The Foundations of Quantum Mechanics in Postwar ... · of quantum mechanics in the rst decades...

The Foundations ofQuantum Mechanics inPostwar Transatlantic

Physics

On the influence of the United States on the lack ofinterest in alternative interpretations of quantum

mechanics, 1950 – 1970

S.L. ten HagenUniversity of Amsterdam

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

Abstract

This text focuses on the lack of success of alternative interpretationsof 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-calledCopenhagen interpretation, while little attention was paid to new ideas onthe 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 Europeanphysics, this paper attempts to answer the question to what extent Amer-ican values and ideas have contributed to the reduced attention for thefoundations of quantum mechanics. It is argued that the activities of theU.S. government and private foundations in the rehabilitation of Europeanscience after World War II had a reinforcing effect on the monocracy ofthe Copenhagen interpretation in Europe. In the U.S. efforts to increasethe amount of scientific exchange, the merely philosophical occupationof interpreting quantum mechanics had little urgency, compared to thesupport of internationally minded research institutions like CERN andthe 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 Americanphysics in particular or to physics in general.

Contents

1 Introduction 7

2 Interpreting Quantum Mechanics Before World War II 92.1 Early debates on the foundations of quantum mechanics . . . . . 92.2 The Einstein-Podolsky-Rosen paradox . . . . . . . . . . . . . . . 102.3 The status of the Copenhagen interpretation . . . . . . . . . . . 13

3 Postwar Interpretations of Quantum Mechanics 163.1 Bohm and the hidden variable interpretation of quantum mechanics 163.2 The relative-state interpretation of Hugh Everett III . . . . . . . 203.3 The 1960s: Bell’s theorem and Zeh’s difficulties . . . . . . . . . . 24

4 The Construction of Transatlantic Physics 284.1 A new world leader in physics . . . . . . . . . . . . . . . . . . . . 284.2 American physics and the Cold War . . . . . . . . . . . . . . . . 304.3 U.S.-Europe inequalities after World War II . . . . . . . . . . . . 324.4 American funding of European physics . . . . . . . . . . . . . . . 344.5 U.S. influence on interpreting quantum mechanics in Europe . . . 37

5 Concluding Remarks 405.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405.2 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Figure 1: The key players in the prewar discussions on the interpretation ofquantum mechanics: Albert Einstein and Niels Bohr. Photograph taken byPaul Ehrenfest at the 1930 Solvay conference on magnetism in Paris (NielsBohr Archive).

Figure 2: The participants at the 1927 Solvay conference in Brussels, dedicatedto photons and electrons. Among them are Albert Einstein, Erwin Schrodinger,Werner Heisenberg, Max Born and, for the first time, Niels Bohr.

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 ofthe theory was accompanied by debates about its philosophical significance. Atthe Solvay conference in Brussels in 1927 (see fig.2), some of the period’s mostfamous physicists were involved in a discussion on the interpretation of quan-tum mechanics.1 The key players in the debates from this period were NielsBohr and Albert Einstein. Among their main supporters were, respectively,Werner Heisenberg and Erwin Schrodinger. Although consensus between thesecontenders has never been reached, the Copenhagen interpretation from NielsBohr 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 WarII. Olival Freire jr. has pointed out that in this period, the interpretation ofquantum mechanics was regarded to be a matter of philosophy rather than realphysics: “It was not by chance that in the 1950s the only conference dedicatedto the subject was organised by philosophers rather than by physicists.”3 It hasbeen argued by David Kaiser that during the 1950s and 1960s, it was hard tobe taken seriously as a physicist, while focusing on the foundations of quantummechanics.4 Although it might be incorrect to consider the 1950s and 1960s as a

1Pais, 1991, p. 316.2Camilleri, 2009b.3Freire Jr., 2004, p. 1745.4Freire Jr., 2009; Kaiser, 2011.

1 Introduction

lost period for the interpretation of quantum mechanics –for example, John Bellpublished his influential text on the completeness of quantum theory in 19645 –interpretational questions reappeared into the agendas of physicists only after1970. From this moment, physicists have successfully proposed new ideas on thetopic, while even bestsellers on the interpretation of quantum mechanics havebeen 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 theoryquantum decoherence during the 1970s– and Alain Aspect –who confirmed Bell’stheorem by experiment in 1981– were able to make great contributions to theinterpretation of quantum mechanics. Slowly, The Foundations of QuantumMechanics began to be recognised as a self-contained field of research.6

This renewed interest for the interpretation of quantum mechanics indicatesa change in the climate of theoretical physics between the 1970s and the preced-ing decades. It is worth asking which factors have contributed to the gradualshift in attitude towards quantum interpretations. Olival Freire jr. has defendedthe conceivable idea that the revived attention on quantum interpretations wascaused by the rise of a new generation of physicists. Towards the end of the1960s, young scientists were unsatisfied with the way they learned to deal withquantum 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 explainsthe renewed attention to the interpretational issues of quantum theory. Be-sides, Freire’s analysis does not explain the lack of interest in the topic amongphysicists during the first decades after World War II. For the latter, it seemsworthwhile to study the context of postwar theoretical physics. In the aftermathof World War II, science became one of the most important platforms for theinternational exchange between the United States and Western Europe. There-fore, it is likely that the intensification of the American-European relationshiphas 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 onthe interpretation quantum mechanics took place before (chapter 2) and afterWorld War II (chapter 3). Subsequently, the postwar internationalisation ofscience in general and the Americanisation of European physics in particularwill be treated in chapter 4. In the conclusion, the following question will beanswered: has the Americanisation of European physics reduced attention tothe interpretation of quantum mechanics during the 1950s and 1960s?

5Bell, 1964.6Freire Jr., 2004.7Ibid., p.1757.

8

2 Interpreting Quantum Mechanics Before WorldWar 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 NielsBohr. Since this interpretation of quantum mechanics violated Albert Einstein’sconcept of reality, his dissatisfaction was growing. Before outlining Einstein’smost 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 insufficientbecause of the uncertainty principle. This principle states if one would performan experiment on any quantum object in order to determine its momentum p orits position x, the measurement accuracy of the one value comes at the expenseof the accuracy of the other. As a consequence, it is impossible to simultaneouslymeasure all the properties of a quantum object. Mathematically, things havebeen summarised in the next equation, containing the measurement errors ofthe 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 quantumtheory. Einstein’s discomfort, therefore, was not aimed at the predictions ofquantum mechanics, but at the actual meaning that was given to it by Bohrand his followers.8 In Physik und Philosophie from 1958, Heisenberg has ex-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 atomicevents, is to be described in terms of classical physics . . . Still the applicationof classical concepts is limited by the relations of uncertainty.”9 According toBohr and Heisenberg, the classical world forms the starting point for any in-terpretation, since the range of human observation is limited within the use ofclassical concepts.

What if the uncertainties from the equation above do not only occur in theworld of mathematics? According to Bohr and Heisenberg, exactly this is thecase. The Copenhagen interpretation of quantum mechanics is built upon theassumption that the uncertainty principle is not inherent to quantum theory,but to nature itself. In other words, exact values for physical quantities inthe microscopic quantum world simply do not exist. As a consequence, one islimited 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

8The Copenhagen circle in the 1950s and 1960s particularly consisted of Niels Bohr, WernerHeisenberg, Wolfgang Pauli, Max Born and Leon Rosenfeld.

9Heisenberg, 1958, p. 14.

2 Interpreting Quantum Mechanics Before World War II

the nature of reality. Following the Copenhagen interpretation, reality is notembedded in the quantum object itself; it is just a consequence of our classicalobservations. Depending on the device used to measure some quantity of thequantum object, reality emerges in terms of classical phenomena. Bohr endorsedthis view by introducing the concept of complementarity to the interpretationof quantum mechanics, which has been expressed by Heisenberg as follows: “Bygoing from the one picture to the other and back again, we finally get the rightimpression of the strange kind of reality behind our atomic experiments . . . theknowledge of the position of a particle is complementary to the knowledge of itsmomentum.”10 While one experiment may reveal an electron as having classicalwave properties, another experiment would determine the electron to behavelike 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 ofmeasurement.

Starting from the 1930s, several objections have been formulated to this in-terpretation of quantum mechanics. Einstein’s main objection focused on thesupposed absence of an objective physical reality, due to the decisive role of mea-surement in Bohrian quantum mechanics. Remaining faithful to a deterministicworldview, Einstein thought that the lack of objectivity in the microscopic worldexposed the incompleteness of quantum theory. For his colleague physicist LouisDe Broglie, the need for an extension of the theory was beyond dispute: “Ac-tually this interpretation, by seeking to describe quantum phenomena solely bymeans 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 quantummechanics, it seemed crooked that the possibility to interpret quantum theorywould depend on the a priori assumption of classical concepts. Besides, sincethe macroscopic world is entirely built up from microscopic parts, one wouldexpect 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, ‘CanQuantum 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 quantummechanics would lead to a paradox, if it was assumed that “. . . every elementof the physical reality must have a counterpart in the physical theory.”12 Ein-stein, Podolsky and Rosen briefly repeated this demand in what they calledthe criterion of reality : “If, without in any way disturbing a system, we canpredict with certainty the value of a physical quantity, then there exists an ele-

10Ibid., p.18.11De Broglie, 1954, p.235.12Einstein, 1935, p. 777.

