Few, if any, issues in physics have engendered as much discussion as the measurement problem in quantum mechanics. It is generally agreed that the `normal' dynamical evolution of the state vector in quantum mechanics is given by a unitary map. The linearity of this map implies that the state vector will, in general, be found in a superposition of eigenstates of a given observable (or, similarly, that the density matrix describing a subsystem will not correspond to a definite value of this observable). However, when we make a measurement of an observable, we always obtain a define value—although it is impossible to predict with certainty which value will be obtained. The traditional response to this issue is to postulate that when a measurement is made, the wavefunction `collapses' to an eigenstate of the observable being measured, perhaps due to the inherent classicality of the measuring apparatus (Bohr), or to the consciousness of the observer (Wigner), or possibly to some modification of quantum dynamics that occurs even when observations are not being made. The main motivation for the Everett (`many worlds') interpretation is to avoid introducing any such collapse postulate. This volume commemorates the 50th anniversary of the publication of Everett's paper in 1957 and contains 20 original articles as well as the transcripts of several discussions that took place at meetings devoted to the Everett interpretation at Oxford University and the Perimeter Institute. The attractiveness of the Everett interpretation is very succinctly summarized by a sentence from Vaidman's contribution (p 582): `The collapse, with its randomness, non-locality and the lack of a well-defined moment of occurrence, is such an ugly scar on quantum theory, that I, along with many others, am ready to follow Everett and deny its existence.' But the main drawback of the interpretation is then equally succinctly stated in the next sentence: `The price is the many worlds interpretation, i.e., the existence of numerous parallel worlds.' In other words, if the state vector evolves to have a non-zero amplitude for two different values of an observable that I `measure' (i.e., interact with in an appropriate way), each of these alternatives remains equally `real', i.e., there is a `branch' of the wavefunction in which the observable takes the first value and I believe that it has this first value and a branch of the wavefunction where it takes the second value and I believe that it has this second value. Each of these `me's' sees a definite value for the observable, thus nicely accounting for what I believe I experience without invoking collapse. But then, it seems that I have to believe that there are many other `me's' around—even if they are very difficult for this particular me to access—and no way I can predict which `me' I will be in the future. Indeed, Saunders writes on p 192, `If Alice is a person (of course she is) then we must say, even prior to branching at tj that there are many Alices present, atom for atom duplicates...Alice should be uncertain after all— each Alice should be uncertain—for each as of tj does not (and as a matter of principle, cannot) know which of these branching persons she is.' Thus, if taken seriously, the Everett viewpoint leads directly to questions like `What determines which ``me'' I am now and/or will feel that I am in the future?' The response `You are all of them' somehow doesn't seem satisfying, and further pursuit of these issues would seem to lead to questions as fruitless as questions like `Why am I ``me'' and not ``you''?' I believe that this accounts for why the Everett interpretation does not have universal appeal. But enough of `me' for now (more later); let's discuss the book. Unlike many edited volumes and conference proceedings in physics—which often consist of hastily slapped-together re-hashes of previous papers—this volume meets very high standards in the choice of topics and authors and in the quality of the writing. Although each of the articles is closely tied in with the Everett interpretation, there are many articles of considerable value and interest in their own right, such as the discussion of quasiclassical realms by Hartle and and the discussion of the de Broglie–Bohm interpretation by Valentini. There is also an excellent introduction by Saunders to the quantum measurement problem and the main issues that arise in the Everett interpretation. A central issue in the Everett interpretation is the status of the `Born rule', which asserts that, for state ψ, the probability of obtaining a particular outcome of a measurement is ||Pψ||2, where P is the projection operator onto the eigensubspace associated with the measurement outcome. In traditional interpretations, the Born rule is simply postulated as part of the collapse hypothesis. In the Everett interpretation, it is far from obvious that the Born rule even has any meaning—if all outcomes occur, how can one talk about the probability of a particular outcome? Given the importance of this issue, it is highly appropriate that four chapters of the book (by Saunders, Papineau, Wallace, and Greaves and Myrvold) are devoted to addressing probability and the Born rule from the Everett viewpoint, and three chapters (by Kent, Albert, and Price) are devoted to criticising these views. As indicated by the above quote from Saunders' article, uncertainty and probability can enter the Everett interpretation via each Alice being uncertain as to which Alice she is or will be. However, in view of the difficulties to which this kind of argumentation will lead, it is perhaps not surprising that in the 123 pages of combined discussion by Saunders, Papineau, Wallace, and Greaves and Myrvold, there are only fleeting references to the use of probability along these lines. Rather, the approach taken is reminiscent of that of adept politicians: if you can't answer a question, then answer a different question, and answer it with conviction and detail. Instead of considering probabilities of outcomes—a highly problematical notion when all of the possible outcomes actually occur—consider only decision theory issues. In particular, Wallace gives a precise and detailed mathematical proof that, under certain hypotheses, the only rational strategy that a quantum observer can follow in making decisions is to weight the possible outcomes by Born rule probabilities. To illustrate this more graphically, suppose tomorrow's weather were determined by some quantum mechanical, Stern–Gerlach-like process. I would like to know whether it is likely to rain tomorrow. However, when I ask an Everett follower (who knows the state vector and has done the Born rule calculation), all she will tell me in direct response to my question is that `it will rain and it won't rain.' Nevertheless, she is willing tell me whether or not it is rational for me to take an umbrella. I suppose that if she tells me that it would be highly irrational for me to take an umbrella, I can take the hint and deduce that it is very unlikely to rain. However, there does seem something wrong with the fact that she is not allowed to say this directly. In any case, if the conclusion of a mathematically correct argument is that rational decision strategies require the Born rule, then there must be quite a bit lying in the assumptions. The articles by Kent, Albert, and Price do a good job of fleshing out these assumptions and pointing out the weaknesses and flaws in the probability and decision theory discussions within the Everett framework. I hope it is clear from what I have written above that this is an outstanding volume, which should be of great interest to anyone interested in the foundations of quantum theory. Indeed, I feel quite fortunate to be the particular `me' that happens to exist in the same branch of the wavefunction of the universe in which this book was published.
