Comment on "Velocity and Speed Correlations in Hamiltonian Flocks" by M. Casiulis et al. [arXiv:1911.06042]

TL;DR

This paper critically evaluates a Hamiltonian model proposing rigid rotations as the cause of velocity and speed correlations in bird flocks, demonstrating that it contradicts extensive experimental evidence and is not a suitable explanation.

Contribution

The authors refute the rigid rotation hypothesis by showing it conflicts with established empirical data on bird flock behavior.

Findings

Rigid rotation model contradicts experimental data

Alternative explanations like Goldstone modes are more consistent

Rigid rotation does not explain observed correlations in bird flocks

Abstract

In arXiv:1911.06042v1 M. Casiulis et al. study a Hamiltonian model in which rigid rotations of moving clusters give rise to scale-free correlations of velocity and speed. M. Casiulis et al. compare correlations in their model to those observed in real flocks of birds and claim that rigid-body rotations provide an explanation that stands in contrast with, and it is simpler than, previously proposed explanations of correlations in bird flocks, namely Goldstone modes in the velocity orientations and marginal (or near-critical) modes in the speed. Here, we show that the rigid rotation scenario is completely inconsistent with a large body of well-established experimental evidence on real flocks of birds, and it therefore does not provide an appropriate explanation for the observed phenomenology.