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A Historical Survey of Sir Karl Popper's Contribution to Quantum

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A Historical Survey of Sir Karl Popper's Contribution to Quantum A Historical Survey of Sir Karl Popper’s Contribution to Quantum Mechanics William M. Shields Worcester Polytechnic Institute, Worcester, Massachusetts, United States. E-mail: [email protected] Editors: Ion C. Baianu, Christoph Lehner, Debajyoti Gangopadhyay & Danko Georgiev Article history: Submitted on May 28, 2012; Accepted on July 28, 2012; Published on November 15, 2012. ir Karl Popper (1902–1994), though not 1 Popper in the physics journals trained as a physicist and embarrassed early Sin his career by a physics error pointed out by Sir Karl Popper, by any measure one of the preeminent Einstein and Bohr, ultimately made substantial con- philosophers of the twentieth century, died in 1994 at the tributions to the interpretation of quantum mechan- age of 92. He was productive to the end, publishing in ics. As was often the case, Popper initially formu- the year of his death a criticism of Kuhn’s incommensu- lated his position by criticizing the views of others – rability of paradigms [1]. That debate continues over his in this case Niels Bohr and Werner Heisenberg. Un- many and profound philosophical ideas and opinions is derlying Popper’s criticism was his belief that, first, hardly surprising, almost two decades after his death. The the Copenhagen interpretation of quantum mechan- proliferation of book-length biographies and scholarly ics abandoned scientific realism and second, the as- philosophical articles is testimony to Popper’s standing as sertion that quantum theory was complete (an asser- a philosopher [2–5]. Major conferences are also regularly tion rejected by Einstein among others) amounted held on the thought of Karl Popper [6, 7]. to an unfalsifiable claim. Popper insisted that the It may come as a surprise, however, that beginning most basic predictions of quantum mechanics should in the year 2000, Popper’s name appears prominently in continue to be tested, with an eye towards falsifica- no less than a dozen papers in the journals of theoretical tion rather than mere adding of decimal places to physics, in the majority of cases in the paper’s title [8–20]. confirmatory experiments. His persistent attacks on Several of these papers report the results of Popper’s the Copenhagen interpretation were aimed not at the experiment carried out by physicists at the University uncertainty principle itself and the formalism from of Maryland in 1999 [13–17]. Coincidentally, the final which it was derived, but at the acceptance by physi- section of a recent text on the Einstein-Podolsky-Rosen cists of an unclear epistemology and ontology that left (EPR) paradox in physics deals with this same experi- critical questions unanswered. ment, proposed by Popper in the early 1980’s [21, Sec. Quanta 2012; 1: 1–12. 5.4.3]. How is it that physicists in the new millennium are invoking Karl Popper’s name, conducting experiments suggested by him, and arguing over the meaning of the results? This is an open access article distributed under the terms of the Creative Commons Attribution License CC-BY-3.0, which To suggest answers to this question, I review below permits unrestricted use, distribution, and reproduction in any medium, Popper’s fifty years of contributions to the interpretation provided the original author and source are credited. of quantum mechanics. For reasons that will become clear Quanta j DOI: 10.12743/quanta.v1i1.4 November 2012 j Volume 1 j Issue 1 j Page 1 in later sections of the paper, I trace a line of thought ex- Eventually Popper came to appreciate the core of the dis- periments that Popper proposed to test his own views agreement within physics and was able to sort out in his against the views of the majority of theoretical physicists mind the various positions of Einstein, Bohr, Heisenberg, who created quantum mechanics. In the final section of Schrodinger,¨ and Born. By the time Logik Der Forschung the paper, I offer a possible explanation for why Pop- was well underway, Popper felt qualified to address quan- per’s passionately-held opinions continue to attract the tum theory in the book. attention of physicists, and are worthy of that attention. Never one to tackle a subject halfway, Popper devoted all of Chapter IX of Logik Der Forschung to Some Obser- vations on Quantum Theory [23]. In the English transla- 2 ‘Logik Der Forschung’ and tion, it runs to 35 densely-argued pages wherein Popper Einstein’s refutation sets out the views he was to maintain, with some modifi- cations, for the rest of his life [24]. He makes his purpose The most exciting and fundamental discoveries of quan- clear in the introductory section: tum mechanics were made while Popper was in college and graduate school. Though his dissertation was not What follows here might be