiv.1403.6162
 arXiv:
 arXiv:1403.6162
 Bibcode:
 2014arXiv1403.6162K
 Keywords:

 Condensed Matter  Statistical Mechanics;
 Condensed Matter  Quantum Gases;
 Physics  History and Philosophy of Physics