In Quirks of the Quantum, Samuel Chase Coale argues that the twentieth century is the quantum era, riddled with all of the indeterminacies, blurriness, and entanglements that he attributes to quantum theory, which he describes as the “philosophical basis for quantum physics” (1). He acknowledges that “to truly understand quantum mechanics as a science, one would have to study mathematics,” which he readily admits his book does not undertake. Instead, he relies on the “tropes, metaphors and models” (ibid.) that scientists have published about the implications of quantum theory. He thus distinguishes mathematical formulae from philosophical constructions, privileging the latter over the former. Philosophy, however, does not provide a “basis” for quantum mechanics; rather, quantum mechanics describes the phenomena, and then philosophy struggles with what the physical theory implies.Coale makes wide-ranging references to what quantum physicists, including Neils Bohr, have thought about these implications. Strangely, he does not include in his account Karen Barad's excellent Meeting the Universe Halfway: Quantum Physics and the Entanglement of Matter and Meaning. A former physicist, Barad develops the quantum philosophy of Neils Bohr into an approach she calls agential realism, which insists that humans and nonhumans are ineluctably bound together in the production of meaning. She argues that quantum mechanics demonstrates the “ontological inseparability of intra-acting agencies,” which emerge as separate entities only as a result of “cuts” we make to simplify phenomena and render them susceptible to measurement and analysis (148). Although her conclusions are similar to Coale's, her approach is distinguished by a deep knowledge of quantum mechanics as a mathematical theory, and her formulations are rigorous and precise. Her absence from Coale's book is the more notable because his analysis needs an infusion of the kind of deep thinking that Barad embodies. By contrast, his book engages in a discursive inflation of quantum physics, wherein the rigor and precision it has as a physical theory is buried under an avalanche of verbiage that makes quantum physics appear so broad and vague that almost anything can fit under its umbrella.A good example of this problem is the way in which Coale uses “entanglement.” In quantum mechanics, it refers to the correlation of the quantum characteristics of two subatomic particles. For example, when two photons interact (say, by pumping a laser beam through a nonlinear crystal such as beta-barium borate, which splits some of the photon stream into photon pairs with correlated polarizations), they can become entangled such that their wave functions (i.e., the eigenstates described by their superpositions) are coordinated. If they are then separated, measurement on one of them determines how the wave function for the other will collapse, even if they are by then far apart. The distance (and time intervals) separating the two photons can be quite large; entanglement has been shown to operate as far removed as fourteen miles. Moreover, it also operates on the macroscale, for example in the ability of migratory birds to sense the inclination of the earth's magnetic field (via entangled electrons in the molecules of their eyes). This is mysterious to the extent that no one knows how or why the particles become coordinated, but it is nevertheless quite distinct from such uses of “entanglement” that would include, for example, two lovers becoming entangled with one another or the World Bank becoming entangled in the global economic system. Citing Patrick O'Donnell, Coale appropriates the word to mean, following O'Donnell, “the complex relationship between self, objects, and the world” (164), which stretches the meaning so far that it leaves quantum mechanics (or quantum theory, as Coale would have it) far behind.In similar fashion, Coale assimilates to quantum theory most of deconstruction, especially the indeterminacy, undermining of origins, and undecidability that Jacques Derrida brilliantly developed in Of Grammatology and later works. He does the same with postmodernism, so that it begins to seem almost a branch of quantum physics rather than a philosophical and cultural movement with complex roots in cultural, economic, and technological developments. For example, John Johnston in Information Multiplicity: American Fiction in the Age of Media Saturation argues persuasively that the postmodern American novel has been distinctively shaped by the glut of information that the Internet and contemporary communication media make available, a point also made by Fredric Jameson in his seminal study Postmodernism, or the Cultural Logic of Late Capitalism. Yet for Coale, “that flow of information suggests the quantum flux, constantly in motion with islands of possible comprehension like the tracks and trails of ‘particles’ that are measured in individual experiments” (109). The importance of globalization, the complex economic structures emerging from late capitalism, the transformations the web has wrought—all these in Coale's analysis become subsets of quantum physics.Within the limits of his methodology, which are significant, Coale provides good readings of several postmodern authors, including Joan Didion, Tim O'Brien, Thomas Pynchon, Paul Auster, and Don DeLillo. Although many have discussed the fragmentation, nonlinear narratives, lack of coherent