The recovery of molecular chaos in dense active systems

Molecular chaos plays the central role in kinetic theory by providing a closure to the Boltzmann equation for quantitative description of classic fluids. Yet how to properly extend it to active systems is still an open question in nonequilibrium physics. By means of experiment and simulation, we investigate the emergent behaviors of self-propelled particles that execute active reorientation in analogy to collision avoidance in animal herds, robotic swarms, and traffic flows. It is shown that many-body interactions strongly regulate the rich phase dynamics, which cannot be explained by existing kinetic models that assume molecular chaos directly. To rationalize the different emergent phases observed, we propose a kinetic model where the many-body effect is treated implicitly so that molecular chaos is recovered. The proposed model demonstrates a promising approach extending kinetic theory to dense active systems and leads to the optimal growth rate of flocking.