LETTERS TO PROGRESS IN PHYSICS Further Insight Relative to Cavity Radiation: A Thought Experiment Refuting Kirchhoff’s Law

Kirchhoff’s law of thermal emission demands that all cavities contain blackbody, or normal, radiation which is dependent solely on the temperature and the frequency of observation, while remaining independent of the nature of the enclosure. For over 150 years, this law has stood as a great pillar for those who believe that gaseous stars could emit a blackbody spectrum. However, it is well-known that, under laboratory condi-tions, gases emit in bands and cannot produce a thermal spectrum. Furthermore, all laboratory blackbodies are constructed from nearly ideal absorbers. This fact strongly opposes the validity of Kirchhoff’s formulation. Clearly, if Kirchhoff had been correct, then laboratory blackbodies could be constructed of any arbitrary material. Through the use of two cavities in temperature equilibrium with one another, a thought experiment is presented herein which soundly refutes Kirchhoff’s law of thermal emission. If a space be entirely surrounded by bodies of the same temperature, so that no rays can penetrate through them, every pencil in the interior of the space must be so constituted, in regard to its qual-ity and intensity, as if it had proceeded from a per-fectly black body of the same temperature, and must therefore be independent of the form and nature of the bodies, being determined by temperature alone... In the interior therefore of an opake red-hot body of any temperature, the illumination is always the same, whatever be the constitution of the body in other respects. Gustav Robert Kirchhoff, 1860 [1]