Foundations of Quantum Physics - Springer Link

Foundations of Physics, Vol. 34, No. 11, November 2004 (© 2004) DOI: 10.1007/s10701-004-1314-1

The Historical Roots of “Foundations of Quantum Physics” as a Field of Research (1950–1970) Olival Freire Jr.1 Received October 2, 2004 The rising interest, in the late 20th century, in the foundations of quantum physics, a subject in which Franco Selleri has excelled, has suggested the fair question: how did it become so? The current answer says that experiments have allowed to bring into the laboratories some previous gedanken experiments, beginning with those about EPR and related to Bell’s inequalities. I want to explore an alternative view, by which there would have been, before Bell’s inequalities experimental tests, a change in the views shared by physicists concerning the intellectual status of that issue. I will take three cases which will serve as the threads of our story: the connections between Bohm’s causal interpretation and Bell’s inequalities; Wigner’s ideas on the measurement problem; and finally Everett’s relative states formulation. In the end, I will discuss how those threads were gathered together by creating foundations of quantum physics as a field of research. KEY WORDS: history of quantum physics; measurement problem; scientific controversies.

1. INTRODUCTION The foundations of quantum mechanics, a subject in which Franco Selleri has excelled, became, in the late 20th century, a flourishing field of physical research. Some of the papers in this field ranked more than 1000 citations in the SCI; Physical Review included it as one of its major themes in its centenary issue; physicists are wondering about the technological uses of certain physical effects that were disputable some decades ago; and new words such as “quantum information” entered the professional lexicon. 1 Dibner

Institute for the History of Science and Technology — MIT, Cambridge, MA, 02139; e-mail: [email protected] (On leave of absence from UFBa, Brazil.) 1741 0015-9018/04/1100-1741/0 © 2004 Springer Science+Business Media, Inc.

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So, it is natural that one would ask the fair question: how did it become so? Nevertheless, there are already some answers to that question in physicists’ widespread accounts of those changes. They speak exclusively about theoretical predictions and the role of experiments that have permitted to bring into the laboratories some previous gedanken experiments, beginning with those about EPR and Bell’s inequalities related to it.(1,2) This kind of description tends to become the received view on the subject, due to the bulk of materials concerning Bell’s inequalities and experiments on them in the last two decades.(3,4) That description is akin to the description of physical sciences in which only theory and experiments could play a role, and it would explain the changes in the controversy leading to the creation of the disciplinary field of the foundations of quantum mechanics mainly as a consequence of the role played by experiments in physics. But, is it the only, or even the most interesting, description? I want to explore a possible alternative description, by which there would have been, before Bell’s experiments, a slow change in the views of the physicists concerning the foundations of physics, both as a controversial subject and a field of research, and also the creation of institutional and professional opportunities related to that subject. This change would have happened during the 1950s and 1960s, and it could explain the production and the good reception of Bell’s inequalities rather than be explained by them. Surely, experiments on Bell’s inequalities increased the speed of that change and in later times other factors have played their role. To argue for this change I will take three cases which will work as the threads of our story: the connections between Bohm’s causal interpretation and Bell’s inequalities, Wigner’s ideas on the measurement problem, and Everett’s relative states interpretation. At the end of this paper, I will discuss how those threads were gathered together by creating the network that is at the historical origin of foundations of quantum mechanics as a field of research. These threads are not the only relevant ones for our story. A more comprehensive account should include, for instance, the debate about the range of validity of quantum electrodynamics, among Edwin T. Jaynes and others, in the late 1960s and early 1970s;(5) the Soviet campaign against complementarity, in the 1950s;(6) the alternative interpretation represented by the stochastic interpretation, held by Imre Fényes, and by Friedrich A. Bopp, in the 1950s and Edward Nelson in the 1960s;(7) and the lasting debate among philosophers of science about the philosophy of quantum mechanics. As Franco Selleri began to work on such subjects in 1969, a time at which neither experiments nor even Bell’s inequalities took the center of the stage, it would be appropriate for us to discuss the context he found at that time, through which he contributed to still further change.

