Abstract

The classical (Spitzer-Chandrasekhar) theory of cluster relaxation is unsatisfactory because it involves the Coulomb logarithm and completely neglects the cluster’s self-gravity. A modern alternative is the Balescu-Lenard (BL) equation, which has no such ill-defined parameter and includes self-gravity. The BL equation has recently explained the secular evolution of collisionless stellar discs. In this talk I will review the classical theory and demonstrate its pitfalls, showing at each stage why the BL equation does a better job. I will then present the first application of the BL formalism to spherical systems. I will show how the BL flux through action space differs from the corresponding classical flux, and discuss what a complete theory of cluster relaxation might entail.