10

2.2 The Einstein-Podolsky-Rosen paradox

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

In order to illustrate the contradiction, they introduced a situation consistingof two quantum systems (I and II) able to interact until the moment t = T . Fort > T , i.e. any moment after the interaction between the two systems hasstopped, the wave function of the composed system (Ψ) can be calculated usingSchrodinger’s equation,

ih∂

∂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 touse Schrodinger’s equation for the two systems separately. The wave functionof 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) =

∞∑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 andit 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), andthat 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 toother eigenfunctions for both system I and system II. For example, the measure-ment of quantity B with possible eigenvalues b1, b2, b3, .. and eigenfunctionsv1(x1), v2(x1), v3(x1), would have resulted in an expression for the composedwave function, containing other eigenfunctions for system II as well:

Ψ(x1, x2) =

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

13Ibid., p.777.

11


possible to assign different wave functions to the same reality.”14 Clearly, thelatter is in contradiction with the earlier proposed criterion of reality, statingthat any element in physical reality should be represented by a mathematicalcounterpart in physical theory. Following this, Einstein, Podolsky and Rosendrew the conclusion that ”the quantum-mechanical description of physical real-ity given by wave function is not complete.”15 The EPR paper finishes with anexplicit invitation to future research on the foundations of quantum mechanics:“While we have thus shown that the wave function does not provide a completedescription of the physical reality, we left open the question of whether or notsuch 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 quantummechanics.

For this study, which focuses on the Americanisation of European theoreticalphysics, it might be relevant to look at some responses to the EPR paper withinboth 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 secondsystem] cannot be affected by [a measurement on the first system] and must inall cases constitute ‘the same physical reality’, while “. . . whenever two systemsinteract for a short time there is a correlation between the subsequent behaviourof one system and that of the other.” Soon after his claim, Kemble realised thatin his eagerness to point out the incorrectness of the EPR argument, he missedthe fact that the wave function of the combined system in the EPR paradoxwas not unique.17 Kemble’s spirited though incorrect response to the EPR pa-per becomes comprehensible when one studies his own attitude to the meaningof quantum mechanics, as he has pointed out in The Fundamental Principlesof Quantum Physics18. In this book, Kemble stressed that “the wave functionis merely a subjective computational tool and not in any sense a descriptionof objective reality.” According to Kemble, the interpretation of the wave func-tion fell beyond the scope of the theoretical physicist. Max Jammer has arguedthat Kemble’s view on quantum interpretations as irrelevant for physicists wasinfluenced by the “operationalism of Bridgman, the positivism of Mach, andthe pragmatism of Peirce”19. Kemble and his American contemporaries saw itas the physicist’s main duty ”to describe the experimental facts in his domainas 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 toimpose.”20 In section 4.1, the pragmatic attitude among American theoretical

14Ibid., p. 779.15Ibid., p. 780.16Kemble, 1935.17Jammer, 1974, p. 191.18Kemble, 1937.19Jammer, 1974, p. 191.20Kemble, 1937.

12

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 froma man of whom it could be expected: Niels Bohr. Apart from rejecting thecriterion 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 thevery definition of the physical quantities in question rests.”21 Bohr counteredEinstein’s attack by showing that in quantum mechanics, a measurement can-not be performed without disturbing the system measured upon. Apart fromBohr’s reaction, the EPR paper only gained a few dozen direct citations in thesubsequent decades, indicating that European physicists were not that willingto contribute to the discussion as well.


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 quantumtheory had taken place among a select group of physicists. There were physi-cists who had made important contributions to the –mathematical– theory ofquantum mechanics, but who refrained from the details of the philosophical de-bate between Einstein and Bohr. This can be demonstrated by a phrase froma 1948 letter from Einstein to Max Born,22 commenting on the everlasting de-bate between him and the defenders of the Copenhagen interpretation: “I canquite understand why you take me for an obstinate old sinner, but I feel clearlythat you do not understand how I came to travel my lonely way . . . it would beimpossible for you to appreciate my attitude. I should also have great pleasurein tearing to pieces your positivistic-philosophical viewpoint. But in this lifeit is unlikely that anything will come of it.”23 A tone of frustration resoundsfrom Einstein’s words. Apparently, Einstein felt that Born did not grasp hisobjections to the Copenhagen interpretation. According to Max Jammer, whointerviewed Einstein in 1952 and 1953, Einstein “never abandoned the viewthat quantum mechanics, as presently formulated, is an incomplete descriptionof reality.”24 Yet, he had to rely more and more on his own, as the acceptanceof 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 hadpulled the longest straw in the discussion with Einstein: the vast majority ofphysicists accepted complementarity and the other features of the Copenhageninterpretation. Jammer has stated that most of these adherents were drivenby a pragmatic attitude and, “impressed by the spectacular successes of quan-tum mechanics in all fields of microphysics, they were interested primarily in its

21Bohr, 1935.22The 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 absolutevalue of the wave function |Ψ(x, t)|2 is equal to its probability p.

23As cited by Jammer, 1974, p. 188.24Ibid., p. 188.

13


applications to practical problems and its extension to unexplored regions.”25

The desire for exact results of the theory of quantum mechanics was indeedgreat among physicists of that time, acclaims the Dutch physicist Sander Bais:“. . . quantum theory went from one success to another. For example, the theoryon condensed matter was fully understood all of a sudden . . . the possibility thatsomewhere deep down in the theory, there might still exist some fundamentalissues 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. Butat a certain moment, the founder critics Einstein, Schrodinger and De Brogliewere joined by a generation of young communist physicists from both Europeand the Soviet Union, who felt uncomfortable with the interpretation of quan-tum mechanics as it was.27 Reasoning from a materialistic world view, Sovietphysicists were among the most fanatic opponents of Niels Bohr. Like Einstein,they did not oppose to quantum theory itself – which would be a hard thingto do, considering the accumulation of empirical evidence – but to the meaningthat was given to it by Bohr and his men. Again, the loss of objective realityin the quantum world was situated in the centre of the dispute. According toLeninist tradition, all forms of reality can be reduced to the working of matter.This deterministic foundation of communist ideology prevented many Sovietphysicists from following Bohr’s ideas on complementarity and the uncertaintyof nature. The communist physicists tried to develop a causal interpretation ofquantum mechanics, motivated by the conviction that Bohr’s interpretation ofquantum mechanics was idealistic and positivistic.28 29 The introduction to a1952 article by the Soviet physicist Dimitri Blokhintzev reveals how the cardswere on the table: “The present article is devoted to the unmasking of idealisticand agnostic speculations of [the Copenhagen] school on the basic problems ofquantum mechanics . . . ”30 In line with Einstein’s view on quantum mechanics,Blokhintzev doubted the completeness of the theory. Furthermore, the criticalspirit of the Soviet physicists was not restricted within the borders of the So-viet Union, it transcended to Western Europe as well. Especially among FrenchMarxist physicists, the interest in the interpretation of quantum theory wasrevived by the communist interference in the debate. For example, Jean-PaulVigier adopted Blokhintzev reproach that Bohr’s ideas on the interpretation ofquantum mechanics were subjectivistic and idealistic.31 It has been suggested

25Ibid., p. 247.26Sander 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 andthe author of this text in Amsterdam in May 2014.

27Camilleri, 2009a, p.34.28Ibid.29As 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 ideasof Marxism perfectly, and rejected those of Gregor Mendel.

30Blokhintzev, 1952, p.546.31The communist influence on the debate in Western European physics is described by

14


by Mara Beller and Don Howard that the existence of the term Copenhageninterpretation provides a false idea of unity between the prewar physicists whohave 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 postwarSoviet critics of Bohr’s interpretation of quantum mechanics. He has arguedthat the Copenhagen interpretation has not emerged simultaneously with quan-tum theory, but appeared much later in a political context: “the idea of unitaryinterpretation only emerges in the 1950s in the context of the challenge of SovietMarxist 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 ofcriticism, 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 Leon Rosenfeld belonged tothe adherents of the Copenhagen interpretation, while he was as a Marxist. Aswe will see in the next chapter, much critique during the 1950s and 1960s wasprovided on non-ideological grounds.

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

32Beller, 1999; Howard, 2004.33Camilleri, 2009b, p. 29.34Camilleri, 2009a, p. 38.

15

3 Postwar Interpretations of Quantum Mechan-ics

In the first few decades after World War II, several attempts have been madeto change the mainstream interpretation of quantum mechanics. But in termsof scientific impact, none of these postwar alternatives has ever come close tothe Copenhagen interpretation. Three contributions from the period 1950–1970will be presented in this chapter. First, the alternative interpretations of DavidBohm and Hugh Everett III will be discussed in both physical and historicalcontext. By doing this, it should become clear how the postwar scientific cli-mate has influenced the success of these interpretations. After this, John Bell’stheorem from 1964 will be discussed. These case studies serve as a prelude tochapter 4, where it will be investigated whether the Americanisation of physicsafter World War II has reduced the interest for alternative interpretations ofquantum 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 theoryof quantum mechanics did not cover a complete description of reality. Theyrealised that if the sole existence of a probability distribution for individualparticles could be avoided, determinism and causality would be restored in thedescription of nature on the smallest scale. In general, the opponents of theCopenhagen interpretation desired “to construct a theory to explain the be-haviour of individual systems from the statistics of their ensembles”, says MaxJammer.35 So-called hidden variable theories were among the candidates forextending the theory of quantum mechanics. The motivation for these kindof theories was the alleged existence of dynamical variables hidden from sight,continuously correlating the states of different systems. Such variables wouldfix the EPR paradox, since they would retain the connection between system Iand system II. Although Einstein’s criticism can be regarded as a direct inspi-ration for the emergence of interpretations including hidden variables, he didnot work on them himself. One of the Copenhagen criticasters who did believein a solution provided by the use of hidden variables, was a young Americanphysicist called David Bohm. He took over the baton from Louis de Broglie inthe 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 1951book Quantum Theory, defining them as “a further set of variables, describingthe state of new kinds of entities existing in a deeper subquantum mechanical

35Jammer, 1974, p. 253.36Louis 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 hiddenvariable theory in 1952, David Bohm was not aware of the existence of this interpretation.

