Thermalization of Gases: A First Principles Approach

TL;DR

This paper develops a quantum-first approach to gas thermalization, showing how temperature and Planck distribution emerge from quantum dynamics, and discusses limitations of classical hydrodynamics in describing such gases.

Contribution

It extends emergent thermalization theory to quantum gases, highlighting quantum effects, history dependence, and limitations of classical hydrodynamics in non-ultracold regimes.

Findings

Thermalization arises from quantum and unitary dynamics.

Photon production approximates Planck distribution in optically thick gases.

Classical hydrodynamics may fail to describe far-from-ultracold gases.

Abstract

Previous approaches of emergent thermalization for condensed matter based on typical wavefunctions are extended to generate an intrinsically quantum theory of gases. Gases are fundamentally quantum objects at all temperatures, by virtue of rapid delocalization of their constituents. When there is a sufficiently broad spread in the energy of eigenstates, a well-defined temperature is shown to arise by photon production when the samples are optically thick. This produces a highly accurate approximation to the Planck distribution so that thermalization arises from the initial data as a consequence of purely quantum and unitary dynamics. These results are used as a foil for some common hydrodynamic theory of ultracold gases. It is suggested here that strong history dependence typically remains in these gases and so limits the validity of thermodynamics in their description. These problems are even more profound in the extension of hydrodynamics to such gases when they are optically thin, even when their internal energy is not low. We investigate rotation of elliptically trapped gases and consistency problems with deriving a local hydrodynamic approach. The presence of vorticity that is "hidden" from order parameter approaches is discussed along with some buoyancy intrinsically associated with vorticity that gives essential quantum corrections to gases in the regimes where standard perturbation approaches to the Boltzmann equations are known to fail to converge. These results suggest that studying of trapped gases in the far from ultracold regions may yield interesting results not described by classical hydrodynamics.