Events in Physics

Leading birds by the beak: On the response of flocks to external perturbations

Flocking – the collective motion of many active particles – is a ubiquitous emergent phenomenon that occurs
in many living and synthetic systems over a wide range of scales. Examples range from mammal herds,
fish schools and bird flocks to bacteria colonies and cellular migrations, down to sub-cellular molecular motors and biopolymers. While our knowledge of collective motion in unperturbed, isolated, systems greatly advanced in recent years, little is known concerning the response of moving groups to external perturbations. This is an important question in statistical physics: symmetry breaking systems are often characterized by their response to a small external field, and studying response can also help answer the question of whether a generalized fluctuation dissipation relation of some sort holds in flocks. Ethologists, on the other hand, are interested
in response to external threats and more generally in the biological significance of group response mechanisms. Finally, understanding response is essential for controlling flocking systems, either biological or artificial.

In this talk, I will first discuss the asymptotic response to small external fields, extending a classic equilibrium
field-theoretic results to far-from-equilibrium polar ordered active fluids (“flocks”). Next, I will consider a more realistic set-up, in which finite flocks interact with the external world via the flock own boundary. In this problem, we consider a finite, dynamic perturbation that only affects the flock boundary, and characterize the information inflow towards the flock bulk.