Few, if any, issues in physics have engendered as much discussion as the measurement problem in quantum mechanics. It is generally agreed that the normal' dynamical evolution of the state vector in quantum mechanics is given by a unitary map. The linearity of this map implies that the state vector will, in general, be found in a superposition of eigenstates of a given observable (or, similarly, that the density matrix describing a subsystem will not correspond to a definite value of this observable). However, when we make a measurement of an observable, we always obtain a define value—although it is impossible to predict with certainty which value will be obtained. The traditional response to this issue is to postulate that when a measurement is made, the wavefunction collapses' to an eigenstate of the observable being measured, perhaps due to the inherent classicality of the measuring apparatus (Bohr), or to the consciousness of the observer (Wigner), or possibly to some modification of quantum dynamics that occurs even when observations are not being made. The main motivation for the Everett (many worlds') interpretation is to avoid introducing any such collapse postulate. This volume commemorates the 50th anniversary of the publication of Everett's paper in 1957 and contains 20 original articles as well as the transcripts of several discussions that took place at meetings devoted to the Everett interpretation at Oxford University and the Perimeter Institute. The attractiveness of the Everett interpretation is very succinctly summarized by a sentence from Vaidman's contribution (p 582): The collapse, with its randomness, non-locality and the lack of a well-defined moment of occurrence, is such an ugly scar on quantum theory, that I, along with many others, am ready to follow Everett and deny its existence.' But the main drawback of the interpretation is then equally succinctly stated in the next sentence: The price is the many worlds interpretation, i.e., the existence of numerous parallel worlds.' In other words, if the state vector evolves to have a non-zero amplitude for two different values of an observable that I measure' (i.e., interact with in an appropriate way), each of these alternatives remains equally real', i.e., there is a branch' of the wavefunction in which the observable takes the first value and I believe that it has this first value and a branch of the wavefunction where it takes the second value and I believe that it has this second value. Each of these me's' sees a definite value for the observable, thus nicely accounting for what I believe I experience without invoking collapse. But then, it seems that I have to believe that there are many other me's' around—even if they are very difficult for this particular me to access—and no way I can predict which me' I will be in the future. Indeed, Saunders writes on p 192, If Alice is a person (of course she is) then we must say, even prior to branching at tj that there are many Alices present, atom for atom duplicates...Alice should be uncertain after all— each Alice should be uncertain—for each as of tj does not (and as a matter of principle, cannot) know which of these branching persons she is.' Thus, if taken seriously, the Everett viewpoint leads directly to questions like What determines which ``me'' I am now and/or will feel that I am in the future?' The response You are all of them' somehow doesn't seem satisfying, and further pursuit of these issues would seem to lead to questions as fruitless as questions like Why am I ``me'' and not ``you''?' I believe that this accounts for why the Everett interpretation does not have universal appeal. But enough of me' for now (more later); let's discuss the book. Unlike many edited volumes and conference proceedings in physics—which often consist of hastily slapped-together re-hashes of previous papers—this volume meets very high standards in the choice of topics and authors and in the quality of the writing. Although each of the articles is closely tied in with the Everett interpretation, there are many articles of considerable value and interest in their own right, such as the discussion of quasiclassical realms by Hartle and and the discussion of the de Broglie–Bohm interpretation by Valentini. There is also an excellent introduction by Saunders to the quantum measurement problem and the main issues that arise in the Everett interpretation. A central issue in the Everett interpretation is the status of the Born rule', which asserts that, for state ψ, the probability of obtaining a particular outcome of a measurement is ||Pψ||2, where P is the projection operator onto the eigensubspace associated with the measurement outcome. In traditional interpretations, the Born rule is simply postulated as part of the collapse hypothesis. In the Everett interpretation, it is far from obvious that the Born rule even has any meaning—if all outcomes occur, how can one talk about the probability of a particular outcome? Given the importance of this issue, it is highly appropriate that four chapters of the book (by Saunders, Papineau, Wallace, and Greaves and Myrvold) are devoted to addressing probability and the Born rule from the Everett viewpoint, and three chapters (by Kent, Albert, and Price) are devoted to criticising these views. As indicated by the above quote from Saunders' article, uncertainty and probability can enter the Everett interpretation