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Home , David Bohm, Quantum Reality

Phys Educ Vol 14, 1979 Prlnted in Great Brltaln

CONTROVERSIESIN PHYSICS aggressiveness which resembles more the defence of orthodoxy of one ideology than a spirit of scientific objectivity’ (Jauch 1973, introduction). The hidden- ‘The entirely reasonable question, Are there hidden- variable theories consistent with quantum theory and variables if so whatare their characteristics?,has been unfortunately clouded by emotionalism’ (Ballentine controversy in 1970, p374). Perhaps some of the acrimonv which has surrounded this debate can be seen in the following quantum physics comment on the workof hidden-variable theorists made by Leon Rosenfeld, an eminent and TREVOR J PINCH defender of the orthodoxview of quantum theory: Science Studies Centre, Universityof Bath ‘That suchirrational dogmatists shouldhurl the very accusation of irrationality and dogmatism at the defenders of the common-sense, uncommitted attitude of other scientists is the crowning paradox which gives Recent studies of controversies in science have shown a touch of comedy to a controversy so distressingly that they are not always settled by appeals to Nature pointless and untimely’ (Rosenfeld 1958, p658). alone. Social and political factors as well as scientific These are hardly the sorts of comments we would factors often combine to produce the outcome(see for expect to follow from unproblematicreadings of example Forman 1971, Collins 1975, Frankel 1976 Nature’s secrets. So what was at stake in this dispute? and Wynne 1976). The ‘truth’, it seems, emerges from To answer thisquestion we need to go back to the whatcan be asurprisingly volatile struggle, very revolution in physics brought about by the develop- unlike the rarified atmosphere of noblesse within ment of quantum theoryin the 1920s. which suchdebates are generally thoughtto take place. Traditionally, scientific knowledge is portrayed as Statistical natureof quantum theory stemming from relatively unproblematic ‘readings’ of Thequantum theory is fundamentallya statistical Nature-essentially science is seen as a process in theoryreplacing the causal description given by which humans play a passive role.. This view has been classicalmechanics. The statistical nature of the challenged by those who wish to argue that science is, theory is manifest in manyfundamental processes, first and foremost, a product of human activity-that such as radioactivity.Although it is known thatat is, that scientific knowledge results from the creative some point a radioactive atom will decay, the exact activity of scientists (Kuhn 1970). Scientific moment cannot be specified according to the quantum knowledge is a social product constructed, and indeed theory. Radioactive decay, like all quantum processes, fought for, by scientists in particular social and is essentially statistical, and all that can be given is a historical settings. In this view it is at times of probability that the atom will decay in a certain time scientific controversy, when all orpart of such interval. No such hiatus existed in classical physics. knowledge comesunder challenge, thatthe social The lawsdescribing physical systems were fully causal dimensions of science become clearest. Just as natural and their behaviour could, in principle, be determined scientistsoften learn most aboutthe systemunder throughout space andtime. study when it is experiencing its greatest perturbation When it became apparent that the quantum theory or stress, so toocan the studentof the scientific was astatistical theory,the question arose in the enterprise find the stress produced by a scientific con- minds of the early quantum as to whether troversy most rewarding for revealing the social there wassome causal substratum underlying the processes of science. world of statistical effects. Perhaps Nature did obey The hidden-variables controversy, which raged so deterministiclaws but we did not yethave a good vociferously in the 1950s and early 1960s following enoughtheory toproduce sucha description. The the challenge posed by hidden-variables theories to the phenomenon of Brownian was called upon as orthodox version of quantumtheory, is gooda an analogy. The seemingly random ‘joggling’ motion example of a controversy which was obviously of smokea particle suspended in a gascan be permeated by social influences. Take, for instance, the explained in terms of themany collisions ofgas following statements by physicists concerned with the molecules with thesmoke particle.These gas controversy: molecules moveaccording tothe laws of classical ‘. . . the discussions which surround the quest for and it is only their aggregateproperties hidden variables in have, on both which appear to be random. The purpose of hidden- sides of the camp, often been conducted in a spirit of variablestheorists in quantum mechanicswas to