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2. INTERPRETATION OF QUANTUM PHYSICS — THE FIRST TIMES The sociology of the physicists’ behavior concerning the interpretational problem arisen from quantum mechanics is well described by Jammer’s diagnosis,(7) according to which the period up to the end of the 1940s can be described as the “almost unchallenged monocracy of the Copenhagen school in the philosophy of quantum mechanics”. This description, however, does not tell us how physicists adhered to such a school, nor how they understood complementarity, the conceptual core of that school. Heilbron, discussing the first missionaries of the Copenhagen spirit, gives us some hints on these questions.(8) He suggested that, out of Bohr’s close circle (Heisenberg, Pauli, Jordan, Born, Rosenfeld) ¨ and their brilliant opponents (Einstein, Schrodinger), physicists did not consciously adhere to complementarity or criticize it, but rather used the quantum machinery to scrutinize the microscopic world. Heilbron suggested that the philosophical flavor of Bohr’s view on the interpretation of quantum mechanics was responsible for American and British indifference to complementarity. Schweber would add two American peculiarities, both of them hostile to the idea of philosophizing on quantum mechanics issues, namely the way in which theoretical and experimental physicists were put together in the same departments, reinforcing experiments and application, and American trends toward pragmatism.(9) If one considers that for European physicists, such as Bohr and Pauli, epistemological considerations were part of their way of doing physics, one could get a hint of the complexity of the diffusion of the quantum mechanics, which has become universal by circulating through such different intellectual and professional contexts. As a matter of fact, the textbooks in which physicists learnt quantum mechanics until the 1950s did not “reflect much concern at all about the interpretation of the theory”.(10) According to Kragh, “most textbook authors, even if sympathetic to Bohr’s ideas, found it difficult to include and justify a section on complementarity. Among fortythree textbooks on quantum mechanics published between 1928 and 1937, forty included a treatment of the uncertainty principle; only eight of them mentioned the complementarity principle”.(11) Bohr’s epistemological writings were circulated in papers presented in scientific meetings, and put together in an anthology, which is a vehicle quite different from textbooks. Physicists are formed, as remarked by T. S. Kuhn, mainly via textbooks.(12) So, Jammer’s diagnosis hides a kind of social and intellectual division of work among physicists. “Monocracy of Copenhagen school” meant two types of physicists. A few of them involved with foundational problems, extension of quantum mechanics to new phenomena and its applications

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to old and new problems, and the others involved with extension and applications, but believing that the foundational problems were well solved by the founder fathers of quantum mechanics. If this account seems plausible, and once quantum mechanics in fact brought major conceptual and philosophical problems, one should not wonder at the rising interest on foundational problems after World War II, when a new intellectual and social context emerged amongst physicists everywhere.

3. THE HEATED DISPUTE AND BELL’S INEQUALITIES AS SPIN-OFF OF BOHM’S HIDDEN VARIABLES The 1950s saw the revival of old criticisms by former quantum dissenters — like Einstein — and the appearance of new ones, but none of them was as influential as David Bohm’s hidden variables interpretation. Since one can find a number of philosophical, historical and popular texts about both Bell’s inequalities and Bohm’s interpretation,(3,4,13–17) I will limit myself to two remarks not emphasized enough in the available literature. They intend to describe the climate of the time related to the status of foundations of quantum mechanics, and to present Bell’s inequalities as a spin-off of Bohm’s hidden variables proposal. In the early 1950s, Bohm challenged not only the complementarity interpretation, once his proposal was an alternative interpretation of quantum mechanics, but also von Neumann’s mathematical proof against the existence of hidden variables theories compatible with quantum theory.(18) The challenge was a consequence of his proposal leading to the same results for all non-relativistic physical processes, as does the usual mathematical formalism interpreted from the complementarity point of view. As a matter of fact, not only did he suggest a new interpretation, which he called both hidden variables and causal interpretation, but he also modified the quantum formalism in order to build a model of particles with well-defined trajectories, something that was prevented by standard quantum theory. Although Bohm’s proposal gathered adherents, such as Louis de Broglie, Jean-Pierre Vigier and, for a brief period, Mario Bunge, the causal interpretation by no means became largely accepted. In fact, it was severely criticized by Pauli, Rosenfeld, Heisenberg, and Born, and seen with scepticism by several others. The way in which Bohm’s causal interpretation was received among physicists reinforced the early idea that interpretation of quantum physics was a philosophical subject. First of all because when Rosenfeld described Bohm’s proposal as “metaphysical”,(19) and Heisenberg termed it “ideological”,(20) they meant that the dispute between complementarity and causal interpretations was a controversy


