Kirchhoff-Planck law for freely radiating bodies and fluctuation-dissipation theorem

The interrelation between the Kirchhoff-Planck law (KPL) and the fluctuation-dissipation theorem (FDT) is discussed. It is known that the KPL is valid for freely radiating atoms if (i) the occupation probability of the atomic levels is the Boltzmann equilibrium probability and if (ii) the induced emission is treated as negative absorption. It is shown that the assumption (ii) is also inherent in the definition of the generalized susceptability. The susceptability is directly connected with the corresponding correlation function (FDT). It is proofed that the attached dipole fluctuation spectrum in its normally ordered form and the classical Maxwell equations reproduce the KPL. This result serves as the guideline for the extension of this procedure to macroscopic systems (susceptability ε (ω)). The theory can be used to describe radiative heat transfer in inhomogeneous systems. The validity of this macroscopic theory implies the same assumptions as the validity of the KPL for freely radiating atoms. The freely radiating dielectric half-space is treated as a simple example.