A Note on Spinning Billiards and Chaos

Abstract: We investigate the impact of internal degrees of freedom - specifically spin - on the classical dynamics of billiard systems. While traditional studies model billiards as point particles undergoing specular reflection, we extend the paradigm by incorporating finite-size effects and angular momentum, introducing a dimensionless spin parameter $\alpha$ that characterizes the moment of inertia. Using numerical simulations across circular, rectangular, stadium, and Sinai geometries, we analyze the resulting trajectories and quantify chaos via the leading Lyapunov exponent. Strikingly, we find that spin regularizes the dynamics even in geometries that are classically chaotic: for a wide range of $\alpha$, the Lyapunov exponent vanishes at late times in the stadium and Sinai tables, signaling suppression of chaos. This effect is corroborated by phase space analysis showing non-exponential divergence of nearby trajectories. Our results suggest that internal structure can qualitatively alter the dynamical landscape of a system, potentially serving as a mechanism for chaos suppression in broader contexts.