The other major pillar of the “Rethinking Basic Science” theme is the study of the history of quantum field theory, which focuses on the attempts to bring together the two new great theoretical structures of the early twentieth century: the relativity theories and quantum mechanics. This study thus draws both on the work on the history of relativity, discussed in the last section, as well as on the ongoing work on the foundations and early history of quantum mechanics.

In further studying the early history of quantum mechanics, the project complements earlier work on Erwin Schrödinger’s creation of wave mechanics through its in-depth study of the other transition from classical physics to quantum mechanics: Werner Heisenberg’s creation of matrix mechanics. By studying this transition the project has moved away from considering the old quantum theory as a more or less consistent theoretical framework. Instead we have argued that quantum physics during the 1920s is better understood as a patchwork of concrete physical problems. These problems were loosely connected with each other through the concept of a quantum system in which physical processes were described by means of transitions between stationary states. This generic description needed to be explicated to account for specific problems, and here physicists used several independent theoretical tools, which are usually identified with the old quantum theory in the literature, in particular the so-called correspondence principle.

In several case studies (Jähnert), it was shown that the application of the correspondence principle to specific problems led to the integration of the principle into different theoretical representations and ultimately to its adaption as a research tool. This process of *transformation through implementation* was central for the conceptual development of the correspondence principle and played a key role in the emergence of quantum mechanics.

This, in turn, allowed for a reassessment of the context of Werner Heisenberg’s work leading to matrix mechanics. Heisenberg’s work was fuelled by applications of the correspondence principle in the context of multiple spectroscopy, which had not been taken into account before, and relied heavily on the representations and techniques of problem-solving developed in this context. As such, the transition from classical physics to quantum mechanics can be reinterpreted as a conceptual reflection on the adaptation of the principle and as a transition of the problem solving in a patchwork to the development of an overarching theoretical framework (Blum, Jähnert, Lehner, Renn).

Building on the new perspective on the creation of quantum mechanics, the project investigates the early history of quantum field theory (the merger of special relativity and quantum theory). We have been able to show how initially this was considered an integral part of the new quantum mechanics, but then, through the development of several novel concepts and methods specific to it alone, became a theory apart (Blum, Lehner). We could thereby explain why quantum field theory and quantum mechanics, while generally considered to be parts of the same overarching framework, are taught and practiced very differently to this day. In this manner, we were able to re-evaluate much of the historiography of quantum field theory, which had underestimated this separation and thus failed to identify essential epistemic obstacles, instead blaming mere calculational mistakes for certain retarded developments (Blum).

It was only in the ongoing debate over the interpretation of quantum mechanics that the divorce between quantum mechanics and quantum field theory remained incomplete. In this context, the project pursues a detailed study of the Schrödinger *Nachlass*, in particular his early work on the concept of entanglement, providing a reassessment of the history of this important theme and highlighting Schrödinger’s work as not a mere reaction to Einstein, Podolski, and Rosen’s famous work of 1935, but as an original innovation in which EPR arguments were developed prior to Einstein (Uffink, Lehner). The project further pursues the interpretation debate after World War II, in particular in its relation to quantum field theory, and studies how novel interpretations of quantum mechanics, such as the Bohmian particle interpretation, fared when faced with the rapidly expanding theoretical structures of quantum field theory (Blum, Andrea Oldofredi).

The project also studies the early attempts at merging quantum theory and general relativity and how this search for a theory of quantum gravity emerged (much later than is usually assumed) as the central unsolved problem of theoretical physics. We have reconstructed the origins of the different approaches to the problem and shown how the attempts to bring these approaches together to form a coherent quantum gravity community in the 1950s failed due to the lack of a unifying theoretical and formal framework (Blum, Hartz).

Concerning the history of quantum field theory, the project has also initiated a collaboration with a new Max Planck Partner Group, set up in 2017 at the Capital Normal University (CNU) in Beijing, led by Yin Xiaodong. The aim of the Partner Group is to form a new basis for understanding how knowledge of modern physics was transferred to China and was further developed there, for example, in the attempt to set up a non-atomistic alternative to the Western quark model.