Susceptibilities and electric and magnetic polarization fluctuations in nonoptically active media

Abstract

The previous work which related electric polarization fluctuations to the dielectric constant is extended to include magnetic polarization fluctuations in nonoptically active media. To do this it is necessary to assume that electric fields induce changes only in the electric polarization and magnetic fields induce changes only in the magnetic polarization (in a well defined sense given in the text). This assumption is implemented after determining the relation between the average value of an arbitrary quantity and the average fields in the medium. The phenomenological parameters are related to the susceptibilities which, in turn, are related to the fluctuations by the Callen–Welton theorem. Finally, various approximate relations resulting from the neglect of certain specified electromagnetic interactions in the calculations of the susceptibilities are discussed.