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Active matter is composed by a large number of active particles, each of which consumes energy in order to move or to exert mechanical forces, a situation common for instance to many biological systems. Due to energy consumption, these systems are inherently out of thermodynamic equilibrium, and the study of active matter collective properties is nowadays fast-emerging interdisciplinary research field, which links out-of-equilibrium statistical physics with biological as well as engineering-related topics.
In this seminar, I will concentrate on one of its most spectacular aspects, collective motion. Flocking - or collective motion - is a ubiquitous emergent phenomenon that occurs in many living and synthetic systems over a wide range of scales. Examples range from fish schools and bird flocks to bacteria colonies and cellular migrations, down to sub-cellular molecular motors and biopolymers.
Making use of analytical, numerical and experimental results, I will discuss the universal features common to this wide class of phenomena, showing how its physical properties may be largely understood as the consequence of i) the spontaneous breaking of continuous rotational symmetry and (ii ) the far-from-equilibrium nature of locally interacting moving agents. In particular, I will present results concerning the response of moving group to perturbations, a problem with important consequences for animal group behavior (e.g., response to external threats) and for controlling flocking systems, either biological or artificial. Given time, I will also discuss open issues such as dense active systems and the role of boundaries and surface tension in finite flocks.