subjectivities, and indeterminacy of novels by these writers as postmodern constructions, for Coale they become manifestations of quantum theory (not surprisingly, since he sees postmodernism itself as a branch of the same). With chapters on the quantum flux, quantum style, and death as quantum dissolution, he combines good readings with inflated claims for them as manifestations of quantum theory. The exceptions to this quantum inflation are his discussions of DeLillo's novels. He mentions in the introduction that he found several articles on quantum theory and references of quantum phenomena in DeLillo's notebooks at the Harry Ransom Humanities Research Center at the University of Texas at Austin. These bear fruit in his readings of DeLillo's novels, especially his fine reading of The Body Artist and the section on DeLillo's Underworld, arguably the best interpretation in the book. This section alone almost justifies the price of admission.Were Coale to frame his book as a contribution to postmodern literary criticism, it would take its place among many other such books and articles as a well-written and interesting contribution that makes good observations about a number of important authors and their works. By insisting that all of this fits under quantum theory, he overreaches what the theory can justly claim and, in the process, dilutes the effectiveness of his book. Without a doubt, we need more informed criticism of the interactions between literature, culture, and science, but such work is more effective when it focuses on the specificities of scientific theories and their complex entanglements with cultural and technological practices. Lest it fall under Coale's quantum umbrella, I should add that I do not intend in this formulation a reference to quantum theory!
In Quirks of the Quantum, Samuel Chase Coale argues that the twentieth century is the quantum era, riddled with all of the indeterminacies, blurriness, and entanglements that he attributes to quantum theory, which he describes as the “philosophical basis for quantum physics” (1). He acknowledges that “to truly understand quantum mechanics as a science, one would have to study mathematics,” which he readily admits his book does not undertake. Instead, he relies on the “tropes, metaphors and models” (ibid.) that scientists have published about the implications of quantum theory. He thus distinguishes mathematical formulae from philosophical constructions, privileging the latter over the former. Philosophy, however, does not provide a “basis” for quantum mechanics; rather, quantum mechanics describes the phenomena, and then philosophy struggles with what the physical theory implies.Coale makes wide-ranging references to what quantum physicists, including Neils Bohr, have thought about these implications. Strangely, he does not include in his account Karen Barad's excellent Meeting the Universe Halfway: Quantum Physics and the Entanglement of Matter and Meaning. A former physicist, Barad develops the quantum philosophy of Neils Bohr into an approach she calls agential realism, which insists that humans and nonhumans are ineluctably bound together in the production of meaning. She argues that quantum mechanics demonstrates the “ontological inseparability of intra-acting agencies,” which emerge as separate entities only as a result of “cuts” we make to simplify phenomena and render them susceptible to measurement and analysis (148). Although her conclusions are similar to Coale's, her approach is distinguished by a deep knowledge of quantum mechanics as a mathematical theory, and her formulations are rigorous and precise. Her absence from Coale's book is the more notable because his analysis needs an infusion of the kind of deep thinking that Barad embodies. By contrast, his book engages in a discursive inflation of quantum physics, wherein the rigor and precision it has as a physical theory is buried under an avalanche of verbiage that makes quantum physics appear so broad and vague that almost anything can fit under its umbrella.A good example of this problem is the way in which Coale uses “entanglement.” In quantum mechanics, it refers to the correlation of the quantum characteristics of two subatomic particles. For example, when two photons interact (say, by pumping a laser beam through a nonlinear crystal such as beta-barium borate, which splits some of the photon stream into photon pairs with correlated polarizations), they can become entangled such that their wave functions (i.e., the eigenstates described by their superpositions) are coordinated. If they are then separated, measurement on one of them determines how the wave function for the other will collapse, even if they are by then far apart. The distance (and time intervals) separating the two photons can be quite large; entanglement has been shown to operate as far removed as fourteen miles. Moreover, it also operates on the macroscale, for example in the ability of migratory birds to sense the inclination of the earth's magnetic field (via entangled electrons in the molecules of their eyes). This is mysterious to the extent that no one knows how or why the particles become coordinated, but it is nevertheless quite distinct from such uses of “entanglement” that would include, for example, two lovers becoming entangled with one another or the World Bank becoming entangled in the global economic system. Citing Patrick O'Donnell, Coale appropriates