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without implications for physics. In fact, this label could put critics outside the debates in physics because, in the end, philosophical controversies were not subjects for professional physicists. Secondly, insofar as Bohm and his collaborators did not fulfil their promise “that the interpretation suggested may be needed for the resolution of the difficulties [. . . ] into the domain of distances of the order of 10−13 cm or less”,(18) the dispute was seen at the time as being out of reach of physics. Thus, A. Messiah, in his influential textbook of the 1950s, wrote, “the controversy has finally reached a point where it can no longer be decided by any further experimental observations; it henceforth belongs to the philosophy of science rather than to the domain of physical science proper”.(21) Ironically, in the 1950s, Bohm and collaborators unconsciously reinforced that label since they kept their discourse epistemologically loaded by talking about the philosophical advantages of their “broader conceptual framework [since] it makes possible a precise and continuous description of all processes, even at the quantum level”.(18) It was not by chance that in the 1950s the only conference dedicated to the subject was organized by philosophers rather than by physicists.(22) The idea of a philosophical controversy survived in the common discourse on the subject even when the context changed, as was the case when Max Jammer entitled his book, in 1974, “The Philosophy of Quantum Mechanics”.(7) One should note, however, that at least in a special context, that of the young French Marxist physicists around Jean-Pierre Vigier, the philosophical bias of the dispute was more appealing to them than it was considered a diverting factor.(23) Bohm’s proposal fabricated, however, an important spin-off: it motivated John Bell and others to re-examine von Neumman’s proof against the possibility of introducing new variables in quantum theory. Thirty years later, Bell wrote: “In 1952 I saw the impossible done”.(24) In fact, still in the beginning of the 1950s, he wrote to Pauli,(25) asking for papers on Bohm’s proposal. However, due to some vicissitudes, he only resumed this problem ten years later.(26) Meanwhile, Gleason, in 1957, and Jauch and Piron, in 1963, tried to reformulate the proof. To Jauch and Piron the motivation was clear: criticizing the idea of completing quantum theory suggested by Bohm.(27) It was not haphazardly that Bell resumed his early reflections on von Neumann’s proof motivated by discussions with Jauch, in Geneva. “I thought that I had located the unreasonable assumption in Jauch’s work. [. . . ] I decided that I would get all that down on paper by writing a review article on the general subject of hidden variables”.(26) Bell’s works, however, led the subject in a direction completely different from that tried by Jauch and Piron. He not only showed the flaws of von Neumann’s proof but, additionally, analysed some of Einstein’s former criticisms and evidenced that in effect there is a contradiction between

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quantum theory and hidden variables, but not hidden variables in general, since the contradiction occurs only with a certain family of hidden variables, namely local hidden variables. Bohm’s hidden variables were nonlocal.(28,29) These last results we call nowadays “Bell’s inequalities”. Bell’s works opened the possibility of including experimental physics in order to reject some theories and preserve others; however, physicists did not see the subject in this way so promptly. It was only some years later that this possibility was grasped. It was with independent insights by Shimony and Clauser that this possibility was foreseen.(30) Seen from a retrospective view, Bell’s inequalities carried with them the end of any reminiscence of the idea of a philosophical controversy on the foundations of quantum physics. After all, philosophical controversies do not include experiments to solve them. Since that time, one has been able to speak of a genuine scientific controversy, even if it keeps its philosophical implications. I will return at the end to the profiles of physicists such as Shimony and Clauser, who were able to push Bell’s inequalities to the laboratory benches. 4. E.P. WIGNER: HOW ORTHODOXY BECOMES HETERODOXY1 If the causal interpretation presented the greatest challenge to complementarity coming from outside the circle of the founding fathers of quantum mechanics, another major challenge came from within. In the early 1960s, Wigner published two papers that would become the centerpiece of his views on the foundations of quantum mechanics.(31,32) He revisited a distinction first emphasized by von Neumann between two kinds of evolution of quantum states. The first one, linear and deterministic, is governed ¨ by the Schrodinger equation. The second one, nonlinear, unpredictable, occurs during the measurement processes. Additionally, but still following von Neumann, he treated measuring devices quantum mechanically, instead of treating them classically as suggested by Bohr. The latter choice leads to the propagation of the singular superposition of quantum states from the system under scrutiny to the ensemble system plus the measuring apparatus. As it is impossible to see this bizarre superposition in our macroscopic world, one needs to answer how, where, and when this superposition becomes a vector with just one component. After all, what we get after measurements is related to vectors and probabilities rather than to superposition of vectors. Wigner emphasized this point and arrived at 1I

am abridging Wigner’s story from my “Orthodoxy and Heterodoxy in the Research on the Foundations of Quantum Physics: E. P. Wigner’s Case.” In Boaventura S. Santos (ed), Cognitive Justice in a Global World, University of Wisconsin Press, forthcoming.


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the next conclusion: in order to eliminate this superposition one needs to admit that measurement leads eventually to the role of the observer’s introspection, i.e., when the information enters the mind of the observer. To illustrate his point, he imagined an idealized experiment in order to demonstrate the difference between quantum descriptions of measurements with and without human observers. Nowadays the argument related to Wigner’s idealized experiment is known as “Wigner’s friend”. Furthermore, Wigner’s arguments entailed a more sociological and historical question: to define the orthodox interpretation of quantum mechanics, and to identify its protagonists. He wrote:(32) “The standard view is an outgrowth of Heisenberg’s paper in which the uncertainty relation was first formulated. The far-reaching implications of the consequences of Heisenberg’s ideas were first fully appreciated, I believe, by von Neumann, but many others arrived independently at conclusions similar to this. There is a very nice little book, by London and Bauer, which summarizes quite completely what I shall call the orthodox view”. In Wigner’s account, therefore, Bohr and complementarity are underestimated, and Heisenberg and von Neumann become chief protagonists. This excerpt could be interpreted as a dispute over the intellectual heritage of the founder fathers of quantum mechanics. It was not by chance that Wigner wrote this after the death of von Neumann in 1957 and Bohr in 1962, and while scientists and historians in the U.S. were involved in one of the largest projects ever to collect and store records significant in the creation and evolution of a scientific theory, which would come to be known as the Archives for the History of Quantum Physics.(33) Wigner’s papers drew both support and opposition. From the latter, the most important was Rosenfeld. He was Bohr’s former assistant since the 1930s, and a physicist very sensitive to epistemological matters. Rosenfeld and Wigner had, however, very different profiles on a number of issues. Politically, Wigner was very conservative; in contrast, Rosenfeld was engaged in Marxist philosophy since the thirties. They also displayed significant differences in their approach. We could also speak of different scientific styles. For Wigner, following von Neumann, dissecting the mathematical formalism of a physical theory in order to exhibit its axiomatic structure was a necessary step in grasping the theory’s full implications. But for Rosenfeld, following Bohr, a phenomenological insight into a physical theory was the best way to understand it, and he always emphasized his distrust of the reach of axiomatic treatment of physical theories. Last but not least, Rosenfeld maintained that complementarity was the great epistemological lesson of quantum theory, and for this reason, he could not accept Wigner’s position, according to which, Bohr’s complementarity played no role in the orthodox interpretation of

