Smooth phase transition of energy equilibration in a springy Sinai billiard

TL;DR

This paper investigates how energy equilibration rates in a springy Sinai billiard depend smoothly on system parameters, especially as the size of the obstacle varies, revealing a phase transition in the dynamics.

Contribution

It demonstrates that even with a single ergodic component, the energy equilibration rate can remain positive, and it uncovers a smooth phase transition related to the size of the Sinai billiard obstacle.

Findings

Equilibration rate remains positive with a single ergodic component.

A smooth phase transition occurs as the obstacle size increases.

Decreasing the obstacle size increases the ergodization time.

Abstract

Statistical equilibration of energies in a slow-fast system is a fundamental open problem in physics. In a recent paper, it was shown that the equilibration rate in a springy billiard can remain strictly positive in the limit of vanishing mass ratio (of the particle and billiard wall) when the frozen billiard has more than one ergodic components [Proc. Natl. Acad. Sci. USA 114, E10514 (2017)]. In this paper, using the model of a springy Sinai billiard, it is shown that this can happen even in the case where the frozen billiard has a single ergodic component, but when the time of ergodization in the frozen system is much longer than the time of equilibration. It is also shown that as the size of the disc in the Sinai billiard is increased from zero, thereby leading to a decrease in the time required for ergodization in the frozen system, the system undergoes a smooth phase transition in the equilibration rate dependence on mass ratio.