On the Limits of “Undecidability”: Quantum Physics, Deconstruction, and Anti-realism Christopher Norris (bio) I Philosophers of science have responded in various ways to the conceptual and interpretative problems thrown up by quantum mechanics. These problems emerged a century ago from Planck’s researches into black-body radiation and have scarcely been resolved—rather, if anything, deepened and complicated—by the intensive debate that has surrounded them since that time.1 For some theorists, the resultant dilemmas (of observer “interference,” wave/particle dualism, quantum superposition, nonlocal causality, superluminal [faster-than-light] propagation, etc.) are such as to entail drastic revisions to our very idea of the physical world as existing and exerting its causal powers quite apart from our current best theories or investigative methods.2 For others again, quantum mechanics just is the best theory currently available, whatever the problems that it happens to create for our normal habits of logical thought or consistent reasoning on the evidence. Thus logic itself may have to be revised—right down to putative “laws of thought” such as noncontradiction and excluded middle—if these turn out to conflict with the results of quantum-mechanical theory or experiment.3 This is the view taken by Hilary Putnam in his writings on quantum logic and also by others, von Neumann among them, for whom the working rule in any such trade-off is to keep the physics as simple as possible even if the result is to complicate logic by introducing values other than those of determinate (bivalent) truth or falsehood.4 Very often it consorts with the argument that if quantum mechanics is right—at any rate on the standard Copenhagen interpretation—then we had best give up any hope of retaining a “classical” Einsteinian ontology, that is to say, a Lorentz-invariant account of space-time coordinates where the speed of light is the limiting value for assignments of position, velocity, and causal propagation.5 Hence Einstein’s famous series of debates with Niels Bohr when he devised a whole range of increasingly elaborate thought-experiments designed to prove that the quantum-mechnical description of reality had to be in some way “incomplete,” or not to have reckoned with some crucial factor that would bring it into line with logical reasoning and a causal-realist ontology. For on Einstein’s view there was simply no room in science for a theory—such as Bohr’s—that involved either the suspension of classical truth/falsehood [End Page 407] values or the idea of some “spooky action-at-a-distance” which contravened both commonsense realism and the requirements of Special Relativity.6 However it is Bohr who is generally thought to have carried the day in these debates. In which case—so it is argued—we are left with a choice between three possible “solutions.” These are (1) the hypothesis that quantum mechanics is “incomplete” or open to an alternative (“hidden-variables”) interpretation;7 (2) the adoption of an alternative (many-valued or non-bivalent) quantum logic that effectively conjures such problems away;8 or (3) the conclusion that realism may have to be abandoned in favour of some radically different ontology or worldview which counts this a price worth paying in order to conserve the predictive power of the standard quantum formalisms.9 On option (3) there is a further choice of interpretative views according to the theorist’s preferred choice between the various models (or possible-world scenarios) currently on offer. Thus the model may be one that embraces nonlocal simultaneous particle interaction, or which gives up space-time covariance (relative to the speed of light) as required by Einstein’s Special Theory. Or again it may be one that somehow envisions—strangest of all—the splitting-off of multiple coexistent but non-interacting “worlds” whenever the wave-packet collapses or whenever an act of measurement is performed upon any such quantum-system whose properties must be thought of as strictly indeterminate until that moment.10 For, according to its partisans, this theory is the most economical—i.e., involves least conflict with the standard quantum formalisms—despite its baroque proliferation of alternative worlds and its well-nigh unthinkable implications when applied...