via each Alice being uncertain as to which Alice she is or will be. However, in view of the difficulties to which this kind of argumentation will lead, it is perhaps not surprising that in the 123 pages of combined discussion by Saunders, Papineau, Wallace, and Greaves and Myrvold, there are only fleeting references to the use of probability along these lines. Rather, the approach taken is reminiscent of that of adept politicians: if you can't answer a question, then answer a different question, and answer it with conviction and detail. Instead of considering probabilities of outcomes—a highly problematical notion when all of the possible outcomes actually occur—consider only decision theory issues. In particular, Wallace gives a precise and detailed mathematical proof that, under certain hypotheses, the only rational strategy that a quantum observer can follow in making decisions is to weight the possible outcomes by Born rule probabilities. To illustrate this more graphically, suppose tomorrow's weather were determined by some quantum mechanical, Stern–Gerlach-like process. I would like to know whether it is likely to rain tomorrow. However, when I ask an Everett follower (who knows the state vector and has done the Born rule calculation), all she will tell me in direct response to my question is that it will rain and it won't rain.' Nevertheless, she is willing tell me whether or not it is rational for me to take an umbrella. I suppose that if she tells me that it would be highly irrational for me to take an umbrella, I can take the hint and deduce that it is very unlikely to rain. However, there does seem something wrong with the fact that she is not allowed to say this directly. In any case, if the conclusion of a mathematically correct argument is that rational decision strategies require the Born rule, then there must be quite a bit lying in the assumptions. The articles by Kent, Albert, and Price do a good job of fleshing out these assumptions and pointing out the weaknesses and flaws in the probability and decision theory discussions within the Everett framework. I hope it is clear from what I have written above that this is an outstanding volume, which should be of great interest to anyone interested in the foundations of quantum theory. Indeed, I feel quite fortunate to be the particular `me' that happens to exist in the same branch of the wavefunction of the universe in which this book was published.
Recent years have seen a good deal of work done on one of the most mind blowing and conceptually radical views in quantum mechanics, namely the ‘many worlds’ interpretation of quantum mechanics. Many worlds was first proposed by Hugh Everett III in his long Ph.D. dissertation, which only some years later, and after significant shortening, came to be published as the ‘Relative State Formulation of Quantum Mechanics’ (Everett 1957). To commemorate the 50th anniversary of the first publication of Everett's ideas, two conferences were held at Oxford University and the Perimeter Institute, which brought together some of the most active researchers in the field. The works presented in these two conferences have now been compiled in the volume Many Worlds? Everett, Quantum Theory and Reality, edited by Simon Saunders, Jonathan Barrett, Adrian Kent and David Wallace.1 This is then a comprehensive collection of papers – 23 papers altogether – where the reader will find virtually anything that can be said about the many worlds interpretation today.
Abstract What does realism about the quantum state imply? What follows when quantum theory is applied without restriction, if need be, to the whole universe? These are the questions which are debated in this book. All the chapters start from the point of realism, and based on the need, or the aspiration, for a theory that unites micro- and macroworlds, at least in principle. But the further claim argued by some is that if you allow the Schrödinger equation unrestricted application, supposing the quantum state to be something physically real, then this universe is one of countlessly many others, constantly branching in time, all of which are real. The result is the many worlds theory, also known as the Everett interpretation of quantum mechanics. The contrary claim sees this picture of many worlds as in no sense inherent in quantum mechanics, even when the latter is allowed unrestricted scope and even given that the quantum state itself is something physically real. For this picture of branching worlds fails to make physical sense, let alone common sense, even on its own terms. The status of these worlds, what they are made of, is never adequately explained. Ordinary ideas about time and identity over time become hopelessly compromised. The concept of probability itself is brought into question. This picture of many branching worlds is inchoate, it is a vision, an error. There are realist alternatives to many worlds, some even that preserve the Schrödinger equation unchanged. Twenty chapters, accompanied by commentaries and discussions, examine these claims and counterclaims in depth. They focus first on the question of ontology, the existence of worlds, second on the interpretation of probability, and third on alternatives or additions to many worlds. The introduction offers a helpful guide to the arguments for the Everett interpretation, particularly as they have been formulated in the last two decades.
