Flocking with random non-reciprocal interactions

TL;DR

This paper investigates how collective flocking behaviors emerge and persist in systems with random, nonreciprocal interactions among particles, revealing complex phases like chiral states and stable clusters.

Contribution

It introduces a Vicsek-like model with fully random nonreciprocal interactions, demonstrating the emergence of flocking, chiral, and oscillating states, and formation of self-organized clusters.

Findings

Ordered flocking occurs when alignment bias matches random interactions.

Random interactions support global chiral and oscillating states.

Self-organized medium-sized clusters form without alignment bias.

Abstract

Flocking is ubiquitous in nature and emerges due to short- or long-range alignment interactions among self-propelled agents. Two unfriendly species that antialign or even interact nonreciprocally show more complex collective phenomena, ranging from parallel and antiparallel flocking over run-and-chase behavior to chiral phases. Whether flocking or any of these collective phenomena can survive in the presence of a large number of species with random nonreciprocal interactions remained elusive so far. As a first step here, the extreme case of a Vicsek-like model with fully random nonreciprocal interactions between the individual particles is considered. For infinite-range interaction, as soon as the alignment bias is of the same order as the random interactions, the ordered flocking phase occurs, but deep within this phase, the random nonreciprocal interactions can still support global chiral and oscillating states in which the collective movement direction rotates or oscillates slowly. For short-range interactions, moreover, even without alignment bias self-organized cliques emerge, in which medium-size clusters of particles that have predominantly aligning interactions meet accidentally and stay together for macroscopic times. These results may serve as a starting point for the study of multispecies flocking models with nonrandom but complex nonreciprocal interspecies interactions.