Signatures of irreversibility in microscopic models of flocking

Flocking in d = 2 is a genuine nonequilibrium 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. These asymmetries 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.

Automated summary

This study investigates the irreversibility and entropy production rate in the phenomenon of flocking in 2D systems, and reveals the asymmetry in the steady-state distribution of the flock's microstates as a signature of irreversibility. The findings provide insights into quantifying the departure from equilibrium and extracting information about interaction potentials from data.