This book is a treat, a collection of essays about the relationship between consciousness and quantum mechanics that is guaranteed to move the needle: serious, original, and relatively accessible. The essays develop cutting-edge views by thought-leading authors on questions that matter for the philosophy of physics and the philosophy of mind, and while to follow along you need to know what supervenience is and what an eigenstate is, the majority of essays should be accessible to advanced undergraduates (i.e., those who have taken a rigorous introductory course in quantum mechanics as well as a course or two in the philosophy of mind).Why care about the relationship between consciousness and quantum mechanics? If you are coming from physics, your point of entry is probably the measurement problem. When you let quantum systems evolve according to the Schrödinger equation, they enter into superpositions of mutually incompatible states. How can this be reconciled with the testimony of our senses that such systems are always in one state or the other when we measure them (with a probability distribution given by the Born rule)? This is the measurement problem, and it concerns consciousness because it draws from the testimony of our senses.If you are coming from the philosophy of mind, your primary concern is probably the mind-body problem: the task of accounting for how consciousness arises out of or fits into physical reality. If consciousness is reducible, how might the details of quantum mechanics make it harder, or easier, to establish that reduction? If consciousness is irreducible, how might the details of quantum mechanics make it harder, or easier, to explain why consciousness exists, or to find some work for it to do in the world? You may also be concerned with the structure and function of consciousness: for example, you may hold that minds can do things that classical digital computers cannot, and take it that we must look to the quantum realm to discover how.If you are not already entrenched, you might just wonder what kind of world we live in. The mechanistic philosophy of centuries past seemed to disenchant the world, rendering it a place of barren corpuscles banging into one another, leaving no room for mind or meaning, except perhaps as patterns in the collisions, or useless epiphenomena. But quantum mechanics and its relativistic generalizations force a rethinking of the mechanistic picture from the ground up. Once the dust has settled—if it ever does—we may find reasons to relegate consciousness once again to the shadows. But we may also find reasons to give it pride of place. If you are after scientific reasons to reenchant the world, this is a good place to look.The entries in this volume address these questions and others besides. The volume is organized into three parts. The first part contains four essays about the idea that consciousness causes (or constrains) quantum collapse. The second part contains eight essays discussing other ways that consciousness might contribute to theories of quantum mechanics. The third part contains five essays about how quantum mechanics might contribute to an ultimate theory of consciousness. I’ll follow the book’s order.In chapter 1, David Chalmers and Kelvin McQueen develop a new consciousness-triggered collapse interpretation of quantum mechanics. On their approach, consciousness has a strict physical correlate (perhaps given by quantum integrated information theory), it is approximately superposition-resistant, and collapse dynamics are continuous (à la Philip Pearle). They respond to the standard objections that a consciousness-collapse link makes the collapse mechanism imprecise and requires dualism, as well as to worries about Quantum Zeno, among others. Crucially, their approach allows that superpositions of consciousness are possible: their motivation is to give the view a run for its money and to find work for consciousness to do in the universe, not to rid the world of all traces of superposed consciousness. One question: In taking consciousness itself to be superposition-resistant (rather than the environmental systems it asks questions about, as Henry Stapp suggests), we appear to favor internalism over externalism. What would an externalist variant look like? Another question: How exactly would experimental results—for example, that simple enough systems with maximal Φ remain in coherent superposition—constrain our theoretical options? If to accommodate this we have to impose an experimentally determined threshold for the minimal absolute Φ for consciousness, will that mean that on the physicalist version of the view, the property we privilege for superposition-resistance is just an arbitrary collection of atoms?In chapter 2, Elias Okon and Miguel Angel Sebastian defend a consciousness-collapse theory that they call the Subjective-Objective collapse model. Like Chalmers and McQueen, Okon and Sebastian invoke a Pearle-style continuous collapse dynamics and think of consciousness as only weakly superposition-resistant. A key contribution is the treatment of a worry about multiple realizability (i.e., the way that some property of biological neurons and some other property of artificial neurons might both realize the state of being in pain). The problem is that it is not straightforward to define Hermitian operators whose eigenstates are disjunctions of the eigenstates of other Hermitian operators: you must ensure orthogonality (“being red or being a circle” is not orthogonal to “being red or being square”). In answer, the authors point out that we don’t need each multiply realizable phenomenal state to be an eigenstate of our collapse operator; it suffices that each way of realizing each phenomenal state is an eigenstate. One question is whether this problematizes the dancing qualia argument. A second worry is that we still must find a suitable basis for Hilbert space. But theories of consciousness may constrain both position and momentum: for example, both distances between neurons and how energy is transferred. We can talk about wave packets, but these will not get us eigenstates of a Hermitian operator.In chapter 3, J. Acacio de Barros and Carlos Montemayor suggest that if a mentality-triggered collapse theory is what you want, you should opt for intentionality rather than phenomenality. This is because, first, we have evidence from psychophysics that there is unconscious