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quantum theory. So, for a number of reasons — political, ideological, and philosophical — Rosenfeld could not accept a view like Wigner’s, which assigned a central role to the mind in physical phenomena. Rosenfeld’s strategy for criticizing Wigner’s view was to give strong praise to a certain work, by writing, “these misunderstandings [i.e., that the translation of Bohr’s argument into the formal language of the theory should present unrecognized difficulties], which go back to the deficiencies in von Neumann’s axiomatic treatment, have only recently been completely removed by the very thorough and elegant discussion of the measuring process in quantum mechanics carried out by Daneri, Loinger and Prosperi”.(34) The Italian physicists had used the ergodic theorem to explain quantum measurements as a thermodynamic amplification of a signal, triggered by the interaction between quantum systems and measurement devices.(35) It’s clear that if Rosenfeld’s point of view about the reach of the Italian work were accepted, Wigner’s claims would be considered ungrounded. The dispute lasted throughout the second half of the 1960s, and it was marked by bitter arguments, even though it dealt with rather technical content, i.e., to determine whether the Italian work was a rigorous solution or just an approximation. It eventually ended at the Varenna summer school, dedicated to the Foundations of Quantum Mechanics, in 1970, where Wigner argued for the need of a research program on quantum measurement processes. His point was that both his approach and that of the Italian physicists (Prosperi lectured at the same summer school) would entail a significant modification of the theory current at that time, hence “the inapplicability of quantum mechanics to some part of the measurement process has to be postulated or admitted”.(36) It is important to consider how Wigner’s contemporaries interpreted his dispute with Rosenfeld and the Italian physicists. O.R. Frisch, in a colloquium held in 1968, said: “I understand that at present there exists a controversy, roughly speaking between a group of people which includes Wigner as the best known person and another group centered on Milan in Italy, and that these two have different views on how this reduction happens”.(37) For the first time in the literature, the name “Princeton school” was used to differentiate Wigner’s views from those of the Copenhagen school. According to Ballentine,(38) there were “several versions of the Copenhagen interpretation” and, “although both claim orthodoxy, there now seems to be a difference of upholds between what may be called the Copenhagen school represented by Rosenfeld, and the Princeton school represented by Wigner”. Since then, labeling Copenhagen and Princeton schools has become current in the literature.(39) The monocracy was thus broken, from inside, as was the fate of many other monocracies of the 20th century.


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The portrait of Wigner as just a disputant in the creation of the field of foundations of quantum mechanics is not completely fair to him. He engaged in a variety of activities and assumed a kind of non-dogmatic but highly influential leadership. He formed a group of students to work on the subject, such as Abner Shimony and Michael Yanase. Wigner was also supportive of entrants and non-entrants in the field, such as Bernard d’Espagnat, Henry Margenau, and John Archibald Wheeler. In the late 1960s, Wigner accepted Margenau’s invitation to be a member of the editorial board of a new journal, Foundations of Physics, aimed at fostering research of “disciplined speculations suggestive of new basic approaches in physics”, including those concerning the foundations of quantum mechanics. Wigner not only accepted this invitation but assumed editorial responsibilities of the journal, suggesting papers and influencing the choice of editor who would replace Margenau upon retirement. In a time when T. S. Kuhn’s views about the role of dogmas in the development of science are so widely accepted, it is worthwhile to conclude these comments about Wigner’s style of intellectual influence with a remark about one of his characteristics, namely, the non-dogmatic manner in which he dealt with the subjects related to the foundations of quantum mechanics. See, for example, his reaction to the approach suggested by H. D. Zeh. This approach was critical both of Wigner’s and of the Italian ¨ physicists’ approach because both admitted the validity of Schrodinger’s equation to describe the measurement devices, and according to Zeh measurement devices are not closed systems to which such an equation could be applied. Wigner received a preprint version of Zeh’s paper, supported its publication in the first volume of Foundations of Physics, and opened his Varenna talk with six possible solutions to the measurement problem, Zeh’s solution being the last. Shimony had the insight to record Wigner’s feelings about the attitudinal changes he underwent. These changes may also help us understand changes in the Zeitgeist of physicists of the 1960s and 1970s with respect to the foundations of quantum mechanics. Intending to defend what he considered to be the “quantum orthodoxy”, he in fact helped to legitimize heterodoxy on this subject, and he himself became a dissenter.(40) Today, Wigner’s conjecture about the role of the mind in the quantum measurement process is no longer part of physics, but rather part of the history of physics. In contrast, Wigner’s research program — to understand from a physical point of view what quantum measurement is — has flourished and is part of science, related to what one calls today “quantum information”. Furthermore, in order to create this subfield of physics, foundations of quantum physics, it was necessary to break what Jammer called the “Copenhagen monocracy”. As recently remarked by Aspect, “questioning

