Dc. Cole, Thermodynamics of blackbody radiation via classical physics for arbitrarily shaped cavities with perfectly conducting walls, FOUND PHYS, 30(11), 2000, pp. 1849-1867
An analysis is carried out involving reversible thermodynamic operations on
arbitrarily shaped small cavities in perfectly conducting material. These
operations consist of quasistatic deformations and displacements of cavity
walls and objects within the cavity. This analysis necessarily involves the
consideration of Casimir-like forces. ces. Typically, even for the simples
t of geometrical structures, such calculations become quite complex, as the
y need to take into account changes in singular quantities. Much of this co
mplexity is reduced significantly here by working directly with the change
ill electromagnetic fields as a, result of the deformation and displacement
changes. A key result of this work is the derivation that for such cavity
structures, classical electromagnetic zero-point radiation is the appropria
te spectrum at a temperature of absolute zero to ensure that the reversible
deformation operations obey both isothermal and adiabatic conditions. In a
ddition, a generalized Wien displacement law is obtained from the demand th
at the change in entropy of the radiation in these arbitrarily shaped struc
tures must be a state function of temperature and frequency.