the word to mean, following O'Donnell, “the complex relationship between self, objects, and the world” (164), which stretches the meaning so far that it leaves quantum mechanics (or quantum theory, as Coale would have it) far behind.In similar fashion, Coale assimilates to quantum theory most of deconstruction, especially the indeterminacy, undermining of origins, and undecidability that Jacques Derrida brilliantly developed in Of Grammatology and later works. He does the same with postmodernism, so that it begins to seem almost a branch of quantum physics rather than a philosophical and cultural movement with complex roots in cultural, economic, and technological developments. For example, John Johnston in Information Multiplicity: American Fiction in the Age of Media Saturation argues persuasively that the postmodern American novel has been distinctively shaped by the glut of information that the Internet and contemporary communication media make available, a point also made by Fredric Jameson in his seminal study Postmodernism, or the Cultural Logic of Late Capitalism. Yet for Coale, “that flow of information suggests the quantum flux, constantly in motion with islands of possible comprehension like the tracks and trails of ‘particles’ that are measured in individual experiments” (109). The importance of globalization, the complex economic structures emerging from late capitalism, the transformations the web has wrought—all these in Coale's analysis become subsets of quantum physics.Within the limits of his methodology, which are significant, Coale provides good readings of several postmodern authors, including Joan Didion, Tim O'Brien, Thomas Pynchon, Paul Auster, and Don DeLillo. Although many have discussed the fragmentation, nonlinear narratives, lack of coherent subjectivities, and indeterminacy of novels by these writers as postmodern constructions, for Coale they become manifestations of quantum theory (not surprisingly, since he sees postmodernism itself as a branch of the same). With chapters on the quantum flux, quantum style, and death as quantum dissolution, he combines good readings with inflated claims for them as manifestations of quantum theory. The exceptions to this quantum inflation are his discussions of DeLillo's novels. He mentions in the introduction that he found several articles on quantum theory and references of quantum phenomena in DeLillo's notebooks at the Harry Ransom Humanities Research Center at the University of Texas at Austin. These bear fruit in his readings of DeLillo's novels, especially his fine reading of The Body Artist and the section on DeLillo's Underworld, arguably the best interpretation in the book. This section alone almost justifies the price of admission.Were Coale to frame his book as a contribution to postmodern literary criticism, it would take its place among many other such books and articles as a well-written and interesting contribution that makes good observations about a number of important authors and their works. By insisting that all of this fits under quantum theory, he overreaches what the theory can justly claim and, in the process, dilutes the effectiveness of his book. Without a doubt, we need more informed criticism of the interactions between literature, culture, and science, but such work is more effective when it focuses on the specificities of scientific theories and their complex entanglements with cultural and technological practices. Lest it fall under Coale's quantum umbrella, I should add that I do not intend in this formulation a reference to quantum theory!
Introduction Chapter 1 dealt with one of the deepest mysteries in modern science: how to explain consciousness. This Part deals with another mystery: how to make sense of quantum mechanics. Both involve a problem of reconciliation with the classical worldview, which is one hint that the two mysteries might be related. A second is that, unlike classical physics, which makes no reference to or allowance for consciousness, quantum theory raises the issue of consciousness and its relationship to the physical world in a very direct way. These hints do not necessarily mean there is a connection between the two mysteries, but by the end of this Part I hope to have shown why it would be natural to look for one. My more immediate aim here is to give the social scientific reader an introductory understanding of the experimental findings of quantum theory, its key concepts, and the debates about its interpretation. The discussion assumes no prior knowledge of quantum physics and makes no use of mathematics. Quantum theory without equations might seem like an oxymoron, and certainly reading this Part will not enable anyone to use quantum theory. But to a perhaps surprising degree it is possible to understand quantum theory without being able to use it. Its findings can be communicated in ordinary language, and the same goes for its main concepts and interpretive debates. One consequence of this has been the emergence of a virtual cottage industry of books on quantum theory for a lay audience. Most are written by physicists, and many are quite good. However, it is not just popular treatments that do without equations: much of the professional philosophical literature does as well. (I draw on both kinds of literature below.) Philosophers of physics are trained to understand the math, of course, which is necessary for a full appreciation of the issues.