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the ‘orthodox’ views, including the famous Copenhagen interpretation, might lead to an improved understanding of the quantum mechanics formalism, even though that formalism remained impeccably accurate”.(41) Wigner made major contributions in that questioning.

5. OTHER DISSENTS: THE MANY LIVES OF EVERETT’S INTERPRETATION The third thread of our story concerns Hugh Everett’s relative states interpretation. I will discuss it not only because of its real influence on the creation of the field of foundations of quantum mechanics but also because the history of its two lives can enlighten the changing climate among physicists concerning that subject from the 1950s up to the 1960s. Relative states interpretation was Everett’s doctoral dissertation, at Princeton, under John Archibald Wheeler, in the mid 1950s. Trying to fight out what seemed to him a paradox in quantum mechanics, Everett arrived to take just one kind of evolution of the quantum state — the linear ¨ one described by Schrodinger equation — giving up the second, which happens during measurements. He was able to build a “universal wave function”, without collapses, and to show that the correlation between the many splitting states prevented changes of information between them, assuring that one observer in one of the split states could not know what would be happening in other split states. Everett was a reader of Bohm’s and von Neumann’s textbooks, but not of Bohr’s epistemological writings.2 He put together in his approach Bohm’s realism and von Neumann’s quantum treatment of measurement devices and observers and did not accept Bohr’s complementarity view about measurement. Everett’s dissertation created an affair that left Wheeler walking a tight rope, and Everett himself disillusioned with physics and physicists, as we shall see. Wheeler, who likes strong conjectures, praised Everett’s ideas for his unusual approach and for his consistent and elegant mathematical treatment, but as somebody close to Bohr he could not accept Everett’s inferences against complementarity. Thus, Wheeler had the idea, which retrospectively one could name wishful thinking, of convincing Bohr of the 2 “The

particular difficulties with quantum mechanics that are discussed in my paper have mostly to do with the more common (at least in this country) form of quantum theory, as expressed for example by von Neumann, and not so much with the Bohr (Copenhagen) interpretation. The Bohr interpretation is to me even more unsatisfactory, and on quite different grounds”. Hugh Everett, III, to Aage Petersen, 31 May, 1957. Wheeler Papers. Series I, Box Di - Fermi Award #1, Folder Everett. Wheeler Papers are at the American Philosophical Society, Philadelphia.


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value of Everett’s approach and persuading Everett to clear the epistemological considerations off his dissertation. Wheeler’s plans were far from modest. He wanted to publish Everett’s dissertation, in full, at the Danish Academy of Science as a way of legitimating it among complementarity’s supporters.3 In 1956, with a draft of the dissertation in his luggage, Wheeler went to Copenhagen to review it with Bohr. The discussions there included Aage Petersen and Alexander Stern, besides Wheeler and Bohr. At this point of our story, it’s worth remarking that, in the 1950s, Bohr’s influence on the matters of the interpretation of quantum mechanics was so great that a Princetonian dissertation was being reviewed in Copenhagen before being judged at Princeton University. Bohr could not, and did not, accept Everett’s ideas on the epistemological considerations about observation in quantum mechanics. The discussions were not cold, and the temperature of the debates was so high that Stern reported Everett’s point as theology. Wheeler wrote, “if it is a theological statement to postulate the “universal wave function” it is also a theological statement to refuse to entertain the postulate”.4 Wheeler came back from Copenhagen defeated but not surrendered. His remark to Everett — “So, in one way your thesis is all done; in another way, the hardest part of the work is just beginning” — meant the dissertation would be approved, but the battle to convince Bohr should continue, this time with Everett’s stay in Copenhagen.(42) In fact, the dissertation received its Princetonian formal approval in 1957; and an abridged version of it was modestly published in a special issue of Review of Modern Physics, with the proceedings of a conference to which Everett never went, together with a note of Wheeler talking about the possible convergence between Everett’s ideas and complementarity.(43,44) Everett went to Copenhagen in 1959, but the discussions with Bohr bore no fruit. The main result, according to Everett, was to get a mathematical result not related to quantum mechanics he was already working on.(45) Max Jammer reported Everett’s ideas as “one of the best kept secrets in this century”,(7) astonished by the 10 years they were not taken into account by physicists. What he did not realize at the time was that his statement could have a double meaning. The second one means that the very fact of the dissertation being discussed in Copenhagen before its 3 “Since

