Entropy, dynamics and molecular chaos, Kac's model

We use a model due to Kac to investigate some properties of the generalized entropy proposed by the Brussels group. We show that the entropy production is positive-definite and that the entropy and entropy production per particle are finite in the limit of an infinite system.

The generalized $\text{H}$ theorem is a dynamical theorem and describes the approach to equilibrium of the whole system: generally, correlations play a role in its formulation and cannot be forgotten even if macroscopic variables satisfy the chaos condition. However, a detailed investigation of the evolution equations for the moments shows that correlations reach their equilibrium value faster than the one-particle reduced distribution function and that, asymptotically, there is a regime where one recovers Boltzmann's results.