Quantum mechanics is usually defined in terms of some loosely connected axioms and rules. Such a foundation is far from the beauty of, e.g., the `principles' underlying classical mechanics. Motivated, in addition, by notorious interpretation problems, there have been numerous attempts to modify or `complete' quantum mechanics. A first attempt was based on so-called hidden variables; its proponents essentially tried to expel the non-classical nature of quantum mechanics. More recent proposals intend to complete quantum mechanics not within mechanics proper but on a `higher (synthetic) level'; by means of a combination with gravitation theory (R Penrose), with quantum information theory (C M Caves, C A Fuchs) or with psychology and brain science (H P Stapp). I think it is fair to say that in each case the combination is with a subject that, per se , suffers from a very limited understanding that is even more severe than that of quantum mechanics. This was acceptable, though, if it could convincingly be argued that scientific progress desperately needs to join forces. Quantum mechanics of a closed system was a beautiful and well understood theory with its respective state being presented as a point on a deterministic trajectory in Liouville space---not unlike the motion of a classical N -particle system in its 6N -dimensional phase-space. Unfortunately, we need an inside and an outside view, we need an external reference frame, we need an observer. This unavoidable partition is the origin of most of the troubles we have with quantum mechanics. A pragmatic solution is introduced in the form of so-called measurement postulates: one of the various incompatible properties of the system under consideration is supposed to be realized (i.e. to become a fact, to be defined without fundamental dispersion) based on `instantaneous' projections within some externally selected measurement basis. As a result, the theory becomes essentially statistical rather than deterministic; furthermore there is an asymmetry between the observed and the observing. This is the point where consciousness may come in. Complemented by an introduction and several appendices, Henry Stapp's book consists essentially of three parts: theory, implications, and new developments. The theory part gives a very readable account of the Copenhagen interpretation, some aspects of a psychophysical theory, and, eventually, hints towards a quantum foundation of the brain--mind connection. The next part, `implications', summarizes some previous attempts to bridge the gap between the working rules of quantum mechanics and their possible consequences for our understanding of this world (Pauli, Everett, Bohm, Heisenberg). The last section, `new developments', dwells on some ideas about the conscious brain and its possible foundation on quantum mechanics. The book is an interesting and, in part, fascinating contribution to a field that continues to be a companion to `practical' quantum mechanics since its very beginning. It is doubtful whether such types of `quantum ontologies' will ever become (empirically) testable; right now one can hardly expect more than to be offered some consistent `grand picture', which the reader may find more or less acceptable or even rewarding. Many practicing quantum physicists, though, will remain unimpressed. The shift from synthetic ontology to analytic ontology is the foundation of the present work. This means that fundamental wholes are being partitioned into their ontologically subordinate components by means of `events'. The actual event, in turn, is an abrupt change in the Heisenberg state describing the quantum universe. The new state then defines the tendencies associated with the next actual event. To avoid infinite regression in terms of going from one state of tendencies to the next, consciousness is there to give these events a special `feel', to provide a status of `intrinsic actuality'. The brain of an alert human observer is similar in an important way to a quantum detection device: it can amplify small signals to large macroscopic effects. On the other hand, actual events are not postulated to occur exclusively in brains. They are more generally associated with the formation of records. Records are necessarily part of the total state of the universe: it is obvious that the state of the universe cannot undergo a Schrödinger dynamics and at the same time record its own history. `The full universe consists therefore of an exceedingly thin veneer of relatively sluggish, directly observable properties resting on a vast ocean or rapidly fluctuating unobservable ones.' The present ideas also bear on how the world should be seen to develop. While conventional cosmology encounters problems as to how to define the intial conditions, which would enter the governing equations of motion, here `the boundary conditions are set not at some initial time, but gradually by a sequence of acts that imposes a sequence of constraints. After any sequence of acts there remains a collection of possible worlds, some of which will be eliminated by the next act.' Connected with those acts is `meaning': there has always been some speculation about the special significance of local properties in our understanding of the world. One could argue that correlations (even the quantum correlations found, e.g., in the EPR-experiments) were as real as anything else. But also