On the Limits of “Undecidability”: Quantum Physics, Deconstruction, and Anti-realism Christopher Norris (bio) I Philosophers of science have responded in various ways to the conceptual and interpretative problems thrown up by quantum mechanics. These problems emerged a century ago from Planck’s researches into black-body radiation and have scarcely been resolved—rather, if anything, deepened and complicated—by the intensive debate that has surrounded them since that time.1 For some theorists, the resultant dilemmas (of observer “interference,” wave/particle dualism, quantum superposition, nonlocal causality, superluminal [faster-than-light] propagation, etc.) are such as to entail drastic revisions to our very idea of the physical world as existing and exerting its causal powers quite apart from our current best theories or investigative methods.2 For others again, quantum mechanics just is the best theory currently available, whatever the problems that it happens to create for our normal habits of logical thought or consistent reasoning on the evidence. Thus logic itself may have to be revised—right down to putative “laws of thought” such as noncontradiction and excluded middle—if these turn out to conflict with the results of quantum-mechanical theory or experiment.3 This is the view taken by Hilary Putnam in his writings on quantum logic and also by others, von Neumann among them, for whom the working rule in any such trade-off is to keep the physics as simple as possible even if the result is to complicate logic by introducing values other than those of determinate (bivalent) truth or falsehood.4 Very often it consorts with the argument that if quantum mechanics is right—at any rate on the standard Copenhagen interpretation—then we had best give up any hope of retaining a “classical” Einsteinian ontology, that is to say, a Lorentz-invariant account of space-time coordinates where the speed of light is the limiting value for assignments of position, velocity, and causal propagation.5 Hence Einstein’s famous series of debates with Niels Bohr when he devised a whole range of increasingly elaborate thought-experiments designed to prove that the quantum-mechnical description of reality had to be in some way “incomplete,” or not to have reckoned with some crucial factor that would bring it into line with logical reasoning and a causal-realist ontology. For on Einstein’s view there was simply no room in science for a theory—such as Bohr’s—that involved either the suspension of classical truth/falsehood [End Page 407] values or the idea of some “spooky action-at-a-distance” which contravened both commonsense realism and the requirements of Special Relativity.6 However it is Bohr who is generally thought to have carried the day in these debates. In which case—so it is argued—we are left with a choice between three possible “solutions.” These are (1) the hypothesis that quantum mechanics is “incomplete” or open to an alternative (“hidden-variables”) interpretation;7 (2) the adoption of an alternative (many-valued or non-bivalent) quantum logic that effectively conjures such problems away;8 or (3) the conclusion that realism may have to be abandoned in favour of some radically different ontology or worldview which counts this a price worth paying in order to conserve the predictive power of the standard quantum formalisms.9 On option (3) there is a further choice of interpretative views according to the theorist’s preferred choice between the various models (or possible-world scenarios) currently on offer. Thus the model may be one that embraces nonlocal simultaneous particle interaction, or which gives up space-time covariance (relative to the speed of light) as required by Einstein’s Special Theory. Or again it may be one that somehow envisions—strangest of all—the splitting-off of multiple coexistent but non-interacting “worlds” whenever the wave-packet collapses or whenever an act of measurement is performed upon any such quantum-system whose properties must be thought of as strictly indeterminate until that moment.10 For, according to its partisans, this theory is the most economical—i.e., involves least conflict with the standard quantum formalisms—despite its baroque proliferation of alternative worlds and its well-nigh unthinkable implications when applied...
This book is a critical introduction to the long-standing debate concerning the conceptual foundations of quantum mechanics and the problems it has posed for physicists and philosophers from Einstein to the present. Quantum theory has been a major infulence on postmodernism, and presents significant problems for realists. Keeping his own realist position in check, Christopher Norris subjects a wide range of key opponents and supporters of realism to a high and equal level of scrutiny. With a characteristic combination of rigour and intellectual generosity, he draws out the merits and weaknesses from opposing arguments. In a sequence of closely argued chapters, Norris examines the premises of orthodox quantum theory, as developed most influentially by Bohr and Heisenberg, and its impact on varous philosophical developments. These include the ideas developed by W.V Quine, Thomas Kuhn, Michael Dummett, Bas van Fraassen, and Hilary Puttnam. In each case, Norris argues, these thinkers have been influenced by the orthodox construal of quantum mechanics as requiring drastic revision of principles which had hitherto defined the very nature of scientific method, causal explanati and rational enquiry. Putting the case for a realist approach which adheres to well-tried scientific principles of causal reasoning and inference to the best explanation, Christopher Norris clarifies these debates to a non-specialist readership and scholars of philosophy, science studies and the philosophy of science alike. Quantum Theory and the Flight From Realism suggests that philosophical reflection can contribute to a better understanding of these crucial, current issues.
