It is argued that the lesson we should learn from Bell's inequalities is not that quantum mechanics requires some kind of action at a distance, but that it leads us to believe in parallel worlds.

Introduction

Bell's work [1] led to a revolution in our understanding of nature. I remember attending my first physics conference, on “Microphysical Reality and Quantum Formalism,” in Urbino, 1985. Most of the talks were about Aspect's experiment [2] confirming the nonlocality of quantum mechanics based on experimental violations of Bell's inequalities. Although I did not share the skepticism of many speakers regarding the results of Aspect, I was not ready to accept that a local action in one place can instantaneously change anything at another place. So, while for the majority the lesson from Bell was that quantum mechanics requires some “spooky action at a distance”, I was led by Bell's result to an alternative revolutionary change in our view of nature. I saw no other way than accepting the manyworlds interpretation (MWI) of quantum mechanics [3, 4].

I shall start by presenting the Einstein–Podolsky–Rosen (EPR) [5] argument. Then I will present Bell's presented using the Greenberger–Horne–Zeilinger setup [6] in the form proposed by Mermin [7, 8]. The discussion of nonlocality will suggest that Bell's inequalities are the only manifestation of action at a distance in nature. The demonstration of the necessity of action at a distance will be done through a detailed analysis of the GHZ experiment. Then I shall show how multiple worlds resolve the problem of action at a distance. After discussing the issue of nonlocality in the MWI, I shall conclude by citing Bell's views on the MWI.

EPR–Bell–GHZ

The story of Bell cannot be told without first describing the EPR argument. Instead of following the historical route, I shall use the GHZ setup, which, in my view, is the clearest way to explain the EPR and Bell's discovery.

Abstract

In 1964, John Bell proved that quantum mechanics is “unreasonable” (to use Einstein's term): there are nonlocal bipartite quantum correlations. But they are not the most nonlocal bipartite correlations consistent with relativistic causality (“no superluminal signalling”); maximally nonlocal “superquantum” (or “PR-box”) correlations are also consistent with relativistic causality. I show that – unlike quantum correlations – these correlations do not have a classical limit consistent with relativistic causality. The generalization of this result to all stronger-than-quantum nonlocal correlations is a derivation of Tsirelson's bound – a theorem of quantum mechanics – from the three axioms of relativistic causality, nonlocality, and the existence of a classical limit. But is it reasonable to derive (a part of) quantum mechanics from the unreasonable axiom of nonlocality?! I consider replacing the nonlocality axiom with an equivalent axiom that even Bell and Einstein might have considered reasonable: an axiom of local retrocausality.

In 1964, John Bell [1] proved that quantum mechanics is “unreasonable,” as defined by Einstein, Podolsky and Rosen [2] in 1935: “No reasonable definition of reality could be expected to permit this.” “This” (i.e., violation of “Einstein separability,” to use a technical term, or “spooky action at a distance,” as Einstein put it) turns out to be endemic to quantum mechanics. For example, pairs of photons measured at spacelike separations may yield nonlocal quantum correlations, i.e., correlations that cannot be traced to any data or “programs” the photons carry with them. As Bell [3] put it 20 years later, “For me, it is so reasonable to assume that the photons in those experiments carry with them programs, which have been correlated in advance, telling them how to behave. This is so rational that I think that when Einstein saw that, and the others refused to see it, he was the rational man. The other people, although history has justified them, were burying their heads in the sand. I feel that Einstein's intellectual superiority over Bohr, in this instance, was enormous; a vast gulf between the man who saw clearly what was needed, and the obscurantist. So for me, it is a pity that Einstein's idea doesn't work. The reasonable thing just doesn't work.”

Introduction

Since its inception, fifty years ago, Bell's theorem has had a long history not only of experimental tests but also of theoretical developments. Studying pairs of correlated quantummechanical particles separated in space, in a composite “entangled” state, Bell [1] showed that the joint ascription of hidden variables and locality to the system led to an inequality that is violated by the predictions of quantum mechanics. Fifteen years later, experiments confirmed the predictions of quantum mechanics, ruling out a large class of local realist theories.