Stapp stresses the special role of locality: the `local observable properties, or properties similar to them are the natural, and perhaps exclusive, carriers of meaning in the quantum universe. From this point of view the quantum universe tends to create meaning.' This sounds like an absolute concept: meaning not with respect to something else, but defined intrinsically---not easy to digest. The role of consciousness in the developing quantum universe requires more attention. `The causal irrelevance of our thoughts within classical physics constitutes a serious deficiency of that theory, construed as a description of reality.' This is taken to be entirely different within quantum mechanics. `The core idea of quantum mechanics is to describe our activities as knowledge-seeking and knowledge-using agents.' `21st century science does not reduce human beings to mechanical automata. Rather it elevates human beings to agents whose free choices can, according to the known laws, actually influence their behaviour.' An example with respect to perception is discussed: `Why, when we look at a triangle, do we experience three lines joined at three points and not some pattern of neuron firings?' The brain `does not convert an actual whole triangle into some jumbled set of particle motions; rather it converts a concatenation of separate external events into the actualization of some single integrated pattern of neural activity that is congruent to the perceived whole triangle.' How convincing is this proposal? It is hard to tell. I think Henry Stapp did a good job, but there are tight limitations to any such endeavour. Quantum mechanics is often strange indeed, but it also gives rise to our classical world around us. For the emergence of classicality jumps and measurement projections (the basic phenomena connected with those fundamental events of choice) are not needed. Therefore, I doubt whether the explanation of the evolution of our world really allows (or requires) that much free choice. On the other hand, most scientist will agree that empirical science was not possible without free will: we could not ask independent questions if this asking was part of a deterministic trajectory. The fact that the result of a quantum measurement is indeterminate (within given probabilities) does certainly not explain free will. How about the type of measurment? The experimentalist will have to assume that he can select the pertinent observable within some limits. But given a certain design the so-called pointer basis (producing stable measurement results) is no longer a matter of free choice. `The main theme of classical physics is that we live in a clocklike universe.' Today it is often assumed that the universe was a big (quantum-) computer or a cellular automaton. Many would be all too happy to leave that rather restrictive picture behind. But where to go? Stapp suggests giving consciousness a prominent role: `The most profound alteration of the fundamental principles was to bring consciousness of human beings into the basic structure of the physical theory.' How far we are able to go in this direction will depend on the amount of concrete research results becoming available to support this view.
As probably the most successful scientific theory ever created, quantum theory has profoundly changed our view of the world and extended the limits of our knowledge, impacting both the theoretical interpretation of a tremendous range of phenomena and the practical development of a host of technological breakthroughs. Yet for all its success, quantum theory remains utterly baffling. Quantum Reality: Theory and Philosophy, Second Edition cuts through much of the confusion to provide readers with an exploration of quantum theory that is as authoritatively comprehensive as it is intriguingly comprehensible. The book has been fully updated throughout to include the latest results in quantum entanglement, the theory and practical applications of quantum computing, quantum cosmology and quantum gravity. Needing little more than a school level physics and mathematics background, this volume requires only an interest in understanding how quantum theory came to be and the myriad ways it both explains how our universe functions and extends the reach of human knowledge. Written by well-known physics author and teacher Dr. Jonathan Allday, this highly engaging work: • Presents a thorough grounding in the theoretical machinery of quantum physics • Offers a whistle-stop tour through the early part of the 20th century when the founding fathers of quantum theory forever altered the frontiers of human thought • Provides an example-filled interpretation of the theory, its applications, and its pinnacle in quantum field theory (QFT), so crucial in shaping ideas about the nature of reality • Separates fact from speculation regarding quantum physics’ ability to provide a starting point for philosophical queries into ultimate understanding and the limits of science The world beneath the one that we experience with our senses is profoundly mysterious, and while we may never completely unravel that mystery, quantum theory allows us to come closer than ever to understanding where the science leaves off and the mystery begins. Quantum Reality: Theory and Philosophy, Second Edition makes that understanding accessible to anyone possessing a quest for knowledge and a sense of awe.