3.1 Bohm and the hidden variable interpretation of quantum mechanics

level and obeying qualitatively new types of individual laws.”37 While the majorpart 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 quantumtheory “should be able to describe the process of observation itself in terms ofthe wave functions of the observing apparatus and those of the system underobservation.”38 This claim shows Bohm’s discomfort with the exclusive role ofmeasurement in the Copenhagen interpretation. In subsequent years, Bohmdeveloped his own hidden variable interpretation of quantum mechanics, aspublished in the 1952 article Suggested Interpretation of the Quantum Theoryin Terms Of ”Hidden Variables” I, II.39

In Bohm’s interpretation of quantum mechanics, both the wave and particleaspects of matter are regarded as real. The wave function Ψ represents an actualphysical 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 Schrodinger equation, Bohmconcluded that the momentum of a quantum system must equal

p = ∇S(x) (6)

By defining exact values for momentum and position, Bohm in fact providedquantum 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 realityon the smallest scale is built on merely probability: “[In this alternative inter-pretation], quantum mechanical probabilities are regarded as only a practicalnecessity . . . 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 paperBohm thus suggested a deterministic interpretation to quantum theory, by in-troducing precise values for both p and x as hidden variables to Schrodinger’sequation. After a thorough comparison between the “usual interpretation” ofquantum mechanics, and his own “alternative interpretation”, Bohm endorsedthat “as long as we assume that ψ satisfies Schrodinger’s equation, that v =∇S(x)/m,42 and that we have a statistical ensemble with a probability densityequal to |Ψ(x)|2, our interpretation of quantum theory leads to physical resultsthat 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

37Bohm, 1951.38Ibid., p. 583.39Bohm, 1952a; Bohm, 1952b.40Bohm, 1952a, p. 170.41Ibid., p. 166.42v and m are the velocity and the mass of the quantum system, respectively.43Bohm, 1952a, p. 179.

17

3 Postwar Interpretations of Quantum Mechanics

the inclusion of hidden variables could be consistent with the theory of quantummechanics. He claimed that the same experimental results could be obtained,when treating the actual values of position x and momentum p as extra variablesin the yet existing equations of quantum mechanics: “In our interpretation, weassert that the at present ‘hidden’ precisely definable particle positions andmomenta determine the results of each individual measurement process, butin a way whose precise details are so complicated and uncontrollable, and solittle known, that one must for all practical purposes restrict oneself to a sta-tistical description of the connection between the values of these variables andthe directly observable measurements.”44 Bohm thus stressed that the uncer-tainty relation and the squared wave function are indeed necessary in order togain experimental results from quantum mechanics. But, he endorsed that theexistence of real values for position and momentum on the quantum level donot need to be excluded, if these properties are treated as hidden variables. Be-sides, Bohm appointed that his alternative interpretation of quantum mechanicsmight solve some of the practical problems of quantum mechanics, since “theusual mathematical formulation seems to lead to insoluble difficulties when it isextrapolated 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 beenignored by his colleague physicists.46 Some say the minor impact of Bohm’shidden variable proof was caused by the fact that Louis de Broglie had al-ready proposed –and given up– a similar contribution to quantum mechanics25 years earlier. By presenting some of the early objections to Bohm’s theoryfrom the Copenhagen circle, Wayne Myrvold has found that “[Einstein, Pauliand Heisenberg] regarded the discussion as a resumption of the discussion of deBroglie’s theory that had taken place a quarter century earlier.”47 On the otherhand, Olival Freire jr. has argued that cultural factors in postwar theoreticalphysics were the main cause of the poor reception of Bohm’s interpretation.48

Freire illustrates the resistance Bohm had to endure from the worldwide physicscommunity by a quote from the American physicist Isidor Rabi, who arguedthat “the causal interpretation gives us no line to work on other than the useof the concepts of quantum theory.”49 In this judgement, one might be able torecognise a pragmatic attitude towards the development of theoretical physics.In a sense, Rabi’s words are reminiscent of those of Kemble when commentingon the EPR paradox. Rabi wanted to see a situation in which “the things turnaround.” Apparently, the preservation of an objective reality at the quantumlevel was not sufficient for him. The Belgian physicist Leon Rosenfeld wentfurther in rejecting Bohm’s theory, writing the following to Bohm: “I shall notenter into any controversy with you or anybody else on the subject of comple-

44Bohm, 1952b, p. 183.45Bohm, 1952a, In abstract.46Cushing, 1994.47Myrvold, 2003, p. 7.48Freire Jr., 2005.49As cited by ibid., p. 12.

18

3.1 Bohm and the hidden variable interpretation of quantum mechanics

mentarity, for the simple reason that there is not the slightest controversial pointabout it.”50 According to Freire, the absence of “predictions not foreseen by theusual quantum mechanics . . . reinforced the derogatory label of ‘philosophical‘stuck on them by their opponents”51. He claims that this philosophical labelhas 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 lackof encouraging response by the changing attitude of physicists towards the sig-nificance of physical theories: “Physics has changed from its earlier form, whenit tried to explain things and give some physical picture. Now the essence isregarded as mathematical. It’s felt the truth is in the formula’s.” and “Thereis 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 conclusionand 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 inaccepting the new interpretation. Moreover, Bohm’s description of physics as amathematical activity subscribes to a pragmatic attitude. As we have seen insection 2.2, and will see in section 4.1, this kind of attitude is often associatedwith American scientific ideals. Again, it seems worth asking if the American-isation of physics might have influenced the attitude among physicists towardsalternative interpretations of quantum mechanics.

50Letter from Rosenfeld to Bohm, 20 May 195251Freire Jr., 2005, p. 24.52Interview with David Bohm, conducted by F. David Peat and John Briggs, originally

published in Omni, January 1987

19

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 interpretationof 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 informedin the debates between David Bohm and the Copenhagen circle, might wonderwhy 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 middleof his work on the hidden variable interpretation of quantum mechanics, be-cause of McCarthyism in the United States. However, there is no proof thatthe moderate reception of Bohm’s interpretation had anything to do with hiscommunist symphathies.53 Besides, it is of interest that David Bohm was notthe only young physicist in the 1950s who was confronted with struggles whilecalling the Copenhagen interpretation into question. To illustrate this, the com-ing section will cover the context and content of Hugh Everett III’s alternativeinterpretation 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 theorydemanded 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

53Freire Jr., 2005.54Jammer, 1974.


should be part of a total continuous quantum system. In such a description ofquantum events, there would be no longer a need for the ‘collapse’ of the wavefunction as a result from measurement. Hugh Everett III, a physics studentat Princeton at that time, was triggered by the possibility to get rid of themeasurement problem. Under supervision of John Archibald Wheeler, Everettstarted on his dissertation in 1955, which eventually resulted in the “relativestate” formulation of quantum mechanics. Like David Bohm, Everett presentedhis 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 quantumtheory, which has demonstrated its usefulness in an overwhelming variety ofproblems, but rather to supply a new, more general and complete formulation,from which the conventional interpretation can be deduced.”55 It is worth notingthat the cautiousness with which Everett confronts the conventional interpre-tation might have been the result of a laborious struggle between him and thekey defenders of the Copenhagen interpretation in the preceding years. Beforeoutlining the difficulties that Everett met during the proposal of his ideas, thecontent 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 quantummechanics, by referring to Einstein’s theory of general relativity: “How is oneto apply the conventional formulation of quantum mechanics to the space-timegeometry itself? The issue becomes especially acute in the case of a closed uni-verse.” Furthermore, Everett endorsed the need for the inclusion of the observerinto the formulation of quantum mechanics: “How are a quantum description ofa closed universe, of approximate measurements, and of a system that containsan observer to be made? These questions have one feature in common, that theyall inquire about the quantum mechanics that is internal to an isolated system.No way is evident to apply the conventional formulation of quantum mechanicsto a system that is not subject to external observation.” In his paper, Everettproposed to “regard pure wave mechanics as a complete theory.” In his eyes, itwas possible to formulate quantum mechanics in a way that the wave function“obeys a linear wave equation everywhere and at all times supplies a completemathematical 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 centralisingthe concept of measurement in quantum mechanics, he proposed that the wavefunction served as the basic physical entity. Thus, in his relative state formu-lation of quantum mechanics, Everett centralised the mathematical concepts ofthe theory itself.

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

55Everett, 1957, p. 454.56Ibid., p. 455.

