The problem of computing the radiative portion of a field is considered. By considering the process of emitting radiation, an operational interpretation of the wave equation is developed. This interpretation allows one to write down immediately the inhomogeneous wave equation expressing the radiation from a velocity field in a compressible medium. Guided by the concepts developed in the discussion of the emission of radiation, the inhomogeneous wave equation is deduced from the Navier-Stokes equations. This computation also yields the scattering of the radiative field by the nonradiative flow.

• Received 30 June 1952

DOI:https://doi.org/10.1103/PhysRev.90.240