Spectral Optical Theorem and Radiative Transport Equation in Random Media with Large Scale Fluctuations

The optical theorem is a general law in scattering theory that relates the total scattering cross section of a scatterer to the imaginary part of its forward scattering amplitude. It turns out that this law is so fundamental that it extrapolates and permeates across several disciplines and physical processes with far-reaching implications. In the case of multiple scattering in random media the optical theorem relates the mass operator to the intensity operator. Note that the mass operator governs the mean Green's function while the intensity operator governs the coherence function in the Bethe-Salpeter equation (BSE). Tatarskii was among the first to derive the multiple scattering optical theorem for random media. We notice that the optical theorem that he used in applications is in the spatial domain. This is the version widely used in the literature.