Comment on “‘Many Minds’ Interpretations of Quantum Mechanics by Michael Lockwood”
by David Deutsch
Centre for Quantum Computation, The Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, U.K.
This article appeared in British Journal for the Philosophy of Science 47 222-8 (1996)
At the philosophical foundations of our best and deepest theory of the structure of reality, namely quantum mechanics, there is an intellectual scandal that reflects badly on most of this century’s leading physicists and philosophers of physics. One way of making the nature of the scandal plain is simply to observe that this paper[1] by Lockwood is untainted by it. Lockwood gives us an up-to-date investigation of metaphysics, and discusses the implications of quantum theory for some of the bread-and-butter concepts of philosophy, such as reality, the self and causality. The scandal is that there is very little other work of that description in the literature, and what little there is, is systematically disregarded by mainstream thinking in both philosophy and physics. Despite the unrivalled empirical success of quantum theory, the very suggestion that it may be literally true as a description of nature is still greeted with cynicism, incomprehension and even anger.
The particular implication of quantum theory that all the fuss is about is of course, as Lockwood puts it, “the simultaneous existence of distinct … experiences” (of a single person). For instance, as I write this, I am having the experience of drinking tea. Quantum theory implies that vast numbers of other experiences of mine, including the experience of drinking coffee at this moment, are also taking place. The reason why I do not have an experience of having all those experiences simultaneously is that the laws of quantum mechanics restrict the operation of our brains so as to confine, as Lockwood puts it, “the gaze of consciousness to a kind of ‘tunnel vision’ directed downwards in the experiential manifold. We cannot look ‘sideways’ through the manifold, any more than we can look ‘upwards’, into the future.”
All experiences are presumably associated with measurements, and as Lockwood explains, measurements create quantum entanglement between the observer and the measured system. This means that in those parts of the “experiential manifold” where I was having the experience of drinking tea, I also had the experience of reporting that experience to you in the paragraph above. Likewise for coffee. Consequently, whenever I believe that I am perceiving something real – for instance, if I take it that there really is a cup of tea here on the table – quantum theory obliges me to believe the same of those other perceptions which it says that I am also having. In other words, the coffee, and virtually every other physical object that I am capable of perceiving and reporting to you, must be on the table too, though I cannot see them from here. Thus the “simultaneous existence of distinct experiences” is a special case of a general multiplicity in physical reality at large.
I have just said both that I cannot see the coffee, and that I am having the perception of seeing coffee. This is no contradiction, merely two different uses of the word ‘I’. The problem here is that ordinary language implicitly makes the false assumption that our experiences (and observable events in general) have a single-valued history. To help resolve the ambiguities created by this assumption, Lockwood introduces the term Mind to denote the multiple entity that is having all the (“maximal”) experiences that I am in reality having, and reserves the term mind for an entity that is having anyone of those experiences. So I (the Mind) am both seeing tea and seeing coffee, and am simultaneously reporting both experiences, but I (the mind), who am writing “tea”, am seeing only tea. Similarly, we call multi-valued physical reality as a whole the multiverse, to distinguish it from the universe of classical physics in which observables can take only one value at a time.
Quantum entanglement makes my experience of drinking tea go with my experience of seeing and reporting tea, and therefore presumably also with the presence of actual tea, but not with my seeing or reporting coffee, nor with the presence of actual coffee. Both the tea and the coffee, and many other drinks, together with all the associated experiences, are equally present in reality. But quantum entanglement connects them in ‘layers’ – including a ‘tea’ layer and a ‘coffee’ layer. In each layer, the experiences correspond (roughly) with each other and with the physical objects that they are experiences of, but in any one layer there is no experience of any other layer (except indirectly, through interference phenomena). This is what motivates referring to each of these layers as a universe, and to layers of the multiverse collectively as parallel universes[2].
Lockwood has “quite deliberately avoided … reference to ‘many worlds’” (i.e. parallel universes) in his paper, and points out that Everett used no such term in any of his published writings. But it is not at all clear why.Perhaps Everett was anticipating the powerful taboo that subsequently arose against directly asserting that the universe we see around us is only one of many that exist in reality. This is “the astounding truth which Schrödinger suspected – and which Everett first had the courage wholeheartedly to embrace”, a truth which, to this day, is acknowledged by only a minority of physicists and a smaller minority of philosophers. In any case, I can testify from conversations with Everett in 1977 that, by then at least, he was robustly defending his theory in parallel-universes terms.
Lockwood’s preference for the term ‘many minds’ over ‘parallel universes’ risks giving the impression that it is only minds that are multiple, and not the rest of reality. Nothing could be further from the truth, or from Lockwood’s theory. As he says, the multiplicity of reality at large is “an inescapable consequence of [quantum theory’s] allowing superpositions of what classical physics would regard as mutually exclusive alternatives”. His argument for the multiplicity of minds is a special case of this. Indeed it is of the essence of Lockwood’s metaphysics that minds are physical systems, and have no preferred status under the universal laws of physics.