0031-9120/79/010048+05501 00 0 1979Thelnst~iuteofPhysics postulate a substratum ofsuch ‘hidden’ variables physicist came to talk to him about the theory. This analogous to the gas molecules in classical statistical physicist was and it is he who has been mechanics.These new variables would allow the atthe centre of the hidden-variablescontroversy. statistical theoryto be embedded in a morefunda- Bohm was teaching at Princeton having first obtained mentaldeterministic theory. The problemwas the his PhD at the University of California, Berkeley. He reverse of that encountered in statisticalmechanics had become interested in the foundationsof the theory where the behaviour of ensembles of systems has tobe when attending J Robert Oppenheimer’slectures on generalised from the behaviour of individual systems. quantum mechanics at Berkeley. Bohm wasnot In quantum physics the aim of hidden-variables altogether satisfied with the theory, so he that theoristswas to provide an explanationfor the the best way to get to understand it was to write a behaviour of individual systems based on the statistics book about it. Hisbook, entitled Quantum Theory, of their ensembles. was published in 1951 (Bohm 1960)and, was Hidden-variables approaches thus do not attempta favourably reviewed by several eminent quantum radical break from conventional quantum mechanics. physicists (it has since become one of the standard Quantum mechanics is much too successful a theory texts). Einstein in particular liked thebook and to be completely incorrect. Rather a hidden-variables thought it was the best presentation of the theory that theory will supplement or refine theconventional could be had,and he invited Bohm tocome and viewpoint and will account for most if not all of the discuss it. Having written the book and come down in data explained by the usual theory. The hope was that favour of the orthodox version of the theory, Bohm the level at which the hidden variables operated would still found quantum mechanics hardto understand. eventually become accessible to experiment and then a Stimulated by Einstein’s criticisms and by the crucial test for the viability of such theories could be criticisms made by the Soviet physicists Blokhintsev made. and Terletzkii, Bohm proceeded attemptto to The early debate over hidden variables in quantum construct a hidden-variables version. He found that he theory was settled by the work of the German math- could produce such a theory and, furthermore, that it ematician, . In the sametreatise in was logically consistent and accounted for all the data which he outlined the quantum theory as an operator which the normal version explained. Bohm sent out a calculus in a Hilbert space he showed by his famous preprint of the paper to several quantum physicists. ‘impossibility proofthat no hidden-variablesinter- Thepaper was eventually published in Physical pretationcould be consistent with theestablished Review (Bohm 1952). theory (von Neumann 1955). von Neumann’sproof Meanwhile Bohm found himself a victim of senator was based on four axioms which he used to deduce the McCarthy’s Committee on unAmerican Activities and normal statistical predictions of quantum theory. He as a result he lost his post at Princeton and took up a showed thatthe type of ensembles implied by the new post at the University of Sao Pauloin . As if existence of hiddenvariables could not be derived Bohm did not havetroubles enough, he found that from these axioms. The proof consisted of a highly physicists were rejecting his hidden-variables version esoteric, mathematicalargument but the conclusion of the theory. In particular both Pauli and de Broglie was plain enough: referred him back to de Broglie’s attempts to construct ‘It is thereforenot, as often assumed, a question of a a similar theory (the ‘double solution’ theory) in 1927. reinterpretation of quantum mechanics: the present Even Einstein regarded Bohm’s theoryas unsatis- systemof quantum mechanics would have to be factory. Bohm had been unaware of the earlier theory objectively false, in order that another description of of de Broglie which had had to be abandoned because theelementary processes thanthe statistical one be of technical objections. When Bohm looked up these possible’ (von Neumann 1955, p325). objections he found he couldovercome them by Most quantum physicists accepted this and felt that extending his theory to account for the measurement it put an end to the matter. However, there were some process. Bohm’s hidden-variable account of measure- dissenters who hoped that a more satisfactory version ment was published in Physical Review as a sequel to of the theory would be produced. The most notable of the main paper. these was, of course, Einstein. He never accepted the quantumtheory as the last word in microscopic physics-aview he summarised in his famous Bohm’s hidden-variable theory aphorism, ‘I do notbelieve that Godplays dice’. The theory centred on a new physical interpretation of Schrodinger’s equation. By writing the wavefunction ty as ty = R exp (ish), where R and S are real functions, Bohm’s work on hidden variables Bohm was able to express Schrodinger’s equation in a Einstein spent his lasttwenty yearsat Princeton, form similar to a classical Hamilton-Jacobi equation. isolated from mainstream thought in quantum theory, By giving every particle of the ensemble a position x and it was in this later part of his life that a young and momentuma mu, hecould then define a

