Pathwise Convergence of the Hard Spheres Kac Process

Abstract

We derive two estimates for the deviation of the $N$-particle, hard-spheres Kac process from the corresponding Boltzmann equation, measured in expected Wasserstein distance. Particular care is paid to the long-time properties of our estimates, exploiting the stability properties of the limiting Boltzmann equation at the level of realisations of the interacting particle system. As a consequence, we obtain an estimate for the propagation of chaos, uniformly in time and with polynomial rates, as soon as the initial data has a $k^{\mathrm{th}}$ moment, $k>2$. Our approach is similar to Kac's proposal of relating the long-time behaviour of the particle system to that of the limit equation. Along the way, we prove a new estimate for the continuity of the Boltzmann flow measured in Wasserstein distance.

arXiv commentSupersedes the previous version. A version of the main theorem has been added which only requires $k$ initial moments, for any $k>2$. The continuity of the Boltzmann flow has been strengthened, and is now displayed as a separate theorem