Lockwood is reluctant to use ‘many-universes’ terminology because of the classical connotations of the word ‘universe’. He points out that the picture of the multiverse as being simply a collection of entities each of which is similar to the universe of classical physics, misrepresents some important features of the multiverse’s structure. In particular, describing the multiverse in terms of different, incompatible sets of observables slices it into different, inequivalent sets of ‘universes’. So (Lockwood and other many-minds theorists argue) the ‘layering’ structure of the multiverse as a whole is highly arbitrary. By contrast, the ‘layering’ structure for states of mind (given that they are associated with certain observables) is in principle unique. The distinctive assertion of many-minds theories is that the universe perceived by any one mind is not an objectively separate‘layer’ of the multiverse. It is merely the view of the multiverse from the perspective of that mind. Other theories of the multiverse assert or assume that the ‘layering’ structure has some observer-independent basis as well as delimiting what observers can perceive in regions of the multiverse where they are present.
However, I must stress that whatever position one takes on the objectivity of the layering structure, quantum theory leaves no room for any doubt that multiple universes exist. It is as if there were a debate about whether our partitioning of the surface of the Earth into latitudes has an objective basis or is merely a human convention. Whichever view one takes, the physical fact remains: the Earth is not confined to a single latitude but really does extend over many parallel latitudes.
Although Lockwood is quite right that there is more to the multiverse than a stack of quasi-classical layers,‘parallel-universes’ terminology nevertheless provides an extremely accurate description of reality. Let us keep this matter in perspective. Even classical terminology – as, for instance, when I say that there is a cup of tea on the table – often describes reality very well. For most practical purposes it is unnecessary to explain that this is a statement about only one small region of the multiverse, and that the contrary statement “there is no tea on the table” is almost certainly true of some nearby region, and so on. The same is true of the classical term ‘universe’ itself. Objects such as planets or cups of tea interact with each other approximately according to laws of motion that refer only to one value (at a time) for each observable, so it is for many purposes an extremely accurate description to refer to a ‘layer’ of such mutually interacting entities as a universe. This has nothing to do with the presence of any observers. Moreover, insofar as it is accurate to speak of-one universe as existing in reality, quantum theory implies that it is necessarily equally accurate to speak of other, different universes as existing in reality too. So other universes exist in exactly the same sense that the single universe that we see exists. This is not a matter of interpretation. It is a logical consequence of quantum theory, albeit one that somehow, seventy years after the theory was discovered, is still in dispute.
It is also, by the way, a logical consequence of Bohm’s ‘pilot-wave’ theory[3] and its variants[4]. Their proponents think of them as single-universe theories. The idea is that the ‘pilot wave’, i.e. the wave function of the multiverse, guides Bohm’s single universe along its trajectory. This trajectory occupies one of the ‘grooves’ in that immensely complicated multi-dimensional wave function. The question that pilot-wave theorists must therefore address, and over which they invariably equivocate, is what are the unoccupied grooves?It is no good saying that they are merely a theoretical construct and do not exist physically, for they continually jostle both each other and the‘occupied’ groove, affecting its trajectory. For example, we may in principle arrange for complex computations to be performed in vast numbers of‘unoccupied grooves’ (i.e.in parallel universes), and then observe the results directly. So the ‘unoccupied grooves’ must be physically real. Moreover they obey the same laws of physics as the ‘occupied groove’ that is supposed to be ‘the’ universe. But that is just another way of saying that they are universes too. (Cf. Lockwood’s discussion of the “mindless hulk” objection to any single-mind theory.) In short, pilot-wave theories are parallel-universes theories in a state of chronic denial.
This is no coincidence. Pilot-wave theories assume that the quantum formalism describes reality. The multiplicity of reality is a direct consequence of any such theory.
I have been referring throughout to this multiplicity as a consequence of quantum theory, as does Lockwood. Let me deal here with an important objection that could be raised against that assertion. The objection is that we have confused an uncontroversial physical theory, quantum theory, with its controversial parallel-universes ‘interpretation’ for which (as for any‘interpretation’) there could not possibly be any experimental evidence. Thus we could be accused of appealing to the authority of a scientific theory to justify an optional metaphysical overlay which philosophers and physicists are surely entitled to resist, or indeed to reject out of hand if it suits them. But there is a false assumption behind this objection: the assumption that there is more than one interpretation of quantum theory. This assumption has traditionally been made, not only by those who wish to deny the implications of quantum theory, but also by those who do not, such as Lockwood, and myself in the past, and even Everett.
But in fact, there is only one known interpretation of quantum theory. Nor should we find this surprising. It is quite exceptional in science for there to be a dispute about the interpretation of a theory. The only example I can think of in modern physics concerns the‘spin-two-field’ re-interpretation of the General Theory ofRelativity (which involves replacing the curvature of Einstein’s spacetime by a force field that produces gravity in a flat spacetime). The creationist re-interpretation of the fossil record as having been fabricated by God in 4004 BC also comes to mind. In addition to these disputes over rival conceptions of reality, there have sometimes been disputes between a realistic theory and an instrumentalist doctrine that denies that the theory describes reality. For example the Inquisition in Galileo’s time permitted advocacy of the heliocentric theory if it was regarded purely as a means of predicting astronomical observations, but not if it was interpreted as a factual theory of where and what the planets and the Earth are. Similar instrumentalist doctrines have been applied to quantum theory. What these miscellaneous revisionist views of scientific theories have in common is a loss of philosophical nerve in situations where, as Lockwood puts it, “there are no conservative options”. That is, they are not so much bona fide rival ontologies struggling to be heard, as psychological manoeuvres whose purpose is to blind their defenders to evidence of something unwelcome: the motion of the Earth, the curvature of spacetime, dinosaurs, or other universes.