One of the most meaningful theoretical developments that followed Bell's work was the Greenberger–Horne–Zeilinger (GHZ) theorem, also known as Bell's theorem without inequalities. In 1989, the American physicists Daniel Greenberger and Michael Horne, who had been working on Bell's theorem since the late 1960s, together with the Austrian physicist Anton Zeilinger, introduced a novelty into the testing of entanglement, extending Bell's theorem in a different and interesting direction. According to Franck Laloë [2],

The trio analyzed the Einstein, Podolsky, and Rosen 1935 argument once again and were able to write what are now called Greenberger–Horne–Zeilinger (GHZ) entangled states, involving three or four correlated spin-1/2 particles, leading to conflicts between local realist theories and quantum mechanics. However, unlike Bell's theorem, the conflict now was not of a statistical nature, as was the case with Bell's theorem, insofar as measurements on a single GHZ state could lead to conflicting predictions with local realistic models [3–5]. In this paper we present the history of the creation of this theorem and analyze its scope.

Introduction: The Theory of Local Beables

John Bell's most celebrated contribution to the foundations of physics is his famous theorem. The theorem demonstrates that any physical theory capable of generating the predictions of the standard quantum-mechanical algorithm, in particular the prediction of violations of Bell's inequality for experiments done at spacelike separation, cannot be local. The sense of “locality” used here is the same sense that Einstein had in mind when he pointed out that the standard interpretation of the quantum algorithm was committed to “spooky action at a distance” To this day, the import of Bell's theorem is not universally appreciated. I have written about this elsewhere [1], and others in this volume will take up that task. It is properly the main focus during this 50th anniversary of that great achievement.

But it is also important to recall and celebrate Bell's other achievements. In many of his later writings, including “The Theory of Local Beables,” “Quantum Mechanics for Cosmologists,” “On the Impossible Pilot Wave,” “Beables for Quantum Field Theory,” “Six Possible Worlds of Quantum Mechanics,” “Are There Quantum Jumps?” and “Against ‘Measurement,”’ 1 Bell turned his attention to the more general problem of physically construing the mathematical formalism used to derive these predictions. This activity is often denominated “interpreting quantum theory” as if there were some precise physical theory that might somehow be supplemented with an “interpretation” Once the issue is framed this way it is easy to ask: But if I already have a theory in hand, what can be gained by supplementing it with an “interpretation”? Many physicists, at this juncture, are happy to conclude that “interpretations” are not a matter of physics at all – maybe they are only of interest to philosophers – and that therefore the whole enterprise of “interpreting quantum theory” is not within the purview of physics per se.

What then is in the purviewof physics proper?One answer to this question goes under the banner “instrumentalism”: all physics as such, is concerned about is predicting the outcomes of experiments.

Introduction

Important advances in scientific knowledge may, not infrequently, be looked at from several angles. Here, in investigating the question the article title defines, part of our attention will have to be turned on the relationships of Bell's theorem to the old, but still not fully cleared up, question of the nature of causality.

The word “nonlocality” means the violation of local causality, the initial meaning of whichwas that no influence, that is, no causal action, can be propagated faster than light. For a long time it was a received view that actions somewhat akin to the basically time-directed ones that living beings perform also take place within inanimate nature. This conception of causality may be called the causality–action theory. But later, having in view Hume's serious reservations concerning it, renowned philosophers of science claimed it might and should be dropped, since the effects it was meant to account for could be explained just as well by considering that what is at work is simply a conjunction of physical laws expressed by means of differential equations – a view that later Hempel and Oppenheim generalized and called the D-N (deductive–nomological) model of explanation (reprinted in [1]). Here the (widely accepted, as it seems) corresponding conception of causality will, for simplicity, be called the law-centered theory of causality.

But does the latter theory subsume, at least within physics, all the meanings of the word ‘cause’ that true understanding calls for? Among philosophers the matter is not settled. Hempel himself granted that in several respects the D-N model is somehow inadequate for analyzing the cause–effect relationship [2]. And it seems indeed that a significant argument favoring a ‘no’ answer to the question might have been put forward as early as 1907 (although, to our knowledge, it was not), when Einstein pointed out that according to his then two years old special relativity (SR) theory, if we assumed an effect could follow its cause sooner than light velocity permits, then in some other frames of reference it would occur before its cause. For, concerning this consequence of the assumption, he wrote, “in my opinion, regarded as pure logic … it contains no contradiction; however it absolutely clashes with the character of our total experience, and in this way is proved the impossibility of the hypothesis” [3].

