A photon rest mass and the propagation of longitudinal electric waves in interstellar and intergalactic space

The author deals with the effect of a nonzero photon rest mass m on the propagation of longitudinal electric waves in a plasma for which the electrons are treated as a warm fluid with scalar pressure. The Proca equations are used to obtain a dispersion relation which describes two longitudinal wave types; when m=0, one wave type reduces to the usual electron acoustic wave while the second type, the Proca wave, ceases to exist. For frequencies away from resonance, the cold plasma model is approximately valid. Dispersion and absorption are discussed for frequencies both away from and near to resonance; particular attention is paid to Proca waves, and the possibilities of their travelling over interstellar and intergalactic distances are assessed.