Lars Onsager Prize (2020): Swarms, flocks and crowds

Over the past 25 years collective motion has attracted increasing interest in the physics and biology communities. Motivations include the associated novel type of the “collision rule” (for physicists) as well as major technological advances regarding resources such as computational power or the collection of data. Observations of bacteria as they – thousands of them – exhibit rich group-motion patterns under the microscope in a Petri dish inspired an interpretation in terms of statistical physics. It turned out that a simple rule could be used to interpret the behaviour and that variants of this rule could be introduced to describe group motion in a surprisingly wide variety of comoving units, from cells through animals to people, and very recently to swarms of aerial robots. The related results have been obtained by computer simulations, developing theoretical techniques and, naturally, carrying out experiments. After an introduction to the basic examples and aspects of flocking, I shall present two recent case studies related to the role of hierarchical decision-making during collective motion.