21


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

and S2:

ψS =∑i,j

aijξS1i ηS2

j (7)

Subsequently, Everett assigned unique relative states to the second subsystem,corresponding to all possible states of the first subsystem. By choosing ξk asthe state for the first system, the corresponding relative state, ψ(S2; relξk, S1),equals

ψ(S2; relξk, S1) = Nk

∑j

akjηS2j (8)

with Nk a normalisation constant. By combining equation 7 and 8, the state ofthe composite system can be represented as “a single superposition of pairs ofstates, each consisting of a state from the basis {ξi} in S1 and its relative statein S2:

ψS =∑i

1

NiξS1i 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 afundamental relativity of states, which is implied by the formalism of compositesystems.”57

After this, Everett continued by enclosing a mathematical description of theprocess of observation in the theory quantum mechanics. According to Everett,there is a “task of making deductions about the appearance of phenomena toobservers which are considered as purely physical systems and are treated withinthe 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. Afterthe observation of quantity A and B in S1 and S2 respectively, the final totalstate can be written as ∑

i,j

aibjφS1i ηS2

j ψSnψ0[. . . , ai, bj ] (11)

After he had formulated the observation process in quantum mechanics in thisway, 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 ofobservation here is only one physical system representing the observer, yet there

57Ibid., p. 456.58Ibid., p. 457.59Ibid., p. 459.

22


is no single unique state of the observer. . . ”60 Everett concluded that, since allthe possible states are part of the total system, the observer “branches” intodifferent states after every observation. Mathematically, each branch representsa different observation and the corresponding eigenstate for the object-systemstate. Because the everlasting emergence of these branches caused by the pro-cess of observation suggests the co-existence of an infinite number of parallelrealities, Everett’s interpretation is often referred to as the many worlds ormultiverse 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 theobserver and the rejection of a priori probabilities within the mathematical for-mulation of quantum mechanics served important advantages: “Objections havebeen raised in the past to the conventional or “external observation” formulationof quantum theory on the grounds that its probabilistic features are postulatedin advance instead of behind, derived from the theory itself. We believe thatthe present “relative-state” formulation meets this objection, while retaining allof 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 withhim in the 1930s, hoped to be able to unite the Copenhagen interpretation withEverett’s meta-consideration of quantum mechanics. As a follower of Bohr,Wheeler wished to make sure that the principle of complementarity would bemaintained.62 Therefore, he emphasised that Everett’s interpretation should beseen as an extension and not as a refutation of the Copenhagen interpretation.63

To convince the Copenhagen school of Everett’s revolutionary ideas, Wheelervisited Copenhagen already in 1956. However, Bohr and his assistants AagePetersen and Alexander Stern refused to accept Everett’s ideas, while the bynow familiar Leon 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 fromWheeler, Everett eventually visited the Niels Bohr Institute in Copenhagen in1959 himself . For Everett, this visit was a last resort to convince Niels Bohrof the advantages that came with his alternative formulation of quantum me-chanics. However, Bohr rejected Everett’s ideas once again and so the meetingended in disappointment.65 In the end, Everett made the choice to leave physics

60Ibid., p. 459.61Ibid., p. 462.62Freire Jr., 2005.63Everett 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 claimsthat “the Copenhagen interpretation is hopelessly incomplete because of its a priori relianceon classical physics . . . as well as a philosophical monstrosity with a reality concept for theMacroscopic world and denial of the same for the microcosm.”

64The described events surrounding Wheeler’s efforts in Copenhagen have been presentedby Olival Freire jr., Freire Jr., 2005

65Byrne, 2007.

23


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

More than ten years later, Bryce deWitt revived the attention to Everett’sideas.67 It seems that before this time, any possible success for Everett’s rel-ative state formulation of quantum mechanics was frustrated by the defendersof the Copenhagen interpretation. The fate of Everett’s interpretation showsremarkable similarities with a story that we have heard just before. Or, inFreire’s words: “blocked by the Copenhagen monocracy, Everett’s ideas had afate similar to Bohm.”68 Indeed, the status of the Copenhagen interpretationseems 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 inCopenhagen in particular. How come that there were no other places, suitablefor discussing new ideas on the interpretation of quantum mechanics? Perhaps,this had something to do with the general values in postwar physics. Whatdevelopments were occupying the minds of physicists, so that they were notable to react on the proposals of David Bohm and Hugh Everett III? While theCopenhagen monocracy explains the poor reception among the Copenhagen cir-cle, it fails to explain why these few physicists had been determining the fate ofalternative interpretations of quantum mechanics before and after the SecondWorld War.

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

Some other physicists were indeed triggered by the alternative interpretation ofBohm and Everett. In this section, we will see how the work of David Bohminspired John Bell to work further on hidden variables and quantum mechanics.Also, the solitary position of Dieter Zeh, in whose work on decoherence thespirit of Everett dwells, will be highlighted. It is interesting to see whetherthese physicists had to face the same kind of difficulties as their predecessors inthe 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 ideason hidden variables. Despite Bell’s early interest in the foundations of quantummechanics, it was not until the start of the 1960s that he fully dedicated to thetopic. In the meantime, he had successfully worked at CERN69 on acceleratorsand high-energy physics.70 While theories on hidden variables had been in hishead since 1952, his breakthrough paper on the EPR paradox appeared onlyin 1964.71 In this text, Bell showed that the EPR assumptions of locality andrealism were incompatible with the theory of quantum mechanics and that no

66One should be careful to explain Everett’s departure from American physics by the poorreception of his interpretation. As will become evident in chapter 4, a career like Everett’swas definitely no exception for American physicists in the 1950s.

67Freire Jr., 2005.68Ibid., p.31.69Section 4.4 provides a discussion on the role of the United States in the establishment of

CERN70Freire Jr., 2009, p. 283.71Bell, 1964.

24


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 ofthe EPR experiment, which we will now briefly recapitulate.

The experiment included two independently rotating detectors, measuringthe spin of an electron in the direction of the unit vector a, and the spin of apositron in the direction of the unit vector b (see fig. 4). The positron andelectron 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 mechanicalprediction of this experiment states that the average value P of the spin productmust equal

P (a, b) = −a · b (13)

Bell showed in his paper that this result can not be obtained from any localhidden 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 unitvector c, that the inclusion of hidden variables in the experiment results in thefollowing 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 ofthe experiment by quantum mechanics. If, for instance, the unity vectors lie ina plain, making angles of 45◦ with each other, it can be computed using thequantum 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 anincorrect 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 thewhole is incorrect. Considering the amount of empirical evidence in favour ofquantum theory, the latter option seemed impossible. Furthermore, Bell’s work

72This equation is called the Bell inequality

25


provided experimentalists with the possibility to contribute to the foundationsof quantum mechanics. Suddenly, the controversies in quantum mechanics couldnot be regarded as solely philosophical disputes, but as scientific, experimentallysolvable 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 thelight of this study. For instance, how come that Bell waited until 1964 beforefocusing entirely on the problems surrounding hidden variables? In 1952, afterthe publication of Bohm’s previously discussed article, he had already admittedin correspondence with Wolfgang Pauli that he was attracted by Bohm’s ideason hidden variables. Also, he had been suspecting since that time that VonNeumann’s proof against their existence was not waterproof.73 What were thefactors 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, asFreire 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 possibilitiesfor physicists to get involved in the foundations of quantum mechanics were stillrestricted. It is exemplary for the bad status of the topic that the physicist whoearlier tried to perform an experiment on Bell’s theorem, John Clauser, nevergot a permanent position in physics.75

Maybe the case of the American John Clauser can be partly explained dueto 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 gettinginvolved 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, theCopenhagen monocracy was arriving in its final days.76 One would thereforesuggest that the road was free for anyone who wanted to tackle the interpreta-tional questions surrounding quantum theory. In 1967, Zeh directed his researchtowards the measurement problem in quantum mechanics, drawing the conclu-sion that macroscopic entities in quantum mechanics could not be describedas closed systems.77 Zeh’s requests to get his first paper on the measurementproblem published were repeatedly turned down. Next to this, Zeh had to dealwith a rather offensive climate towards his new interest at his own HeidelbergUniversity. 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 tobe done behind the scenes: “. . . it was absolutely impossible at that time to

73Freire Jr., 2005, p. 32.74Freire Jr., 2009, p. 284.75Kaiser, 2011.76Niels Bohr died on 18 November 1962, and there was an unsolved conflict between Eugene

Wigner and Leon Rosenfeld, according to Freire Jr., 200977Ibid., p. 281.

26


discuss these ideas with colleagues, or even to publish them. An influentialHeidelberg Nobel prize winner frankly informed me that any further activitieson this subject would end my academic career!”78 Just as the theories of Bohmand Everett gained attendance decades after their first publication, Zeh’s ideasbecame successful only in the 1980s. Zeh has subscribed the unfavourable atmo-sphere for making new contributions to the interpretation of quantum mechanicsbefore this time to the authority of the Copenhagen circle. In 1980, he wrote toJohn 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 fundamentalproblems of the quantum.”79 It is remarkable that Zeh has repeatedly referredto the Copenhagen interpretation as “irrational” and “pragmatic”, stating thathis theory of decoherence, which reached maturity during the 1980s, “is an at-tempt to replace the pragmatic irrationalism that is common in quantum theorytextbooks (complementarity, dualism, fundamental uncertainty etc.).”80

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

78Zeh, 2006, p. 4.79H.D. Zeh to J.A. Wheeler, 20 October 1980, as quoted by Freire Jr., 200980Zeh, 2006, p. 18.

