The Foundations of Quantum Mechanics in Postwar Transatlantic Physics: On the in uence of the United States on the lack of interest in alternative interpretations of quantum mechanics, 1950-1970

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The Foundations of

Quantum Mechanics in

Postwar Transatlantic

Physics

On the influence of the United States on the lack of

interest in alternative interpretations of quantum

mechanics, 1950 – 1970

S.L. ten Hagen

University of Amsterdam

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18 July 2014

Universiteit van Amsterdam

Faculteit der Natuurwetenschappen, Wiskunde en Informatica Insitute for Theoretical Physics

Bachelorproject Natuur- en Sterrenkunde (12 EC)

Author: Sjang ten Hagen Student ID: 6375162

Project supervisor: Jeroen van Dongen Second corrector: Bernard Nienhuis

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Abstract

This text focuses on the lack of success of alternative interpretations of quantum mechanics in the first decades after World War II. In this pe-riod, physicists from both Europe and the U.S. stayed true to the so-called Copenhagen interpretation, while little attention was paid to new ideas on the foundations of quantum mechanics. A corresponding pragmatic atti-tude towards the role of physics is often associated with American physics. Regarding the increasing power of the United States in postwar European physics, this paper attempts to answer the question to what extent Amer-ican values and ideas have contributed to the reduced attention for the foundations of quantum mechanics. It is argued that the activities of the U.S. government and private foundations in the rehabilitation of European science after World War II had a reinforcing effect on the monocracy of the Copenhagen interpretation in Europe. In the U.S. efforts to increase the amount of scientific exchange, the merely philosophical occupation of interpreting quantum mechanics had little urgency, compared to the support of internationally minded research institutions like CERN and the Niels Bohr Institute, which could contribute to the sense of a united, democratic and transatlantic community. As a final point of discussion, the question is raised whether a pragmatic tradition suits to American physics in particular or to physics in general.

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1 Introduction

In the 1920s, a glow of excitement surrounded European theoretical physics. It was caused by the rapid development of a new theory called quantum me-chanics, which focused on nature on the smallest scale. The development of the theory was accompanied by debates about its philosophical significance. At the Solvay conference in Brussels in 1927 (see fig.2), some of the period’s most famous physicists were involved in a discussion on the interpretation of quan-tum mechanics.1 The key players in the debates from this period were Niels Bohr and Albert Einstein. Among their main supporters were, respectively, Werner Heisenberg and Erwin Schr¨odinger. Although consensus between these contenders has never been reached, the Copenhagen interpretation from Niels Bohr has emerged as the conventional interpretation of quantum mechanics.2

Despite lively prewar discussions on the topic, new contributions to the in-terpretation of quantum mechanics were scarce in the years after World War II. Olival Freire jr. has pointed out that in this period, the interpretation of quantum mechanics was regarded to be a matter of philosophy rather than real physics: “It was not by chance that in the 1950s the only conference dedicated to the subject was organised by philosophers rather than by physicists.”3 _{It has}

been argued by David Kaiser that during the 1950s and 1960s, it was hard to be taken seriously as a physicist, while focusing on the foundations of quantum mechanics.4_{Although it might be incorrect to consider the 1950s and 1960s as a}

1_{Pais, 1991, p. 316.} 2_{Camilleri, 2009b.} 3_{Freire Jr., 2004, p. 1745.} 4_{Freire Jr., 2009; Kaiser, 2011.}

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1 Introduction

lost period for the interpretation of quantum mechanics –for example, John Bell published his influential text on the completeness of quantum theory in 19645– interpretational questions reappeared into the agendas of physicists only after 1970. From this moment, physicists have successfully proposed new ideas on the topic, while even bestsellers on the interpretation of quantum mechanics have been published. For example, Fritjof Capra’s 1975 The Tao of Physics has ap-peared in 23 languages, attracting the attention of a wide audience. Meanwhile, physicists such as Heinz-Dieter Zeh –who laid the groundwork for the theory quantum decoherence during the 1970s– and Alain Aspect –who confirmed Bell’s theorem by experiment in 1981– were able to make great contributions to the interpretation of quantum mechanics. Slowly, The Foundations of Quantum Mechanics began to be recognised as a self-contained field of research.6

This renewed interest for the interpretation of quantum mechanics indicates a change in the climate of theoretical physics between the 1970s and the preced-ing decades. It is worth askpreced-ing which factors have contributed to the gradual shift in attitude towards quantum interpretations. Olival Freire jr. has defended the conceivable idea that the revived attention on quantum interpretations was caused by the rise of a new generation of physicists. Towards the end of the 1960s, young scientists were unsatisfied with the way they learned to deal with quantum mechanics during their education, namely by ‘shutting up and cal-culate’. Freire has argued that the dissatisfaction among them was growing, caused by the fact that they “hardly grasped the theory without an interpre-tation”.7 _{It is questionable whether this generation shift sufficiently explains}

the renewed attention to the interpretational issues of quantum theory. Be-sides, Freire’s analysis does not explain the lack of interest in the topic among physicists during the first decades after World War II. For the latter, it seems worthwhile to study the context of postwar theoretical physics. In the aftermath of World War II, science became one of the most important platforms for the international exchange between the United States and Western Europe. There-fore, it is likely that the intensification of the American-European relationship has effected the agendas of scientists from both continents.

This text examines the role of the postwar Americanisation of European the-oretical physics as a possible explanation for the lack of attention to the founda-tion of quantum mechanics. First, it will be presented which developments on the interpretation quantum mechanics took place before (chapter 2) and after World War II (chapter 3). Subsequently, the postwar internationalisation of science in general and the Americanisation of European physics in particular will be treated in chapter 4. In the conclusion, the following question will be answered: has the Americanisation of European physics reduced attention to the interpretation of quantum mechanics during the 1950s and 1960s?

5_{Bell, 1964.} 6_{Freire Jr., 2004.} 7_{Ibid., p.1757.}

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2 Interpreting Quantum Mechanics Before World

War II

2.1 Early debates on the foundations of quantum mechanics

During the early stages of quantum mechanics, physicists were increasingly ac-cepting an interpretative framework on the theory, which was provided by Niels Bohr. Since this interpretation of quantum mechanics violated Albert Einstein’s concept of reality, his dissatisfaction was growing. Before outlining Einstein’s most urgent objections, let me first summarise the content of Bohr’s conven-tional or orthodox interpretation, which has later became known as the Copen-hagen interpretation.

In quantum mechanics, a classical description of matter proves insufficient because of the uncertainty principle. This principle states if one would perform an experiment on any quantum object in order to determine its momentum p or its position x, the measurement accuracy of the one value comes at the expense of the accuracy of the other. As a consequence, it is impossible to simultaneously measure all the properties of a quantum object. Mathematically, things have been summarised in the next equation, containing the measurement errors of the two physical quantities px and x, and Planck’s constant, ¯h:

∆x∆px≥

¯ h

2 (1)

This equation has proven its correctness by countless applications of quantum theory. Einstein’s discomfort, therefore, was not aimed at the predictions of quantum mechanics, but at the actual meaning that was given to it by Bohr and his followers.8 _{In Physik und Philosophie from 1958, Heisenberg has}

plained Bohr’s position on the meaning of the uncertainty principle: “Any ex-periment in physics, whether it refers to phenomena of daily life or to atomic events, is to be described in terms of classical physics . . . Still the application of classical concepts is limited by the relations of uncertainty.”9 _{According to}

Bohr and Heisenberg, the classical world forms the starting point for any in-terpretation, since the range of human observation is limited within the use of classical concepts.

