Nonreciprocal wave-meditated interactions power emergent activity

Active matter uses energy from its environment to power its own motion. Systems of passive particles ordinarily lack the capacity to transduce environmental energy. They can become active, however, if the individual particles scatter waves. By reformulating the theory of classical wave-matter interactions, we demonstrate that the wave-mediated interaction between pairs of passive scatterers generically is nonreciprocal. The resulting unbalanced center-of-mass force propels the pair, enabling them to extract energy from the wave and transforming them into an active system. This form of activity is an emergent property of the passive particles' interactions and state of organization within the wave, Systems of passive scatterers therefore constitute an especially prevalent example of emergently active matter in which nonreciprocal interactions enable collective motion. Emergent activity flips the script on conventional active matter whose nonreciprocal interactions generally emerge from their activity, and not the other way around. We combine theory, experiment and simulation to explain how emergent activity arises in wave-matter composite systems and to explore the phenomenology of emergent activity in experimentally accessible model systems. This preliminary studies suggest that heterogeneity is a singular perturbation to the dynamics of wave-matter composite systems, and induces emergent activity under all but the most limited conditions.