27

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, thedismissive attitude of physicists towards the interpretation of quantum mechan-ics has been associated with pragmatism. If indeed a pragmatic attitude wasprevailing among postwar physicists in both Europe and America, then wheredoes this attitude come from? Should pragmatism be regarded as inherent tophysics in general, or has it perhaps originated from another source? In dealingwith this question, the remaining part of the text concentrates on the effects ofAmerican interference in European physics, and the consequences for the statusof interpreting quantum mechanics. First, American postwar physics in generaland 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 ideaswere 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 bothindustrial and scientific level, while Europe was left behind with the challengesof reconstruction after the war. In line with the enormous inequalities betweenEurope and the United States at this time, the political and economic powershifted towards the latter. Fitting this new order, the United States took overWestern Europe’s role as the world leader in physics as well. Hence, Europe wasno longer the centre of theoretical physics after World War II. A non-economicalfactor that contributed to the transfer of scientific power from Europe to theUnited States, can be illustrated by Albert Einstein’s relocation to PrincetonUniversity in 1933. In subsequent years, many physicists followed his exampleand resumed their careers in the United States, where they were safe for therise of fascism, holding Europe in its grip.81 Before World War II, the arrival ofrefugee scientists from Europe caused a quick building up of scientific expertisein the United States. Next to this impulse, American physics had already madea qualitative leap during the interbellum. According to Schweber, this was theresult of financial support of the Carnegie, Guggenheim and Rockefeller founda-tions, on the request of America’s leading experimental physicists on universitiessuch as Harvard and Caltech. “Support from these foundations enabled theseelite universities to expand their activities in physics”, says Schweber.82 Thedevelopments in the above enabled the United States to conquer its position asthe world leader in physics already before World War II.

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

81Kaiser, 2011, p.15.82Schweber, 1986, p. 56.

4.1 A new world leader in physics

difference between American and European universities in terms of their internalorganisation. 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 characterof American theoretical physics.”83 As a consequence, theory and experimentwere never separated from one another, while in Europe the separation betweentheory and experiment was the most common thing. Schweber also appointedthe authoritarian structures of European universities, and puts it in contrastto the omnipresent democratic spirit and variety at American institutions: “Inaddition to democracy, American departments had variety: whereas in Germanuniversities, theoretical physicists and experimental physicists usually occupiedseparate institutes, each directed by a single professor who controlled its ac-tivities, in the United States theoreticians not only shared a department withexperimentalists, but were also trained in large part by them.”84

While Schweber considers the connection between theory and experiment asa 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. Nextto the arrival of refugee scientists from Europe in the United States, some ofthe most influential American physicists around World War II had been partlyeducated at universities in Europe. For instance, Robert Oppenheimer andIsidor 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 thetwo cultures of physics to have merged during this period. Schweber deniesthat this was the case, by stating that American physics was already highlydeveloped at the time the refugee physicists arrived, and that the scarcity ofpositions at universities forced the Europeans to fit in the American system,rather than vice versa. “Refugee theoreticians oriented toward the prevailingAnglo American empirical tradition found university positions more readily andintegrated more easily than their more philosophical fellows”, he claims.86

Still, it seems that the contrast between the pragmatic American physicistand the philosophical European physicist blurs among those who were involvedin the interpretation of quantum mechanics after the Second World War. Forinstance, the distinction between the American pragmatic - and the Europeanphilosophical physicist cuts no ice when one studies the case of Niels Bohr. Onthe one hand, Bohr was one of the most philosophically engaged scientists ofhis era, and thus satisfies the European stereotype.87. When structuring hisown institute however, Bohr was following the American example very closely,emphasising the need for an integration of theory and experiment (see section4.4). As another example, this is a 1938 judgement from the American physi-

83Ibid., p.58.84Ibid., p.74.85Ibid., p.74.86Ibid., p.80.87Bohr’s dedication to philosophy has been outlined in his biography by Pais, 1991

29


cist John Slater on the value of interpreting physical theories: “. . . questionsabout a theory which do not affect its ability to predict experimental resultscorrectly 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 somesatisfaction from them.”88 In a first consideration, this empiricist exclamationmight be regarded as a typical form of American pragmatism. But as we haveseen in section 3.3, mentioning the difficulties of Heinz-Dieter Zeh, this opinionresounded 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 thatthere were some factors from American society that influenced the characterof American physics in the 1950s and 1960s. During the Cold War, the bondbetween American physics and society was reflected in the realisation of labora-tories dedicated to the development of military equipment. It was in these placesthat the pragmatic spirit and the unity of theory and experiment in Americanphysics worked best, argues Schweber: “Many factors were responsible for thesuccess of the war-time laboratories . . . but surely one of the key factors was thesymbiotic relationship that had existed between theoreticians and experimen-talists, their shared pragmatism, the ease with which they could communicateand collaborate.”89 The embedding of American physics in society can also beillustrated by displaying the development of the number of American graduat-ing physicists through the 20th century. Figure 5, taken from a 2002 study ofDavid Kaiser, shows the number of physics PhD’s at U.S. research institutions.The graphic marks two moments of dramatic increase, which can be explainedby taking note of the developments in American society that related to theCold 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 calculatedthat “between 1945 and 1951, the number of physics Ph.D’s awarded annuallyby U.S. institutions doubled every 1.7 years”.90 After five years of temporarystabilisation, caused by the lack of office space, the increasing number of Ph.D’sresumes from the beginning of the 1960s. In the light of the arms race againstthe Soviet Union, it was important to train as much physicists as possible andto produce as much military equipment as might be necessary. Triggered byhostile events such as the surprise launch of Sputnik rocket in the Soviet Unionin 1957 and the supposed outnumbering of American physicists by their Sovietassociates, there was “. . . a renewed and widespread emphasis on the need forimproving the quantity and quality of highly trained scientists and engineersboth at home and abroad.”91 In figure 5, the American fear of Communism isillustrated by the second peak in the 1960s.

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

88Slater, 1938.89Schweber, 1986, p. 92.90Kaiser, 2002, p. 136.91Krige, 2008, p. 192.

30

4.2 American physics and the Cold War

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

and education in American physics. After World War II, physicists were first ofall needed to “maintain the West’s scientific and technological supremacy”, saysJohn Krige.92 By developing industrial and military tools, they could help Amer-ican society forward. In addition to this analysis, David Kaiser has stressed thatalong with the increasing importance of training physicists, the style of educa-tion in American universities was affected as well. Fitting the generally assumedpragmatical tradition of American physics, and parallel to the tendency to fo-cus preferably on industrial and military benefits, the education programme ofphysics became mainly dedicated to skills and less to wisdom. In reaction tothe growing demand for physicists, American universities were confronted withan overflow of students. According to Kaiser, “the tremendous, unprecedenteddemographic shift helped to drive a pedagogical emphasis upon efficient, repeat-able – and thereby trainable – techniques of calculation, .. solidifying a prewarinstrumentalist trend.”93

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

92Ibid., p. 194.93Kaiser, 2002, p. 153.

31


sort of thing that Einstein has worked on for many years (without any impor-tant results)”94 Kaiser claims that Birge’s attitude represented a wider trendin American physics, since “most physics departments in the United States re-ported having too many graduate students for their faculty to handle.”95 It islikely that research on the foundations of quantum mechanics forms an exampleof a topic which was, according to influential people like Birge, not suitable forthe education of Ph.D. students. Kaiser states that as a result of the Cold Wardevelopments, “epistemological musings or the striving for ultimate theoreticalfoundations never a strong interest among American physicists even before thewar fell beyond the pale for the postwar generation and their advisors.” Onthe basis of the events at Berkeley university, Kaiser concludes that “. . . duringthe manpower build-up phase of the Cold War, physicists had to choose theirresearch so as to support throngs of graduate students on their coattails.”96

Apparently, also the research agenda’s of American physicists were shaped bythe demand for scientific manpower in American society. This last observationcan serve as an important tool for answering the main question posed in thistext. Namely, it seems that it was not to expect from American physicists toprovide supplements to the interpretation of quantum theory after World WarII. Next to this, we have already seen that the monocracy of the Copenhageninterpretation formed an obstacle for the proposal of Bohm and Everett’s ideas.These two factors together provide a fairly plausible explanation for the limitedfocus on the interpretation of quantum mechanics in the United States duringthe first decades of the Cold War.

We are still left with the question whether the pragmatic attitude of Ameri-can physics, reinforced by the demand for scientific manpower, has been trans-ferred to Europe as well. It is clear that the Copenhagen monocracy had im-peded the emergence of alternative interpretations until approximately 1950.But how was the status of the Copenhagen interpretation influenced by the riseof the United States as the world leader in physics? In order to answer thisquestion, it is worthwhile to examine the way in which American and Europeanphysics were related to one another after World War II. As we will see, theaftermath of World War II has proven to be an excellent chance for the U.S. totighten the bonds with Western Europe.

4.3 U.S.-Europe inequalities after World War II

After World War II, the United States had to deal with two problems. The firstwas the critical state of Western Europe, which had to be rebuilt after the War.The second was the possibility of Western European countries falling into thehands of Communism. If the communist world would expand, this would under-mine the position of the democratic United States as the world most powerfulcountry. In its efforts to tackle both problems at once, the U.S. saw an impor-tant role for the rehabilitation of Western European Science, which significantly

94As quoted by ibid., p. 155.95Ibid., p. 156.96Ibid., p.156.

32

4.3 U.S.-Europe inequalities after World War II

lagged behind American science. A comparison of the number of scientists inEurope and the U.S., gives an idea of the postwar scientific inequality betweenWestern Europe and the United States. A 1956 NATO consulting publicationby the Committee of Three reveals that the number of scientists and engineersper million inhabitants in the U.S. was almost 3 times higher than in Europe.Also in absolute number, the estimated 27.200 European scientists and engineerswere outnumbered by their 47.700 American colleagues.97 These figures showthat there existed an immense scientific and technological inequality betweenthe United States and its befriended Western European countries.