What if the uncertainties from the equation above do not only occur in the world of mathematics? According to Bohr and Heisenberg, exactly this is the case. The Copenhagen interpretation of quantum mechanics is built upon the assumption that the uncertainty principle is not inherent to quantum theory, but to nature itself. In other words, exact values for physical quantities in the microscopic quantum world simply do not exist. As a consequence, one is limited to the use of probabilities when describing objects at the quantum level. This statement raises deep questions about the way we have to think about

8_{The Copenhagen circle in the 1950s and 1960s particularly consisted of Niels Bohr, Werner}

Heisenberg, Wolfgang Pauli, Max Born and L´eon Rosenfeld.

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2 Interpreting Quantum Mechanics Before World War II

the nature of reality. Following the Copenhagen interpretation, reality is not embedded in the quantum object itself; it is just a consequence of our classical observations. Depending on the device used to measure some quantity of the quantum object, reality emerges in terms of classical phenomena. Bohr endorsed this view by introducing the concept of complementarity to the interpretation of quantum mechanics, which has been expressed by Heisenberg as follows: “By going from the one picture to the other and back again, we finally get the right impression of the strange kind of reality behind our atomic experiments . . . the knowledge of the position of a particle is complementary to the knowledge of its momentum.”10_{While one experiment may reveal an electron as having classical}

wave properties, another experiment would determine the electron to behave like a particle. In the end however, an electron is neither a particle nor a wave, since quantum objects only appear to have properties in the classical context of measurement.

Starting from the 1930s, several objections have been formulated to this in-terpretation of quantum mechanics. Einstein’s main objection focused on the supposed absence of an objective physical reality, due to the decisive role of mea-surement in Bohrian quantum mechanics. Remaining faithful to a deterministic worldview, Einstein thought that the lack of objectivity in the microscopic world exposed the incompleteness of quantum theory. For his colleague physicist Louis De Broglie, the need for an extension of the theory was beyond dispute: “Ac-tually this interpretation, by seeking to describe quantum phenomena solely by means of the continuous Ψ-function, whose statistical character is certain, logi-cally ends in a kind of subjectivism akin to idealism in its philosophical meaning, and it tends to deny the existence of a physical reality independent of observa-tion.”11 For other critics of Bohr and Heisenberg’s interpretation of quantum mechanics, it seemed crooked that the possibility to interpret quantum theory would depend on the a priori assumption of classical concepts. Besides, since the macroscopic world is entirely built up from microscopic parts, one would expect a possibility to derive the one from the other.

2.2 The Einstein-Podolsky-Rosen paradox

The 1935 paper from Albert Einstein, Boris Podolsky and Nathan Rosen, ‘Can Quantum Mechanical Description of Physical Reality be Considered Complete?’, includes one of the earliest and most profound objections to the Copenhagen in-terpretation. The authors demonstrated that Bohr’s interpretation of quantum mechanics would lead to a paradox, if it was assumed that “. . . every element of the physical reality must have a counterpart in the physical theory.”12 Ein-stein, Podolsky and Rosen briefly repeated this demand in what they called the criterion of reality: “If, without in any way disturbing a system, we can predict with certainty the value of a physical quantity, then there exists an

ele-10_{Ibid., p.18.}

11_{De Broglie, 1954, p.235.} 12_{Einstein, 1935, p. 777.}

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2.2 The Einstein-Podolsky-Rosen paradox

ment of physical reality corresponding to this physical quantity.”13In the case of quantum mechanics, the three physicists argued that the criterion of reality was in contradiction with with the assumption that the wave function provides a complete description of the physical reality of a corresponding system state.

In order to illustrate the contradiction, they introduced a situation consisting of two quantum systems (I and II) able to interact until the moment t = T . For t > T , i.e. any moment after the interaction between the two systems has stopped, the wave function of the composed system (Ψ) can be calculated using Schr¨odinger’s equation, i¯h∂ ∂tΨ(x, t) = [ −¯h2 2m∇ 2_{+ V (x, t)]Ψ(x, t)} ₍₂₎

Now, if one desires to determine some physical quantity of one of the two sys-tems, it is necessary to perform a measurement, since it is then not possible to use Schr¨odinger’s equation for the two systems separately. The wave function of the composed system Ψ(x1,x2) can be expressed in terms of the

eigenfunc-tions of the possible measurement outcomes on the first system, u1(x1), u2(x1),

u3(x1), .. and the eigenfunctions of the second system, ψ1(x2), ψ2(x2), ψ3(x2),

.. : Ψ(x1, x2) = ∞ X n=1 ψn(x2)un(x1) (3)

Einstein et al. explain how the wave function of the composed system is manip-ulated by a measurement: “Suppose now that the quantity A is measured and it is found that it has the value ak. It is then concluded that after the

measure-ment the first system is left in the state given by the wave function uk(x1), and

that the second system is left in the state given by the wave function ψk(x2)”

Now, if instead of A, another quantity was measured, this would have led to other eigenfunctions for both system I and system II. For example, the measure-ment of quantity B with possible eigenvalues b1, b2, b3, .. and eigenfunctions

v1(x1), v2(x1), v3(x1), would have resulted in an expression for the composed

wave function, containing other eigenfunctions for system II as well: Ψ(x1, x2) =

∞

X

s=1

φs(x2)vs(x1) (4)

Comparing equation 3 to equation 4 shows that in Bohrian quantum mechanics, it is possible to express the unique wave function Ψ(x1,x2) in two different, but

mathematically equivalent ways. It is here that the paradox comes into play, Einstein et al. argue: “We see therefore that as a consequence of two different measurements performed upon the first system, the second system may be left in states with two different wave functions.” It thus follows from the equations that the outcome of the measurement on system I influences the state of system II, while the two systems earlier had stopped interacting. Apparently, “it is

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possible to assign different wave functions to the same reality.”14 Clearly, the latter is in contradiction with the earlier proposed criterion of reality, stating that any element in physical reality should be represented by a mathematical counterpart in physical theory. Following this, Einstein, Podolsky and Rosen drew the conclusion that ”the quantum-mechanical description of physical real-ity given by wave function is not complete.”15_{The EPR paper finishes with an}

explicit invitation to future research on the foundations of quantum mechanics: “While we have thus shown that the wave function does not provide a complete description of the physical reality, we left open the question of whether or not such a description exists. We believe, however, that such a theory is possible.” The authors left it in the middle whether the complete description of quan-tum mechanics could be compatible with the yet existing theory of quanquan-tum mechanics.