perception. Second, the first-personal data that motivate collapse theory concern intentionality: we remember only recording definite values; our decisions about what to measure affect the outcomes we see. First question: Can the reasons we have for thinking that there is unconscious intentionality extend into a case for unconscious primitive intentionality? Another question is how the proposal here would differ from Stapp’s treatment of what questions a mind asks of the universe. A third is how we contend with Quantum Zeno, the effect that forces both Chalmers-McQueen and Okon-Angel Sebastian to say that consciousness is only weakly superposition-resistant. Those authors note the possibility that the relevant superposed states of consciousness might overflow access consciousness (in Ned Block’s sense). We may have an easier time ensuring that access conscious states only rarely enter into superposition, if we set them back from our front line of superposition-resistant states.In chapter 4, Adrian Kent explores objective collapse theories—mass-dependent continuous spontaneous localization models. Consciousness may not be in the spotlight on these theories, but they should at least ensure that collapse happens faster than the time it takes for a stimulus (e.g., a system of several photons) to make it from a retinal impact to consciousness. Angelo Bassi, Dirk-André Deckert, and Luca Ferialdi (2010) argue that for a range of theories this constraint is respected, with collapse for a system of several photos happening at around 100 ms. Kent problematizes their argument, showing that their estimation may be off by an order of magnitude or more. Kent’s point is compelling: more work is needed to refine the estimate and to conceptually clarify the import of the constraints from perception.Part 2 consists of eight essays on various ways that consciousness might figure in or constrain quantum theories. In chapter 5, Phillip Goff argues that 3N-dimensional wave function monism cannot accommodate ground truths about consciousness, pressing on worries that the view imposes too few structural constraints to allow for the unambiguous recovery of ordinary 3D reality. While I agree that wave function monistic accounts of ordinary reality (e.g., Albert’s functionalism or Ney’s priority monism) yield an explanatory gap (even given the 3D-ish structure of GRW jumps), I was puzzled here. All type-B physicalists confront an explanatory gap where consciousness is concerned. Isn’t this a point in support of wave function monism, since it amounts to a companions-in-guilt argument against opposition from other type-B materialists?In chapter 6, Peter Lewis raises some sensible worries about the use of phenomenal consciousness as an unanalyzed primitive or special case in quantum theory. He objects to the suggestion made by some Bohmians that we can be directly aware of the particles in our brains. Another target is John von Neumann, who appeals to intuitions about psychophysical connections to justify a collapse hypothesis. These are nice points: I would be curious to know whether Lewis thinks they apply to the justifications for mentalistic collapse theories from part 1.In chapter 7, Jenann Ismael first argues that the hard problem of consciousness is of no concern to physics per se: “If consciousness enters the problem space of physics, it does so by making a difference to the behavior of physical objects.” She then offers some clarifying remarks about how we might reconcile Van Gelder-style dynamicist conceptions of mentality with (virtual) machine functionalism. I find much to agree with here, though I do wonder how strictly to take Ismael’s claims about the irrelevance of the hard problem. If there is a tie between two theories of physics over predictive accuracy, elegance, and naturalness, but one yields a more satisfying analysis of consciousness than the other, are we not even allowed to treat that as a tie-breaker?In chapter 8, Shan Gao defends a mentalistic characterization of the measurement problem in reply to Andrea Oldofredi (2019). The question is whether the key premise of the measurement problem should be that measurements take on definite values or only that we (definitely) experience them as taking on definite values. Of course, our reasons for thinking that they do arise from our seeing or remembering that they do, but we can distinguish between the claim and our evidence for it. Gao argues that the underlying mentalistic claims are more inextricable from the dilemma than that. Many dilemmas involve premises for which the evidence is perceptual, though they make no mention of perception. Here, however, solutions to the problem require special attention to the nature of that evidence.In chapter 9, Paul Skokowski illustrates one reason that we must pay special attention to the nature of that evidence. Jeffrey A. Barrett’s (1999) bare theory is a minimalistic reading of Everett on which measurement outcomes are only definite if they obtain on all branches. One question is whether this means that observers have blatantly false disjunctive beliefs: let someone measure an electron’s spin and then ask, Do you believe that you got a definite measurement outcome, either “UP” or “DOWN”? The observer will say yes, because on each branch they saw one and not the other. However, by the lights of the theory, this disjunctive belief is false. Skokowski’s insight is that while the observer definitely says yes, it does not follow (by the lights of the bare theory) that the observer has a (definite) false belief, since the neural realizer for the disjunctive belief will be different on each branch and therefore not correspond to a common eigenstate (compare the worry from chapter 2 about multiple realizability). But isn’t this just to move from the frying pan to the fire, as it seems now that our observer has no (definite) beliefs at all?In chapter 10, Michael Silberstein and W. M. Stuckey advocate for a psi-epistemicist perspective that vindicates many phenomenological intuitions, including the one that experimental outcomes are definite (and intersubjectively consistent). The approach requires that we rethink naïve realism about the wave function and that we rethink mind-body duality, in favor of a neutral monist ontology governed by constraints. This is an exciting approach, though I’m not fully clear on how to differentiate neutral monism from its