Prologue

A memorial conference for John Bell, open to the public, was held at Rutgers University around 20 years ago. I gave a short talk there on Bell's views about David Bohm's “hidden variables” formulation of quantum mechanics, a version of quantum mechanics often called the de Broglie–Bohm theory or Bohmian mechanics. This theory was in fact discovered by Louis de Broglie in 1927. In 1952, it was rediscovered and developed by Bohm, who was the first to appreciate its connection to the predictions of standard quantum mechanics. I did not publish the talk.

I have decided that it would be appropriate to publish a very lightly edited version of it here, in this volume devoted to the work of Bell on the foundation of quantum mechanics. One reason for doing so is that the connection between Bell and Bohm continues to be somewhat underplayed, with the strength of his advocacy of Bohmian mechanics not properly appreciated. For example, about half of the papers in Bell's collected works on the foundations of quantum mechanics deal with Bohmian mechanics. But in his fine introduction to the revised edition of this great book [1], Alain Aspect mentions this theory in only a single sentence, and parenthetically at that.

For several decades after Bell proved his nonlocality theorem, based in part on Bell's inequality, it was widely claimed that Bell had shown that hidden variables – and Bohmian mechanics in particular – were impossible, that they were incompatible with the predictions of quantum mechanics. For example, the great physicist Eugene Wigner, who, unlike most of his contemporaries, was profoundly concerned with the conceptual foundations of quantum mechanics and usually wrote on the subject with great clarity and insight, has written that

It is a pleasure to contribute to this anthology some of my recollections of John Bell.

I first met him at SLAC, where we were visitors during 1964–5 when he was on leave from CERN, and I was on leave from the Columbia University Physics Department. Soon I found that we had a common interest in the foundations of quantum mechanics, and we had lively discussions on this subject. We were concerned with the “reduction of the wave packet” after an experiment in quantum mechanics was completed, and wrote a tongue-incheek article on this subject for a festschrift in honor of Viki Weisskopf [1], stating that

Now, 50 years later, this topic, known as the measurement problem, has become the source of numerous articles representing many different viewpoints. At the time, however, I confess that I did not realize that the main flaw in John von Neumann's presentation of this problem was his assumption that a measuring device can be represented by a pointer with only two quantum states. Actually, a measuring device must be able to record the outcome of an experiment, and for this purpose it has to have an enormous number of quantum states, leading to an irreversible macroscopic transition in the device. A correct discussion of the measurement problemwas given, for example, by Nico vanKampen [2], but recently he told me that he was unable to persuade Bell, who continued to be concerned about this problem for the rest of his life. Indeed, shortly before he died, he wrote a diatribe [3] entitled “Against Measurement,” where he continued to argue that this problem constituted a fundamental flaw in quantum mechanics. Kurt Gottfried, who referred to this problem as Bell's second major theme, called his article “a fervent jeremiad summarizing a lifetime of reflection on what he saw as the fatal flaws of the orthodox theory” [4].

Abstract

In papers published in the 25 years following his famous 1964 proof, John Bell refined and reformulated his views on locality and causality. Although his formulations of local causality were in terms of probability, he had little to say about that notion. But assumptions about probability are implicit in his arguments and conclusions. Probability does not conform to these assumptions when quantum mechanics is applied to account for the particular correlations Bell argues are locally inexplicable. This account involves no superluminal action and there is even a sense in which it is local, but it is in tension with the requirement that the direct causes and effects of events be nearby.

Introduction

I never met John Bell, but his writings have supplied me with a continual source of new insights as I read and reread them over 40 years. As I worked toward a rather different understanding of quantum mechanics he was foremost in my mind as a severe but honest critic of such attempts. We all would love to know what Einstein would have made of Bell's theorem. I confess that the deep regret I feel that Bell cannot respond to this paper is sometimes assuaged by a sense of relief.