For this study, which focuses on the Americanisation of European theoretical physics, it might be relevant to look at some responses to the EPR paper within both Western Europe and the United States. In the U.S., there was an immedi-ate reaction from Edwin Kemble.16_{In a letter to the editor of Physical Review,}

Kemble attacked the EPR paradox by pointing out a ‘fallacy’ in the argument. According to Kemble, “Einstein, Podolsky and Rosen argue that [the second system] cannot be affected by [a measurement on the first system] and must in all cases constitute ‘the same physical reality’, while “. . . whenever two systems interact for a short time there is a correlation between the subsequent behaviour of one system and that of the other.” Soon after his claim, Kemble realised that in his eagerness to point out the incorrectness of the EPR argument, he missed the fact that the wave function of the combined system in the EPR paradox was not unique.17 _{Kemble’s spirited though incorrect response to the EPR}

pa-per becomes comprehensible when one studies his own attitude to the meaning of quantum mechanics, as he has pointed out in The Fundamental Principles of Quantum Physics18. In this book, Kemble stressed that “the wave function is merely a subjective computational tool and not in any sense a description of objective reality.” According to Kemble, the interpretation of the wave func-tion fell beyond the scope of the theoretical physicist. Max Jammer has argued that Kemble’s view on quantum interpretations as irrelevant for physicists was influenced by the “operationalism of Bridgman, the positivism of Mach, and the pragmatism of Peirce”19_{. Kemble and his American contemporaries saw it}

as the physicist’s main duty ”to describe the experimental facts in his domain as accurately and simply as possible, using any effective procedure without re-gard to such a priori restrictions on his tools as common sense may seek to impose.”20 _{In section 4.1, the pragmatic attitude among American theoretical}

14_{Ibid., p. 779.} 15_{Ibid., p. 780.} 16_{Kemble, 1935.} 17_{Jammer, 1974, p. 191.} 18_{Kemble, 1937.} 19_{Jammer, 1974, p. 191.} 20_{Kemble, 1937.} 12

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2.3 The status of the Copenhagen interpretation

physicists will be further discussed.

In Europe, an important reaction to the EPR paper in Europe came from a man of whom it could be expected: Niels Bohr. Apart from rejecting the criterion of reality in the first place, Bohr pointed out that the concept of mea-surement was incorrectly treated by Einstein, Podolsky and Rosen: “the proce-dure of measurements has an essential influence on the conditions on which the very definition of the physical quantities in question rests.”21 _{Bohr countered}

Einstein’s attack by showing that in quantum mechanics, a measurement can-not be performed without disturbing the system measured upon. Apart from Bohr’s reaction, the EPR paper only gained a few dozen direct citations in the subsequent decades, indicating that European physicists were not that willing to contribute to the discussion as well.

2.3 The status of the Copenhagen interpretation

In general, the majority of physicists had not been involved in the prewar quan-tum interpretation debates. In fact, the discussion on the meaning of quanquan-tum theory had taken place among a select group of physicists. There were physi-cists who had made important contributions to the –mathematical– theory of quantum mechanics, but who refrained from the details of the philosophical de-bate between Einstein and Bohr. This can be demonstrated by a phrase from a 1948 letter from Einstein to Max Born,22_{commenting on the everlasting}

de-bate between him and the defenders of the Copenhagen interpretation: “I can quite understand why you take me for an obstinate old sinner, but I feel clearly that you do not understand how I came to travel my lonely way . . . it would be impossible for you to appreciate my attitude. I should also have great pleasure in tearing to pieces your positivistic-philosophical viewpoint. But in this life it is unlikely that anything will come of it.”23 _{A tone of frustration resounds}

from Einstein’s words. Apparently, Einstein felt that Born did not grasp his objections to the Copenhagen interpretation. According to Max Jammer, who interviewed Einstein in 1952 and 1953, Einstein “never abandoned the view that quantum mechanics, as presently formulated, is an incomplete description of reality.”24 Yet, he had to rely more and more on his own, as the acceptance of Bohr’s interpretation went on during the 1930s and 1940s. It seems that be-fore World War II, there has been a consensus among physicists that Bohr had pulled the longest straw in the discussion with Einstein: the vast majority of physicists accepted complementarity and the other features of the Copenhagen interpretation. Jammer has stated that most of these adherents were driven by a pragmatic attitude and, “impressed by the spectacular successes of quan-tum mechanics in all fields of microphysics, they were interested primarily in its

21_{Bohr, 1935.}

22_{The German physicist and 1954 Nobel laureate Max Born proposed the statistical}

inter-pretation of the wave function in quantum mechanics. It states that the squared absolute value of the wave function |Ψ(x, t)|2 _{is equal to its probability p.}

23_{As cited by Jammer, 1974, p. 188.} 24_{Ibid., p. 188.}

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applications to practical problems and its extension to unexplored regions.”25 The desire for exact results of the theory of quantum mechanics was indeed great among physicists of that time, acclaims the Dutch physicist Sander Bais: “. . . quantum theory went from one success to another. For example, the theory on condensed matter was fully understood all of a sudden . . . the possibility that somewhere deep down in the theory, there might still exist some fundamental issues regarding its interpretation, was dismissed quite easily. . . ”26

Despite several failed prewar attempts of Louis De Broglie to present a de-terministic hidden − variable theory, few physicists proposed alternative inter-pretations of quantum mechanics after the appearance of the EPR paper. But at a certain moment, the founder critics Einstein, Schr¨odinger and De Broglie were joined by a generation of young communist physicists from both Europe and the Soviet Union, who felt uncomfortable with the interpretation of quan-tum mechanics as it was.27 _{Reasoning from a materialistic world view, Soviet}

physicists were among the most fanatic opponents of Niels Bohr. Like Einstein, they did not oppose to quantum theory itself – which would be a hard thing to do, considering the accumulation of empirical evidence – but to the meaning that was given to it by Bohr and his men. Again, the loss of objective reality in the quantum world was situated in the centre of the dispute. According to Leninist tradition, all forms of reality can be reduced to the working of matter. This deterministic foundation of communist ideology prevented many Soviet physicists from following Bohr’s ideas on complementarity and the uncertainty of nature. The communist physicists tried to develop a causal interpretation of quantum mechanics, motivated by the conviction that Bohr’s interpretation of quantum mechanics was idealistic and positivistic.28 29 _{The introduction to a}

1952 article by the Soviet physicist Dimitri Blokhintzev reveals how the cards were on the table: “The present article is devoted to the unmasking of idealistic and agnostic speculations of [the Copenhagen] school on the basic problems of quantum mechanics . . . ”30 _{In line with Einstein’s view on quantum mechanics,}

Blokhintzev doubted the completeness of the theory. Furthermore, the critical spirit of the Soviet physicists was not restricted within the borders of the So-viet Union, it transcended to Western Europe as well. Especially among French Marxist physicists, the interest in the interpretation of quantum theory was revived by the communist interference in the debate. For example, Jean-Paul Vigier adopted Blokhintzev reproach that Bohr’s ideas on the interpretation of quantum mechanics were subjectivistic and idealistic.31 It has been suggested

25_{Ibid., p. 247.}

26_{Sander Bais is a Dutch theoretical physicist who has worked at universities in both the}

U.S. and Western Europe. Bais’ quotes have been taken from an interview between him and the author of this text in Amsterdam in May 2014.

27_{Camilleri, 2009a, p.34.} 28_Ibid.

29_{As had been the case with the infamous Lysenko affair in the 1920s. The Soviet}

agricul-tural specialist Trofim Lysenko invented a fallacious theory on genetics which suited the ideas of Marxism perfectly, and rejected those of Gregor Mendel.