For over nine decades, quantum mechanics has profoundly shaped our understanding of nature, offering explanations for the stability of atoms, the formation of mole cules, the properties of materials, and phenomena such as light or radioactivity. It is essential to modern technologies—from microchips to lasers—and underpins the physics of elementary particles. Yet, despite its successes, it remains a deeply mysterious theory, confronting a central enigma: why does the quantum world behave so differently from everyday reality? Elements such as superposition and quantum entanglement—which allow particles to exist simultaneously in multiple states or remain interconnected regardless of distance—contradict classical world intuitions. This gap between microscopic and macroscopic behavior has given rise to what is known as the “measurement problem,” raising fundamental questions about the nature of reality. Diverging interpretations have led to two major schools of thought: anti-realism, promoted by Niels Bohr, which holds that reality depends on the observer and their interaction with measurement instruments; and realism, supported by Albert Einstein, which argues that quantum mechanics is incomplete and that eality exists independently of observation. Bohr’s complementary philosophy and Heisenberg’s uncertainty principle were central to the formulation of the “Copenhagen interpretation,” which became dominant in the 20th century. However, the persistent opposition of scientists like Einstein and Schrödinger inspired generations of physicists to seek a unification between quantum mechanics and relativity, culminating in new theories such as loop quantum gravity and string theory. Thus, the debate between realism and anti-realism remains alive, and philosophy continues to play a crucial role in exploring the new foundations of physics.
This book is a critical introduction to the long-standing debate concerning the conceptual foundations of quantum mechanics and the problems it has posed for physicists and philosophers from Einstein to the present. Quantum theory has been a major infulence on postmodernism, and presents significant problems for realists. Keeping his own realist position in check, Christopher Norris subjects a wide range of key opponents and supporters of realism to a high and equal level of scrutiny. With a characteristic combination of rigour and intellectual generosity, he draws out the merits and weaknesses from opposing arguments. In a sequence of closely argued chapters, Norris examines the premises of orthodox quantum theory, as developed most influentially by Bohr and Heisenberg, and its impact on varous philosophical developments. These include the ideas developed by W.V Quine, Thomas Kuhn, Michael Dummett, Bas van Fraassen, and Hilary Puttnam. In each case, Norris argues, these thinkers have been influenced by the orthodox construal of quantum mechanics as requiring drastic revision of principles which had hitherto defined the very nature of scientific method, causal explanati and rational enquiry. Putting the case for a realist approach which adheres to well-tried scientific principles of causal reasoning and inference to the best explanation, Christopher Norris clarifies these debates to a non-specialist readership and scholars of philosophy, science studies and the philosophy of science alike. Quantum Theory and the Flight From Realism suggests that philosophical reflection can contribute to a better understanding of these crucial, current issues.
Why is so shop-soiled a topic as scientific realism once again prominent in the philosophy of science? Realism fought a great battle when Copernican and Ptolemaic world views were at issue long ago. Towards the end of the nineteenth century worries about atomism strongly contributed to anti-realism among philosophers of science. Is there a comparable scientific issue today? Maybe. One way to understand quantum mechanics is to take an idealist line. Some people argue that human observation plays an integral role in the very nature of a physical system, so that the system changes simply when it is measured. Talk of ‘the measurement problem in quantum mechanics’, the ‘ignorance interpretation’, and ‘the collapse of the wave packet’ make it no accident that contributions to the philosophy of quantum mechanics play an important part in the writings of the more original figures in the realist debate. A number of the ideas of Hilary Putnam, Bas van Fraassen or Nancy Cartwright seem to result from taking quantum mechanics as the model of all science. Conversely, numerous physicists wax philosophical. Bernard d'Espagnat has made one of the most important recent contributions to a new realism. He is partly motivated by the dissolution, in some parts of modern physics, of old realist concepts such as matter and entity. He is especially driven by some recent results, that bear the general name of Bell's inequality, and which have been thought to call in question concepts as various as logic, the temporal order of causation, and action at a distance.
