comment on lockwood - david deutsch

8/13/2019 Comment on Lockwood - David Deutsch

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too, though I cannot see them from here. Thus the 'simultaneous existenceof distinct

experiences'is a

specialcase of a

general multiplicityin

physicalreality 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 differentuses of the word 'I'. The problem here is that ordinary language implicitlymakes the false assumption that our experiences (and observable events in

general) have a single-valued history. To help resolve the ambiguitiescreated by this assumption, Lockwood introduces the term Mind todenote the multiple entity that is having all the ('maximal') experiencesthat I am in reality having, and reserves the term mind for an entity that is

having any one of those experiences. So I (the Mind) am both seeing teaand seeing coffee, and am simultaneously reporting both experiences, but I

(the mind), who am writing 'tea', am seeing only tea. Similarly, we callmulti-valued physical reality as a whole the multiverse, to distinguish itfrom the universe of classical physics in which observables can take onlyone value at a time.

Quantum entanglement makes my experience of drinking tea go with myexperience of seeing and reporting tea, and therefore presumably also withthe presence of actual tea, but not with my seeing or reporting coffee, norwith the presence of actual coffee. Both the tea and the coffee, and manyother drinks, together with all the associated experiences, are equallypresent 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 physicalobjects that they are experiences of, but in any one layer there is no

experience of any other layer (except indirectly, through interferencephenomena). This is what motivates referring to each of these layers as auniverse, and to layers of the multiverse collectively as parallel universes

(DeWitt [1970], p. 9).Lockwood has 'quite deliberately avoided . . . reference to many

worlds ' (i.e. parallel universes) in his paper, and points out that Everettused 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 subse-

quently arose against directly asserting that the universe we see aroundus is only one of many that exist in reality. This is 'the astounding truthwhich Schr6dinger suspected-and which Everett first had the couragewholeheartedly to embrace', a truth which, to this day, is acknowledgedby only a minority of physicists and a smaller minority of philosophers.In any case, I can testify from conversations with Everett in 1977 that, bythen at least, he was robustly defending his theory in parallel-universesterms.

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Lockwood's preference for the term 'many minds' over 'paralleluniverses' risks

givingthe

impressionthat it is

onlyminds that are

multiple, and not the rest of reality. Nothing could be further from thetruth, or from Lockwood's theory. As he says, the multiplicity of reality at

large is 'an inescapable consequence of [quantum theory's] allowingsuperpositions of what classical physics would regard as mutuallyexclusive alternatives'. His argument for the multiplicity of minds is a

special case of this. Indeed it is of the essence of Lockwood's metaphysicsthat minds are physical systems, and have no preferred status under theuniversal laws of

physics.Lockwood is reluctant to use 'many-universes' terminology because ofthe classical connotations of the word 'universe'. He points out that the

picture of the multiverse as being simply a collection of entities each ofwhich is similar to the universe of classical physics misrepresents some

important features of the multiverse's structure. In particular, describingthe multiverse in terms of different, incompatible sets of observables slicesit into different, inequivalent sets of 'universes'. So (Lockwood and other

many-minds theorists argue) the 'layering' structure of the multiverse as awhole is highly arbitrary. By contrast, the 'layering' structure for states ofmind (given that they are associated with certain observables) is in

principle unique. The distinctive assertion of many-minds theories isthat 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 assumethat 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 objec-

tivity of the layering structure, quantum theory leaves no room for anydoubt that multiple universes exist. It is as if there were a debate aboutwhether 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 singlelatitude but really does extend over many parallel latitudes.

Although Lockwood is quite right that there is more to the multiversethan a stack of quasi-classical layers, 'parallel-universes' terminologynevertheless provides an extremely accurate description of reality. Let us

keep this matter in perspective. Even classical terminology-as, forinstance, when I say that there is a cup of tea on the table-often describesreality very well. For most practical purposes it is unnecessary to explainthat this is a statement about only one small region of the multiverse, andthat the contrary statement 'there is no tea on the table' is almost certainly







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true of some nearby region, and so on. The same is true of the classical term'universe' itself.