the strongest present opposition to some parts of it [Everett’s dissertation] comes from Bohr, I feel that acceptance in the Danish Academy would be the best public proof of having passed the necessary tests”. J. A Wheeler to A. G. Shenstone, 28 May, 1956, Wheeler Papers. Series I — Box Di-Fermi#2, folder Everett. Idem. 4 Wheeler to A Stern, 25 May 1956. Wheeler Papers. Series 5 — Relativity Notebook 4, p. 92. The sentence is handwritten on the typed letter. It is also written “CWM”, which suggests C.W. Misner was its author.

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approval at Princeton was also kept in secret. Neither Jammer nor other writers discovered it. Disillusioned with the whole affair and seduced by his works on game theory and computers, with the Pentagon, Everett abandoned physics, and never again wrote even one word on the interpretation of quantum mechanics, even when his ideas were revived ten years later, and he succeeded in a research career related to American defense. His paper received no more than 20 citations in the next ten years.(46) Blocked by the monocracy around Copenhagen’s views on quantum mechanics, one could say that Everett’s ideas virtually did not have a first life in the 1950s; they were almost stillborn. Ten years later, the cosmologist Bryce DeWitt revived Everett’s idea for its second and enduring life. Some factors were influential to the success of that revival. DeWitt used it in an intellectual context that was different from that of the dispute concerning the interpretations of quantum mechanics. Interested in quantizing the state vector of the whole evolving universe, a state afforded by general relativity, he arrived at the conclusion that Everett’s was the best interpretation candidate to account for it.(47) DeWitt’s paper became a reference in cosmology, because it introduced the so-called DeWitt-Wheeler Equation. It received more than 1000 citations.(46) But DeWitt was not only interested in using Everett’s idea in the context of cosmology. It is curious to remark that as early as 1957, he had read a preprint of Everett’s paper in Review of Modern Physics and was very skeptical about its epistemological implications.(48) Ten years later his attitude changed. He wanted to advertise Everett’s ideas among physicists and, in fact, found a favorable ambiance to do it. He recruited Neil Graham, his PhD student, to work on the subject; published popular and bibliographical review papers; started lecturing on that interpretation; and managed to publish the whole of Everett’s dissertation.(49) He went on to rechristen it as the “many-worlds” interpretation, a label far from Everett’s own goals, but responsible for popularizing that interpretation outside the circle of professional physicists. The interest in Everett’s ideas, about 1970, came not only from their implications for cosmology but also from the changing views shared by many physicists about the matter of the foundations of quantum mechanics. DeWitt’s paper was well received both among cosmologists and the flourishing community of foundations of quantum mechanics.(46) He was invited to lecture on the relative states interpretation at the Varenna’s summer school, a question we will resume, and his proposal to publish a bibliographical review on the foundations of physics, which was part of Graham’s dissertation, was well received by the American Journal of Physics’ editors, who published it in a prestigious section of that journal, its


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“Resource Letters”.(50) Additionally, he approached R. Hobart Ellis, Jr., then editor of Physics Today, asking about the interest of the journal in “initiating another vigorous debate [the previous one concerned tachyons] in a different area, which is also of keen interest to most physicists, namely the interpretation of quantum mechanics,” and suggesting himself to write about Everett’s interpretation.(51) He was surprised at the strong interest of Hobart Ellis in the subject. The editor wrote, “Your letter of 21 October strikes a very responsive chord. For a long time I personally have been dissatisfied with the apparent contradictions that physicists appear to be ready to live with in quantum mechanics and its interpretation. Someone has compared the present situation with that in which cycles and epicycles could explain all the movements in the heavens and science was well satisfied with the view until the Copernican theory took over. I feel the comparison is particularly apt”. Next, Hobart Ellis mentioned his previous project for publishing physicists’ replies to a questionnaire on the subject, a project that failed due to the scanty time available. Finally, he concluded that “it seems to me that the article you propose would be a very interesting and useful contribution to Physics Today”, but added that “in fact I think a general review of different interpretations of quantum mechanics without special emphasis on any one would be of interest”.(52) Some time after the publication of DeWitt’s paper, “Quantum mechanics and reality”,(53) the new editor, Harold L. Davies, undertook a typical editorial procedure, that of putting emphasis on a certain subject. Physics Today published in the same article, under the title “Quantum-mechanics debate”, six large letters, by L. E. Ballentine, Philip Pearle, Evan H. Walker, Mendel Sachs, Toyoki Koga, and Joseph Gerver, with different but critical points of view on Everett’s interpretation, besides DeWitt’s reply.(54) That article was followed by several letters debating the theme, one of which remarked very incisively on the changing mood among physicists concerning the subject, a mood quite different from that in which Everett’s interpretation had been born ten years before. In fact, M. Hammerton, from the Medical Research Center, Cambridge, UK, wrote:(55) “The very interesting contributions to the quantum mechanics debate in your April issue, and the paper by DeWitt which triggered them, exemplify the highly complex and subtle ways in which scientific opinion can change. When I was an undergraduate reading physics 20 years ago, [. . . ] the Copenhagen line was “scientific,” anything else was meaningless, mumbo-jumbo, or, at best, mistaken. Now the curious thing is that, as far as I am aware, there has been no major finding or theoretical insight that could be held to demolish or supersede this interpretation. Nevertheless, there is now considerable dissatisfaction with it, and a willingness to regard other points of view — for example, hidden variables — as being at least respectable.”