THE conceptual problems raised by the discovery of quantum effects have not been fully resolved after half a century. Part of the reason for this is undoubtedly to be found in the mutual ignorance which prevails between physicists and philosophers. In his book Heisenberg brings together a philosophically inclined temperament with an unrivalled knowledge of physics. The result is a book of very great interest, however much one might disagree with his conclusions. The collection of essays of which the other book is made up is intended to bring crucial conceptual problems in physics especially to the notice of philosophers. The design and intention of the collection is admirable for this is the proper way to overcome the departmentalization of interest that has left the deep conceptual problems still unresolved. Men like Heisenberg who combine philosophical temperament with scientic knowledge of great distinction are rare, and yet the difficulties can only be cleared up by the application of philosophical techniques of analysis to problems which are an intimate and difficult part of physics. A great deal of the interest of Heisenberg's book lies in the very comprehensive account he gives of the origin and growth of the Copenhagen school, and the way the new phenomena forced a largely unwelcome picture of nature upon Bohr and his friends. The most important chapter from a philosophical point of view is that in which Heisenberg defends this interpretation against the attacks to which it has been subjected. The essence of his argument for the interpretation can be summed up in two propositions. (i) The Copenhagen interpretation of physical theory, which depends upon the acceptance of the so-called Principle of Complementarity, is an inevitable and final interpretation. The reason for this is that there are only two possible general models for understanding given physical phenomena; a particle model or a wave model. These are exhaustive and exclusive because the former depends upon discontinuous and the latter upon continuous (using substance here in the traditional philosophical sense). If we find, as we do, that the two fundamental constituents of the universe, matter and radiation, each exhibit two classes of phenomena, one explainable only on a particle model and one on a wave model, then we are compelled to adopt a complementarity hypothesis and give up the idea of finding a single and uniform model for the understanding of phenomena. There are only two kinds of models; they are mutually exclusive; both are required; hence we either despair before an insoluble problem or adopt the Copenhagen interpretation. (2) Any serious criticism of this interpretation must offer an alternative
The difficulties of the old quantum theory were largely dispelled by the development of quantum mechanics in the 1920s. First Heisenberg (1925) developed the rather abstract matrix mechanics and soon after Schro¨dinger (1926; see also Eckart, 1926) developed the physically more appealing wave mechanics. Subsequently he showed them to be mathematically equivalent. The physicists were greatly relieved; now they had once again a mathematical formalism that enabled the results of experiments to be calculated. It was possible to move forward. It is certainly a very elegant and indeed beautiful formalism, and does enable the results of many physical measurements to be calculated with great accuracy. It is used daily by atomic and nuclear physicists; there is no other way to make calculations. From the earliest days there has been intense debate about the interpretation of quantum mechanics, and it shows no signs of being settled; if anything it is becoming more intense over the years. As Penrose has remarked, no one understands quantum mechanics. Feynman (1967; see also Sykes, 1994) also admitted that ‘nobody understands quantum mechanics’, and Einstein once remarked: ‘I have thought a hundred times as much about the quantum problems as I have about general relativity theory’ (Pais, 1994, p. 57). It should therefore be evident that the most that can be done here is to try to show where the real difficulties lie, and how the failure to recognize that quantum mechanics is an essentially statistical theory has led to a series of confusing quantum paradoxes. It is just these paradoxes that have often been used to support theological speculations. It is useful to ask what ‘understanding quantum mechanics’ actually means. Certainly it means being able to calculate the results of many measurements. The same applies to Newton’s theory of gravitation, which we understand in the sense that we can use it as a basis of calculation, although we do not really understand the gravitational force itself. Very often, especially in modern physics, the mathematical formalism does not give us the sort of intuitive physical feeling for what is going on that we often obtain from classical mechanics. Perhaps we can never achieve more than this for quantum mechanics. In classical mechanics we can assume that a property that is measured objectively exists prior to the interaction of the measuring apparatus with the observed system. Quantum mechanics, however, ‘is incompatible with the proposition that measurement discovers some unknown but pre-existing property’ (Peres, 1997, p. 14). There is an important distinction between mysteries and absurdities. We may be forced to live with mysteries, but there is no reason why we should