Locality and Local Causality

In his seminal 1964 paper [1], John Bell expressed locality as the requirement

This seems to require that the result of a measurement would have been the same, no matter what operations had been performed on such a distant system. But suppose the result of a measurement were the outcome of an indeterministic process. Then the result of the measurement might have been different even if exactly the same operations (if any) had been performed on that distant system. So can no indeterministic theory satisfy the locality requirement? Bell felt no need to address that awkward question in his 1964 paper [1], since he took the EPR argument to establish that any additional variables needed to restore locality and causalitywould have to determine a unique result of ameasurement.

Abstract

While it is widely agreed that Bell's theorem is an important result in the foundations of quantum physics, there is much disagreement about what exactly Bell's theorem shows. It is agreed that Bell derived a contradiction with experimental facts from some list of assumptions, thus showing that at least one of the assumptions must be wrong; but there is disagreement about what the assumptions were that went into the argument. In this paper, I make a few points in order to help clarify the situation.

Introduction

Different authors have expressed very different views about what Bell's theorem shows about physics. The disagreement concerns particularly the question of which assumptions go into the argument. Since we have to give up one of the assumptions leading to the empirically violated Bell inequality, knowing what the assumptions were is crucial. For example, if author X believes that the argument requires only one assumption, A1, while author Y believes that it requires two, A1 and A2, then X will conclude that A1¬ must be abandoned, while Y will conclude that either A1 or A2 must abandoned, so A1 may well be true in our world if A2 is false. In this paper, I consider several assumptions that have been mentioned in connection with Bell's theorem, and I look into their roles in the proof of Bell's theorem. While I do not say anything here that has never been said before, I hope that my remarks can nevertheless be helpful to some readers.

For the sake of definiteness, the version of the relevant experiment that I will consider involves two spin-1\2 particles, initially in the singlet spin state ψ = 2−1/2(|↑↓ ⟩ − |↓↑ ⟩). Two widely separated experimenters, Alice and Bob, have one particle transported to each of them. At spacelike separation, they each choose a direction in space, represented by the unit vectors a and b, and carry out a quantum measurement of the spin observable in this direction on their particle, (a · σ) ⊗ I and I ⊗ (b · σ), with I the 2 × 2 unit matrix and σ the triple of Paulimatrices. The possible outcomes, A and B, are±1 on each side. Bell's theorem is the statement that under the assumptions we are about to discuss, Bell's inequality holds; quantum theory predicts violations of Bell's inequality, and experiments have confirmed these predictions.

Introduction

Fifty years after the publication of Bell's theorem, there remains some controversy regarding what the theorem is telling us about quantum mechanics, and what the experimental violations of Bell inequalities are telling us about the world. This chapter represents my best attempt to be clear about what I think the lessons are. In brief: There is some sort of nonlocality inherent in any quantum theory, and, moreover, in any theory that reproduces, even approximately, the quantum probabilities for the outcomes of experiments. But not all forms of nonlocality are the same; there is a distinction to be made between action at a distance and other forms of nonlocality, and I will argue that the nonlocality needed to violate the Bell inequalities need not involve action at a distance. Furthermore, the distinction between forms of nonlocality makes a difference when it comes to compatibility with relativistic causal structure.

The Bell locality condition is a condition which, if satisfied, renders possible a completely local account of the correlations between outcomes of spatially separated experiments. Bell's theorem tells us that the probabilities that we compute from quantum mechanics do not admit of such an account. Modulo auxiliary assumptions that, in my opinion, should be noncontroversial, this yields the conclusion that there must be something nonlocal about any theory that yields the quantum mechanical probabilities or anything close to them.

One way to violate the Bell locality condition – and this is the way that any deterministic theory must do it – is via straightforward action at a distance. However, we should also consider the possibility of a stochastic theorywith probabilistic laws that involve irreducible correlations. The sort of nonlocality inherent in such a theory involves relations between events that are significantly different from causal relations as usually conceived. Perhaps surprisingly, such a theory, unlike its deterministic cousins, need not invoke a distinguished relation of distant simultaneity.

