On the Boltzmann-Grad limit for the two dimensional periodic Lorentz gas

TL;DR

This paper investigates the long-term behavior of a complex integro-differential equation describing the two-dimensional periodic Lorentz gas in the Boltzmann-Grad limit, focusing on equilibrium states and convergence properties.

Contribution

It provides the first detailed analysis of the dynamical properties of the integro-differential equation governing the Lorentz gas in this limit, including explicit transition probabilities and equilibrium analysis.

Findings

Identification of equilibrium states

Proof of an H Theorem for the equation

Discussion of the approach to equilibrium over time

Abstract

The two-dimensional, periodic Lorentz gas, is the dynamical system corresponding with the free motion of a point particle in a planar system of fixed circular obstacles centered at the vertices of a square lattice in the Euclidian plane. Assuming elastic collisions between the particle and the obstacles, this dynamical system is studied in the Boltzmann-Grad limit, assuming that the obstacle radius $r$ and the reciprocal mean free path are asymptotically equivalent small quantities, and that the particle's distribution function is slowly varying in the space variable. In this limit, the periodic Lorentz gas cannot be described by a linear Boltzmann equation (see [F. Golse, Ann. Fac. Sci. Toulouse 17 (2008), 735--749]), but involves an integro-differential equation conjectured in [E. Caglioti, F. Golse, C.R. Acad. Sci. S\'er. I Math. 346 (2008) 477--482] and proved in [J. Marklof, A. Str\"ombergsson, preprint arXiv:0801.0612], set on a phase-space larger than the usual single-particle phase-space. The main purpose of the present paper is to study the dynamical properties of this integro-differential equation: identifying its equilibrium states, proving a H Theorem and discussing the speed of approach to equilibrium in the long time limit. In the first part of the paper, we derive the explicit formula for a transition probability appearing in that equation following the method sketched in [E. Caglioti, F. Golse, loc. cit.].