Self-propelled particles align with neighbors, exhibiting a disorder-to-order phase transition. The Vicsek model shows a flocking transition at critical noise η_c, breaking continuous rotational symmetry in 2D — in violation of Mermin-Wagner for this non-equilibrium system.
Vicsek et al. (1995): phase transition to collective motion. Toner-Tu (1995): long-range order survives in 2D active systems (unlike equilibrium). Density-wave bands emerge near transition.