title: Entropy meters and the entropy of non-extensive systems.
abstract: In our derivation of the second law of thermodynamics from the relation of adiabatic accessibility of equilibrium states, we stressed the importance of being able to scale a system's size without changing its intrinsic properties. This leaves open the question of defining the entropy of macroscopic, but unscalable systems, such as gravitating bodies or systems where surface effects are important. We show here how the problem can be overcome, in principle, with the aid of an 'entropy meter'. An entropy meter can also be used to determine entropy functions for non-equilibrium states and mesoscopic systems.
authors.affiliation: Departments of Mathematics and Physics , Princeton University , Princeton, NJ 08544, USA.
authors.forename: Elliott H
authors.initials: EH
authors.lastname: Lieb
authors.affiliation: Faculty of Physics, University of Vienna, Vienna, Austria ; Erwin Schrödinger Institute for Mathematical Physics, Vienna, Austria.
authors.forename: Jakob
authors.initials: J
authors.lastname: Yngvason
keywords: entropy meters; non-extensive systems; second law of thermodynamics