Quantum theory is a phenomenally successful cornerstone of physics. But a recent poll among quantum physicists shows that there is still little agreement on what the theory tells us about physical reality. Yet precious bits of consensus are emerging. As always, Albert Einstein was ahead of his time. Quantum theory was still in its infancy, its formalism a mere sketch, and its implications only beginning to be realized. Yet Einstein, in 1917, already spotted what would trouble him throughout his lifetime. He concluded that the nascent theory, which he had helped initiate, seemed to leave “time and direction of elementary processes,” such as the spontaneous emission of radiation from a molecule, “to chance” 1. Einstein's conclusion was visionary. It was also incredibly bold, for it suggested a radical departure from the powerful, centuries-old deterministic and causal worldview of classical physics. But Einstein also immediately expressed his discomfort with the idea of fundamental chance. In fact, his worries were just the seed of what would turn into one of the longest and most heated debates in the history of physics. What does quantum theory mean, and what does it tell us about the nature of reality? Some of the brightest minds in physics locked horns over these issues. Besides Einstein, there were Niels Bohr, Erwin Schrödinger, Werner Heisenberg, Pascual Jordan, Carl Friedrich von Weizsäcker, Wolfgang Pauli, and Richard Feynman, just to name a few. Ask anyone today working on foundational questions in quantum theory and you are likely to hear that there is still no consensus on many of these questions—all the while, of course, everybody seems to be in perfect agreement on how to apply the quantum formalism when it comes to making experimental predictions. Remarkably, there has never been any serious attempt to find out what the current opinions of the community really are and how they may be correlated. We jumped on the opportunity and polled 33 participants at a 2011 conference on “Quantum Physics and the Nature of Reality” (Fig. 1) 2. (We hasten to add that our poll is to be understood as merely a snapshot and certainly cannot claim to be representative.) As soon as we had collected the questionnaires, the respondents eagerly asked to hear the results. Evidently, we had not been the only curious party. What did we find? The fundamental questions, it seems, still elicit radically different responses. Take the quantum state—represented by the wavefunction—as an illustration. The wavefunction is the central mathematical symbol of quantum mechanics. It allows us to make incredibly accurate predictions, be it in atomic, molecular, or solid-state physics. But how are we to interpret it? Is it itself a physical property, or does it merely represent our knowledge? Our poll revealed a tie between these two positions. And not only that: a third of respondents preferred a mix of both viewpoints—perhaps a sign that a more nuanced view, or even a synthesis of views, may be on the horizon. Only 3% of those polled thought that quantum states never refer to a single system but only to ensembles of systems. Perhaps this departure from the ensemble view—a view that, we suspect, remains more popular in the physics community at large—has to do with the fact that experiments with single quantum systems have become routine in today's quantum-information laboratories. Sharp disagreement was in evidence on another deep question. Abraham Pais, Einstein's biographer, recalled that “during one walk Einstein suddenly stopped, turned to me and asked whether I really believed that the moon exists only when I look at it” 3. Putting the issue differently, are the physical properties of objects well defined before we actually measure them? Half of the respondents in our poll believed that they sometimes were, while the other half said that they never were. These two positions correspond to radically different ways of understanding the function of measurement: either as simply ascertaining—at least sometimes—of what is already there, or as an act of genuine creation. The role of measurement is also at the heart of another perennial favorite of foundational discussions, the measurement problem. The measurement problem highlights a curious fact about quantum theory: while the act of measurement assumes a fundamental role, it is not described—not without interpretive contortions, anyway—by the dynamical law of quantum theory, the Schrödinger equation. For decades, the measurement problem was seen by many as the main stumbling block for quantum theory, sparking a storm of rivaling attempts to deal with this perceived issue. Today, the waters seem to have calmed and attention shifted elsewhere, as only less than a fourth of respondents consider the measurement problem “a severe difficulty threatening quantum mechanics.” Changing attitudes are also evident when it comes to the issue of chance Einstein had brought to light in 1917. Today, the question of the randomness of quantum events no longer seems to be all that contentious. Most of our respondents accepted randomness as irreducible or a fundamental concept in nature (or both). Less than one-tenth of respondents thought that quantum randomness was merely apparent rather than fundamental, and no one believed in a hidden determinism governing quantum events. We wish we could hear Einstein's opinion today. Why have attitudes and priorities changed? It's hard to say, but we think there may be several driving forces. The rapid ascent of quantum information science has not only led to applications such as secure cryptography and the promise of lightning-fast quantum computers, but has also shed new light on foundational problems and handed us new tools for addressing them. Our poll shows that these tools are considered a welcome addition, with three-fourths seeing quantum information as a breath of fresh air for foundational investigations. There is