Time-(ir)reversibility in active matter: from micro to macro

TL;DR

This review explores how active matter systems break time-reversibility at micro and macro scales, analyzing the roles of energy dissipation, fluctuations, and interactions, and discusses the potential for equilibrium methods to inform active material design.

Contribution

It provides a comprehensive analysis of time-reversibility in active matter, highlighting the conditions under which effective reversible descriptions are possible across scales.

Findings

Active matter exhibits unique dynamical phenomenology not seen in passive systems.

Effective time-reversible models can describe active systems at various scales.

Interactions and external influences reveal the non-equilibrium microscopic energy sources.

Abstract

Active matter encompasses systems whose individual consituents dissipate energy to exert propelling forces on their environment. This rapidly developing field harbors a dynamical phenomenology with no counterpart in passive systems. The extent to which this is rooted in the breaking of time-reversibility has recently triggered an important theoretical and experimental activity which is the focus of this review. Building on recent progress in the field, we disentangle the respective roles of the arrow of time and of the non-Boltzmann nature of steady-state fluctuations in single- and many-body active systems. We show that effective time-reversible descriptions of active systems may be found at all scales, and discuss how interactions, either between constituents or with external operators, may reveal the non-equilibrium nature of the microscopic source of energy. At a time when the engineering of active materials appears within our reach, this allows us to discuss to which extent methods stemming from equilibrium statistical mechanics may guide us in their design.