Einstein's paper, "Planck's Theory of Radiation and the Theory of Specific Heat of Solids" (1907) is famous for that it marks the beginning of the quantum theory of solids. In contrast to classical statistics, a simple quantum oscillator model of lattice vibrations yields a decreasing specific heat with decreasing temperature, in agreement with the then few experimental findings. This brought him to the attention of Nernst, who became one of the driving forces to attract Einstein to Berlin. Less known is, however, that this paper contains two other fundamental insights. First, it shows that the differences between classical and quantum statistical distribution functions are related to the energetic spectra of systems, rather than to the (in)distinguishability of particles. Second, the problem of quantization is (indirectly) formulated as the problem to select the set of quantum states out of the set of classical states.

Albert Einstein (1879-1955) was the first to take Max Planck's (1858-1947) quantum hypothesis seriously, and he believed in the existence of universal physical laws. His efforts to realize his visions culminated in his Annus Mirabilis 1905, the centenary of which has been celebrated in the World Year of Physics 2005. Corresponding to the experimental material, he sought for applications of quantum theory, first in the field of light-matter interaction (Einstein, 1905 and 1906), where he found the first explanations of Stoke's rule, of the external photo effect (the main point in the reasoning for awarding him the Nobel prize and of the photo-ionization of gases. His seminal paper, "Planck's Theory of Radiation and the Theory of Specific Heat of Solids" (Einstein, 1907), published two years later, founded the quantum theory of solids; it was the first to apply the quantum hypothesis to ponderable matter (Klein, 1965). Again, Albert Einstein took an atomistic positionin the context of the recent centenary of the death of Ludwig Boltzmann (1844-1906), it may be remembered that then the acceptance of atomism was still not prevailing in the physics community.

As in his 1905 papers, Albert Einstein brought forward most simple and striking arguments. In a few lines, he demonstrated that Wilhelm Wien's (1864-1928) classical distribution law is obtained when using the continuous energy spectrum of a classical oscillator, while Max Planck's non-classical distribution law results from the discrete energy spectrum of a quantum oscillator. He found supporting evidence in the fact, that the frequency of the hypothetical quantum oscillator representing the lattice vibrations was often close to the experimental reststrahlen frequency. Twowidely underestimatedimplications will be discussed in this paper. The first concerns the notion of state being one of the most important notions of physics, while the second sheds a non-traditional light on the nature of quantization. These implications had not been realized by his contemporaries, however, and also not in Martin Klein's appraisal (Klein, 1965). For this, it should be useful to supplement my paper (Enders, 2008), (hereafter referred to as I) by a more detailed account of them.

Physics Journal, Einstein's 1907 Paper, Planck's Theory of Radiation, Specific Heat of Solids, State, Quantization, Classical Mechanics, Quantum Mechanics, Energy, Photo-Ionization, Photo Effect, Continuous Energy Spectrum, Classical Oscillator, Momentum.