This makes a difference for the situation of the two sorts of theories vis-a-vis relativity. Though a deterministic theory can achieve the appearance of being relativistic, at the level of observable phenomena, it can do so only by introducing a preferred foliation and with it a causal structure at odds with relativistic causal structures. Things are otherwise with dynamical collapse theories.

Introduction

John S. Bell, of course, is best known for the result now referred to simply as “Bell's theorem,” which removed from consideration the class of so-called local hidden-variable theories that at the time of its publishing appeared to be the most natural class of theories among those that would render quantum mechanics a form of statistical mechanics. If, as this and other results suggest, quantum theory is to serve as a truly fundamental theory, conceptual precision in its interpretation is not only desirable but paramount. John Bell was accordingly concerned about what he viewed as conceptual imprecision, from the physical point of view, within the standard approaches to the theory. He saw this as most acute in the case of their treatment of measurement at the level of principle. His concerns were strongly expressed in the article “Against measurement” [1]. This was published in 1990 in a volume of proceedings of the 1989 Erice meeting “Sixty-Two Years of Uncertainty,” during which it was my pleasure to meet and eat with Bell, and to listen to him present this paper. He pointed out that this conceptual imprecision is reflected in the terminology of the foundations of quantum theory, a great deal of which he explicitly deemed worthy of banishment from discussions of principle, because it corresponds to a set of what he saw as vague and, in some instances, outright destructive concepts. His concern was thus not one regarding mathematics so much as regarding basic concepts used in contemporary quantum physics, which he viewed as failing the needs of natural philosophy and so of fundamental physics, despite their practical adequacy. Here, I consider John Bell's critique of standard quantum measurement theory and some alternative treatments wherein he saw greater conceptual precision, and make further suggestions as to how to improve conceptual precision, as he advocated.

That the source of difficulties is to be understood specifically as a problem of imprecision of physical concepts that stands in the way of the achievement of an exact fundamental mechanical theory is pointed out at the outset of “Against measurement”: Bell wished to make it clear “at once that it is not mathematical precision, but physical” that caused him such great concern.

The end of history?

The Regius Professor of History, on being asked, ‘When does history end?’, responded, ‘When they can no longer sue!’ More pertinently, I was deeply involved in managing the Laboratory from the time ofmy return to Cambridge in 1991 until the present. Any pretense at objectivity would scarcely be credible. As a consequence, this chapter is more personal than the rest of the text, but I will endeavour to stick to the facts.

The changes in the named professorships through this period were as follows:

1. • Richard Friend appointed Cavendish Professor in 1995 at the age of 42 in succession to Sam Edwards

2. • Peter Littlewood appointed to the 1966 Professorship of Theoretical Physics in succession to Volker Heine in 1997; in 2013, this chair was transferred to Didier Queloz

3. • Henning Sirringhaus appointed to the Hitachi Professorship of Electron Device Physics in 2005

4. • Ullrich Steiner appointed John Humphrey Plummer Professor of Physics in 2005

5. • Jeremy Baumberg appointed Professor of Nanoelectronics, proleptically filling the Chair of Physics held by Michael Pepper in 2007

6. • James Stirling appointed Jacksonian Professor of Natural Philosophy following my retirement from the chair in 2008

7. • Ben Simons appointed to the Herchel Smith Professorship of the Physics of Medicine in 2011

8. • Roberto Maiolino appointed Professor of Astrophysics in succession to Richard Hills in 2012.

The Heads of Department during this period were:

1. • Archie Howie: 1990–97

2. • Malcolm Longair: 1997–2005

3. • Peter Littlewood: 2005–11

4. • James Stirling: 2011–13

5. • Andy Parker: 2013–present.

The days when Rutherford could manage the Cavendish Laboratory as a single dominating presence or Thomson manage the finances of the Laboratory from his personal cheque-book were long gone. The role of the Head of Department was much more that of the chief executive officer of a middle-sized company. By 2015 the number of persons on site was approaching 1000 and the turnover about £30 million. All those who have held the post of Head of Department since 1995 have found that, in the research funding and educational climate of the late twentieth and early twenty-first centuries, the role has become very much more demanding and time-consuming than might be expected of an academic institution. How did this come about?