If you have a restless intellect, it is very likely that you have played at some point with the idea of investigating the meaning and conceptual foundations of quantum mechanics. It is also probable (albeit not certain) that your intentions have been stopped on their tracks by an encounter with some version of the "Shut up and calculate" command. You may have heard that everything is already understood. That understanding is not your job. Or, if it is, it is either impossible or very difficult. Maybe somebody explained to you that physics is concerned with "hows" and not with "whys"; that whys are the business of "philosophy" ---you know, that dirty word. That what you call "understanding" is just being Newtonian; which of course you cannot ask quantum mechanics to be. Perhaps they also added some norms: The important thing a theory must do is predict; a theory must only talk about measurable quantities. It may also be the case that you almost asked "OK, and why is that?", but you finally bit your tongue. If you persisted in your intentions and the debate got a little heated up, it is even possible that it was suggested that you suffered of some type of moral or epistemic weakness that tend to disappear as you grow up. Maybe you received some job advice such as "Don't work in that if you ever want to own a house". I have certainly met all these objections in my short career, and I think that they are all just wrong. In this somewhat personal document, I try to defend that making sense of quantum mechanics is an exciting, challenging, important and open scientific endeavor. I do this by compulsively quoting Feynman (and others), and I provide some arguments that you might want to use the next time you confront the mentioned "opinions". By analogy with the anti-rationalistic Copenhagen command, all the arguments are subsumed in a standard answer to it: "Shut up and let me think"
If you have a restless intellect, it is very likely that you have played at some point with the idea of investigating the meaning and conceptual foundations of quantum mechanics. It is also probable (albeit not certain) that your intentions have been stopped on their tracks by an encounter with some version of the "Shut up and calculate" command. You may have heard that everything is already understood. That understanding is not your job. Or, if it is, it is either impossible or very difficult. Maybe somebody explained to you that physics is concerned with "hows" and not with "whys"; that whys are the business of "philosophy" ---you know, that dirty word. That what you call "understanding" is just being Newtonian; which of course you cannot ask quantum mechanics to be. Perhaps they also added some norms: The important thing a theory must do is predict; a theory must only talk about measurable quantities. It may also be the case that you almost asked "OK, and why is that?", but you finally bit your tongue. If you persisted in your intentions and the debate got a little heated up, it is even possible that it was suggested that you suffered of some type of moral or epistemic weakness that tend to disappear as you grow up. Maybe you received some job advice such as "Don't work in that if you ever want to own a house". I have certainly met all these objections in my short career, and I think that they are all just wrong. In this somewhat personal document, I try to defend that making sense of quantum mechanics is an exciting, challenging, important and open scientific endeavor. I do this by compulsively quoting Feynman (and others), and I provide some arguments that you might want to use the next time you confront the mentioned "opinions". By analogy with the anti-rationalistic Copenhagen command, all the arguments are subsumed in a standard answer to it: "Shut up and let me think"
Fifteen years ago, I mused in a Reference Frame column on how different generations of physicists differed in the degree to which they thought that the interpretation of quantum mechanics remains a serious problem. I declared myself to be among those who feel uncomfortable when asked to articulate what we really think about the quantum theory, adding that “If I were forced to sum up in one sentence what the Copenhagen interpretation says to me, it would be ‘Shut up and calculate!’” In the intervening years, I've come to hold a milder and more nuanced opinion of the Copenhagen view, but that should be the subject of another column. The subject of this one is the habit of misquotation or misattribution that afflicts our profession, a rather different example of which I pointed out a few months ago. Given my capacity for intellectual development (“inconsistency,” in the terminology deployed in the current American political season), it's fortunate that I've now reached an age at which I tend to forget about things I've written more than a few years ago. Indeed, I find it downright irritating when somebody asks me questions about papers I wrote a mere half dozen years ago, naively identifying me with the author of those ancient texts. Until quite recently, I had no memory of ever having written such a childishly brusque dismissal of such an exquisitely subtle point of view, much less of having published it in so widely read a venue. This amnesia, combined with the evolution in my thinking that had distanced me from my long-forgotten words, may explain why I was initially somewhat puzzled by the slight sensation of discomfort that passed over me when, browsing the e-print archive earlier this year, I read a characterization of Max Born's probability rule as “the favorite ingredient of what has been nicknamed, after Feynman's famous dictum, the shut up and calculate interpretation of quantum mechanics.” I yield to nobody in my admiration for Richard Feynman's aphorisms on the nature of quantum mechanics. Indeed, long ago I published a poem consisting of nothing more than a resetting as verse of a paragraph Feynman had written about his own attitude toward the quantum theory, in his now (but not then) famous article that launched the whole field of quantum computation. I like to think I have devoured everything Feynman ever wrote on the character of quantum mechanics.
