Under the supervision of Rubens, Hermann Beckmann measured the temperature dependence of far-infrared blackbody radiation for the first time in 1898 using the reststrahlen bands of fluorite. Thus, although Planck did introduce the quantum hypothesis in 1900, this hypothesis remained in epistemic isolation until a more thorough exploration of Planck’s theory allowed the physics community to appreciate its nonclassical meaning. This plurality of representation is another case of the same underdetermination that had been treated with the HNR. The reason was that Planck’s symbolic practices are compatible with two radically different combinatorial models both to be found in Boltzmann’s 1877 work. By carefully analyzing Planck’s combinatorial procedure between October and December 1900, I argue that he took a noncommittal stance respect to the question whether the quantization concerned the nature of the resonators or a way to distribute a continuum of energy. However, some month later, Rubens’ experiments showed that Wien’s law was untenable. In March 1900, he tightened this link even more.
But it was necessary to buttress his definition of entropy in order to conclude his second argument for irreversibility. Ironically, he wasn’t looking for any proof of the law to begin with. In the Pentalogy, Planck had bounded his theory tight with Wien’s radiation law. This chapter concludes the analysis of Planck’s theory of radiation, and discusses the fateful leap to the quantum hypothesis. Planck obtained his argument for irreversibility, but he had to pay a prize for it: his entire program depended essentially on the validity of Wien’s law. The central move of the reorganization of his theory was the introduction of the hypothesis of natural radiation as a way to draw a divide between the macroscopic and microscopic state. He modified the morphology of his theory to accommodate new resources and gave a more central role to some symbolic practices, notably Fourier series. In the second part of the Pentalogy, Planck changed strategy. Boltzmann jumped in and showed that this could not possibly be the case. Initially, Planck thought that this statement could be demonstrated as a consequence of the electromagnetic features of the problem. He wanted to prove that electromagnetic radiation in a cavity, when suitably stimulated, reaches irreversibly a form of stable thermal equilibrium. Instead, he considered heat radiation as an ideal case to support his strict view of thermal irreversibility.
In particular, Planck was not primarily interested in deriving a radiation law. Planck viewed the problem of the black-body radiation very differently from Wien and the majority of his contemporaries. In this chapter I explore Planck’s radiation theory from his preliminary studies (1896) through his more mature Pentalogy (1897–1899). On the historiographical side, this approach entails that we have to pay more attention to the mathematical practices and to the way in which they shape the physical representation of phenomena. I distinguish between the representational, the transformational, and the explanatory dimension and I argue that a theory produces knowledge through the epistemic cooperation of these dimensions. By systematically turning the dogmas upside down, I argue that we should use reflectively our theoretical knowledge to shed light on theories as knowledge-production devices. In the final part of the chapter, I propose my view. These dogmas have hindered a reconciliation between the analysis of the internal structure of theories and their being historical-cultural objects. In the course of time, this attitude has crystallized in three theses or dogmas about what a theory is, how it works, and how we should approach it. The reason of this separation, I argue, is that philosophy of science has maintained an intellectualistic stance toward scientific theories. I claim that philosophy and historiography of theories are currently utterly separated and this situation is detrimental to both. This chapter sets the historiographical framework for the rest of the book.
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