Scattered waves fuel emergent activity

When a particle scatters electromagnetic or acoustic waves, the exchange of incident and scattered momentum on its surface yields a force. If other particles are present, their scattered waves mediate interparticle forces. Wave-mediated interparticle forces are not inherently reciprocal: the difference in scattered momentum from heterogeneous particles is carried away into the medium. These nonreciprocal forces allow inert particles to consume energy from the incident wave to power their own collective motion. We show in experiments that 3 dissimilar particles immersed in an acoustic standing wave rotate, and in simulations that hundreds of particles form clusters whose activity increases as a power of particle heterogeneity. Passive, or homogenous, systems are characterized by exponentially decaying velocity temporal correlation functions consistent with equilibrium systems, while heterogeneous systems are characterized by slower temporal correlation decay.