Signatures of irreversibility in microscopic models of flocking

TL;DR

This paper investigates irreversibility in microscopic flocking models, using entropy production rate to quantify non-equilibrium behavior and identifying asymmetries in steady state distributions as signatures of irreversibility.

Contribution

It introduces a method to quantify irreversibility in flocking models via entropy production and links asymmetries in microstates to time reversal symmetry breaking.

Findings

EPR peaks near the order-disorder transition

Asymmetries in microstate distributions indicate irreversibility

Pairwise asymmetries reveal local irreversibility signatures

Abstract

Flocking in $d=2$ is a genuine non-equilibrium phenomenon for which irreversibility is an essential ingredient. We study a class of minimal flocking models whose only source of irreversibility is self-propulsion and use the entropy production rate (EPR) to quantify the departure from equilibrium across their phase diagrams. The EPR is maximal in the vicinity of the order-disorder transition, where reshuffling of the interaction network is fast. We show that signatures of irreversibility come in the form of asymmetries in the steady state distribution of the flock's microstates. They occur as consequences of the time reversal symmetry breaking in the considered self-propelled systems, independently of the interaction details. In the case of metric pairwise forces, they reduce to local asymmetries in the distribution of pairs of particles. This study suggests a possible use of pair asymmetries both to quantify the departure from equilibrium and to learn relevant information about aligning interaction potentials from data.