49 continuoustrajectory for the particle if its initial view thatsuch disturbances were inherently position was known. In practice, experiments did not unanalysable. allow the initial position to be precisely determined and hence x was a ‘hidden variable’. Using the Hamilton-Jacobi analogy, Bohmconcluded thatthe Response to Bohm’s theory particlecould be regarded as being subject to a In general the reaction to Bohm’s work was negative, U(x)as well asthe classical although he did win over some important allies. The potential V(x).The quantumpotential was given by: most important of these was who responded to Bohm’s paper by readopting the hidden- U(x>= -AZv2R/2mR. variables approach he hadabandoned twenty-five Thus, inthis theory, U/ wasnot an abstract math- years previously. Apart from his own theoretical work ematicalsymbol from which certainprobabilities and encouragement, de Broglie publicly endorsed the could be derived but an objectively real field which attempts to reconstruct the foundations of quantum exerted a force - VUon the particle in a similar way mechanics when he wrote the foreword to the book tothat in which an electromagnetic field affects that Bohm produced on hidden variables in 1957 charges through the Lorentz force. Bohm was able to (Bohm 1957). de Broglie also defended Bohm in showhow this theory treated several standard Nature against attacks made by Leon Rosenfeld (de problems of nonrelativistic quantum mechanics, such Broglie 1958).However, therewas very little active asthe , the many-body problem, support shown for the hidden-variables enterprise and scatteringproblems andthefamous double-slit it was mainly Bohm, de Broglie and other physicists at experiment.Because it wasbased on Schrodinger’s the Institute Henri Poincari in Paris (the most notable equation, Bohm’s theory was not at variance with the being Jean-Paul Vigier) who carried the banner of the conventional formalism and thus accounted for all the radical physicists. The hidden-variables approach was samedata. However, if thehidden variables (the developed by thesephysicists throughoutthe 1950s positions of the particles) were no longer hidden, then and early 1960s but met with little success. the theory could be directlytested. Bohm held out The main type of criticism to be levelled at Bohm’s thehope that,as physicsentered new domains theory was that it was ‘metaphysical’ because it did (dimensions of orderthe cm or less), his not lead toany experimentaldivergences fromthe approach would lead to novel predictions but, as in orthodox theory. Bohm was able to respond to this practice the position of particlea could not be type of criticism by pointing out that the orthodox accuratelydetermined, it was experimentally indis- theory itself relies on metaphysical assumptions, viz tinguishable from conventional quantum mechanics. It that the mostcomplete possible specification of an was, however, conceptually very different. The type of individual system is in terms of a wavefunction that descriptionof quantum systems it gavewas at gives onlyprobable results of actual measurement variance with theusual viewpoint. As Bohm put processes.Bohm at leastreplaced such untestable it: ‘In contrasttothe usualinterpretation, this assumptions by ones which might be testableas alternativeinterpretation permits us to conceive of physics advanced intonew domains. each individual system as being in a precisely Another frequent objection was that Bohm’s theory definable state, whose changes with time are deter- destroyed the symmetry which, in the words of one mined by definite laws, analogous to (but notidentical critic, constituted ‘the powerand the glory of quantum with) the classical equations of motion’ (Bohm 1952, theory’ (Hanson1963, p88). It seems that this p 166). criticism was directed towards what was held to be a Thethrust of the orthodox view wasthat any range of nonsymmetrical properties of Bohm’s theory modelling of was impossible. All that such as,for instance, its removal of particle-wave could be hoped for was a mathematically consistent duality (particles were acted upon by real force fields), theory which gave the probable outcomes of certain and the different roles attributed to pairs of variables experimentalmeasurements. By modelling quantum such as position (which was now a hidden variable) systems in terms of hiddenvariables, Bohm was andmomentum (which was left defined as in the attempting to circumvent this restriction on theorising. orthodox theory). It was also felt that a suggestion of Although Bohm’s approachattempted to restorea Bohm’s for an additional term in Schrodinger’s classical view of quantum reality, his theory was itself equation, which might become important as physics nonclassical because, in his treatment of the quantum entered new domains, destroyed the linearity of the measurement process, he associated hidden variables theory.However, such criticisms seem unconvincing with the measuring apparatus as well as the quantum