put up with absurdities such as the wave-particle duality, tunnelling through potential barriers, acausal events and the collapse of the wave function. These are the quantum paradoxes that will be discussed later on. They arise within the Copenhagen interpretation of quantum mechanics and are solved by other interpretations. These alternative interpretations enable us to make conjectures about what is going on, even though we cannot understand it in detail. This is acceptable and a stimulus to further research. It is certainly difficult enough to understand quantum mechanics, but there is no need to try to visualize logical absurdities in the effort to do so. Quantum mechanics is a mathematical formalism that makes it possible to calculate the results of some experiments. Anything more than this, such as questions about the interpretation of what is measured and whether it tells us what is actually going on in the system being studied, belongs to philosophy. Thus we are concerned here with what is essentially a philosophical debate; if it were just a matter of physics the debate would have been settled long ago. One of the difficulties is that it invokes the results of experiments and uses the language of quantum mechanics. Inevitably this language uses philosophical terms and then it is easy to give the impression that the philosophical propositions are in some way entailed by the physical results. These in turn can influence theological thinking. In order to be quite clear which arguments are justified and which are not, it is essential to separate the three levels of discourse: physical, philosophical and theological. Our task is to relate these together, and this immediately raises the traditional problems of the philosophy of science, including the relation of experiment to theory, and of theory to explanation. We begin by discussing experiments, distinguishing between those that are real and those that are imaginary.
Most philosophical work on quantum physics has concerned simple systems. And for good reason. Even one or two particle systems can exhibit the striking features we have come to associate with quantum physics—features such as entanglement, interference, and nonlocality. But as far as physics goes, such systems barely scrape the surface. In particular, they have only a finite number of degrees of freedom, which means their states are characterized by a finite number of independent parameters. The systems studied in fields such as high-energy particle physics and many-body physics, meanwhile, have uncountably many degrees of freedom. They are far more complicated than the usual philosophical fare.This is not troubling as long as the conceptual heart of quantum physics can be effectively condensed down. But what if philosophers' toy systems are fundamentally different from realistic ones? That would be a problem. And indeed, as Laura Ruetsche convincingly argues in Interpreting Quantum Theories, systems with uncountable degrees of freedom—what Ruetsche collectively calls QM∞—are fundamentally different from the systems that have become the stock and trade of philosophy of physics, in ways that change the interpretational project.Ruetsche's book does two things exceedingly well. The first is to provide a (comparatively) accessible and philosophically oriented introduction to the algebraic approach to quantum physics, which is essential for understanding QM∞. This is a valuable contribution. The algebraic approach is the setting for much recent philosophical work. And yet learning it is notoriously difficult, as even pedagogical texts are aimed at research-level mathematicians. Ruetsche's book is technically demanding, but it works to bring nonmathematicians along. It is, without a doubt, the best place to enter this literature.Second, the book sets the agenda for future work on QM∞, by effectively and judiciously drawing important foundational questions out of a sprawling and difficult physics literature. In this regard, it is a close cousin to John Earman's (1995) excellent Bangs, Crunches, Whimpers, and Shrieks, which has had a significant influence on subsequent philosophy of spacetime physics. Philosophers of physics will be addressing the questions Ruetsche has raised, in many cases for the first time, for years to come.Interpreting Quantum Theories centers on what might be called the problem of inequivalent representations. The quantum theories that philosophers are accustomed to are set in a mathematical structure known as a (separable) Hilbert space, which is an at-most countably infinite dimensional complex vector space. Rays in a Hilbert space represent possible states of a physical system, and self-adjoint operators on that Hilbert space correspond to properties of the system.To treat QM∞, meanwhile, physicists pass to a generalization of this formalism. Now one begins with an abstract algebra of operators corresponding to the various properties associated with the system. States in this more general framework are linear functionals on the algebra whose value at each operator is interpreted as the expectation value of a measurement of that property in that state.The trouble with this more general framework is that the interpretational tools of Hilbert space quantum theories do not directly apply. Some solace comes from recognizing that any (appropriate) algebra of operators admits a representation in terms of the operators on a Hilbert space. This