Electromagnetic fluctuations from a nonlocal continuum point of view

In the present paper it is shown that the theory of electromagnetic fluctuations in arbitrary media developed by Landau and Lifshitz using the methods of quantum field theory in statistical physics can also be derived by the nonlocal theory of electromagnetic solids without resorting to what is known as random fields or stochastic inductions introduced by the former approach. The electrodynamics of memory-dependent nonlocal continua is shown to lead in a natural way to the partial differential equation, associated with the Maxwell-Ampere's law, for the retarded photon Green's function, in terms of which Casimir forces can be calculated for neutral bodies. In the process of derivation, the inverse of nonlocal electric conduction moduli is found to be matched suitably with the generalized susceptibilities of the fluctuation-dissipation theorem of statistical physics.