30_{Blokhintzev, 1952, p.546.}

31_{The communist influence on the debate in Western European physics is described by}

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2.3 The status of the Copenhagen interpretation

by Mara Beller and Don Howard that the existence of the term Copenhagen interpretation provides a false idea of unity between the prewar physicists who have later become known as the advocates of the Copenhagen interpretation.32 In line with this view, Kristian Camilleri has stressed that the term Copen-hagen interpretation was an invention of the 1950s, first used by the postwar Soviet critics of Bohr’s interpretation of quantum mechanics. He has argued that the Copenhagen interpretation has not emerged simultaneously with quan-tum theory, but appeared much later in a political context: “the idea of unitary interpretation only emerges in the 1950s in the context of the challenge of Soviet Marxist critique of quantum mechanics.”33

It would not be correct to consider ideology as a decisive factor for all oppo-nents of quantum interpretation. In France, there had been a continuous spirit of criticism, driven by the Frenchman and first-hour-critic Louis de Broglie, who re-garded the Copenhagen interpretation as subjectivistic himself.34_Furthermore,

not all communists regarded themselves as opponents from the Copenhagen in-terpretation. For example, the Belgian physicist L´eon Rosenfeld belonged to the adherents of the Copenhagen interpretation, while he was as a Marxist. As we will see in the next chapter, much critique during the 1950s and 1960s was provided on non-ideological grounds.

Camilleri on the basis of historical work of Andrew Cross, documenting the 1950s support in France or the Soviet critique on quantum mechanics. Camilleri, 2009a, p. 37.

32_{Beller, 1999; Howard, 2004.} 33_{Camilleri, 2009b, p. 29.} 34_{Camilleri, 2009a, p. 38.}

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3 Postwar Interpretations of Quantum

Mechan-ics

In the first few decades after World War II, several attempts have been made to change the mainstream interpretation of quantum mechanics. But in terms of scientific impact, none of these postwar alternatives has ever come close to the Copenhagen interpretation. Three contributions from the period 1950–1970 will be presented in this chapter. First, the alternative interpretations of David Bohm and Hugh Everett III will be discussed in both physical and historical context. By doing this, it should become clear how the postwar scientific cli-mate has influenced the success of these interpretations. After this, John Bell’s theorem from 1964 will be discussed. These case studies serve as a prelude to chapter 4, where it will be investigated whether the Americanisation of physics after World War II has reduced the interest for alternative interpretations of quantum mechanics.

3.1 Bohm and the hidden variable interpretation of quantum

me-chanics

Some physicists were triggered by the EPR paper’s conclusion that the theory of quantum mechanics did not cover a complete description of reality. They realised that if the sole existence of a probability distribution for individual particles could be avoided, determinism and causality would be restored in the description of nature on the smallest scale. In general, the opponents of the Copenhagen interpretation desired “to construct a theory to explain the be-haviour of individual systems from the statistics of their ensembles”, says Max Jammer.35 So-called hidden variable theories were among the candidates for extending the theory of quantum mechanics. The motivation for these kind of theories was the alleged existence of dynamical variables hidden from sight, continuously correlating the states of different systems. Such variables would fix the EPR paradox, since they would retain the connection between system I and system II. Although Einstein’s criticism can be regarded as a direct inspi-ration for the emergence of interpretations including hidden variables, he did not work on them himself. One of the Copenhagen criticasters who did believe in a solution provided by the use of hidden variables, was a young American physicist called David Bohm. He took over the baton from Louis de Broglie in the 1950s.36 _{In this section, the work of David Bohm will be broadly outlined,}

and placed in the perspective of this study.

Bohm dropped his ideas on hidden variables for the first time in his 1951 book Quantum Theory, defining them as “a further set of variables, describing the state of new kinds of entities existing in a deeper subquantum mechanical

35_{Jammer, 1974, p. 253.}

36_{Louis de Broglie proposed the pilot-wave interpretation of quantum mechanics in 1927. It}

serves as the first example of a hidden variable theory. Until the publication of his own hidden variable theory in 1952, David Bohm was not aware of the existence of this interpretation.

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3.1 Bohm and the hidden variable interpretation of quantum mechanics

level and obeying qualitatively new types of individual laws.”37While the major part of Quantum Theory had been in line with Bohr’s interpretation of quan-tum mechanics, Bohm stated in the final chapters of his book that quanquan-tum theory “should be able to describe the process of observation itself in terms of the wave functions of the observing apparatus and those of the system under observation.”38 _{This claim shows Bohm’s discomfort with the exclusive role of}

measurement in the Copenhagen interpretation. In subsequent years, Bohm developed his own hidden variable interpretation of quantum mechanics, as published in the 1952 article Suggested Interpretation of the Quantum Theory in Terms Of ”Hidden Variables” I, II.39

In Bohm’s interpretation of quantum mechanics, both the wave and particle aspects of matter are regarded as real. The wave function Ψ represents an actual physical constitution and can be written down as:

Ψ = R expiS ¯

h (5)

In this equation, R and S are both real and “codetermine each other”.40

Ap-plying this expression for the wave function to the Schr¨odinger equation, Bohm concluded that the momentum of a quantum system must equal

p = ∇S(x) (6)

By defining exact values for momentum and position, Bohm in fact provided quantum objects with definite trajectories. In contrast to the Copenhagen inter-pretation, Bohm rejected the complementarity of the wave and particle prop-erties of a quantum object, together with the idea that the nature of reality on the smallest scale is built on merely probability: “[In this alternative inter-pretation], quantum mechanical probabilities are regarded as only a practical necessity . . . and not as a manifestation of an inherent lack of complete de-termination in the properties of matter at the quantum level.”41 _{In his paper}

Bohm thus suggested a deterministic interpretation to quantum theory, by in-troducing precise values for both p and x as hidden variables to Schr¨odinger’s equation. After a thorough comparison between the “usual interpretation” of quantum mechanics, and his own “alternative interpretation”, Bohm endorsed that “as long as we assume that ψ satisfies Schr¨odinger’s equation, that v = ∇S(x)/m,42_{and that we have a statistical ensemble with a probability density}

equal to |Ψ(x)|2, our interpretation of quantum theory leads to physical results that are identical with those obtained from the usual interpretation.”43 _.

While it is beyond the scope of this study to present Bohm’s complete deriva-tions mathematically, it should be clear that it was Bohm’s goal to prove that

37_{Bohm, 1951.} 38_{Ibid., p. 583.}

39_{Bohm, 1952a; Bohm, 1952b.} 40_{Bohm, 1952a, p. 170.} 41_{Ibid., p. 166.}

42_{v and m are the velocity and the mass of the quantum system, respectively.} 43_{Bohm, 1952a, p. 179.}

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3 Postwar Interpretations of Quantum Mechanics

the inclusion of hidden variables could be consistent with the theory of quantum mechanics. He claimed that the same experimental results could be obtained, when treating the actual values of position x and momentum p as extra variables in the yet existing equations of quantum mechanics: “In our interpretation, we assert that the at present ‘hidden’ precisely definable particle positions and momenta determine the results of each individual measurement process, but in a way whose precise details are so complicated and uncontrollable, and so little known, that one must for all practical purposes restrict oneself to a sta-tistical description of the connection between the values of these variables and the directly observable measurements.”44 _{Bohm thus stressed that the}

uncer-tainty relation and the squared wave function are indeed necessary in order to gain experimental results from quantum mechanics. But, he endorsed that the existence of real values for position and momentum on the quantum level do not need to be excluded, if these properties are treated as hidden variables. Be-sides, Bohm appointed that his alternative interpretation of quantum mechanics might solve some of the practical problems of quantum mechanics, since “the usual mathematical formulation seems to lead to insoluble difficulties when it is extrapolated into the domain of distances of the order of 10−13 cm or less.”45