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6. THE THREADS GATHERED TOGETHER — WEAVING FOUNDATIONS AS A REGULAR FIELD OF RESEARCH In the late 1960s, those three threads, besides others cited in the beginning of this paper, were gathered together by weaving the web that is at the origin of the taking up of foundations of quantum mechanics as a regular field of research. The protagonists were physicists, such as Wigner, Bohm, Bell, DeWitt, Shimony, Clauser, D’Espagnat, Vigier, de Broglie, Margenau, Yanase, Jauch, Prosperi, and others not portrayed in our story. They did not agree on the content of their research, but on its status. In fact, at least among those physicists there was a new mood as far as that status was concerned. Foundations and interpretations of quantum physics were no longer considered solved problems, or a matter to be restricted to philosophical debates. Instead, it was considered a matter to professional physicists, even if it was a controversial subject with strong philosophical implications. The same changing mood can be seen from another perspective. The “almost unchallenged monocracy of the Copenhagen school in the philosophy of quantum mechanics”, in the early 1950s, was not only challenged. It was, in fact, broken. To those physicists, complementarity remained as one of the possible interpretations, but no more the only one. It was perhaps still considered the strongest candidate, but it ought to compete with its rivals; and they largely shared the idea that there were real and relevant problems to be solved in the foundations of quantum physics. I would say that those physicists recognized the existence of a scientific controversy concerning the foundations and interpretations of quantum mechanics and managed to treat it as a field of regular research. Three events about 1970 can evidence that change of mood. This paper has already presented one of them, related to Physics Today. Let us see the two others. The first was the creation of Foundations of Physics, and the second was the theme of the International School of Physics ‘Enrico Fermi’. The scientific journal Foundations of Physics appeared in 1970 with the aim of being the vehicle for debates in the field designated by its title and, above all, for theoretical debates related to quantum physics. The initiative of creating that journal came from Wolfgang Yourgrau, who invited Henry Margenau to join him as its first editor. The Editorial Board of Foundations of Physics comprised physicists who, two decades before, had been on opposite sides in quantum disputes. Thus we find David Bohm, Louis de Broglie, and Eugene Wigner, side by side with V. A. Fock, who was close to the complementarity interpretation. Even though the journal was aimed at foundations of physics in general, 16 out of the 18 papers in the first volume dealt with quantum themes. The purpose of the Editorial Preface was to define what the editors understood for ‘foundations’, evidencing how far this word was from the dominant


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preoccupations among physicists at the time. The editors presented some arguments meant to convince physicists that a journal such as Foundations of Physics could be useful to physics in general and not only to the foundations of physics. They went on to say that: “Some believe that the risk of overlooking such defects [infelicities in scientific foundations] is great in our days when the need and the actualities of public support place an excess of emphasis upon the pragmatic aspects of science. If this is true, compensating attention to matters of foundations is an important necessity.”(56) More than 30 years later, the time has confirmed how fortunate was Yourgrau and Margenau’s initiative. Almost 20 years after its creation, Alwyn van der Merwe, then its editor, and urged by Franco Selleri, created Foundations of Physics Letters. The journals have become hallmarks of the field of foundations of quantum mechanics. Also in 1970, between 29 June and 11 July, the Italian Society of Physics held one of its traditional courses of the International School of Physics “Enrico Fermi” with the title “Foundations of Quantum Mechanics”. Summer courses had been regularly carried out since 1953, in Varenna, on Como Lake, but none of them were dedicated to such a theme. The 1970 course had its origin in a proposal put forward by Franco Selleri, at the time a councillor of that society, a reputable practitioner of high-energy physics, but a new entrant in the matter of foundations.5 The proposal was supported by G. Toraldo di Francia, then the president of the society, who was sensitive to the subject and was worried about the threat of the splitting of the society due to the turmoil in Italian universities between 1968 and 1969. He realized that that summer school could better put together physicists than contribute to their disunity, since a balanced physicist headed it.6 So, he chose the French physicist Bernard d’Espagnat to be the head of the course. It had 84 participants, and its “proceedings”,(36) with the lectures given there, reveal a diversified spectrum of themes and lecturers, including some of the protagonists of the controversy, such as Wigner, Jauch, Shimony, d’Espagnat, Bell, De Broglie, Selleri, and Bohm. In choosing d’Espagnat to lead the course, Toraldo di Francia could not have made a better choice because it meant the right man at the right place. His interest in Philosophy and Science dated back to high school. After succeeding in a Physics career, he began to spend time on the subjects related to the foundations of quantum physics.(57) He sent a preliminary invitation to the participants, which is an exemplary evidence of how the controversy was being dealt with. He criticized the 5 Franco 6 Toraldo

Selleri, Bari, 2003, interviewed by the author. di Francia, Florence, 2003, interviewed by the author.

