As described in earlier sections, any two material bodies will interact across an intermediate substance or space. This interaction is rooted in the electromagnetic fluctuations—spontaneous, transient electric and magnetic fields—that occur in material bodies as well as in vacuum cavities. The frequency spectrum of these fluctuations is uniquely related to the electromagnetic absorption spectrum, the natural resonance frequencies of the particular material. In principle, electrodynamic forces can be calculated from absorption spectra.

Lifshitz's original formulation in 1954 (see Prelude, note 17) used a method, due to Rytov, to consider the correlation in electromagnetic fluctuations between two bodies separated by a vacuum gap. The force between the bodies is derived from the Maxwell stress tensor corresponding to the spontaneous electromagnetic fields that arise in the gap between boundary surfaces—the walls of the Planck–Casimir box. His result, for the case of two semi-infinite media separated by a planar slab gap, reduces in special limits to all previous valid results, specifically those of Casimir and of Casimir and Polder for, respectively, the interaction between two metal plates or between two point particles. In 1959, Dzyaloshinskii, Lifshitz, and Pitaevskii (DLP) published a derivation that used diagram techniques of quantum field theory to allow the gap between the two bodies to be filled with a nonvacuous material.

The DLP result can be derived as well through an intuitive and heuristic method wherein the energy of the electromagnetic interaction is viewed as the energy of electromagnetic waves that fit between the dielectric boundaries of the planar gap.