suggests that one can always move back to the more familiar Hilbert space formalism. But this is a pyrrhic victory. In general these Hilbert space representations are neither unique nor empirically equivalent. In particular, these inequivalent representations, interpreted in the familiar way, yield disagreements about which algebraic states are physically realizable.Faced with this situation, Ruetsche presents several principled options—what she calls “pristine” interpretations of QM∞. The two most significant are what she calls “Hilbert Space Conservatism” and “Algebraic Imperialism.” The Conservative claims that, for a given system, there is one true Hilbert space representation, which should be interpreted in the familiar ways. The Imperialist, meanwhile, insists that only the abstract algebra matters: the physical states are the algebraic states, properties correspond to the operators in the algebra, and if our old methods do not apply, so much the worse for our methods.At this level of description, Imperialists may appear to have the advantage, since they need not fret over which representation to choose. But there's a catch. As we have seen, there is a sense in which Imperialists generally recognize a broader range of possible physical states. But Conservatives allow for more properties. This is because when one represents the abstract algebra as operators on a Hilbert space, the operators in the representation naturally approximate additional operators on that Hilbert space, in a way that is unavailable in the abstract setting. Thus each interpretation suffers from limitations that the other does not.To adjudicate between the interpretations, Ruetsche takes them to the trenches, to see which provides the resources to support the actual explanatory and representational practices of physics. After a masterful survey, she concludes that neither interpretation has all of the requisite resources. In particular, she argues, physicists regularly make essential use of operators that are only available to Conservatives, while making equally essential use of multiple inequivalent representations—which only Imperialists can support. More, the particular mix of Conservatism and Imperialism varies between applications, apparently ruling out a stable interpretation of the formalism that can be articulated independently of the messy details. We are left with what Ruetsche calls “adulterated” interpretations.Philosophers' practice notwithstanding, it might seem unsurprising that we cannot separate our interpretation of physical theories from the pragmatics of their application to the world. After all, we are interested in these theories only insofar as they are successfully applied. But Ruetsche believes the problem is more acute. It is not merely that pragmatic considerations enter the debate between Conservatives and Imperialists. Ruetsche argues that there may not be any consistent, general interpretation of QM∞ compatible with practice. And this, she claims, makes realism about QM∞ untenable.The idea that the applications of a broad class of our best physical theories undermine their consistent interpretation in a way that renders realism incoherent is striking. Insofar as Ruetsche's case is convincing, it should have serious repercussions for general philosophy of science—both because it bears on important debates and because it shows how generalist questions depend on the details of particular scientific theories and their applications. Indeed, Ruetsche's skepticism about the terms of the realism debate and her radical particularism should influence general philosophy of science. But that said, her argument ultimately leaves room for disagreement.Ruetsche's basic claim is that Conservatism and Imperialism each support practices that the other cannot and that both kinds of practice are essential. But there is an important asymmetry here. Conservatives suffer because they fail to represent putative physical possibilities appealed to in practice. Imperialists, meanwhile, need to justify that in certain applications, they may limit attention to a subset of the physical possibilities their interpretation admits, and then use resources that only make sense in those limited regimes.The trouble is that the Imperialists' opportunism is neither unusual in physics nor difficult to motivate. In spacetime physics, for instance, properties such as the total mass of the universe or global time can be defined only in very special cases. And yet such properties are used in practice, because often the salient spacetimes are precisely the ones in which these quantities make (approximate) sense. To make her case that adulteration in QM∞ undermines realism, Ruetsche would need to establish that the representation-dependent quantities used in applications of QM∞ are different in kind from analogous quantities that appear throughout physics. And here it is not clear that she succeeds.None of this undercuts the importance of the book, however. Indeed, Ruetsche acknowledges that her argument may depend on how one understands idealization in physics. And even if one takes the view just described, there is a great deal of work to do on how Imperialism can support the practices Ruetsche identifies—work that will surely proceed within the framework Ruetsche has laid out, by addressing questions Ruetsche has raised. And so, wherever our study of QM∞ takes us, Ruetsche's book is sure to play a central role.