Apr 07, 2006
Can quantum mechanics explain consciousness?
Via Brain Ethics
In the most recent issue of Nature (March 30) Christof Koch and Klaus Hepp challenge the hypothesis that human consciousness invokes quantum principles:
We challenge those who call upon consciousness to carry the burden of the measurement process in quantum mechanics with the following thought experiment. Visual psychology has caught up with magicians and has devised numerous techniques for making things disappear. For instance, if one eye of a subject receives a stream of highly salient images, a constant image projected into the other eye is only seen infrequently. Such perceptual suppression can be exploited to study whether onsciousness is strictly necessary to the collapse of the wave function. Say an observer is looking at a superimposed quantum system, such as Schrödinger’s box with the live and dead cat, with one eye while his other eye sees a succession of faces. Under the appropriate circumstances, the subject is only conscious of the rapidly changing faces, while the cat in the box remains invisible to him. What happens to the cat? The conventional prediction would be that as soon as the photons from this quantum system encounter a classical object, such as the retina of the observer, quantum superposition is lost and the cat is either dead or alive.This is true no matter whether the observer consciously saw the cat in the box or not. If, however, consciousness is truly necessary to resolve the measurement problem, the animal’s fate would remain undecided until that point in time when the cat in the box becomes perceptually dominant to the observer. This seems unlikely but could, at least in principle, be empirically verified. The empirical demonstration of slowly decoherent and controllable quantum bits in neurons connected by electrical or chemical synapses, or the discovery of an efficient quantum algorithm for computations performed by the brain, would do much to bring these speculations from the ‘far-out’ to the mere ‘very unlikely’. Until such progress has been made, there is little reason to appeal to quantum mechanics to explain higher brain functions, including consciousness.
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Against Nature
Thanks to Nature for publishing "Quantum Mechanics in the Brain," by Koch and Hepp. As one who has pursued this topic for 35 years, I feel confident that this article will come to be seen as a watershed in the evolution of the debate.
I must report that the authors (along with most of their peers) are thoroughly mistaken. They ask whether there is room for quantum computation in the brain. One might well reply, "Is there room for anything else?"
Thus, in his article on "Field Theory," Freeman Dyson tells us pretty plainly that "There is nothing else except these [quantum] fields: the whole of the material universe is built of them." [1] The brain is presumably part of the material universe; it seems to follow that the brain just is a collection of quantum fields—and, particularly, electromagnetic (EM) fields. Abdus Salam writes: "all chemical binding is electromagnetic in origin, and so are all phenomena of nerve impulses." [2] If conscious processes are phenomena of nerve impulses, then it would seem to follow that those processes are EM in origin.
Dyson informs us further: "A classical field is just a special large-scale manifestation of a quantum field." So, how might classical information survive, absent quantum information, as Koch and Hepp assert? They appear to subscribe to the discredited notion that there are somehow two realms, the quantum and the classical. Umezawa, in his highly readable work on 'Advanced Field Theory,' explains otherwise:
"The incorrect perception that the quantum system has only microscopic manifestations considerably confused this subject. As we have seen in preceding sections, manifestation of ordered states is of quantum origin. When we recall that almost all of the macroscopic ordered states are the result of quantum field theory, it seems natural to assume that macroscopic ordered states in biological systems are also created by a similar mechanism."
They write that physicists are ignorant of neurobiologists' work, and conversely. An important exception is Karl Pribram, who writes in a classic modern work that "the mathematical formulations that have been developed for quantum mechanics and quantum field theory can go a long way toward describing neural processes." [3]
The authors rightly suggest that questions remain as to how brains compute. (They nonetheless feel confident that QM can't help us.)
In this connection it is intriguing to note that the Churchlands argue for the tensor network theory of Pellionisz and Llinas. [4, 5] How are those tensors realized in the physics of the brain? The operator formalism of quantum theory would seem to fit the bill admirably.
Given the fractal character of neural nets, we might expect to find this kind of self-similarity across spatio-temporal scales.
Koch and Hepp repeat the dogma that quantum mechanics (QM) is fundamentally indeterministic—a view nearly unquestioned, until recently. [6] Later, they note that the problem of qualia remains. Max Born, who fathered the statistical interpretation of QM, wrote at the time that "Anyone dissatisfied with these ideas may feel free to assume that there are additional parameters not yet introduced into the theory which determine the individual event." [7] Nowadays these parameters are called "hidden variables" and have lately been revived by such luminaries as Hartle, Holland, 't Hooft, Peres and Smolin—a development of the highest importance, given that those qualia known as secondary qualities are not yet incorporated into the body of science and are only "hidden" in plain view.
Finally, why the (now familiar) rush to shut down this line of inquiry? Who can say, before the fact, what discoveries await us?
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[1] Dyson, Freeman J., "Field Theory," pp. 58-60, Scientific American, 188: 1953.
[2] Salam, Abdus, Unification of Fundamental Forces. Cambridge, 1990.
[3] Pribram, Brain and Perception. Hillsdale, NJ: Lawrence Erlbaum, 1991.
[4] Churchland, P. M. A Neurocomputational Perspective. Cambridge, MA: MIT Press, 1989.
[5] Churchland, P. S. Neurophilosophy : Toward a Unified Understanding of the Mind-Brain. Cambridge, MA: MIT Press, 1986.
[6] Wheeler & Tegmark, "100 Years of Quantum Mysteries," Scientific American, 2/2001.
[7] Holland, Peter R. The Quantum Theory of Motion . Cambridge University Press, 1993.
Posted by: Brian Flanagan | Apr 22, 2006
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