The theories that are known as rival “interpretations of quantum theory” fall into three categories:
- Various ways of expressing, envisaging or elaborating the parallel-universes character of physical reality. As I said, many-minds theories are in this category (despite Lockwood’s terminological reservations), along with Everett’s original relative-state theory, several other parallel-universes variants due to DeWitt[2], myself and others, and the‘many-histories’ variants due to Hartle[5] and others.
- Various ways of denying that quantum theory is a true description of reality, including the ‘statistical interpretation’, other instrumentalist stratagems, theories relying on an ‘external observer’ or a ‘classical level’, and ‘dynamical-collapse’ theories[6].
- Confusion, obfuscation or inconsistency, including non-dynamical ‘wave function collapse’ theories, the ‘Copenhagen interpretation’ (which is in some sense still the canonical view of quantum theory, though it has few actual defenders left), and much of the informal discussion of the meaning of quantum theory that appears in textbooks. To these we must add countless concoctions of pseudo-science and mysticism to which this whole regrettable and unnecessary controversy has inevitably opened the door.
As I said, Bohm’s pilot-wave theory is in category 1 if we accept its internal logic, though many of the claims that its supporters make for it are in categories 2 and 3. There is some overlap between categories 2 and 3, since obfuscation about the nature of reality often retreats, when criticised, into instrumentalism or anti-realism.
Perhaps one reason why the dichotomy between ‘formalism’ and ‘interpretation’ has been accepted so uncritically is that the debate has been conducted almost exclusively among theorists. Thus it has revolved around the question “what exactly does quantum theory imply about reality”. Putting it that way can make it seem natural to try to separate the ‘scientific’ (mathematical, predictive) core of the theory from its explanatory structure, and to keep the former fixed while adjusting the latter according to one’s philosophical prejudices. But no good can come of such an exercise. The formalism of quantum theory did not come out of nowhere. It is the solution of a scientific problem, and as always in science, the problem was not primarily what mathematical formula best predicts the outcomes of experiments. It was what mathematical structures correspond best to reality. If we alter the ‘interpretation’ of the theory without regard to the second question, we can conjure up virtually any world we like. But it will not be the real world. The real world is the multiverse, and it does contain many universes.
The point that theorists tend to miss is that the multiplicity of reality is not only, or even primarily, a consequence of quantum theory. It is quite simply an observed fact. Any interference experiment (such as the two-slit experiment),when performed with individual particles one at a time, has no known interpretation in which the particle we see is the only physical entity passing through the apparatus. We know that the invisible entities passing through obey the same phenomenological equations of motion (e.g. geometrical optics) as the single particle we do see. And we know from Einstein-Podolski-Rosen-type experiments, such as that of Aspect, that these not-directly-perceptible particles are arranged in extended ‘layers’ each of which behaves internally like an approximately classical universe. Admittedly all these observations detect other universes only indirectly. But then, we can detect pterodactyls and quarks only indirectly too. The evidence that other universes exist is at least as strong as the evidence for pterodactyls or quarks.
Lockwood is one of very few philosophers who have defied conventional philosophical wisdom by taking the trouble to learn what the fundamental theories of physics actually say. A few physicists are likewise beginning to realise that the sheer philosophical naïvety that still prevails in the profession has prevented our most important theories from being properly understood and has seriously impeded progress. The twentieth century has been a veritable dark age for metaphysics – indeed it has been characterised by the explicit repudiation of metaphysics both by philosophers and by physicists. Now that the lights seem to be coming on again,we are in a position to enjoy the one beneficial side-effect of the long darkness: a backlog of wonderful, urgent philosophical problems, raised by scientific advances in the intervening period. The problems raised by quantum theory are among the most conspicuous of these. Lockwood and a few others have made a start at addressing them. Let us hope that we are witnessing a return to rationality in these matters.
Copyright © 1996 by David Deutsch and Oxford University Press
References
[1] Lockwood, M.J. 1995 ‘ManyMinds’ Interpretations of Quantum Mechanics (to appear in Brit.J. Phil. Sci.)
[2] DeWitt, B. 1970 Physics Today 23 9
[3] Bohm, D.1952 Phys. Rev. 85 166
[4] Bell, J.S. 1986 in Quantum Concepts in Space and Time, Isham, C. and Penrose, R., eds. Oxford University Press
[5] Hartle, J.B. 1991 Phys. Rev. D44 10 3173
[6] Ghirardi, G.C., Rimini, A., Weber, T. 1986 Phys. Rev. D34 470