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instrumentalistic view of physical theories, defining the controversy: “[But] it will probably become quite clear, in a few days, that under a superficial agreement on how to use the rules we have learnt, we entertain real differences of opinion as to what these rules refer to”. d’Espagnat proposed then an agreement that one could see as diplomatic rules to be followed in order to have a pacific and creative coexistence amidst a situation of scientific controversies. Here is the suggested agreement: “(1) we should not take as our goals the conversion of the heretic but rather a better understanding of his standpoint; (2) we should not suggest that we consider as a stupid fool anybody in the audience (lest the stupid fools should in the end appear clearly to be ourselves!); (3) we should try to cling to facts; (4) nevertheless, we should be prepared to hear without indignation very nonconformist views which have no ‘immediate’ bearing on facts.”(36) It is noteworthy that, in spite of the differences between the Foundations of Physics and the Varenna’s course, both needed to face the same task in order to justify their existence: to argue against the intrumentalistic view of science, which suggests that monocracy around complementarity and instrumentalistic views reinforced the division of labor among physicists we have discussed in this paper.

7. CONCLUSIONS The time Selleri went on the stage of the research into foundations of quantum mechanics was also the time proposals for experiments on Bell’s inequalities appeared. At the very Varenna’s course where Selleri spoke about realism and defended the necessity of experiments to settle those controversies, Shimony discussed the proposals for experiments on Bell’s inequalities he, Clauser, and Michael Horne had published the year before. However, it seems that, when experiments went on the stage, there already was a new mood among physicists, or, at least, a certain number of people willing to consider foundations of quantum physics as a relevant field of research. If the existence of a changing mood before the appearance of Bell’s inequalities tests seems plausible, we need to put a question to the history of science. What factors contributed to that change? That question is still more relevant if one considers that the change cannot be framed into the historical accounts of American physics formatted by pragmatism or by the trend towards applications as a consequence of the kind of physics Cold War times needed.(9,58) Here is not the place to proceed with this discussion, but I would like to suggest some lines of research one should consider to address that question. The background of the changes was the growing dissatisfaction coming from the new generations of physics


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students who hardly grasped the theory without an interpretation, or from those physicists compromised with a realistic view, or still from those leaning towards axiomatizing scientific theories. All of those dissatisfactions ran, in the medium term, against complementarity. Other generational effects came from the disappearance of most of the founder fathers. It may have stimulated disputes for their intellectual heritage and renewed willingness to analyze foundational questions. A comprehensive understanding of those changes will require, however, taking into account the context of the second half of the 1960s, especially in American physics. It was a time in which public opinion support of the increasing budgets of physics, typical of Cold War times in the 1950s, declined.(59) Physicists tried to accommodate themselves to the new context. Physicists from high energies, for instance, changed their discourse on the importance of their field from its value to national defense and the scientific race to the cultural value of their subjects.(60) May that effect have been influential in creating open-mindedness among physicists concerning the foundations of what was believed to be the best-founded physical theory? It is a conjecture to be investigated. In a weaker sense, however, one could find traces, even in this paper, of the changing political and cultural mood of the time. Opening a debate on quantum physics was a minor problem – to editors of Physics Today – if compared with the strong debate they needed to conduct about the political role of the American Physical Society, a debate mainly related to the widespread idea of physics being closely related to the military efforts in the Vietnam War. Toraldo di Francia, when supporting that summer school on foundations of quantum mechanics was also attentive to the preservation of the unity of Italian physicists, threatened as it was by the spread of the universities’ rebellion among physicists. Let us come to an end by resuming the case of the two physicists, Shimony and Clauser, who were responsible for driving Bell’s inequalities into the laboratories. The former was ready, due to his path in physics, under Wigner, and in philosophy, to succeed that way. The latter was a student who did not agree with complementarity in his graduate studies, concluded his doctoral dissertation far from foundations of quantum mechanics, in astrophysics, but who, influenced by l’air du temps of the protests against Vietnam War, wanted to shake the world, and quantum mechanics was one of the targets of his desire.(13)

ACKNOWLEDGEMENTS I am thankful to CNPq — Brazil (Grant 303967/2002-1), American Institute of Physics, American Philosophical Society (Slater Fellowship),

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Dibner Institute for the History of Science and Technology (Senior Fellowship) for the grants and aids which supported this research, and Joan Bromberg for her remarks on this paper.

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