It is not very well known that the philosopher Karl Popper has been one of the foremost critics of the orthodox interpretation of quantum physics for about six decades. This paper reconstructs in detail most of Popper's activities on foundations of quantum mechanics (FQM) in the period of 1950s and 1960s, when his involvement in the community of quantum physicists became extensive and quite influential. Thanks to unpublished documents and correspondence, it is now possible to shed new light on Popper's central - though neglected - role in this "thought collective" of physicists concerned with FQM, and on the intellectual relationships that Popper established in this context with some of the protagonists of the debate over quantum physics (such as David Bohm, Alfed Land\'e and Henry Margenau, among many others). Foundations of quantum mechanics represented in those years also the initial ground for the embittering controversy between Popper and perhaps his most notable former student, Paul Feyerabend. I present here novel elements to further understand the origin of their troubled relationship.
Karl R. Popper (1902Popper ( -1994) ) has been one of the most preeminent philosophers of science of the twentieth century. He is generally known for his views on scientific method, having introduced falsifiability as the demarcation criterion between scientific and non-scientific theories, and as a matter of fact still today "many scientists subscribe to some version of simplified Popperianism" (Kragh 2013).
However, it has been essentially neglected by contemporary historiography that Popper contributed to the scientific discourse in at least another manner: for about six decades he was in the front line of the controversy over quantum physics, side by side with some of the most eminent physicists of his time. Indeed, since as early as 1934, Popper was among the few opponents of the leading anti-realistic and instrumentalist Copenhagen interpretation of quantum mechanics (CIQM). 1 Pushing forward a line of research introduced in a recent work (Del Santo 2017), this paper, without any pretense of being exhaustive, aims at proving a historical account of Popper’s activities in the foundations of quantum mechanics (FQM) between 1950s and 1960s, with a focus on the intellectual relationships that Popper established with many influential physicists.
In this respect, I deem it advantageous to (loosely) base my reconstruction of Popper’s work on Fleck’s idea of Denkkollektiv. In 1935, Ludwig Fleck proposed the concept of Denkkollektiv (thought collective), as the “community of persons mutually exchanging ideas or maintaining intellectual interaction” (Fleck 1935). Although I am aware that it can be pointless and even pernicious to try to trace a sharp line between disciplines, I wish to point out the existence of an underlying structure of 1 The authorship of the CIQM, and whether it is meaningful to speak of a Copenhagen interpretation at all, has been often questioned.
Interestingly, Don Howard, in his insightful paper on the history of CIQM, even points out that “it was Popper, an old critic of quantum orthodoxy […], who did more than anyone else, starting in the late 1950s, to cement in the popular mind the idea that Bohr and Heisenberg were together committed to a subjectivist interpretation of quantum mechanics.” (Howard 2004). In what follows I still maintain the historical definition of CIQM as an anti-realistic and instrumentalist interpretation of the quantum formalism, which shares some fundamental principles (e.g. an observer-dependant collapse of the wave function, the Born rule, the wave-particle duality, the correspondence principle, etc.). social nature in the intellectual communities, that strongly conditions the development of particular directions of research, introducing a specific methodology, terminology, “mood” and “thought style” (Fleck 1935).
In fact, Popper has been, in certain periods, an important link that joined the Denkkollektiv of philosophers of science -of which he was one of the most renowned members -with the Denkkollektiv of those illustrious outsider physicists concerned with reclaiming realism in quantum mechanics (QM). 2 These physicists were the “dissidents” who -having a common ground in a realistic interpretation of quantum physics -challenged orthodoxy (see Freire 2014). After the early critiques put forward by Einstein, Schrödinger, de Broglie, etc., the instrumentalist CIQM became widely accepted, also due to the pragmatic paradigm that Cold War demands had enforced in science. In the post-war period, research on FQM was kept alive -although with drastic conceptual and ideological differencesprimarily by physicists the likes of D. Bohm, J. P. Vigier, H. Everett, B. DeWitt and J. Bell and slowly experienced a revival in the seventies thanks to other groups, especially in Italy, initiated by F. Selleri (see Baracca, Bergia, and Del Santo 2016), and in a peculiar way in the US too (see Kaiser 2011).
In the period here investigated, Popper’s role in the debate regarding FQM transformed tremendously. I will show that from 1948 -when he came back to quantum mechanics -up until late 1960s, Popper gave innovative contributions to FQM. However, his activities were conducted mainly in the context of a philosophical Denkkollektiv. Indeed, Popper’s rare publications on the subject matter until late sixties were diffused mainly in the distribution channels of philosophers (conferences, specialized journals, etc.). At the same time, Popper’s regular interlocutors amongst professional physicists interested in the FQM were sparse, and essentially restricted to Alfred Landé, David Bohm and Hermann Bondi (who was an illustrious cosmologist, but never contributed to FQM).
From 1966 onwards, however, thanks to new acquaintanceships, such as Mario Bunge and Wolfgang Yourgrau and their editorial initiatives, Popper found room in the Physics Denkkollektiv. It is the case of Popper’s first paper deliberately though for an audience of physicists, Quantum Mechanics without the Observer
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