A Review of The Physics of Consciousness by Evan Harris Walker
Matthew J. Donald
The Cavendish Laboratory
Madingley Road
Cambridge CB3 0HE
U.K.
matthew.donald@phy.cam.ac.uk
Copyright (c) Matthew J. Donald 2001
PSYCHE, 7(15), October 2001
Previously held: http://psyche.cs.monash.edu.au/v7/psyche-7-15-donald.html
KEYWORDS: quantum theory, consciousness.
REVIEW OF: Evan Harris Walker (2000). The Physics of Consciousness: The Quantum Mind and the Meaning of Life. Cambridge, MA: Perseus. 368 pp.
ISBN 0-7382-0436-6.
At least three books struggle to emerge from this volume. One book, at
the level of popular science, leads us through the development of
physics, from Newton's laws to Bell's inequalities, in order to argue
for the relevance of consciousness to the understanding of quantum
theory. This is followed by a sketch of an interpretation of quantum
mechanics. Interwoven with both is a memoir of Walker's teenage
girlfriend, who died of Hodgkin's disease nearly fifty years ago. The
theme which holds the volume together is Walker's insistence on the
importance of looking beyond materialism.
In this review, I intend mainly to criticize Walker's interpretation of
quantum theory. It seems to me that popular science is very important,
and that Walker's effort in this direction is, on the whole,
interesting, well-written, and competent. Nevertheless, I feel that
there is a problem, albeit one which is not uncommon, with attaching to
popular science what amounts to no more than a collection of undeveloped
proposals, requiring a very different audience for its evaluation.
Walker expresses materialism as the claim that, "We need only enumerate
the basic pieces of matter, write down the forces acting on them, and
turn the crank. Out will come the answer to every question about the
nature of our reality" (p.68). Such a claim might be attacked not only
by those philosophers who are unhappy with the idea of explaining
consciousness in entirely physical or functional terms, but also by some
physicists who have doubts about whether we really do have such perfect
theories that all we have left to do is to "turn the crank". The most
interesting possibility is that new solutions to problems within physics
might help resolve philosophical problems with materialism; perhaps, for
example, by radically altering the physical terms at our disposal.
Indeed, some of us do believe that quantum theory, our most powerful,
wide-ranging, and accurate physical theory, has huge conceptual gaps,
and that those gaps can be interpreted as indicating that
"consciousness" may refer to more than just a way in which it is natural
for some particularly-complex physical systems to talk about themselves.
Walker does a good job of reviewing these issues; very much from the
point of view of a modern physicist. The general reader who is more
inclined to philosophy than physics might like to try, as an
alternative, either Lockwood (1989) or Chalmers (1996). Albert (1992)
introduces more mathematics but also provides better coverage of recent
work in the foundations of quantum theory. All three of these authors
pay more careful attention than does Walker to the many-worlds idea.
Walker's rejection of this idea is based on a caricature which bears
very little relation either to modern work on the subject, or to
anything published by Everett himself.
There is nothing new in the suggestion that there are difficulties with
quantum mechanics and that those difficulties may have something to do
with the idea of an "observer". It is not easy to go further. Walker
starts with a discussion of Zen Buddhism. Like psychoanalysis, Zen is a
system which can repudiate any comment, but Walker seems to invoke it
merely to underline the primacy of the subjective over the objective.
Even this is a dangerously radical move for a physicist, as it threatens
to make the detailed technical triumphs of science irrelevant. Indeed,
the major problem as I see it, for anyone who wants to look beyond
conventional materialism is to explain the successes of neuroscience.
Walker attempts to tie together quantum theory and neuroscience by
arguing that quantum tunnelling has a vital role in synaptic
transmission. This depends upon very specific and technical assumptions
about the mechanism involved, for which he refers to Walker (1977). In
that paper, he claims that his theory "predicts specific results for
future experimental work. Its utility will be measured by the validity
of these predictions." It is disturbing, therefore, that his book gives
no more recent references to work in this area, despite the fact that
synaptic structure and function are among the most studied topics in
neuroscience. A magnificent survey of the entire field which has just
appeared (Cowan, Suedhof, & Stevens, 2001), certainly leaves no space
for his hypotheses.