Bohm’s publication did not have the impact that he had hoped for. More-over, there exists an image that Bohm’s causal interpretation has largely been ignored by his colleague physicists.46 _{Some say the minor impact of Bohm’s}

hidden variable proof was caused by the fact that Louis de Broglie had al-ready proposed –and given up– a similar contribution to quantum mechanics 25 years earlier. By presenting some of the early objections to Bohm’s theory from the Copenhagen circle, Wayne Myrvold has found that “[Einstein, Pauli and Heisenberg] regarded the discussion as a resumption of the discussion of de Broglie’s theory that had taken place a quarter century earlier.”47_{On the other}

hand, Olival Freire jr. has argued that cultural factors in postwar theoretical physics were the main cause of the poor reception of Bohm’s interpretation.48

Freire illustrates the resistance Bohm had to endure from the worldwide physics community by a quote from the American physicist Isidor Rabi, who argued that “the causal interpretation gives us no line to work on other than the use of the concepts of quantum theory.”49In this judgement, one might be able to recognise a pragmatic attitude towards the development of theoretical physics. In a sense, Rabi’s words are reminiscent of those of Kemble when commenting on the EPR paradox. Rabi wanted to see a situation in which “the things turn around.” Apparently, the preservation of an objective reality at the quantum level was not sufficient for him. The Belgian physicist L´eon Rosenfeld went further in rejecting Bohm’s theory, writing the following to Bohm: “I shall not enter into any controversy with you or anybody else on the subject of

comple-44_{Bohm, 1952b, p. 183.} 45_{Bohm, 1952a, In abstract.} 46_{Cushing, 1994.}

47_{Myrvold, 2003, p. 7.} 48_{Freire Jr., 2005.} 49_{As cited by ibid., p. 12.}

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3.1 Bohm and the hidden variable interpretation of quantum mechanics

mentarity, for the simple reason that there is not the slightest controversial point about it.”50According to Freire, the absence of “predictions not foreseen by the usual quantum mechanics . . . reinforced the derogatory label of ‘philosophical‘ stuck on them by their opponents”51. He claims that this philosophical label has damaged the status of interpreting quantum mechanics among physicists, especially since Bohm’s publication.

When looking back at this period himself, Bohm tried to explain the lack of encouraging response by the changing attitude of physicists towards the sig-nificance of physical theories: “Physics has changed from its earlier form, when it tried to explain things and give some physical picture. Now the essence is regarded as mathematical. It’s felt the truth is in the formula’s.” and “There is a long history of belief in quantum mechanics, and people have faith in it. And they don’t like having this faith challenged.”52 _{Both Freire’s conclusion}

and Bohm’s quote indicate that in the poor reception of Bohm’s interpreta-tion, the scientific culture in postwar theoretical physics has played a role in accepting the new interpretation. Moreover, Bohm’s description of physics as a mathematical activity subscribes to a pragmatic attitude. As we have seen in section 2.2, and will see in section 4.1, this kind of attitude is often associated with American scientific ideals. Again, it seems worth asking if the American-isation of physics might have influenced the attitude among physicists towards alternative interpretations of quantum mechanics.

50_{Letter from Rosenfeld to Bohm, 20 May 1952} 51_{Freire Jr., 2005, p. 24.}

52_{Interview with David Bohm, conducted by F. David Peat and John Briggs, originally}

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Figure 3: In 1954, Everett (right) and Bohr (middle) met at Princeton Univer-sity. In 1955, Everett started to write down his own ideas on the interpretation of quantum mechanics.

3.2 The relative-state interpretation of Hugh Everett III

Any person who has read the previous two sections and who is well informed in the debates between David Bohm and the Copenhagen circle, might wonder why it has not yet been mentioned that David Bohm was a communist. In-deed, Bohm was banned from his position at Princeton University in the middle of his work on the hidden variable interpretation of quantum mechanics, be-cause of McCarthyism in the United States. However, there is no proof that the moderate reception of Bohm’s interpretation had anything to do with his communist symphathies.53 _{Besides, it is of interest that David Bohm was not}

the only young physicist in the 1950s who was confronted with struggles while calling the Copenhagen interpretation into question. To illustrate this, the com-ing section will cover the context and content of Hugh Everett III’s alternative interpretation of quantum mechanics.

In the beginning of the 1950s, a group of physicists concentrating at Prince-ton University realised that the concept of measurement in quantum theory demanded a serious modification if it was to be united with the theory of gen-eral relativity.54 _{It was the thought that both observer and measuring device}

53_{Freire Jr., 2005.} 54_{Jammer, 1974.}

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3.2 The relative-state interpretation of Hugh Everett III

should be part of a total continuous quantum system. In such a description of quantum events, there would be no longer a need for the ‘collapse’ of the wave function as a result from measurement. Hugh Everett III, a physics student at Princeton at that time, was triggered by the possibility to get rid of the measurement problem. Under supervision of John Archibald Wheeler, Everett started on his dissertation in 1955, which eventually resulted in the “relative state” formulation of quantum mechanics. Like David Bohm, Everett presented his interpretation as an addition to the consisting theory of quantum mechanics, or as a meta theory. In the opening sentences of his paper, it reads as follows: “The aim is not to deny or contradict the conventional formulation of quantum theory, which has demonstrated its usefulness in an overwhelming variety of problems, but rather to supply a new, more general and complete formulation, from which the conventional interpretation can be deduced.”55_{It is worth noting}

that the cautiousness with which Everett confronts the conventional interpre-tation might have been the result of a laborious struggle between him and the key defenders of the Copenhagen interpretation in the preceding years. Before outlining the difficulties that Everett met during the proposal of his ideas, the content of his interpretation will be presented.

In his 1957 paper called Relative State Formulation of Quantum Mechanics, Everett addressed the problem of the conventional interpretation of quantum mechanics, by referring to Einstein’s theory of general relativity: “How is one to apply the conventional formulation of quantum mechanics to the space-time geometry itself? The issue becomes especially acute in the case of a closed uni-verse.” Furthermore, Everett endorsed the need for the inclusion of the observer into the formulation of quantum mechanics: “How are a quantum description of a closed universe, of approximate measurements, and of a system that contains an observer to be made? These questions have one feature in common, that they all inquire about the quantum mechanics that is internal to an isolated system. No way is evident to apply the conventional formulation of quantum mechanics to a system that is not subject to external observation.” In his paper, Everett proposed to “regard pure wave mechanics as a complete theory.” In his eyes, it was possible to formulate quantum mechanics in a way that the wave function “obeys a linear wave equation everywhere and at all times supplies a complete mathematical model for every isolated physical system without exception.”56 In contrast to the Copenhagen interpretation, Everett did not treat the classi-cal world as a necessary conduit for conceiving reality. Instead of centralising the concept of measurement in quantum mechanics, he proposed that the wave function served as the basic physical entity. Thus, in his relative state formu-lation of quantum mechanics, Everett centralised the mathematical concepts of the theory itself.

