ABSTRACT
This chapter presents a simple example which considers a fluid of uncharged particles with spin 1/2. The essential assumption which makes this system so simple, is that the particles interact through a velocity- and spin-independent force. The spin of each particle can be taken to point either parallel or antiparallel to some arbitrary direction of quantization. The spin susceptibility reduces to the well-known Pauli susceptibility, and is independent of temperature. The diffusion of magnetization takes place via spin-flip interaction between neighbors. The chapter utilizes a hit-and-run technique to obtain the correlation function from our hydrodynamic analysis. Transverse momentum transport in a normal fluid also follows a hydrodynamic diffusion law, and everything we said above applies, mutatis mutandis, to this process, too. However, the longitudinal behavior which has to do with compressions and temperature fluctuations, is complicated by the fact that several hydrodynamic modes are coupled.