Walker claims that "only in the most exceptional circumstances [...] do
we ever find quantum mechanical effects entering the macroscopic world"
(p.194). By a "quantum mechanical effect", he seems to mean an effect
which he cannot explain in classical terms. In view of this, he might
seem required, if he is to demonstrate that quantum effects underpin
consciousness, to argue that something like quantum tunnelling has a
role to play in the brain. But, in fact, if the fundamental laws of
physics are quantum mechanical, then every physical effect is quantum
mechanical. Quantum mechanics infects every physical structure at every
level. In particular, as I have argued in Donald (1990), the
unpredictability of the detailed functioning of a living human brain
requires an explanation compatible with our understanding of quantum
theory.
The firing of an individual neural synapse on any single occasion is
certainly unpredictable (Regehr & Stevens, 2001). In classical terms,
this unpredictability is caused by unknown thermal molecular motions. In
quantum terms, on the other hand, even under the conventional hypothesis
in which quantum tunnelling is not involved and vesicle release is an
ordinary biochemical process triggered by an electrochemically-driven
influx of calcium into the pre-synaptic neuron (Suedhof & Scheller,
2001), the unpredictability lies deeper, stemming from an entire history
of uncertain scatterings and interactions at the molecular level and
below. But a central task of an interpretation of quantum mechanics is
to explain how and at what level quantum unpredictability is resolved.
Walker only attempts a resolution at the level of synaptic function.
Molecular interactions lie below this level, and therefore he could
argue for the relevance of quantum theory to consciousness, even without
invoking quantum tunnelling. In conventional quantum-mechanical terms,
if it never "collapsed" at any other moment, the quantum state of the
brain would have to "collapse" at a great number of synaptic firings in
order to make each of those firings definitely happen at moments
definite in biological terms.
Walker says, however, that this is only "talking about quantum noise
randomly affecting things in the brain" (p.220). He wants to "integrate[
] synaptic firings into a single quantum mechanical conscious existence"
(p.228). To do this, he builds on the idea of quantum tunnelling at
synapses to produce what seems to me to be an even more fantastic
mechanism. This requires the electrons which according to him are
involved in synaptic tunnelling, going on to jump from synapse to
synapse using soluble RNA molecules as stepping stones. Not only does
this mechanism strike me as biologically implausible, but also I cannot
understand how it is supposed to act as an integrating process. Walker's
argument seems to depend on an unexplained confusion between a classical
picture of an electron as a hopping object and a quantum picture of its
wavefunction. The electron wavefunction in the brain is an irreducibly
many-body object. Neural electrons are indistinguishable not just in the
sense that they are all identical, but also in the sense that, on
biological timescales, they are inseparably entangled. Walker refers to
"interlaced collections of quantum potentialities weaving together the
possibilities" (p.237). What he does not explain is how the individual
"possibilities" are characterized. Are they vesicle releases? Are they
electron hops or trajectories? This is a version of the preferred basis
problem; a problem which in one guise or another is utterly fundamental
for most interpretations of quantum theory. An argument which simply
assumes a solution to this problem can hardly be convincing.
Nevertheless, a solution to the preferred basis problem is assumed, not
only here, but also in Walker's subsequent invocation of a non-linear
modification to the Schroedinger equation, based on Walker (1988).
According to Walker, his treatment of consciousness is dualist. The
problem is that his theory seems to require mental abilities which go
well beyond any sort of parallelism between mind and physical brain
structures. Walker claims that quantum possibilities allow us a
non-illusory free will, stating that "for will to have any meaning, it
must be possible for the mind to affect events -- for the mind to
control the body" (p.259). This gives a picture in which mind decides
among the elements of a quantum superposition. What is not made clear,
however, is where the mind keeps the computational power required for
this decision making. If it is in the brain, then the brain should be
capable of detecting and analysing the structure of an uncollapsed
superposition so as to match the willed choice to the collapsed outcome.