Everett presented his interpretation of quantum theory by constructing the concept of observation and the relative state mathematically. To start with, Everett introduced an equation for the quantum state of a composite system S,

55_{Everett, 1957, p. 454.} 56_{Ibid., p. 455.}

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consisting of the complete orthonormal sets of the states of its subsystems S1

and S2: ψS =X i,j aijξiS1η S2 j (7)

Subsequently, Everett assigned unique relative states to the second subsystem, corresponding to all possible states of the first subsystem. By choosing ξk as

the state for the first system, the corresponding relative state, ψ(S2; relξk, S1),

equals

ψ(S2; relξk, S1) = Nk

X

j

akjηjS2 (8)

with Nk a normalisation constant. By combining equation 7 and 8, the state of

the composite system can be represented as “a single superposition of pairs of states, each consisting of a state from the basis {ξi} in S1and its relative state

in S2: ψS =X i 1 Ni ξS1 i ψ(S2; relξk, S1) (9)

Everett concluded from the correlation of the subsystems that, on mathemat-ical grounds that emerged from quantum theory itself, “we are faced with a fundamental relativity of states, which is implied by the formalism of composite systems.”57

After this, Everett continued by enclosing a mathematical description of the process of observation in the theory quantum mechanics. According to Everett, there is a “task of making deductions about the appearance of phenomena to observers which are considered as purely physical systems and are treated within the theory.”58 Everett ascribed the following state function to an observer 0:

ψ0[A, B, . . . , C] (10) where A, B, . . . , C represent the events that the observer has experienced. After the observation of quantity A and B in S1 and S2 respectively, the final total

state can be written as X i,j aibjφSi1η S2 j ψ Sn_ψ0_{[. . . , a} i, bj] (11)

After he had formulated the observation process in quantum mechanics in this way, Everett started searching for the interpretation of these final total states. Everett claimed that “in each element of the superposition . . . the observer-system state describes the observer as definitely perceiving that particular sys-tem state.”59_{Subsequently, he determined that “throughout all of a sequence of}

observation here is only one physical system representing the observer, yet there

57_{Ibid., p. 456.} 58_{Ibid., p. 457.} 59_{Ibid., p. 459.}

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3.2 The relative-state interpretation of Hugh Everett III

is no single unique state of the observer. . . ”60Everett concluded that, since all the possible states are part of the total system, the observer “branches” into different states after every observation. Mathematically, each branch represents a different observation and the corresponding eigenstate for the object-system state. Because the everlasting emergence of these branches caused by the pro-cess of observation suggests the co-existence of an infinite number of parallel realities, Everett’s interpretation is often referred to as the many worlds or multiverse interpretation of quantum mechanics.

In the discussion of the paper, Everett compared the relative state formu-lation to the Copenhagen interpretation. In his opinion, the inclusion of the observer and the rejection of a priori probabilities within the mathematical for-mulation of quantum mechanics served important advantages: “Objections have been raised in the past to the conventional or “external observation” formulation of quantum theory on the grounds that its probabilistic features are postulated in advance instead of behind, derived from the theory itself. We believe that the present “relative-state” formulation meets this objection, while retaining all of the content of the standard formulation.”61_{Still Everett believed, like Bohm,}

that a probabilistic interpretation could serve “as an aid to make practical pre-dictions.”

Wheeler, who was still in contact with Niels Bohr after having worked with him in the 1930s, hoped to be able to unite the Copenhagen interpretation with Everett’s meta-consideration of quantum mechanics. As a follower of Bohr, Wheeler wished to make sure that the principle of complementarity would be maintained.62Therefore, he emphasised that Everett’s interpretation should be seen as an extension and not as a refutation of the Copenhagen interpretation.63 To convince the Copenhagen school of Everett’s revolutionary ideas, Wheeler visited Copenhagen already in 1956. However, Bohr and his assistants Aage Petersen and Alexander Stern refused to accept Everett’s ideas, while the by now familiar L´eon Rosenfeld also expressed aversion towards Everett’s theory.64

The objections of Stern and Rosenfeld ranged from accusing Everett from “the-ology” to “axiomatising any part of physics”, respectively. With the help from Wheeler, Everett eventually visited the Niels Bohr Institute in Copenhagen in 1959 himself . For Everett, this visit was a last resort to convince Niels Bohr of the advantages that came with his alternative formulation of quantum me-chanics. However, Bohr rejected Everett’s ideas once again and so the meeting ended in disappointment.65_{In the end, Everett made the choice to leave physics}

60_{Ibid., p. 459.} 61_{Ibid., p. 462.} 62_{Freire Jr., 2005.}

63_{Everett in his turn seemed not so satisfied with the prevailing interpretation of quantum}

mechanics. Byrne, 2007 has quoted a letter from Everett to Bryce deWitt, in which he claims that “the Copenhagen interpretation is hopelessly incomplete because of its a priori reliance on classical physics . . . as well as a philosophical monstrosity with a reality concept for the Macroscopic world and denial of the same for the microcosm.”

64_{The described events surrounding Wheeler’s efforts in Copenhagen have been presented}

by Olival Freire jr., Freire Jr., 2005

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and to go work for the U.S. military Pentagon.66

More than ten years later, Bryce deWitt revived the attention to Everett’s ideas.67 _{It seems that before this time, any possible success for Everett’s}

rel-ative state formulation of quantum mechanics was frustrated by the defenders of the Copenhagen interpretation. The fate of Everett’s interpretation shows remarkable similarities with a story that we have heard just before. Or, in Freire’s words: “blocked by the Copenhagen monocracy, Everett’s ideas had a fate similar to Bohm.”68 Indeed, the status of the Copenhagen interpretation seems to have obstructed the success of Bohm’s and Everett’s interpretations. However, one can ask himself the question why Everett tried to plug his ideas in Copenhagen in particular. How come that there were no other places, suitable for discussing new ideas on the interpretation of quantum mechanics? Perhaps, this had something to do with the general values in postwar physics. What developments were occupying the minds of physicists, so that they were not able to react on the proposals of David Bohm and Hugh Everett III? While the Copenhagen monocracy explains the poor reception among the Copenhagen cir-cle, it fails to explain why these few physicists had been determining the fate of alternative interpretations of quantum mechanics before and after the Second World War.

3.3 The 1960s: Bell’s theorem and Zeh’s difficulties

Some other physicists were indeed triggered by the alternative interpretation of Bohm and Everett. In this section, we will see how the work of David Bohm inspired John Bell to work further on hidden variables and quantum mechanics. Also, the solitary position of Dieter Zeh, in whose work on decoherence the spirit of Everett dwells, will be highlighted. It is interesting to see whether these physicists had to face the same kind of difficulties as their predecessors in the 1950s.

When the Northern Irish John Stewart Bell was a physics student in Birm-ingham, during the beginning of the 1950s, he became excited with Bohm’s ideas on hidden variables. Despite Bell’s early interest in the foundations of quantum mechanics, it was not until the start of the 1960s that he fully dedicated to the topic. In the meantime, he had successfully worked at CERN69_{on accelerators}

and high-energy physics.70 _{While theories on hidden variables had been in his}

head since 1952, his breakthrough paper on the EPR paradox appeared only in 1964.71 _{In this text, Bell showed that the EPR assumptions of locality and}

realism were incompatible with the theory of quantum mechanics and that no

66_{One should be careful to explain Everett’s departure from American physics by the poor}

reception of his interpretation. As will become evident in chapter 4, a career like Everett’s was definitely no exception for American physicists in the 1950s.

67_{Freire Jr., 2005.} 68_{Ibid., p.31.}

69_{Section 4.4 provides a discussion on the role of the United States in the establishment of}

CERN

70_{Freire Jr., 2009, p. 283.} 71_{Bell, 1964.}

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3.3 The 1960s: Bell’s theorem and Zeh’s difficulties

Figure 4: Schematic presentation of the EPR experiment by John Bell. Illus-tration from Griffiths, 2005, p. 424

local hidden variable theory could ever reproduce all quantum mechanical pre-dictions. Bell reached this conclusion by working out an amplified version of the EPR experiment, which we will now briefly recapitulate.

