Consistent description of fluctuations requires negative temperatures

TL;DR

This paper compares two definitions of temperature in statistical mechanics, emphasizing the role of $T_B$ in describing fluctuations and how it can be measured dynamically, especially in systems with bounded energy.

Contribution

It clarifies the physical meaning of $T_B$ and $T_G$, demonstrating that $T_B$ accurately describes fluctuations and can be measured dynamically in certain systems, unlike $T_G$.

Findings

$T_B$ describes fluctuations even when negative.

$T_B$ can be measured dynamically via long trajectories.

Numerical simulations confirm the relation between $T_B$ sign and configurational order.

Abstract

We review two definitions of temperature in statistical mechanics, $T_B$ and $T_G$, corresponding to two possible definitions of entropy, $S_B$ and $S_G$, known as surface and volume entropy respectively. We restrict our attention to a class of systems with bounded energy and such that the second derivative of $S_B$ with respect to energy is always negative: the second request is quite natural and holds in systems of obvious relevance, i.e. with a number $N$ of degrees of freedom sufficiently large (examples are shown where $N \sim 100$ is sufficient) and without long-range interactions. We first discuss the basic role of $T_B$, even when negative, as the parameter describing fluctuations of observables in a sub-system. Then, we focus on how $T_B$ can be measured dynamically, i.e. averaging over a single long experimental trajectory. On the contrary, the same approach cannot be used in a generic system for $T_G$, since the equipartition theorem may be spoiled by boundary effects due to the limited energy. These general results are substantiated by the numerical study of a Hamiltonian model of interacting rotators with bounded kinetic energy. The numerical results confirm that the kind of configurational order realized in the regions at small $S_B$, or equivalently at small $|T_B|$, depends on the sign of $T_B$.