idealist, dual aspect, or panpsychist cousins.In chapter 11, Michael Bitbol suggests a phenomenology-first approach to interpreting quantum mechanics. Bitbol favors a QBist participatory realism, tempered by phenomenology, that could perhaps also be described as neutral monist. He notes that the phenomenology of embodiment (à la Merleau-Ponty) acquaints us with how something can be both active and passive, both minded and incarnate, and he traces the error of believing in a separate, physical world (an error participatory realism corrects) to the tendency of self-separation: the tendency to try to project ourselves beyond ourselves, discussed in Heidegger, Sartre, and Michel Henry. These phenomenological insights are enlightening, though I am still not quite clear on how the underlying (neutral?) ontology works. Who, precisely, are the observers? Do electrons have feelings, or are they just fictions we use to cope with ours?In chapter 12, Lucien Hardy describes an experiment in which humans might be used to determine the settings in a Bell experiment. Hardy suggests that the outcome may be a violation of quantum theory consistent with the Bell inequality. He argues that this result would support (superdeterministic) mind-body dualism and also could serve as a new more objective Turing test. This is a fascinating proposal: it remains only to do the experiment.This brings us to part 3: accounts that move from quantum mechanics to consciousness, rather than conversely. In chapters 13 and 14, Roger Penrose and Stuart Hameroff give a comprehensive account of the current state of their ORCH-OR theory of quantum collapse and consciousness. Among other things we find here a nice presentation of Goodstein’s theorem as an example of a theorem that is noncomputable but easy enough for humans to understand, and a nice discussion of how retroactivity might equip us with something to say to Benjamin Libet about the role of consciousness in action selection. I’ll note, though, that I asked ChatGPT to explain Goodstein to me and it did a fairly good job, suggesting something like understanding. On Libet, I’m not sure what exactly we need retrocausality for: Why not just content ourselves that awareness of the action plan leaves time for a veto, as Libet and others have suggested?In chapter 15, Basil J. Hiley and Paavo Pylkkänen explore another route to a comprehensive theory of consciousness, drawing from the materials of a theory of quantum reality. Here, that theory is the Bohmian implicate order picture, with special attention to the notion of active information that arises there. This is another approach with neutral monist overtones, but here the wave function of a system behaves something like a customized hylomorphic form for the system while at the same time imbuing it with intentionality. There are interesting claims here about the continuity between life and mind, the unity of consciousness, and protopanpsychism, as well as some speculation of how the theory might align with the various scientific theories of consciousness inspired by psychophysics, like higher-order or global workspace theories.In chapter 16, William Seager offers a helpful discussion of the kinds of intelligibility at issue in these debates, distinguishing between mundane, mathematical, and metaphysical intelligibility, weaving together insights from Leibniz and others to argue that a Bohmian approach offers the best bet for a metaphysically intelligible world view. I sympathize with the positive aspects of his argument, though I think that other papers in this volume showcase the metaphysical intelligibility of rival approaches.Finally, in chapter 17, Lee Smolin describes a completion of quantum mechanics, also echoing Leibniz, which could support a theory of quantum gravity and which offers a new conception of consciousness: the causal theory of views, alongside the principle of precedence. This yields a novel sort of protopanpsychism in which the world is a collection of (background-independent) causal views, where views are conscious when they have a few extra features, including being unprecedented and being maximal. This is an exciting approach. One question is how we reconcile the claim that experiential states are maximal with the (mental) problem of the many, the problem that there are ties between composites (that differ over a stray ion or two) over which has best claim to be me.In closing, this is a fantastic volume. I commend Shan Gao for his editorial work: he would make a wonderful DJ. There are subtle symmetries in the way that each entry segues to the next, and the collection has a remarkable coherence.
David Lewis famously quipped that he was willing to take metaphysical lessons from quantum mechanics only when it cleaned up its own act by providing interpretations or modifications of the formalism shorn of appeal to consciousness, irreducibly macroscopic notions, or outright instrumentalism. By the first decade of the twenty-first century, multiple approaches to quantum mechanics had been developed to a degree which met Lewis’s challenge: dynamical-collapse theories, Bohm-style hidden-variable theories, and Everett-style many-worlds theories, for all their respective problems and challenges, all offer ways of understanding quantum mechanics that metaphysicians can reasonably engage with. And so the last twenty years have seen a resurgence in the metaphysics of quantum mechanics, the question of what ontology and ideology is appropriate to a quantum universe. The metaphysics of quantum mechanics is not independent of the quantum measurement problem but does not coincide with it, either: there is a widespread view that adopting, say, the Everett interpretation still leaves significant ontological questions undecided, and conversely, a given answer to those questions might answer them for a dynamical-collapse theory too.Wave function realism has become a leading contender for the metaphysics of quantum mechanics, particularly in the case of dynamical-collapse and Everettian approaches. The starting point is nonrelativistic quantum mechanics expressed in configuration space: mathematically, the subject matter of the theory is a complex function of 3N variables, evolving under the Schrödinger equation (perhaps interrupted by occasional stochastic collapses). Traditionally those 3N variables are interpreted as the three coordinates of position of N particles—but