As pointed out in section 4.2, the training of scientists, and physicists andengineers in particular, was seen as a basic necessity for American society. If theU.S. wished to construct a scientific community capable of competing with theSoviet Union, any help would be welcome. Namely, the earlier mentioned NATOreport had shown that when the scientific population of all NATO countrieswould be added to the American number of scientists and engineers, it wouldequal the amount of scientists and engineers in the Soviet Union, approximately75.000. Furthermore, there was a fear among the Americans that if they wouldnot act persuasively in setting up scientific collaborations with Europe, theSoviet Union would. By creating closer ties with its allies, it would be possibleto solidify the position as the main economical and political power in the world.

Now, if European science was to be united with American science, its charac-ter needed to be modified to American standards. John Krige has defended theidea that “in the first decade or two after 1945, the United States attempted touse its scientific and technological leadership, in conjunction with its economic,military and industrial strength, to shape the research agendas, the institutions,and the allegiances of scientists in Western Europe in line with U.S. scientific,political, and ideological interests in the region.”98 If the U.S. has indeed actedin this way, it seems naive to think that the effects on science of the Cold Warwere limited within the U.S. borders. But how does it make sense that the Euro-pean countries, which such a long and strong scientific tradition, would have letthis happen? According to Krige, the Western European countries were awarethat they would have to catch up as soon as possible with the new world leader,and thus had little choice in accepting any form of American support. Krigestates that all governments “quickly became convinced that their countries’ in-terests, and perhaps their own political fortunes, were best served by alignmentin the new field of US strength.”99 In addition, it has been stated by Hoeneveldand Van Dongen that especially in small Western European nations “connec-tions with the USA were actively sought” and ”institutions were . . . lined up andwilling to co-construct US hegemony in physics.”100 It can be concluded that in

97These estimates are derived from Recruitment and Training of Scientists, Engineers andTechnicians in NATO Countries and the Soviet Union, a report by NATO consultant RobertMajor on 26 November 1956.

98Krige, 2008, p.3.99Ibid., p.5.

100Commenting on the common attitude in Norway and the Netherlands, respectively Hoen-eveld, 2013, p. 274.

33


most Western European countries, there was a positive attitude regarding thegrowing involvement of the United States in European physics.

The basis of the American support to the reconstruction of Western Europeafter World War II was recorded in the Marshall Plan, which aimed at recon-structing Europe according to American ideas and values. In achieving the goalsof the Marshall Plan, the U.S. saw the rehabilitation of European science as acrucial step. A powerful scientific community in Western Europe provided theUnited States with three main advantages:101 First, the international characterof basic science would create a platform where the scientific elites of both sidesof the Atlantic were able to communicate. Second, scientific development wouldcontribute to economic prosperity in Europe. Without poverty and social un-rest, the Americans especially hoped to ban out the communist influences on theContinent. Finally, improving Western European science would eventually leadto a further reinforcement of the American scientific and technological capacityas well. This last advantage has been pointed out unambiguously by the U.S.Atomic Energy Commissioners in 1947: “With its superior technological poten-tial, the United States can expect to profit more quickly and more fully thanany other nation from the exploitation of published findings from Europe.”102

4.4 American funding of European physics

There were two main ways in which the ”reshaping” of Western Europeanscience took place. First, there was an increasing collaboration between thegovernments of the United States and European countries, illustrated by theformation of The North Atlantic Treaty Organisation (NATO) in 1949.103 TheUnited States wished to build a transatlantic scientific community which shouldcover at least all the European NATO countries.104 These efforts suited theAmerican ideal of a unified democratic world, because it provided the Westernworld with a continuous exchange of scientific ideas and results. Apart fromthe NATO countries’ governments, private American foundations such as theFord Foundation and the Rockefeller Foundation played an important role inthe distribution of the American democratic ideals. These organisations, origi-nally founded in order to “promote the well being of mankind throughout theworld”105, started to interfere with the scientific exchange between the UnitedStates and Western Europe from the beginning of the 20th century. Like theAmerican government, the foundations believed in internationalism as a toolfor global prosperity, and like the American government, their financial possi-bilities were impressive. Both the American government and foundations wereled by officers who had to decide for themselves whether a scientific project was

101As summed up by Krige, 2008, p.10, 11.102As cited by ibid., p. 13.103It was followed by the installation of the NATO Science Committee in 1958.104Belgium, Denmark, France, Iceland, Italy, Luxembourg, Netherlands, Norway, Portugal

and the United Kingdom were among the founders in 1949, together with Canada and theUnited States. Greece en Turkey joined in 1952, West-Germany in 1955.105As reads the historical motto of the Rockefeller foundation, see its official website:www.rockfound.org

34

4.4 American funding of European physics

suitable for granting or not. Because they often had a lack of knowledge on thedetails of physics, they enlisted the assistance of renowned American physicistslike Isidor Rabi and Robert Oppenheimer. In this way, American physicists weredirectly influencing the financial flows in Western European physics. As an ad-vantage compared to the U.S. government, the foundations could show off withpolitical independence, which made them more credible in honestly promotingthe exchange of cultural values between the NATO countries.106

Another important factor in the allocation of funding to projects in Euro-pean physics, was the need for exclusion of communist influences, as desiredby the U.S. government. The exclusion of communists provided the U.S. gov-ernment officials with difficulties, since the distinction between communists andnon-communists was vague in Western Europe when compared to the situationin the U.S. For the foundation officers, things were even more complicated. Theyoften found themselves in between the desires of the U.S. government and thoseof the European physicists, who whether or not had communist sympathies. Intheir home country, there was a danger of being accused of supporting com-munism, while in Europe their credibility would be strained if the foundationswould show any political commitment. This tension was built up even moreby anti-communist speeches from American politicians. For example, CongressRepresentative E.E. Cox warned that “funds have been used to finance individ-uals and organisations whose business it has been to get communism into theprivate and public schools of the country, to talk down America and to playup Russia.”107 Although these accusations might have been false, they wererepresentative for the pressure the foundations experienced during their effortsto rebuild European science. Richard Bissel jr., advising the Ford Foundationin 1952, stated that the West should try to “. . . live in the same world as theRussians without going to war with them, despite profound and continuing dif-ferences of philosophy and interest.”108 This quote endorses the foundations’preference for basic science, getting around the differences and focusing on thesimilarities between scientists from different countries. In this way, the risk ofbeing accused of communist support was diminished.

By the establishment of summer schools, the financial support of outstandingEuropean students and the funding of long term international research projects,both the U.S. government and the foundations tried to redirect European sci-ence towards American standards. Sander Bais recognises the effectiveness ofthis American strategy: ”The Americans have collected, educated and finan-cially supported all the academic talent in the world. Only the best stayed inAmerica, and those who eventually returned home provided for the formationof an international scientific network, with the United States in the middle ofit.”109 Two examples of European research institutions that were in contactwith American funding be discussed in the following. First, we will study the

106Rupp, 1997, p.74.107Krige, 2008, p. 140.108As quoted by Berghahn, 2002.109Bais left for Caltech university in the beginning of the 70s, after he completed his studies

at TU Delft

35


establishment of the European Organisation for Nuclear Research - or CERN.By doing this, the activities and considerations of both the American govern-ment and the foundations in postwar European physics should become clear.Furthermore, we will try to characterise the type of funding that suited theAmerican ideals, and relate it to the limited attention to alternative interpreta-tions of quantum mechanics. Second, we will see how the Niels Bohr Institutein Copenhagen related to American funding.

John Krige has described the events surrounding the French plans to builda nuclear reactor in the 1950s. He stresses that the well-known American physi-cists Isidor Rabi has played a decisive role in the final implementation of theseplans.110 Different aspects of the plan were reshaped in order to align it withAmerican ideas on the role of European physics. Most importantly, the linkwith military applications was considered too close, and the number of inter-national partners too small. Hence, Rabi transformed the plan in what laterhas become known as the realisation of the European Organisation for NuclearResearch (CERN). Rabi invited the governments of Italy, Yugoslavia and WestGermany to participate to the plan as well. It was important for the U.S. thatthese countries would not to fall in the hands of Communism. By tighteningthe scientific bonds with these countries, the U.S. hoped to ensure their partic-ipation to a transatlantic, democratic community. Not only the participatingcountries were selected by representatives of the U.S. government, also the sci-entific goal of the project was adapted so that it would fit the American desires.According to U.S. desires, this centre would focus solely on the developmentof fundamental knowledge of physics. While the original plan focused mainlyon nuclear physics, Rabi and his fellow diplomats advocated a focus on highenergy physics. Those who had initially planned to build a nuclear reactor inFrance, had little choice in accepting Rabi’s modifications of the original theplan. Namely, they were dependent on the U.S. for the funding of their project.

Earlier, we have seen that pragmatic physicists from the United States likeRabi were sceptical towards alternative interpretations of quantum mechan-ics. If this type of physicist was involved in determining the new direction ofEuropean physics, this was a bad signal for the development of quantum inter-pretations.

Niels Bohr’s own institute for theoretical physics, The Institute for Theoreti-cal Physics of Copenhagen, was founded in 1921. Because of the lack of money inDenmark and the expanding capabilities of the United States, Bohr had shiftedfocus towards the international scientific community for financial support ofhis institute already before World War II. When the Rockefeller foundation’sInternational Education Board (IEB) was founded in 1923, Bohr instantly ap-plied for funding on behalf of the extension of his institute. As he wrote in ahis application later to the IEB: “the present economic depression in Denmarkmakes it difficult to procure considerable sums from private sources.” Readinghis further comments in the letter, it is remarkable how similar Bohr’s visionon science was to the U.S.: “The peculiar character of the Institute is a close

110Krige, 2008, chapter 3.

