LMP Seminar: Robustness and fragility of collective motion

Understanding how self-propelling entities manage to move collectively, such as birds in a flock, is a problem that has puzzled physicists for the last 30 years. I will first present an hydrodynamic theory for the ordered phase of collective motion that allows to derive the scaling exponents of the fluctuations. In particular, this proves the fact, well established by numerical simulations, that these phases possess long-rang order in two dimensions and are thus more robust to fluctuations than their equilibrium counterparts. However, we will see that, compared to equilibrium systems, they are more fragile to the nucleation of topological defects. In many cases, such defects make the ordered state actually metastable.

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H. Chaté and A. Solon arXiv:2403.03804 (2024)