Quasiparticle view of wave propagation

A mixed analytical and computational method of treating the propagation of single mode-type wavepackets or beams in inhomogeneous, time-varying, linear, nonlinear, or turbulent media is described. On averaging out fast space-time variations, one views a wavetrain kinetically as a system of "quasiparticles" whose distribution in position and momentum space permits the calculation of space-time dependent wavetrain properties such as envelope amplitude, phase, etc. Individual quasiparticles, which have an energy (Hamiltonian) descriptive of the mode-type, move along "ray" paths characteristic of the medium. The overall system of quasiparticles evolves in a nonsingular (caustic free) manner dependent on initial configuration and on effects of both trapped and untrapped quasiparticles. Starting from a kinetic description of the distribution of quasiparticles, one derives a "fluid dynamic" description that is equivalent to the Whitham and other averaging procedures for analyzing wavetrain propagation; a "macroparticle" description akin to known "centroid" analyses of localized wavetrains is also derivable. The particle-wave duality implied in the above analysis is discussed both from a quantum mechanical and classical standpoint. A few examples of the use of the quasiparticle method are presented.