Objectssuch as

planetsor

cupsof tea interact with each

other approximately according to laws of motion that refer to only onevalue (at a time) for each observable, so it is for many purposes an

extremely accurate description to refer to a 'layer' of such mutually inter-

acting entities as a universe. This has nothing to do with the presence of

any observers. Moreover, in so far as it is accurate to speak of one universeas existing in reality, quantum theory implies that it is necessarily equallyaccurate to speak of other, different universes as existing in reality too. Soother universes exist in exactly the same sense that the single universe thatwe see exists. This is not a matter of interpretation. It is a logical conse-

quence of quantum theory, albeit one that somehow, seventy years afterthe theory was discovered, is still in dispute.

It is also, by the way, a logical consequence of Bohm's 'pilot-wave'theory (Bohm [1952]) and its variants (Bell [1986]). Their proponents thinkof them as single-universe theories. The idea is that the 'pilot-wave', i.e. thewave function of the multiverse, guides Bohm's single universe along its

trajectory. This trajectory occupies one of the 'grooves' in that immenselycomplicated multidimensional wave function. The question that pilot-wave theorists must therefore address, and over which they invariablyequivocate, is what are the unoccupied grooves? It is no good saying thatthey are merely a theoretical construct and do not exist physically, for theycontinually jostle both each other and the 'occupied' groove, affecting its

trajectory (Tipler [1987], p. 189). For example, we may in principle arrangefor complex computations to be performed in vast numbers of 'unoccupiedgrooves' (i.e. in parallel universes), and then observe the results directly(Deutsch [1985], [1987]). So the 'unoccupied grooves' must be physicallyreal. 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 sayingthat they are universes too. (cf. Lockwood's discussion of the 'mindlesshulk' objection to any single-mind theory). In short, pilot-wave theoriesare parallel-universes theories in a state of chronic denial.

This is no coincidence. Pilot-wave theories assume that the quantumformalism describes reality. The multiplicity of reality is a direct conse-

quence 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 importantobjection that could be raised against that assertion. The objection is thatwe have confused an uncontroversial physical theory, quantum theory,with its controversial parallel-universes 'interpretation' for which (as forany 'interpretation') there could not possibly be any experimentalevidence. Thus we could be accused of appealing to the authority of a







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scientific theory to justify an optional metaphysical overlay which philo-

sophersand

physicistsare

surelyentitled to

resist,or indeed to

rejectout 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 quantumtheory. This assumption has traditionally been made, not only by thosewho wish to deny the implications of quantum theory, but also by those whodo 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 forthere to be a dispute about the interpretation of a theory. The only exampleI can think of in modern physics concerns the 'spin-two-field' reinterpreta-tion of the General Theory of Relativity (which involves replacing thecurvature of Einstein's spacetime by a force field that produces gravity in aflat spacetime). The creationist reinterpretation of the fossil record as

having been fabricated by God in 4004 BCalso comes to mind. In additionto these disputes over rival conceptions of reality, there have sometimesbeen disputes between a realistic theory and an instrumentalist doctrinethat denies that the theory describes reality. For example, the Inquisitionin Galileo's time permitted advocacy of the heliocentric theory if it was

regarded purely as a means of predicting astronomical observations, butnot if it was interpreted as a factual theory of where and what the planetsand the Earth are. Similar instrumentalist doctrines have been applied to

quantum theory. What these miscellaneous revisionist views of scientifictheories 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 ide rival ontologies struggling to be heard, as

psychological manoeuvres whose purpose is to blind their defenders toevidence of something unwelcome: the motion of the Earth, the curvatureof spacetime, dinosaurs, or other universes.

The theories that are known as rival 'interpretations of quantum theory'fall into three categories:

1. Various ways of expressing, envisaging or elaborating the parallel-universe character of physical reality. As I said, many-mindstheories are in this category (despite Lockwood's terminologicalreservations), along with Everett's original relative-state theory,several other parallel-universes variants due to DeWitt ([1970]),myself and others, and the 'many-histories' variants due to Hartle

([1991]) and others.2. Various ways of denying that quantum theory is a true description of

reality, including the 'statistical interpretation', other instrumentaliststratagems, theories relying on an 'external observer' or a 'classicallevel', and 'dynamical-collapse' theories (Ghirardi et al., [1986]).







