Spontaneously emerging dynamics from multibody non-reciprocal interactions

Non-reciprocal interactions are known to generate intriguing dynamics. Traditionally, nonreciprocity has been studied by introducing asymmetry in pairwise interactions, achieved through differences in particle size, rotation preference, or by explicitly programmed interactions between robots. In this work, we demonstrate that non-reciprocal interactions can also arise naturally from multibody effects, even in systems of identical, passive particles. Using acoustically levitated particles as an example, we show that sound scattering and microstreaming forces between particles are inherently multibody, leading to spontaneous behaviors such as self-propulsion, limit cycles, as well as internal engine-like dynamics. Unlike traditional non-reciprocal systems that require designed asymmetry, our findings reveal that interaction symmetry can be broken naturally by the dynamics of the particles themselves. This provides new insights into the broader understanding of systems with non-reciprocal interactions.