My work for the past several years has examined the development of the discipline of theoretical physics in Germany from its multiple and contested beginnings in the late nineteenth century to the end of the Weimar republic. It describes the development of a self-aware discipline not only in terms of a history of ideas, but also in terms of that discipline’s discursive, industrial, and—especially—pedagogical contexts. By identifying and then embedding distinctly different “kinds” of theoretical physics—different modes of practice, different ways of being—in their local and national contexts, the apparent coherence of the discipline by the mid- and late-1920s is rendered a riddle to be explained, rather than the natural outcome of the rise of certain kinds of institutions or the internal development of ideas.

Two particular constructions of theoretical physics are explored in detail: what has been termed “the physics of principles,” and what is here termed “the physics of problems.” The physics of principles received its first coherent representation in the writings of Max Planck in Berlin, but was emphasised by several theoreticians, most prominently Albert Einstein and Niels Bohr. The physics of problems was epitomized by the work of the school around Arnold Sommerfeld in Munich, a school identified as one of the key centres for research in theoretical physics for almost three decades after Sommerfeld’s arrival in 1906. While self-identified physicists of principle like Planck and Einstein promoted a practice of theoretical physics devoted to abstract, de-anthropomorphised, foundational, “pure” principles, Sommerfeld focussed on specific problems, drawing these from a variety of sources, including, even emphasising questions of economic or technological benefit.

Although these two constructions are associated with the names of prominent individuals, they had a much wider—community-wide—impact. Planck’s views were put forward as part of a public, widely received discourse, one that was rapidly picked up, as his scientific status rose within the physics community. Einstein’s distinction between “principle-theories” and “constructive-theories” was first offered in an article for the London Times which followed in the wake of the tremendous publicity that surrounded the experimental verification of the gravitation bending of light predicted by the general theory of relativity. The principle/constructive dichotomy Einstein made would be crucial for the representation of his theory—and theoretical physics more generally—to the British public in the years that followed. Bohr’s practice of developing and thinking through principles—like adiabatic invariance, correspondence, and complementarity—was of key importance for the school that formed around him, with students like Wolfgang Pauli, H. Kramers, and Werner Heisenberg. The Sommerfeld School, finally, trained more theoretical physicists than any other site, at least eight Nobel prize winners among them. Sommerfeld’s many students adopted his way of seeing the physical world and his and their analytic practices represented a dominant strand in what the field came to be.

The principles/problems distinction thus becomes a powerful tool for understanding the different paths via which modern theoretical physics came to be. It offers a means of subtly rendering differences in outlook, and—through the particular focus on the crucially important, but understudied Sommerfeld—of recovering a lost way of doing and teaching physics, one connected far more to the needs of industry than those of philosophy, or knowledge pursued (even nominally) for knowledge’s sake. It provides also, however, a starting point for a deeper analysis than one dealing only with a question of “world-views,” or intellectual preferences. Writing to Einstein in 1922, Sommerfeld claimed that “Ich kann nur die Technik der Quanten fördern, Sie müssen Ihre Philosophie machen.” The question of what “die Technik der Quanten” was, and what techniques and modes of practice were associated with it, are central to this project. An understanding of the theoretical practices of principles and problems, and hence the connection between Weltanschauung and the experiential knowledge and production of theory, are among the primary elements to be won from the studies I have pursued. Most generally, I hope to make evident that it is only by exploring the paths of principles and problems together—in their mutual engagements, interrelations, dialectic development, and sometimes symbiotic growth—that one can begin to understand the formative years of the discipline that, to many, signified the pinnacle of scientific achievement for the twentieth century.