described, perhaps, in physics, he had studied the sciences and mathematics as an inquiry into the foundations of quantum and was qualified to teach them on the secondary level. theory. In this, I shall avoid all mathematical The thesis, completed in 1928, was titled On the Prob- arguments and, with one single exception, all lem of Method in the Psychology of Thinking and it had mathematical formulae. This is possible be- more to do with the methodology of science than psy- cause I shall not question the correctness of the chology. Popper characterized it as a “hasty last minute system of mathematical formulae of quantum affair” [22, p. 78]. By the time he began work in earnest theory. I shall only be concerned with the logi- on Logik der Forschung in the early 1930’s, Popper had cal consequences of its physical interpretation educated himself on the new quantum mechanics, by then which is due to [Max] Born. [24, p. 216] becoming accepted as a major advance in atomic physics. Thereafter Popper lays out his criticism of what is com- Consistent with his interest in the logic and methodol- monly called the Copenhagen Interpretation of quantum ogy of science, Popper focused his study on the philosoph- theory (largely the work of Bohr) and the position of ical underpinnings of the new theory. He was especially Heisenberg that the uncertainty relations must be viewed interested in the disputes that had arisen over how to inter- subjectively, as a “limitation of our knowledge” of phys- pret physically the mathematical formalism of the theory. ical systems [24, p. 220]. In the same passage, Popper Popper explains: notes that Moritz Schlick of the Vienna Circle had ex- At the time (1930) when . I began writing my pressed strong support for Heisenberg’s views (Heisen- book, modern physics was in turmoil. Quan- berg’s explication of his own position can be found in tum mechanics had been created by Werner [25]). Heisenberg in 1925, but it was several more Popper devotes the next several sections to advocacy years before outsiders – including professional of a statistical interpretation of the uncertainty relations. physicists – realized that a major breakthrough He argues that, contra Heisenberg, it does indeed make had been achieved. And from the very begin- sense to attribute well-defined positions and momenta to ning there was dissension and confusion. The individual particles. Experimental results showing wave- two greatest physicists, Einstein and Bohr, per- like behavior of particles (as in slit experiments) can be haps the two greatest thinkers of the twentieth explained as “statistical scatter relations.” The scattering century, disagreed with one another. [22, pp. behavior is calculated using the mathematical machin- 90-91] ery of quantum theory, but it does not imply anything about limitations on knowledge or an actual lack of a Lacking doctoral-level knowledge of physics, Popper well-defined position and momentum at any moment in struggled to grasp the new theory: time [24, p. 225]. With this interpretation in hand, Popper wonders aloud I was working on my own from books and from whether anything has in fact been gained. His conclusion articles; the only physicist with whom I some- emphasizes his aim, which is in essence to succeed where times talked about my difficulties was my friend Einstein had failed in his arguments with Bohr: Franz Urbach. I tried to understand the theory and he had doubts whether it was understand- The statistical elements of quantum theory must able – at least by ordinary mortals. [22, p. 91]. be inter-subjectively testable in the same way Quanta j DOI: 10.12743/quanta.v1i1.4 November 2012 j Volume 1 j Issue 1 j Page 2 as any other statements of physics. And my 3 ‘Postscript to Logik Der simple analysis preserves not only the possibil- Forschung’: thoughts on ity of spatio-temporal descriptions, but also the objective character of physics. [24, p. 234] Gedankenexperiments Events that took place two years after the publication sug- Popper’s Postscript evolved by the mid-1950s into a mas- gested to Popper that he should have ended the chapter sive work that outsized the work it was a postscript to, so there. But he did not. In the next sections of the chapter, large that it had to be published in three volumes [27–29]. Popper describes an imaginary experiment (Gedankenex- Popper’s determination to have an impact on quantum periment) “which shows, in full agreement with quantum theory is evidenced by his devoting the entire third vol- theory, that the precise measurements in question are pos- ume to the subject. If Quantum Theory and the Schism sible” [24, p. 243]. I will omit details of the experimental in Physics [29] shows nothing else, it is proof that in the design here – suffice to say that it had the same idea as twenty years since his embarrassment before Bohr, he had later versions, but was flawed in several key respects.
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