Electrodynamics of anisotropic media with space and time dispersion

The average stress tensor, power-flow density, momentum density, and energy density for electromagnetic waves in a linear stationary anisotropic medium with space and time dispersion are clarified with the help of the second-order nonlinear terms in the equation of motion for polarization. The wave fields are connected with the medium-mass motion through the nonlinear terms; the net force between the fields and the medium mass plays a crucial role in interpreting the stress tensor and the wave momentum. Asymmetry in the space-space part of the energy-momentum tensor, which is quadratic in the field fluctuations, is ascribed to anisotropic restoring forces of the polarizations; asymmetry in the space-time part is attributed to nonvanishing average forces between the fields and the medium-mass motion.