36

4.5 U.S. influence on interpreting quantum mechanics in Europe

cooperation between theory and experiment which is a necessary condition forproductive work in atomic physics, and this explains why many physicists fromother countries, and not least from the United States, wish to study here.”111

Bohr’s application was successful, and he was granted by an American organ-isation as the first European physicist. From this moment, Bohr’s ties withAmerican foundations continued to be close. While Bohr was the head of hisown institute in Copenhagen, and Heisenberg at the Max Planck Institute inGottingen, Leon Rosenfeld and other physicists took over the role of Niels Bohrand Werner Heisenberg as the main defenders of the Copenhagen interpreta-tion. As we have already seen in the cases of Everett and Bohm, the tone ofthe discussion changed simultaneously with those who were involved in it. Bohrhad been of major importance for the developments on the interpretation ofquantum mechanics, by doing the groundwork on the Copenhagen interpreta-tion and swiping Einstein’s objections off the table. But apart from being aleading physicist in Europe, Niels Bohr also had a political agenda. He believedin constructing an open international community for physicists from all overthe world. Already before the war, Bohr had realised that there were chancesfor European physics in collaborating with the Americans. As the director ofone of the most internationally minded European research institutes, Niels Bohrplayed a key role in the fund raising for European physics after the Second WorldWar.112 It was thus in the benefit of the U.S. government and foundations toget together with Bohr, in their plans of the redevelopment of European science.The former chairman of the Ford Foundation, Rowan Gaither, has illustratedthis image by endorsing Bohr’s “dedication to the principles of the free worldand his efforts to improve international understanding in the interests of peace”.Gaither concluded that “his influence transcends national boundary”.113 In de-scribing the relation between Bohr and the emergence of a transatlantic scientificcommunity, Krige has explained Bohr’s position in the United States as “thecharismatic international scientific statesman, the promoter of world peace, andthe sure friend of democracy and the United States.”


The influence of American officials in the final implementation of the plan forCERN has shown that European physicists were not evidently able to choosetheir own line of research. In the realisation of this international project, Amer-ican ideas on the development of physics were decisive. In general, the U.S. gov-ernment and foundations were glad to support internationally minded researchorganisations like CERN and the Niels Bohr Institute. It was the thought thatin these places, physicists from different countries could collaborate in projectscomplementary to American research. Since these projects would preferablyocus on basic, experimental research, it makes sense that the philosophical and

111Both quotes are from a letter from Niels Bohr to International Education Board, 27 June1923, available in the online archives of the Rockefeller foundation112Krige, 2008; Pais, 1991.113These quotes were collected by Krige, 2008, p. 170.

37


ideological component of the debate on the interpretation of quantum mechan-ics did not attract American funding. Meanwhile, the infallible status of theCopenhagen interpretation continued to silence new ideas about interpretativeissues. Let us recall a quote from Einstein from 1928: “The soothing philos-ophy - or religion? - of Heisenberg-Bohr is so cleverly concocted that for thepresent it offers the believers a soft resting pillow from which they are not easilychased away.”114 Noting his last few words, it seems that Einstein had alreadyobserved that he was fighting a lost battle. Maybe, he hoped that the situationwould be turned around 30 years later, but it had not. European physics wasbeing reconstructed under American leadership, but there was very little actionin reviving Einstein’s pending objections. One might then raise the questionwhat role the main figures of prewar quantum mechanics have played duringthe increasing influence of the United States on European physics, for it seemslogical the debate would be sustained between these physicists. As an obviousexample, we have discussed the case of Niels Bohr in particular. For him, ex-ploring the possibilities for the internationalisation of his institute occupied hismind after World War II.

For both American officials like Rabi and the European scientific elite, allof them staying true to the Copenhagen interpretation of quantum mechanics,a continuation of the quantum debate was not evident. Instead, it was in theinterest of both to set up a transatlantic scientific network that provided theEuropean countries with work, and the United States with control.

In addition to the preceding disquisition on the influence of the United Stateson the quantum debate, two remarks on the main line of this story must beaffixed. First, it is important to realise that also under American influence,European physics has not necessarily lost its original character. I have outlinedseveral cases in postwar European physics in which American desires have beendecisive. Stereotyping European science until World War II, one often comes upwith Wilhelm von Humboldt’s bildungideal. In this view on the role of scien-tific education, it is all about cultivation of the individual. This view contrastswith the scientific training which was apparent in American postwar physics.In respect to this dichotomy, and commenting on the Americanisation of Dutchscience, Jan Rupp has acknowledged the increasing influence of American ideasin postwar science, but stressed that the Dutch have always continued buildingfurther on their own traditions as well.115 While the question remains whetherthis conclusion can be extrapolated to other Western European countries, it in-dicates that American influences rather mingled with European tradition, thanthat they replaced it.

Second, in the distinction that has been made between the reception ofprewar and postwar interpretations of quantum mechanics, pragmatism has re-peatedly been identified with American physics. This treatment might deservea nuance, for most physicists in Europe had never been involved in the quantum

114In a letter from Einstein to Schrodinger on 31 may 1928, reprinted in Letters on WaveMechanics, M. Klein, New York: Philosophical Library, 1967115Rupp, 1997, concludes this on the basis of research on scientific and political developments

in the Netherlands.

38


debate, also before World War II. Heilbron has suggested that apart from theCopenhagen circle – consisting of Bohr, Heisenberg, Pauli, Born and Rosenfeld– and their earliest opponents – Einstein and Heisenberg – physicists were notengaged with the details of complementarity, let alone that they criticised it.Instead, the vast majority of physicists would rather focus on the applicationof quantum theory: “Texts on quantum physics and wave mechanics publishedduring the 1930s by people outside Bohr’s circle as a rule do not mention com-plementarity. This is true not only of American and Soviet books, where onemight not expect to find it, but also of German ones.116 Because the discussionbetween Bohr and Einstein mainly developed outside the textbook used to edu-cate physicists, a “social and intellectual division” took place among physicistsbefore World War II, says Freire: “a few of them involved with foundationalproblems . . . and the others involves with extension and applications, but be-lieving that the foundational problems were well solved by the founder fathersof quantum mechanics.”117 Because of the relatively slow rate of scientific ex-change from the U.S. to Europe at this time, it is not evident that this ratherpragmatic attitude among physicists in prewar Europe was caused solely byAmerican influences.118

116Heilbron, 1985, p. 206.117Freire Jr., 2004, p. 1743.118As outlined in section 4.1, the exchange of scientists before World War II mainly took

place in the opposite direction.

39

5 Concluding Remarks

5.1 Conclusion

By outlining the stories of David Bohm, John Bell, Dieter Zeh and Hugh Ev-erett III, it became clear that it was not evident to focus on the interpretationsof quantum mechanics as a postwar physicist. Bohm and Everett found bigdifficulties in spreading their ideas in the 1950s and 1960s. For both of them,their careers would have possibly developed more smoothly if they would havefocused on other research topics. It is remarkable that eventually, Everett re-treated from science and made a successful career in the American militaryindustry, matching the Cold War context of American physics. In both caseshowever, the objections to the Copenhagen interpretation proved to be relevantin later decades and in a different context, when physicists outside the Copen-hagen circle brought them under renewed attention. We have also seen that ittook John Bell until 1964 to arrive to the foundations of quantum mechanicswith full dedication, while his attention to the topic had been already caughtin 1952. In the mean time, his mind was being occupied by all the possibili-ties in Europe’s newest and finest research institute, CERN. Finally, the storyof Dieter Zeh has shown that even during the end of the 1960s, the WesternEuropean community of physicists seemed not willing yet to conduct a seriousdiscussion about new ideas on the interpretation of quantum mechanics. Fromthe stories of these Copenhagen critics, we can conclude that during the 1950sand 1960s, there has indeed been an unfavourable climate for the proposal ofalternative interpretations of quantum mechanics. A pragmatic attitude amongphysicists towards the theory of quantum mechanics has apparently caused scep-ticism regarding the proposal of alternative interpretations in both Europe andthe United States.

We have asked ourselves whether this attitude might have been derived fromAmerican democratic values. In the United States during the 1950s and 1960s,the increasing number of students had induced an emphasis on the training andapplication of mathematical techniques rather than the discussion philosophicalquestions on the interpretation of physical theories such as quantum mechanics.The pragmatic attitude among American physicists was reflected in the type oforganisation, where theory and experiment were unified. As we have seen, thesekind organisational structures have been exported to Western Europe in the1950s and 1960s. For example by the establishment of CERN and the increas-ing international exchange of both students and researchers. Those who hadthe opportunity to invest in postwar European physics – the U.S. governmentand private foundations – preferred to support the establishment and consolida-tion of internationally minded research institutes such as CERN and the NielsBohr Institute in Copenhagen. What was more, both CERN and the NBI wereconstructed to the American model of unified physical theory and experiment,based on pragmatic ideals.

