Many Minds - Quantum Mechanics
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 any
one 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 of Relativity (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.
Copright © 1996 by David Deutsch and Oxford University Press
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