in wave function realism, they are interpreted as points in a new, high-dimensional space. Quantum mechanics, according to the wave function realist, fundamentally concerns the undulations of the wave function in this space; three-dimensional goings-on are emergent, high-level, derivative.Alyssa Ney has been a leading figure in the development of wave function realism for most of its existence. Her articles advocating and developing the position have been highly influential, and the present book can be seen as the culmination of this line of her research and is the definitive statement of wave function realism as of 2021. Despite its comparative brevity, the book is too rich to be adequately described—far less critiqued—in this brief review; my goal is to give an overview of the main themes and arguments, and to make a few critical remarks along the way.The World in the Wavefunction is not formally divided into parts, but it can profitably be thought of that way. Chapters 1 through 3 develop the core case for wave function realism. Chapter 4 defends the position from the frequent criticism that it is restricted to nonrelativistic physics, by providing a version of wave function realism appropriate to quantum field theory. And chapters 5 through 7 are an extended exploration of how the observed three-dimensional world can be understood as emerging from a high-dimensional fundamental reality.The case Ney makes in chapters 1 through 3 for wavefunction realism is modest and cautious. In chapter 1 she provides a fairly standard—though elegantly expressed—case for the prima facie plausibility of wave function realism simply based on the fact that standard formulations of quantum mechanics are written in 3N dimensions, and on the (related) fact that entanglement means we cannot interpret quantum mechanics simply in terms of spatially localized particles and nothing else. But in chapter 2 she explicitly argues against any attempt to strengthen these plausibility arguments into reasons why wave function realism must be adopted; indeed, she provides a detailed and sympathetic taxonomy of the alternatives to wave function realism that have been developed, all of which she regards as entirely viable. (Chapter 2 of the book could profitably be used as introductory reading for a general graduate course on quantum metaphysics.) For Ney, the question of which metaphysics to adopt is a standard metaphysician’s question, to be answered by weighing strengths and weaknesses of the various alternatives against one another.In chapter 3, Ney identifies two such strengths for wave function realism. First, it is separable: we can specify the complete state of the universe point-by-point on the fundamental space, without any need to appeal to irreducible holism. Second, it is dynamically local: expressed in 3N-dimensional space, the Schrödinger dynamics involve no action at a distance.Ney makes the case for separability at length. Her case for dynamical locality is brief: she regards it as a straightforward consequence of the fact that the Schrödinger equation is a differential equation, so that the rate of change of the wave function at a point x depends only on the wave function arbitrarily close to x. But I think there is more worth investigating here: after all, the nonrelativistic Schrödinger equation may be a differential equation, but it is a parabolic differential equation, with no maximum speed of propagation: the rate of change of the wave function at x may depend only on a neighborhood of x, but the value of the wavefunction at x an arbitrarily short time later depends on the wave function everywhere in space. (And this is not—or not straightforwardly—just an artifact of ignoring relativity.)But in any case, why think that a separable, local theory is preferable to the alternative? Ney claims that separability and locality are more intuitive and lead to a more comprehensible ontology—and she makes an unabashed defense of intuitiveness and explicability as theoretical virtues. She is quite explicit that her reasons are pragmatic: intuitive theories benefit students and teachers; they are better suited to help us develop new theories; they speak better to the reasons why many students (Ney included) want to study physics in the first place. The implied metaphilosophy here is fascinating and it would be good to see it developed further in subsequent work: Ney seems to reject the mainstream modern approach to ontology (in which these pragmatic virtues are not obviously relevant to the truth of wave function realism) in favor of something neo-Carnapian where (so far as I can see) the goal of a quantum ontology is to help us get a deeper understanding of quantum mechanics, and that’s all there is to it. This also fits naturally with the pluralism she urges, in which approaches other than hers can also be valuable.Chapter 4—on relativistic generalizations of wavefunction realism—attempts both a conceptual and a technical task. The conceptual task is to argue that wavefunction realism per se is not wedded to any particular theory or choice of Hilbert-space basis within the theory: rather, wave function realism is any strategy that reifies a high-dimensional space in order to produce a separable local quantum ontology. If so, there is no block in principle to a relativistic generalization of the approach: we just need to find some appropriate high-dimensional space to describe the quantum state in relativistic quantum theory.The general philosophical point is well taken and convincingly defended (though it’s important to note that while arbitrary choices of Hilbert-space basis will give a separable theory, there is no reason to assume they will give a dynamically local theory: even something as simple as shifting from the position to the momentum basis in nonrelativistic quantum mechanics transforms the Schrödinger equation into something explicitly nonlocal). Unfortunately, I don’t think Ney succeeds in giving a clear statement of the high-dimensional space she has in mind for quantum field theory. This is one place where her laudable goal of making the book intelligible to nonspecialists becomes problematic: she makes many suggestive and interesting comments on quantum field theory, but there is no explicit, precise, mathematical statement of her view that can be contrasted with the explicit, precise, mathematical representation of the state in nonrelativistic