Especially in Copenhagen, the place where complementarity had been in-vented, it was unlikely that the discussion of alternative interpretations of quan-

5.2 Discussion

tum mechanics was to be found high on the research agenda. Indeed, we haveseen that Everett and Bohm found big difficulties in discussing their ideas withphysicists from the Copenhagen circle, such as Leon Rosenfeld and WolfgangPauli. While the monocracy of the Copenhagen interpretation explains the littlesuccess of those who have tried to contribute to the debate during this period,it is important to note that the interference of the American government andprivate foundations have contributed to a climate with little focus on the philos-ophy of physics as well. In short, the American interference in European physicsafter World War II seems to have reinforced the indifference among Europeanphysicists with the interpretation of quantum mechanics. The career of JohnBell serves as a perfect example for this development. He only began to focuson the foundations of quantum mechanics, once he had gained his position atCERN, where he had worked for 12 years. As has been shown in section 4.4, theresearch that was performed at this institute had been broadly determined bythe American government, in consultation with renowned American physicists.In this postwar climate of European physics, young talents like John Bell werenot stimulated to pursue a hermeneutic tradition of doing physics. Instead, theywere driven towards a more pragmatical attitude.

When these developments are put in the perspective of interpreting quantummechanics, the American interference in Western European physics thus appearsto have contributed in maintaining the Copenhagen monocracy.

Meanwhile, it seems necessary to weaken this conclusion, when comparingthe prewar and postwar developments on the interpretation of quantum mechan-ics. Also before World War II, the vast majority of physicists was not involvedin the debate on the foundations of quantum mechanics. The discussion in thisperiod mainly developed behind the scenes between Niels Bohr, Albert Einstein,and a couple of philosophically inclined physicists. Noting this, the question israised whether a pragmatic tradition suits to American physics in particular, orto physics as a scientific discipline in general.

5.2 Discussion

To understand the process of Americanisation of physics in general, a study ofthe correspondence between the key figures of European and American physicsis recommended. Besides, the reception of Bohm, Everett and Bell’s contribu-tions to the interpretation of quantum mechanics by the active adherents tothe Copenhagen interpretation, deserves to be further investigated. Perhapsthen, other clues for American influence in European institutions for physicscould be detected. A complicating factor in this type of research is the fact thatAmericanisation of European physics manifests itself implicitly.

Another interesting way of testing the American influence, would be to com-pare the context of European theoretical physics before and after 1970. Fromthe 1970s, the foundations of quantum mechanics gradually shifted towards themainstream of theoretical physics. As another perspective to the research ques-tion of this study, one might wonder what factors have contributed to the revivedattention to the foundations of quantum mechanics after 1970. Olival Freire jr.

41

5 Concluding Remarks

has mentioned the effect of the rise of a new generation of physicists, and theirdissatisfaction with the way they had learned quantum theory at university.119

In line with Freire, David Kaiser has stated that the generation of the hippieshas revived the attention to the foundations of quantum mechanics in the be-ginning of the 1970s.120 As an opposed view, one could state that the attentionto the foundations of quantum mechanics only got revived, once experimentsgot involved. In the 1970s and 1980s, exactly this was done by John Clauserand Alain Aspect, continuing on the work of John Bell. Parallel to these de-velopments, also the ideas of Everett and Bohm came back in the mainstreampicture of physics. Whether a priori extant in physics or influenced by Americandemocratic ideals, the valuation of experiment suits to a pragmatic attitude.

119Freire Jr., 2004.120Kaiser, 2011.

42

Acknowledgements

First of all, I would like to thank prof. dr. Jeroen van Dongen for intro-ducing me to this fascinating subject, and for providing me with an interestingresearch question. Also, his comments on some previous versions of this texthave been highly appreciated. Second, special thanks go to prof. dr. DennisDieks and prof. dr. ir. Sander Bais for giving me the possibility to interviewthem on behalf of this study. Their knowledge and experiences have been provenvaluable.

References

Aaserud, F. (1990). Redirecting Science. Cambridge University Press.Bell, J. (1964). “On the Einstein Podolsky Rosen Paradox”. In: Physics 1,

pp. 195–200.Beller, M. (1999). Quantum dialogue: the making of a revolution. The University

of Chicago Press, Chicago.Berghahn, V. R. (2002). America and the Intellectual Cold Wars in Europe.

Princeton University Press.Blokhintzev, D. (1952). “Criticism of the Philosophical Views of the so-called

Copenhagen School in Physics”. In: Philosophical Problems of Modern Physics1, pp. 95–129.

Bohm, D. (1951). Quantum Theory. Prentice Hall: New York.— (1952a). “A Suggested Interpretation of the Quantum Theory in Terms Of

”Hidden Variables”. I”. In: Physical Review 85, pp. 166–179.— (1952b). “A Suggested Interpretation of the Quantum Theory in Terms Of

”Hidden Variables”. II”. In: Physical Review 85, pp. 180–193.Bohr, N. (1935). “Can Quantum Mechanical Description of Physical Reality be

Considered Complete?” In: Physical Review 48, pp. 696–702.Byrne, P. (2007). “The Many Worlds of Hugh Everett”. In: Scientific American.Camilleri, K. (2009a). “A history of entanglement: Decoherence and the inter-

pretation problem”. In: Studies in History and Philosophy of Modern Physics40, pp. 290–302.

— (2009b). “Constructing the Myth of the Copenhagen Interpretation”. In:Perspectives on Science 17, pp. 26–57.

Cushing, J. T. (1994). Quantum Mechanics: Historical Contingency and theCopenhagen Hegemony.

Davies, P. (1989). Introduction to Physics and Philosophy by Werner Heisenberg.Penguin books, London.

De Broglie, L. (1930). An Introduction to the Study of Wave Mechanics. E. P.Dutton and Company: New York.

— (1954). The Revolution in Physics: A Non-Mathematical Survey of Quanta.London: Kegan and Paul.

De Tocqueville, A. (1840). De la dmocratie en Amrique. London: Saunders andOtley.

Dieks D., Vermaas P. E. (1998). Modal Interpretations of Quantum Mechanics.Kluwer Academic Publishers.

Einstein A., Podolsky B. Rosen N. (1935). “Can Quantum Mechanical Descrip-tion of Physical Reality be Considered Complete?” In: Physical Review 47,pp. 777–780.

Everett, H. (1957). ““Relative State” Formulation of Quantum Mechanics”. In:Reviews of Modern Physics 29, pp. 454–462.

Freire Jr., O. (2004). “The Historical Roots of Foundations of Quantum Physicsas a Field of Research”. In: Foundations of Physics 34, pp. 1741–1760.

REFERENCES

— (2005). “Science and exile: David Bohm, the Cold War, and a New Inter-pretation of Quantum Mechanics”. In: Historical Studies in the Physical andBiological Sciences 36, pp. 1–34.

— (2009). “Quantum dissidents: Research on the foundations of quantum the-ory circa 1970”. In: Studies in History and Philosophy of Modern Physics40, pp. 280–289.

Gadamer, H. (2014). Waarheid en Methode: hoofdlijnen van een filosofischehermeneutiek, nl. vert. Vantilt.

Griffiths, D. J. (2005). Introduction to Quantum Mechanics, Second Edition.Pearson Education.

Heilbron, J. L. (1985). “The earliest missionaries of the Copenhagen spirit ”.In: Revue d’histoire des sciences 38, pp. 195 –230.

Heisenberg, W. (1958). Physik und Philosophie: Weltperspektiven. Ullstein Taschen-bcher.

Hoeneveld F., van Dongen J. (2013). “Out of a Clear Blue Sky? FOM, TheBomb, and The Boost in Dutch Physics Funding after World War II”. In:Centaurus 55, pp. 264–293.

Howard, D. (2004). “Who Invented the ”Copenhagen Interpretation”?” In: Phi-losophy of Science 71, pp. 669–682.

Jammer, M. (1974). The philosophy of quantum mechanics: The interpretationof quantum mechanics in historical perspective. New York: Wiley.

Kaiser, D. (2002). “Cold War requisitions, scientific manpower, and the produc-tion of American physicists after World War II”. In: Historical Studies inthe Physical and Biological Sciences 33, pp. 131–159.

— (2011). How the Hippies Saved Physics; Science, Counterculture, and theQuantum Revival. W. W. Norton.

Kemble, E. C. (1935). “The correlation of wave functions with the states ofphysical systems”. In: Physical Review 47, pp. 973–974.

— (1937). The Fundamental Principles of Quantum Mechanics. McGraw-Hill:New York.

Krige, J. (2008). American Hegemony and the Postwar Reconstruction of Sci-ence in Europe. MIT Press.

Myrvold, W. C. (2003). “On some early objections to Bohm’s theory”. In: In-ternational Studies in the Philosophy of Science 17, pp. 7–24.

Pais, A. (1991). Niels Bohr’s Times in Physics, Philosophy and Polity. OxfordUniversity Press.

Rupp, J.C.C. (1997). Van oude en nieuwe universiteiten. De verdringing vanDuitse door Amerikaanse invloeden op de wetenschapsbeoefening en het hogeronderwijs in Nederland, 1945-1995. Sdu.

Schweber, S. S. (1986). “The Empiricist Temper Regnant: Theoretical Physicsin the United States 1920-1950”. In: Historical Studies in the Physical andBiological Sciences 17, pp. 55–98.

Slater, J. C. (1938). “Electrodynamics of ponderable bodies”. In: Franklin In-stitute Journal 225, pp. 277 –287.

Zeh, H. D. (2006). “Roots and Fruits of Decoherence”. In:

45

The Foundations of Quantum Mechanics in Postwar ... · of quantum mechanics in the rst decades...

Documents

Transcript of The Foundations of Quantum Mechanics in Postwar ... · of quantum mechanics in the rst decades...