ABSTRACT
This chapter analyzes fluctuations in a normal liquid or gas. Of interest are fluctuations of several physical quantities: the particle number, momentum, energy and entropy densities, and a few others. In superfluid helium, as well as in superconductors, there is macroscopic quantum phase coherence, of which one consequence is the fact that longitudinal and transverse fluctuations are distinct even at infinite wavelength, or more precisely. The chapter aims to extend the results from the collision-dominated regime of hydrodynamics to larger frequency and wavenumber. The continuity equations are microscopically rigorous but they do not yet make a complete theory. As in the spin diffusion case, they have to be supplemented by macroscopic constitutive relations. The hydrodynamic expressions exhaust all the thermodynamic sum rules. The total light scattering intensity becomes enormous because of the thermodynamic sum rule. The chapter considers the transverse momentum density correlation function.