when used to reject Bohm’s theory because they are system. Thus, as in the conventional view, Bohm held inevitably a matter of taste. For instance, in a different that measurementa disturbs thesystem being context, nonlinear field theories were actively sought measured. By modelling this disturbance using hidden as a way of avoiding thenotorious divergencies in variables, he claimed to be able to go beyond the usual quantum field theory. Although principles suchas symmetryand linearity are undoubtedly important sufficient to automatically elicit fromthe elect the guiding principles in physics, they hardly seem strong remark that John von Neumann gave absolute proof enough to use in order to reject a theory-after all, that this way outis not possible’. Nature might not obey our aesthetic tastes. Although Whatmore definitive waywas there to dismiss some criticsdid attempt to focus on moreconcrete Bohm’s work than to declare it to be impossible? The points Bohm was able to reply to these criticisms. esoteric mathematical proof which had been used to It is important to realise that Bohm’s theory was prevent speculation over the foundations of quantum not one thatcould be quietly forgotten: Bohm himself theorytwenty years earlier was oncemore revived. had a good reputation as a physicist (even his critics However, there was onecrucial difference between the were forced to acknowledge this), and with some earlier situation and that of 1952. Bohm’s theory did supportfrom oldthe guard (references to exist-and no one had yet managed to show where (if conversations with Einstein in his papers plus de anywhere) it was inconsistent or which data it did not Broglie’s support) and a lengthy paper in the leading explain. Thus, physicists relying on vonNeumann’s physics journal, such a challenge to the establishment proof were, in effect, declaring anactual existing would not go unnoticed. With the issues at stakebeing theory to be impossible without showing why this was no less than the foundations of one of the most so. important theories of physics, it is not surprising that Much of the ‘heat’generated by the hidden- defenders of orthodoxy felt compelled to expose the variables controversycan, Ithink, be tracedto the mistake which Bohm must havemade. The future extraordinary faith which some people placed in von direction of research in quantum mechanics could not Neumann’s arguments.Clearly, hidden-variable be left to a matter of taste. theorists were likely to interpret this as a blank refusal After the earlier battles to get the Copenhagen inter- to believe, based on prejudice and dogma, while the pretation accepted at the famed Solvay congresses in establishment, on the other hand, was likely to see the the1920s it washardly likely that the physics elite refusal to acceptvon Neumann’s arguments (which would let the foundations of thetheory be under- after all had in turn never been shown to be invalid) as mined by the post-war generation of physicists. And irresponsible and likely to open once more the doors there is no doubt that several of the younger physicists to pointless speculationover the foundations of the (i.e. those of Bohm’s generation) were becoming theory. increasingly dissatisfied with quantumtheory, as is Clearly, if von Neumann’s proof did make hidden evidenced by the whole host of novel interpretations variables impossible and Bohm’s theory did exist, then which appeared in the 1950s and 1960s. However, to one of the two must be invalid. Over the next fifteen dismiss Bohm’s work in aconvincing mannerwas years considerable efforts were devoted to the exam- noteasy, for, as the establishment was quick to ination of von Neumann’s proof in attempts to find acknowledge, Bohm’s theorywas very cleverly where it was in error and totighten up the reasoning in constructed so asto be logically consistent andto order to rule out theoriessuch as Bohm’s. The cul- predict all the same results as the quantumtheory. The mination of theseefforts was apaper by J S Bell usual way of dismissing an invalid theory is to show (1966), which showed that one of the axioms adopted that it is empirically inadequate or based on mistaken by von NeumaM in his proof implicitly ruled out reasoning-but here both paths were closed. Yet there hidden variables (Bell 1966). Thus it turned out that all was one way to reject the theory. This was to recall that von Neumann had done was to show that hidden von Neumann’s impossibility proof which had served variables as defined by him were impossible. von well fortwenty years inpreventing any hidden- Neumann’s mathematical reasoning was correct but variables theory emerging as it was felt that such a he had overgeneralised fromone particular class of task was foredoomed-if von Neumann’s arguments hidden-variables theory (and not a very likely one) to were correct. all such theories. Thus it seems as if the error lay in the application of von Neumann’sproof ratherthan Bohm’s theory. Indeed, to this day, neither the logical von Neumann’s impossibility proof inconsistency nor empirical inadequacy of Bohm’s One of the main responses to Bohm’s theory was