Even if the structure is merely guessed at, the matching process would
seem to require a new type of physical interaction; and anyway the
choice seems to be made before that interaction comes into play. On the
other hand, if the decision making is extra-physical, then Walker has
merely invented a homunculus in a Cartesian theatre (Dennett 1991) and
is denying all the vast range of evidence which indicates that our
physical brains provide the mechanisms by which we think. The theory of
consciousness proposed by Eccles (1986) also suggests an influence of
mind on quantum uncertainties at synapses and is vunerable to similar
criticism.
In his final chapter, Walker elevates his homunculus to the status of a
god. He invokes this mind, apparently without a brain, to explain how
the structure of the universe arose from its initial quantum state. I
suspect that, even in the framework of his own theory, he is making a
mistake here in assuming that "collapse" must happen at the moment when
our conventional picture of the universe would require it. If, as Walker
supposes, it is "observers" who bring about "collapse", then our
conventional picture of the universe may be radically incorrect. The
universe could have continued as a vastly complicated superposition
until such time as entirely physical processes allowed observers to
evolve within some part of that superposition. Not until that time would
it seem to be necessary for any "collapse" to occur.
Although I disagree with many of Walker's proposals, there are only a
couple of points where I feel he has made mistakes which should never
have been published. One is his assertion that the square of the
absolute value of the complex number R+iS is R^2 - S^2 rather than R^2 +
S^2. This could be ignored as a misprint were it not both discussed and
repeated. Another repeated claim which might make one lose confidence in
Walker's mastery of the wide range of subjects on which he depends is
his estimate of 2.35 times 10^13 for the number of synapses in the human
brain. I have no idea what the best estimate should be, but I am sure
that to give three, or even two, significant figures is absurd, and that
any such accuracy would be swamped by variation between individuals. By
comparison, Churchland and Sejnowski (1992, p. 51), give the number of
synapses as "about 10^15".
Walker raises many interesting and important questions. Even if
incomplete, his answers, in general, are thoughtful, provocative, and
original. I disagree with these answers, but then of course I do have my
own alternative theory (Donald 1999), so I may be biased.
References
Albert, D.Z. (1992). Quantum Mechanics and Experience. Cambridge:
Harvard University Press.
Chalmers, D.J. (1996). The Conscious Mind. Oxford University Press.
Churchland, P.S. and Sejnowski, T.J. (1992). The Computational Brain.
Cambridge MA: Bradford.
Cowan, W.M., Suedhof, T.C., and Stevens, C.F. (2001). Synapses.
Baltimore: John Hopkins.
Dennett, D.C. (1991). Consciousness Explained. Boston: Little Brown.
Donald, M.J. (1990). Quantum theory and the brain. Proc. R. Soc. Lond.
A 427, 43-93 and http://www.poco.phy.cam.ac.uk/~mjd1014
Donald, M.J. (1999). Progress in a many-minds interpretation of quantum
theory. quant-ph/9904001 and http://www.poco.phy.cam.ac.uk/~mjd1014
Eccles, J.C. (1986). Do mental events cause neural events analogously to
the probability fields of quantum mechanics? Proc. R. Soc. Lond. B
227, 411-428.
Lockwood, M. (1989) Mind, Brain, and the Quantum. Oxford: Blackwell.
Regehr, W.G. and Stevens, C.F. (2001). Physiology of synaptic
transmission and short-term plasticity. Chapter 3 of Cowan, Suedhof,
and Stevens (2001).
Suedhof, T.C. and Scheller, R.H. (2001). Mechanism and regulation of
neurotransmitter release. Chapter 4 of Cowan, Suedhof, and Stevens
(2001).
Walker, E.H. (1977). Quantum mechanical tunneling in synaptic and
ephaptic transmission. Int. J. Quant. Chem. 11, 103-127.
Walker, E.H. (1988). Information measures in quantum mechanics. Physica
B 151, 332-338.
