Reinvestigation of the thermodynamics of blackbody radiation via classical physics.

Abstract

A key part of the early thermodynamic work on blackbody radiation by Wien, St\'efan, Boltzmann, Planck, and others involved the thermodynamic behavior of a movable piston sliding in a cylinder containing classical electromagnetic thermal radiation. This early work used only classical physics concepts. Here, this analysis is reinvestigated with the change that the implicit assumption is not made that the thermal radiation spectrum reduces to zero at the temperature T=0. Previous work has shown that this consideration may be an important one to yield agreement, or at least better agreement, between classical physical theory and the actual physical behavior of molecular, atomic, and subatomic systems in nature. Indeed, the present analysis on ``cavity thermodynamics'' accounts for the thermodynamic behavior of Casimir forces between the walls of the cavity. Using only the traditional thermodynamic definition of T=0, the form of the classical electromagnetic zero-point (ZP) radiation spectrum is deduced. From the second law of thermodynamics, two forms of Wien's displacement law are obtained and generalized to include the possibility of ZP radiation. The entropy is then explicitly calculated for the parallel-plate case. Also, the limiting situation of high-temperature radiation between the plates and low temperature outside is examined to recover the early analysis by Wien, Planck, and others. Most of the analysis is carried out for two parallel conducting plates bathed in thermal radiation; the Appendix extends this analysis to a rectangular conducting box with a movable interior wall.