Paper detail

Critique of the Gibbs volume entropy and its implication

Dunkel and Hilbert, "Consistent thermostatistics forbids negative absolute temperatures," Nature Physics, {\bf 10}, 67 (2014), and Hilbert, Hänggi, and Dunkel, "Thermodynamic laws in isolated systems," Phys. Rev. E {\bf 90}, 062116 (2014) have presented an unusual view of thermodynamics that sets aside several properties that have traditionally have been assumed to be true. Among other features, their results do not satisfy the postulates of thermodynamics originally proposed by Tisza and Callen. In their theory, differences in the temperatures of two objects cannot determine the direction of heat flow when they are brought into thermal contact. They deny that negative temperatures are possible. We disagree with most of their assertions, and we present arguments in favor of a more traditional interpretation of thermodynamics. We show explicitly that it is possible to deduce the thermodynamic entropy for a paramagnet along the lines of classical thermodynamics and that it agrees with Boltzmann entropy in equilibrium. A Carnot engine with efficiency larger than one is a matter of definition and is possible for inverted energy distributions, regardless of whether negative temperatures are used in the analysis. We elaborate on Penrose's argument that an adiabatic and reversible process connecting systems with Hamiltonian $H$ to $-H$ is possible, thus negative temperatures logically must exist. We give a demonstration that the Boltzmann temperature determines the direction of heat flow while Gibbs temperature does not, for sufficiently large systems.