Planck’s Radiation Law and Einstein’s Riddle

In 1909, A. Einstein criticised Planck’s derivation of the radiation law, arguing that it relied on a seemingly absurd ('ungeheuerlich erscheinenden') assumption. The essence of his criticism lay in the observation that, within a blackbody, energy quanta corresponding to visible light are exceedingly rare even at comparatively high temperatures when measured against the number of molecules. Consequently, Planck’s entropy-based derivation appears unjustified, since at any given instant only an infinitesimal fraction of the possible energy distributions is actually realised. From this perspective, Planck’s approach should, in principle, lead to a result incompatible with experimental observation—yet it does not. In the present paper, Einstein’s criticism is analysed and quantified in detail. It is shown that the argument applies not only to Planck’s original derivation but also to alternative derivations of the radiation law, including Einstein’s own formulation of 1916. It therefore appears that Einstein’s riddle has not yet been solved. This is of particular relevance insofar as it may indicate that the distribution of energy among molecules is not governed by a stochastic process, but rather by an underlying deterministic mechanism. Such a conclusion would also have important implications for the interpretation of entropy. Further explanatory approaches are discussed. Finally, the role and relevance of intuitive models in physics are examined.