On pulse spreading in dissipative and nonequilibrium media

Quasi-monochromatic theory is used to analyze the spatial diffusion with time of a wave packet in a dissipative medium. It is shown that diffusion occurs regardless of the sign of dissipation (absorption or amplification), and that rate of diffusion is determined by the value of the imaginary part of the complex group velocity, whereas the real part of this velocity characterizes migration of the wave packet in the space as a whole. The analysis is carried out for both one-dimensional and multidimensional cases.