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3. 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 informaldiscussion of the meaning of quantum theory that appears in text-books. To these we must add countless concoctions of pseudo-science and mysticism to which this whole regrettable and unneces-

sary 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, sinceobfuscation about the nature of reality often retreats, when criticized, intoinstrumentalism or anti-realism.

Perhaps one reason why the dichotomy between 'formalism' and 'inter-

pretation' has been accepted so uncritically is that the debate has beenconducted almost exclusively among theorists. Thus it has revolvedaround the question, 'what exactly does quantum theory imply about

reality?' Putting it that way can make is seem natural to try to separatethe 'scientific' (mathematical, predictive) core of the theory from its expla-natory structure, and to keep the former fixed while adjusting the latter

according to one's philosophical prejudices. But no good can come of suchan exercise. The formalism of quantum theory did not come out ofnowhere. It is the solution of a scientific problem and, as always in science,the problem was not primarily what mathematical formula best predictsthe outcomes of experiments. It was what mathematical structures corre-

spond best to reality. If we alter the 'interpretation' of the theory withoutregard to the second question, we can conjure up virtually any world welike. But it will not be the real world. The real world is the multiverse, and itdoes contain many universes.

The point that theorists tend to miss is that the multiplicity of reality isnot only, or even primarily, a consequence of quantum theory. It is quitesimply an observed fact. Any interference experiment (such as the two-slit

experiment), when performed with individual particles one at a time, has

no known intepretation in which the particle we see is the only physicalentity passing through the apparatus. We know that the invisible entitiespassing through obey the same phenomenological equations of motion(e.g. geometrical optics) as the single particle we do see. And we know fromEinstein-Podolski-Rosen-type experiments, such as that of Aspect, thatthese 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.







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But then we can detect pterodactyls and quarks only indirectly too. Theevidence that other universes exist is at least as

strongas 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 fundamentaltheories of physics actually say. A few physicists are likewise beginning torealize that the sheer philosophical naivete that still prevails in the profes-sion has prevented our most important theories from being properlyunderstood and has seriously impeded progress. The twentieth centuryhas been a veritable dark age for metaphysics-indeed it has been char-acterized by the explicit repudiation of metaphysics both by philosophersand by physicists. Now that the lights seem to be coming on again, we arein a position to enjoy the one beneficial side-effect of the long darkness: a

backing of wonderful, urgent philosophical problems, raised by scientificadvances in the intervening period. The problems raised by quantumtheory are among the most conspicuous of these. Lockwood and a fewothers have made a start at addressing them. Let us hope that we are

witnessing a return to rationality in these matters.

Quantum Computation and Cryptography Research GroupThe Clarendon Laboratory

University of OxfordParks Road

Oxford OX1 3PUUK

ReferencesBell, J.S. [1986]: n C. Isham and R. Penrose eds), Quantum oncepts n Space and

Time, Oxford, Oxford University Press.

Bohm, D. [1952]: A Suggested nterpretation f the Quantum Theory n Terms ofHidden Variables, and II', Phys. Rev., 85, p. 166.

Deutsch, D. [1985]: Quantum Theory, the Church-Turing rinciple, and theUniversal Quantum Computer', Proc. R. Soc. Lond. A400, p. 97.

Deutsch, D. [1997, o appear]: The Fabric of Reality, Viking Penguin, London.

DeWitt, B. [1970]: Quantum Mechanics nd Reality', Physics Today, 23, 9.

Ghirardhi, G.C., Rimini, A., and Weber, T. [1986]: Unified Dynamics or Micro-scopic and Macroscopic Systems' Phys. Rev., D34, p. 470.

Hartle, J.B. [1991]: Spacetime Coarse-Grainings n Nonrelativistic QuantumMechanics' Phys. Rev., D44, 10, p. 3173.

Lockwood, M.J. [1996]: Many Minds nterpretations f Quantum Mechanics',Brit. J. Phil. Sci., this number.

Tipler, F.J. [1987]: 'Non-Schr6dinger Forces and Pilot Waves in QuantumCosmology' Classical nd Quantum Gravity , 5, p. 189.

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