Entropic forces in a non-equilibrium system: Flocks of birds

TL;DR

This paper explores how entropic forces arising from velocity distribution narrowing can oppose flock cohesion in bird groups, with implications for understanding non-equilibrium systems and biological structures.

Contribution

It introduces the concept that entropic forces influence flock stability depending on interaction range, extending to biological systems like protein structures.

Findings

Entropic forces can weaken flock cohesion, especially with long-range interactions.

Interaction range determines the strength of entropic forces in bird flocks.

Similar entropic effects influence the stability of protein structures.

Abstract

When birds come together to form a flock, the distribution of their individual velocities narrows around the mean velocity of the flock. We argue that, in a broad class of models for the joint distribution of positions and velocities, this narrowing generates an entropic force that opposes the cohesion of the flock. The strength of this force depends strongly on the nature of the interactions among birds: if birds are coupled to a fixed number of neighbors, the entropic forces are weak, while if they couple to all other birds within a fixed distance, the entropic forces are sufficient to tear a flock apart. Similar entropic forces should occur in other non-equilibrium systems. For the joint distribution of protein structures and amino-acid sequences, these forces favor the occurrence of "highly designable" structures.