quantum mechanics as a complex function on 3N-dimensional space.In the last part of the book—concerned with the emergence of ordinary three-dimensional ontology—Ney begins (chapter 5) by rebutting influential arguments by Tim Maudlin, Valia Allori, Sheldon Goldstein, and others that it is empirically incoherent for a physical theory not to make precise, nonemergent, nonderivative statements about matter in three-dimensional space (for a theory not to have a primitive ontology, to use the standard term for it). Ney argues—persuasively, to me—that it is perfectly compatible with the way science usually works for a theory to make statements about the observable only in an indirect, derivative way, so that while it might be a desideratum for a theory to have a primitive ontology, it is not compulsory for it to do so in order to be empirically adequate. She goes on, though, to note that a satisfactory quantum metaphysics had better recover the existence of ordinary three-dimensional objects, on Moorean grounds: we are really confident that those objects exist!Chapter 6 is largely critical: an extended exegesis of the functionalist strategy for recovering higher-order ontology developed by David Albert, myself, and others. She raises two main criticisms of the functionalist strategy: it fails to distinguish between the wave function simulating three-dimensional objects and constituting those objects; it is restricted to recovering classical three-dimensional ontology, whereas we want to recover quantum-mechanical phenomena in three dimensions too. (The former objection is a development of criticisms by Bradley Monton, Tim Maudlin, and others; the latter is novel so far as I know.) For reasons of space, I won’t attempt to reply on behalf of the functionalist, but Ney certainly puts the challenge with admirable clarity.The final chapter of the book presents Ney’s positive account of emergent ontology, based not on functionalism but on considerations of symmetry (though she concedes that the separation is not completely sharp). Simplifying Ney’s subtle position: the permutation symmetries of quantum mechanics give a clear dynamical reason to regard certain decompositions of 3N-dimensional space (intuitively: those that decompose it into the N-fold product of Euclidean space) as preferred; that then licenses the metaphysical hypothesis that a localized wave function in 3N-dimensional space has, as its parts, particles in three-dimensional space. If the wave function is not localized (as will be the case generically, at least for the Everett interpretation), then we can think of those particles as being partially localized in various different regions of space.I have to say that I am not convinced this works, at least without a dynamical-collapse mechanism: in the Everett interpretation, it will yield an ontology where all the particles are completely delocalized. We only see a somewhat localized description of matter in three dimensions when we restrict to an Everett branch, and that restriction is normally motivated on functionalist grounds. But this may simply reflect subtleties of Ney’s intriguing proposal that I am not fully grasping at present.In summary, I congratulate Ney for a rich, subtle, and provocative account of wave function realism. Her edited collection of papers (coedited with Albert), The Wave Function, set the agenda for much of the metaphysics of quantum mechanics in the 2010s; The World in the Wave Function has a good chance of doing the same for the 2020s.
How does it come about then, that great scientists such as Einstein, Schrödinger and De Broglie are nevertheless dissatisfied with the situation? Of course, all these objections are levelled not against the correctness of the formulae, but against their interpretation. […] The lesson to be learned from what I have told of the origin of quantum mechanics is that probable refinements of mathematical methods will not suffice to produce a satisfactory theory, but that somewhere in our doctrine is hidden a concept, unjustified by experience, which we must eliminate to open up the road. (Born [1954], pp. 8, 11) It is truly surprising how little difference all this makes. Most physicists use quantum mechanics every day in their working lives without needing to worry about the fundamental problem of its interpretation. (Weinberg [1992], p. 66) I endorse the view that it may be of no relevance to the acceptability of the Everett interpretation of quantum mechanics as a physical theory whether or not an informed observer can be uncertain about the outcome of a quantum measurement prior to its having occurred. However, I suggest that the very possibility of post-measurement, pre-observation uncertainty has an essential role to play in both confirmation theory and decision theory in a branching universe. This is supported by arguments which do not appeal to van Fraassen’s Reflection Principle. 1. Dendritic Unitary Evolution 2. The Born–Vaidman Rule 3. Greaves on Uncertainty 4. Retroactive Counterfactual Uncertainty 5. Odds and Ends
As we saw in the previous chapter, the Everett interpretation seems to dispense with probabilities in quantum mechanics. Instead of describing a world for which many things are possible, but only some of those possibilities are actualised, it suggests a world in which all possibilities actually happen . Moreover, this is something of which we can be absolutely certain . Obviously, this does not straightforwardly fit our experience. We do not see multiple possibilities becoming actual. Whenever we measure a superposed particle, we observe only one property or another. Moreover, we have a very useful probabilistic rule to help us to predict what we will see. How can the success of this probabilistic rule be explained, if an Everett world does not involve any uncertainty? There are two main moves that are employed in response to this challenge. The first – what I call Stage A – is to show that there is a relevant sort of uncertainty, even in a universe where we are certain that everything will happen. The second move – Stage B – is to try to vindicate the probability of the Born rule in particular. That is, to show that we should not merely be uncertain about the future, but that we have good reason to attach the particular probabilities dictated by the Born rule to the possible outcomes of a quantum experiment. We will consider these two stages in turn.