also optimism about the future technological development of quantum information, with more than 75% of respondents expecting a working and useful quantum computer to be realized within the next fifty years. Such optimism might be rooted in the astonishing experimental progress we have witnessed over the past decades, another driving force behind changing attitudes. We are now at stage where almost perfect experimental control of single quantum systems has been achieved. The 2012 Nobel Prize in physics was awarded for just such experiments. This has put at our fingertips the ability to actually carry out the thought experiments—or approximations thereof, at any rate—that the founders of quantum theory had dreamed up. Take Schrödinger's cat. In 1935, Schrödinger wanted to illustrate what seemed to him an absurd prediction of the new theory: that the strange quantum superpositions of the atomic world can be amplified to everyday objects. Schrödinger's object of choice was a cat in a specifically prepared box. Quantum theory then suggested that the state of the cat should become transformed into a bizarre quantum superposition of being simultaneously alive and dead. But what Schrödinger had intended as a “burlesque” 4 thought experiment is now approaching experimental reality. Over the last decades, researchers have become increasingly skilled in breeding small versions of Schrödinger's cat. While they don't involve actual cats, they do already involve large biological molecules. Our respondents, it seems, were inspired by such developments. A solid two-thirds majority declared that they don't expect any fundamental limit in this enterprise of observing larger and larger objects in superpositions of macroscopically distinct states. No limit, then, for quantum theory—something that Bohr had already suggested when he, in some of his early thought experiments, considered apparatuses that included macroscopic parts behaving in quantum-mechanical ways. There are other new trends. Why do people seem to be much more at ease with relativity theory than with quantum theory? For one, both special and general relativity theory are grounded in a few simple physical principles. By contrast, quantum theory is lacking such a foundation and is instead based on a rather abstract mathematical body. Today, several researchers are out to change this situation, trying to derive quantum theory from a small set of physical or information-theoretic principles. They hope that in this way, we will understand why we have this peculiar theory to begin with. A majority in our poll regarded such “reconstructions” of quantum theory as giving useful insights, indicating once more that any new tool that promises fresh insights is embraced with an open mind. To be sure, many (albeit not all) of the questions we asked in the poll are interpretive questions. This means that unless we should find out some day that quantum theory is in fact not always valid—in which case we would have to rethink the whole game—experiments arguably can't play the role of the arbiter. Some physicists will conclude from this that we need not bother with such questions, and relegate them to philosophy. We think this is too quick. Many ideas in physics didn't come out of thin air, but came about when someone started looking at a problem with a certain philosophical disposition. In this sense, we may think of the plurality of views in quantum mechanics as productive. Every interpretation highlights some aspect of quantum theory, is particularly suited for explaining particular phenomena, and may inspire new technologies—such as the quantum computer—or even new physics. What, then, does the future hold? Half of the respondents in our poll believe that in fifty years from now, we will probably still be coming together for conferences on the fundamental questions of quantum theory. But what kinds of questions will be on the agenda? Will we still be worrying about the same problems as today? Or will these problems have been solved—or found to be less relevant—and our focus shifted to new questions once again? Who knows. But the location of the conference where we conducted the poll may tell a story. It was the place of a monastery, and a central position in the battle between Catholics and Protestants during the seventeenth century. Many of the questions that were hotly debated back then are no longer of interest to many people today, and other, new questions have since taken center stage in our society. Maybe some of the battles over the interpretation of quantum theory will meet a similar fate.
Abstract The book starts with a description of classical mechanics then discusses the quantum phenomena that require us to give up our commonsense classical intuitions. We consider the physical and conceptual arguments that led to the standard von Neumann-Dirac formulation of quantum mechanics and how the standard theory explains quantum phenomena. This includes a discussion of how the theory’s two dynamical laws work with the standard interpretation of states to explain determinate measurement records, quantum statistics, interference effects, entanglement, decoherence, and quantum nonlocality. A careful understanding of how the standard theory works ultimately leads to the quantum measurement problem. We consider how the measurement problem threatens the logical consistency of the standard theory then turn to a discussion of the main proposals for resolving it. This includes collapse formulations of quantum mechanics like Wigner’s extension of the standard theory and the GRW approach and no-collapse formulations like pure wave mechanics, the various many-worlds theories, and Bohmian mechanics. In discussing alternative formulations of quantum mechanics we pay particular attention to the explanatory role played by each theory’s empirical ontology and associated metaphysical commitments and the conceptual trade-offs between theoretical options.