The experiment included two independently rotating detectors, measuring the spin of an electron in the direction of the unit vector a, and the spin of a positron in the direction of the unit vector b (see fig. 4). The positron and electron were provided by the decay of a neutral pi meson:

π0→ e+_{+ e}− ₍₁₂₎

In the case of arbitrary positions of the detectors, the quantum mechanical prediction of this experiment states that the average value P of the spin product must equal

P (a, b) = −a · b (13)

Bell showed in his paper that this result can not be obtained from any local hidden variable theory. He came to this conclusion by supposing that the com-plete system state would be given by such a hidden variable, for example λ. Bell worked out, after introducing the assumption of locality and a third unit vector c, that the inclusion of hidden variables in the experiment results in the following expression for the average values of a, b and c:72

| P (a, b) − P (a, c) |≤ 1 + P (b, c) (14) Subsequently, Bell compared this result to the earlier presented prediction of the experiment by quantum mechanics. If, for instance, the unity vectors lie in a plain, making angles of 45◦ with each other, it can be computed using the quantum mechanical equation 13 that P (a,b) = 0 and that P(a, c) = P(b, c) = -0.707. Plugging these numbers into equation 14, inevitably leads to an incorrect expression.

Bell’s result meant that either the existence of local hidden variable exten-sions of quantum theory should be excluded, or that quantum theory on the whole is incorrect. Considering the amount of empirical evidence in favour of quantum theory, the latter option seemed impossible. Furthermore, Bell’s work

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provided experimentalists with the possibility to contribute to the foundations of quantum mechanics. Suddenly, the controversies in quantum mechanics could not be regarded as solely philosophical disputes, but as scientific, experimentally solvable questions as well.

Though Bell’s theorem was a breakthrough in the development of the inter-pretation of quantum mechanics, we are left with a few pressing questions in the light of this study. For instance, how come that Bell waited until 1964 before focusing entirely on the problems surrounding hidden variables? In 1952, after the publication of Bohm’s previously discussed article, he had already admitted in correspondence with Wolfgang Pauli that he was attracted by Bohm’s ideas on hidden variables. Also, he had been suspecting since that time that Von Neumann’s proof against their existence was not waterproof.73 What were the factors that thwarted an earlier involvement of Bell in the discussion? Also, when Alain Aspect was planning to take Bell’s experiment into the laborato-ries in 1975, Bell remained cautious about Aspect’s perspectives in physics, as Freire has pointed out: “. . . Bell asked him, “Have you a permanent position?” After Aspect’s positive answer, Bell warmly encouraged him to publish the idea, but warned him that this was considered by most physicists a subject for crack-pots.”74_{Apparently, 10 years after Bell’s successful publication, the possibilities}

for physicists to get involved in the foundations of quantum mechanics were still restricted. It is exemplary for the bad status of the topic that the physicist who earlier tried to perform an experiment on Bell’s theorem, John Clauser, never got a permanent position in physics.75

Maybe the case of the American John Clauser can be partly explained due to the poor state of U.S. physics during the beginning of the 1970s. However, there are also examples of European physicists who had difficulties in getting involved in the foundations of quantum mechanics during the end of the 1960s. Another physicist who started to focus on the interpretation of quantum me-chanics in the late 1960s was Heinz-Dieter Zeh. Particularly in this period, the Copenhagen monocracy was arriving in its final days.76 _{One would therefore}

suggest that the road was free for anyone who wanted to tackle the interpreta-tional questions surrounding quantum theory. In 1967, Zeh directed his research towards the measurement problem in quantum mechanics, drawing the conclu-sion that macroscopic entities in quantum mechanics could not be described as closed systems.77 _{Zeh’s requests to get his first paper on the measurement}

problem published were repeatedly turned down. Next to this, Zeh had to deal with a rather offensive climate towards his new interest at his own Heidelberg University. Looking back at this early stage of his career in 2006, Zeh empha-sised that initially, any work on the foundations of quantum mechanics had to be done behind the scenes: “. . . it was absolutely impossible at that time to

73_{Freire Jr., 2005, p. 32.} 74_{Freire Jr., 2009, p. 284.} 75_{Kaiser, 2011.}

76_{Niels Bohr died on 18 November 1962, and there was an unsolved conflict between Eugene}

Wigner and L´eon Rosenfeld, according to Freire Jr., 2009

77_{Ibid., p. 281.}

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3.3 The 1960s: Bell’s theorem and Zeh’s difficulties

discuss these ideas with colleagues, or even to publish them. An influential Heidelberg Nobel prize winner frankly informed me that any further activities on this subject would end my academic career!”78Just as the theories of Bohm and Everett gained attendance decades after their first publication, Zeh’s ideas became successful only in the 1980s. Zeh has subscribed the unfavourable atmo-sphere for making new contributions to the interpretation of quantum mechanics before this time to the authority of the Copenhagen circle. In 1980, he wrote to John Wheeler that he had “always felt bitter about the way how Bohr’s author-ity together with Pauli’s sarcasm killed any discussion about the fundamental problems of the quantum.”79 _{It is remarkable that Zeh has repeatedly referred}

to the Copenhagen interpretation as “irrational” and “pragmatic”, stating that his theory of decoherence, which reached maturity during the 1980s, “is an at-tempt to replace the pragmatic irrationalism that is common in quantum theory textbooks (complementarity, dualism, fundamental uncertainty etc.).”80

The success of Bell’s theorem in the early 1960s, while the Copenhagen monocracy was still intact, and the difficulties of Zeh in the late 1960s, while the Copenhagen circle was falling apart, once again indicate that it is worth studying if there were other factors in physics determining the success of new contributions to the interpretation of quantum mechanics.

78_{Zeh, 2006, p. 4.}

79_{H.D. Zeh to J.A. Wheeler, 20 October 1980, as quoted by Freire Jr., 2009} 80_{Zeh, 2006, p. 18.}

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4 The Construction of Transatlantic Physics

We have seen in the previous section that it was not evident for a postwar physi-cist to focus on the foundations of quantum mechanics. For several times, the dismissive attitude of physicists towards the interpretation of quantum mechan-ics has been associated with pragmatism. If indeed a pragmatic attitude was prevailing among postwar physicists in both Europe and America, then where does this attitude come from? Should pragmatism be regarded as inherent to physics in general, or has it perhaps originated from another source? In dealing with this question, the remaining part of the text concentrates on the effects of American interference in European physics, and the consequences for the status of interpreting quantum mechanics. First, American postwar physics in general and its attitude towards the foundations of physics in particular will be charac-terised. After this, the question is raised if and how American values and ideas were transferred to European physics in the 1950s and 1960s.

4.1 A new world leader in physics

After the Second World War, the United States was strongly developing on both industrial and scientific level, while Europe was left behind with the challenges of reconstruction after the war. In line with the enormous inequalities between Europe and the United States at this time, the political and economic power shifted towards the latter. Fitting this new order, the United States took over Western Europe’s role as the world leader in physics as well. Hence, Europe was no longer the centre of theoretical physics after World War II. A non-economical factor that contributed to the transfer of scientific power from Europe to the United States, can be illustrated by Albert Einstein’s relocation to Princeton University in 1933. In subsequent years, many physicists followed his example and resumed their careers in the United States, where they were safe for the rise of fascism, holding Europe in its grip.81_{Before World War II, the arrival of}

refugee scientists from Europe caused a quick building up of scientific expertise in the United States. Next to this impulse, American physics had already made a qualitative leap during the interbellum. According to Schweber, this was the result of financial support of the Carnegie, Guggenheim and Rockefeller founda-tions, on the request of America’s leading experimental physicists on universities such as Harvard and Caltech. “Support from these foundations enabled these elite universities to expand their activities in physics”, says Schweber.82 _The

developments in the above enabled the United States to conquer its position as the world leader in physics already before World War II.

