We review the past decade s theoretical and experimental studies of flocking: the collective, coherent motion of large numbers of self-propelled ‘‘particles’’ (usually, but not always, living organisms). Like equilibrium condensed matter systems, flocks exhibit distinct ‘‘phases’’ which can be classified by their symmetries. Indeed, the phases that have been theoretically studied to date each have exactly the same symmetry as some equilibrium phase (e.g., ferromagnets, liquid crystals). This… CONTINUE READING
Fig. 6. Structures of condensed phases for a small system of 128 particles, and L = 32. (A) Immobile ‘‘solid’’ at a = 1.0 and b = 100.0 (20 timesteps superimposed). (B)‘‘Flying crystal’’ at a = 3.0 and b = 100.0 (three snapshots, separated by 120 timesteps). (C) Stationary fluid droplet at a = 1.0 and b = 2.0 (20 consecutive timesteps). (D) Moving droplet at a = 3.0 and b = 3.0 (20 consecutive timesteps). (B and D) The arrow indicates the (instantaneous) direction of motion.