Origins and diagnostics of the nonequilibrium character of active systems

TL;DR

This paper develops a Langevin formalism to analyze the nonequilibrium behavior of active matter systems, clarifying how time-reversal symmetry affects their thermodynamic properties and predicting experimentally testable entropy production signatures.

Contribution

It introduces a detailed Langevin approach to distinguish nonequilibrium features in active systems based on their time-reversal properties and provides testable predictions for entropy production.

Findings

Distance from equilibrium depends on time-reversal implementation.

Characteristic frequency-resolved entropy production forms are predicted.

Formalism clarifies sources and signatures of nonequilibrium in active matter.

Abstract

We present in detail a Langevin formalism for constructing stochastic dynamical equations for active-matter systems coupled to a thermal bath. We apply the formalism to clarify issues of principle regarding the sources and signatures of nonequilibrium behaviour in a variety of polar and apolar single-particle systems and polar flocks. We show that distance from thermal equilibrium depends on how time-reversal is implemented and hence on the reference equilibrium state. We predict characteristic forms for the frequency-resolved entropy production for an active polar particle in a harmonic potential, which should be testable in experiments.