Kinetic and hydrodynamic theories of nonequilibrium fluctuations

A unified formulation of both transport and fluctuations in a low density nonequilibrium gas is described. The method is based on an analysis of a generating functional for fluctuations of the single particle phase space density. It is shown that the first functional derivative obeys an inhomogeneous nonlinear Boltzmann equation, from which the dynamics of multi-space and -time fluctuations may be obtained by suitable functional differentiation. In particular, the equations for two-time and equal time fluctuations are obtained to illustrate the method. In this way, the description of nonequilibrium fluctuations in a low density gas is put on the same theoretical basis as the usual Boltzmann kinetic theory. A hydrodynamic theory of fluctuations also may be derived from the kinetic theory; the procedure and results are indicated.