When European physicists arrived in the United States, they would note that business was regulated in quite a different way than in Europe. Schweber has put the pragmatic attitude of the typical American scientist in contrast to the more philosophically inclined European physicist. First, Schweber described the

81_{Kaiser, 2011, p.15.} 82_{Schweber, 1986, p. 56.}

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4.1 A new world leader in physics

difference between American and European universities in terms of their internal organisation. In the United States, all physicists, both theoretical and exper-imental, were to be found in the same institutions. “This integration of theo-reticians and experimentalists under one roof, a reflection of American demo-cratic aspirations, molded the empirical, pragmatic, instrumentalist character of American theoretical physics.”83 _{As a consequence, theory and experiment}

were never separated from one another, while in Europe the separation between theory and experiment was the most common thing. Schweber also appointed the authoritarian structures of European universities, and puts it in contrast to the omnipresent democratic spirit and variety at American institutions: “In addition to democracy, American departments had variety: whereas in German universities, theoretical physicists and experimental physicists usually occupied separate institutes, each directed by a single professor who controlled its ac-tivities, in the United States theoreticians not only shared a department with experimentalists, but were also trained in large part by them.”84

While Schweber considers the connection between theory and experiment as a symbol of American pragmatism, one could wonder if either the typical Amer-ican or European scientist still existed in the years after World War II. Next to the arrival of refugee scientists from Europe in the United States, some of the most influential American physicists around World War II had been partly educated at universities in Europe. For instance, Robert Oppenheimer and Isidor Rabi went to Western Europe as young men. During their later Ameri-can careers, they used the knowledge and methods they had gained overseas.85

Considering these internationally minded developments, one would expect the two cultures of physics to have merged during this period. Schweber denies that this was the case, by stating that American physics was already highly developed at the time the refugee physicists arrived, and that the scarcity of positions at universities forced the Europeans to fit in the American system, rather than vice versa. “Refugee theoreticians oriented toward the prevailing Anglo American empirical tradition found university positions more readily and integrated more easily than their more philosophical fellows”, he claims.86

Still, it seems that the contrast between the pragmatic American physicist and the philosophical European physicist blurs among those who were involved in the interpretation of quantum mechanics after the Second World War. For instance, the distinction between the American pragmatic - and the European philosophical physicist cuts no ice when one studies the case of Niels Bohr. On the one hand, Bohr was one of the most philosophically engaged scientists of his era, and thus satisfies the European stereotype.87_{. When structuring his}

own institute however, Bohr was following the American example very closely, emphasising the need for an integration of theory and experiment (see section 4.4). As another example, this is a 1938 judgement from the American

physi-83_{Ibid., p.58.} 84_{Ibid., p.74.} 85_{Ibid., p.74.} 86_{Ibid., p.80.}

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4 The Construction of Transatlantic Physics

cist John Slater on the value of interpreting physical theories: “. . . questions about a theory which do not affect its ability to predict experimental results correctly seem to me quibbles about words, rather than anything more substan-tial, and I am quite content to leave such questions to those who derive some satisfaction from them.”88 In a first consideration, this empiricist exclamation might be regarded as a typical form of American pragmatism. But as we have seen in section 3.3, mentioning the difficulties of Heinz-Dieter Zeh, this opinion resounded in postwar European institutions for theoretical physics as well.

4.2 American physics and the Cold War

Studying the character of postwar American physics further, it is relevant that there were some factors from American society that influenced the character of American physics in the 1950s and 1960s. During the Cold War, the bond between American physics and society was reflected in the realisation of labora-tories dedicated to the development of military equipment. It was in these places that the pragmatic spirit and the unity of theory and experiment in American physics worked best, argues Schweber: “Many factors were responsible for the success of the war-time laboratories . . . but surely one of the key factors was the symbiotic relationship that had existed between theoreticians and experimen-talists, their shared pragmatism, the ease with which they could communicate and collaborate.”89 _{The embedding of American physics in society can also be}

illustrated by displaying the development of the number of American graduat-ing physicists through the 20th century. Figure 5, taken from a 2002 study of David Kaiser, shows the number of physics PhD’s at U.S. research institutions. The graphic marks two moments of dramatic increase, which can be explained by taking note of the developments in American society that related to the Cold War. The first boost for the number of physicists took place directly af-ter World War II, parallel to the start of the Cold War. Kaiser has calculated that “between 1945 and 1951, the number of physics Ph.D’s awarded annually by U.S. institutions doubled every 1.7 years”.90 _{After five years of temporary}

stabilisation, caused by the lack of office space, the increasing number of Ph.D’s resumes from the beginning of the 1960s. In the light of the arms race against the Soviet Union, it was important to train as much physicists as possible and to produce as much military equipment as might be necessary. Triggered by hostile events such as the surprise launch of Sputnik rocket in the Soviet Union in 1957 and the supposed outnumbering of American physicists by their Soviet associates, there was “. . . a renewed and widespread emphasis on the need for improving the quantity and quality of highly trained scientists and engineers both at home and abroad.”91 In figure 5, the American fear of Communism is illustrated by the second peak in the 1960s.

This political interference had an impact on the development of both research

88_{Slater, 1938.}

89_{Schweber, 1986, p. 92.} 90_{Kaiser, 2002, p. 136.} 91_{Krige, 2008, p. 192.}

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4.2 American physics and the Cold War

Figure 5: From Kaiser, 2002, p. 135: “Total number of Ph.D’s in physics granted each calendar year by U.S. institutions, 1890-1979. Based on data in Adkins (ref. 7), 278-281, and National Research Council (ref. 9), 79.

and education in American physics. After World War II, physicists were first of all needed to “maintain the West’s scientific and technological supremacy”, says John Krige.92_{By developing industrial and military tools, they could help}

Amer-ican society forward. In addition to this analysis, David Kaiser has stressed that along with the increasing importance of training physicists, the style of educa-tion in American universities was affected as well. Fitting the generally assumed pragmatical tradition of American physics, and parallel to the tendency to fo-cus preferably on industrial and military benefits, the education programme of physics became mainly dedicated to skills and less to wisdom. In reaction to the growing demand for physicists, American universities were confronted with an overflow of students. According to Kaiser, “the tremendous, unprecedented demographic shift helped to drive a pedagogical emphasis upon efficient, repeat-able – and thereby trainrepeat-able – techniques of calculation, .. solidifying a prewar instrumentalist trend.”93

Subsequently, Kaiser has posed the question if the Cold War context has influenced the research agendas of American physicists as well. Kaiser describes that for young physicists at Berkeley, the guidance of graduate students be-came of higher importance than their own theoretical work. Exemplary is a quote of Raymond Birge, the head of Berkeley’s physics department at Berke-ley, explaining the forced departure of a young physicist: “In general [he] is interested in field theories of a rather abstract nature. It is somewhat the

92_{Ibid., p. 194.} 93_{Kaiser, 2002, p. 153.}

The Foundations of Quantum Mechanics in Postwar Transatlantic Physics: On the in uence of the United States on the lack of interest in alternative interpretations of quantum mechanics, 1950-1970