theory has ever been demonstrated. simply to declare that it was invalid becausevon Of course, physics has moved on since the 1950s Neumann had shown that all hidden-variables theories and Bohm’s work on hiddenvariables no longer were impossible. Inother words, no theory which attracts muchinterest today, butthis particular introduced hidden variables could both be consistent episode in the history of physics gives an invaluable with quantum theory and explain all the same lesson to those who maintain that science, especially empirical data.The use of vonNeumann’s proof physics, is not permeated by social factors. For how against Bohm’s work was succinctly described by P W else are we to explain the faith placed in von Bridgman in a review published in 1960: Neumann’s arguments? It seems unlikely that parts of ‘Now the mere mention of concealed parameters is thephysics establishment should suffer a collective

51 Phys Fduc Vol 14 1979 PrNnted ~n GreatBrtla~n hallucination. I have offered elsewhere an explanation (1974). For atechnical account ofhidden-variables forthe power that vonNeumann’s arguments were theories see Belinfante (1973). For more details of the thought to have (Pinch 1977). Briefly I claim that it is use of impossibility proofsagainst hidden-variable the esoteric mathematical nature of these arguments theories see Pinch (1977). which gave them their authority and which madeit difficult to see their physical basis. In viewof the establishment’s need to defend itself against Bohm’s References challenge, vonNeumann’s proof was the most Ballentine L E 1970 Rev. M. Phys.42 358-81 definitive weapon at hand. Belinfante F J 1973 A Survey of Hidden Variables Theories It should be pointed out that I am not criticising the (Oxford: Pergamon) rationality of the scientific enterprise. The point is that, Bell J S 1966 Rev. Mod. Phys.38 447-52 when science is viewed as essentially a human activity, Bohm DJ 1952 Phys. Rev. 85 166-93 it makessense for scientists, like the rest of us, to Bohm D J 1957 Causality and Chance in Modern Physics defend our interests by whatever means are possible. (: Routledge and Kegan Paul) Theinterests of the establishmentclearly lay in the Bohm1960DJ QuantumTheory 8th edn (London: Prentice Hall) orthodox view of quantum theory which had proved Bridgman P W 1960 Sci. Am. 203 206 to be such a successful theory, while Bohm’s interests Collins H M 1975 Sociology 9 205-24 were in trying to establish a radical new basis to the de Broglie L 1958 Nature 181 1874 quantum theory which might prove more successful as Forman P 1971 in McCormrnach R (ed) Historical Studies physics entered new domains. Such a clash of interests in the Physical Sciences vol. 3 (Philadelphia: University is almost boundto produce the type of acrimony of Press) ppl-l15 which we normallyassociate with overtly social Frankel E 1976 Social Studies of Science 6 141-84 conflicts but which is generally hidden in science. The Hanson N R 1963 The Concept of the Positron (London: significance of suchclashes in science is that they Cambridge University Press) show that scientific knowledge, produced by humans, JammerM 1974 The Philosophy of QuantumMechanics is ipso facto permeated by social influences. (New York: Wiley) JauchJM 1973 Are Quanta Real? (London: Indiana University Press) Kuhn T S 1970 The Structure of Scientific Revolutions Acknowledgment (Chicago: University of Chicago Press) The author would like to thank Professor Bohm for a Pinch T J 1977 in Mendelsohn E, Weingart P and Whitley R D (eds) The Social Production of Scientific Knowledge helpful conversation. (Dordrecht: Reidel) pp172-215 Rosenfeld L 1958 Nature 181 658 vonNeumann J 1955 Mathematical Foundations of Further reading QuantumMechanics (Princeton: For more details of the debates over the foundations Press) first published in German in 1932 of quantum theory the reader is referred to Jammer Wynne B 1976 Social Studies of Science 6 307-48

CONTROVERSIES IN PHYSICS scientific knowledge entails a constant tension between the stability of established theories and methods, and new ideasand approaches which develop or even C G Barkla overturn-and always to some degree threaten-the intellectual status quo. Not all conflicts and potential and the conflicts in science becomecontroversial of course. Many are resolved or at least set aside without any strong mobilisation and polarisation of opposing J phenomenon schools of thought, and without too much emotional heat being generated by polemical pyrotechnics. BRIAN WYNNE In the last decade or so it has been recognised by School of Independent Studies, many historians and sociologists of science interested University of Lancaster in thenature of scientific knowledge that scientific controversiesillustrate in particularlyclear profile some of the generalconnections between the development of knowledge and the social processes by Scientific controversies arethe bread and butter of which that development takes place and by which scientific development. Thedynamic nature pf knowledge and ‘nonknowledge’ (error, ideology,

GG31-912G/79~GlG052+04$Gl GO 0 1979The Instlluleof PhwCS