This commentator format is quite unfamiliar to me, and somewhat uncomfortable. I don't know whether I'm supposed to flatter the speakers, criticize them, grade them, or merely parrot them. Or should I go off on my own? This latter is tempting. If unleashed I could provide you with true answers to all the questions before us, philosophical, historical, physical, mathematical, physical-mathematical, and so on. But I'll bite my tongue and try to stick to the commentator role. First, a few words on the topic of the opening session this afternoon - why are philosophers interested in quantum field theory? Quantum mechanics undoubtedly abounds in genuine, deep and still unresolved philosophical questions. These are usually posed, at least in popular accounts, in the context of finite systems of nonrelativistic point particles. In preparation for the conference, I asked myself: does relativistic QFT introduce any really distinctive philosophical problems? There are several possibilities, which I can only express in low-brow form. For one thing, a field is really a collection of infinitely many degrees of freedom, one for each point in space. I can well suppose that infinity raises interesting questions for philosophers. Certainly for physicists, especially in the field theory context, it has been a great preoccupation. As we'll hear later from Jackiw, infinity has its useful aspects. Next, for systems of relativistic particles in interaction, it is no longer so easily possible to speak of measurements of position and momentum of individual particles, except perhaps in asymptotic regions.
Attitudes toward quantum mechanics differ interestingly from one generation of physicists to the next. The first generation are the founding fathers, who struggled through the welter of confusing and self-contradictory constructions to emerge with the modern theory of the atomic world and supply it with the “Copenhagen interpretation.” On the whole they seem to have taken the view that while the theory is extraordinarily strange (Bohr is said to have remarked that if it didn't make you dizzy then you didn't really understand it), the strangeness arises out of some deeply ingrained but invalid modes of thought. Once these are recognized and abandoned the theory makes sense in a perfectly straightforward way. The word “irrational,” which appears frequently in Bohr's early writings about the quantum theory, is almost entirely absent from his later essays. The second generation, those who were students of the founding fathers in the early postrevolutionary period, seem firmly—at times even ferociously—committed to the position that there is really nothing peculiar about the quantum world at all. Far from making bons mots about dizziness, or the opposite of deep truths being deep truths, they appear to go out of their way to make quantum mechanics sound as boringly ordinary as possible. The third generation—mine—were born a decade or so after the revolution and learned about the quantum as kids from popular books like George Gamow's. We seem to be much more relaxed about it than the other two. Few of us brood about what it all means, any more than we worry about how to define mass or time when we use classical mechanics. In contemplative moments some of us think the theory is wonderfully strange and others think it isn't; but we don't hold these views with great passion. Most of us, in fact, feel irritated, bored, or downright uncomfortable when asked to articulate what we really think about quantum mechanics. I'm one of the uncomfortable ones. If I were forced to sum up in one sentence what the Copenhagen interpretation says to me, it would be “Shut up and calculate!” But I won't shut up. I would rather celebrate the strangeness of quantum theory than deny it, because I believe it still has interesting things to teach us about how certain powerful but flawed verbal and mental tools we once took for granted continue to infect our thinking in subtly hidden ways.
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.
Richard Feynman famously declared, "I think that I can safely say that nobody really understands quantum mechanics." Sean Carroll lamented the persistence of this sentiment in a recent opinion piece entitled, "Even Physicists Don't Understand Quantum Mechanics. Worse, they don't seem to want to understand it." Quantum mechanics is arguably the greatest achievement of modern science and by and large we absolutely understand quantum theory. Rather, the "understanding" to which these two statements evidently refer concerns the ontological status of theoretical constructs. For example, "Do quantum wave functions accurately depict physical reality?" The quantum measurement problems represents a collection of such queries and the conundrums to which they lead. Most physicists are content with foregoing such metaphysical issues, falling back on Bohr's Copenhagen interpretation, and then get on with the business of doing physics. I suspect that Carroll would criticize these physicists as being too dismissive of the wonderful mysteries of quantum mechanics by relegating its role to that of an algorithm for making predictions. Quite to the contrary, I maintain that those who still pursue a resolution to the measurement problem are burdened with a classical view of reality and fail to truly embrace the fundamental quantum aspects of nature.