This is a topic which seems to come up with depressing regularity, so I figure I should put my stance on the record. The bulk of this post is recycled from a Memoirs of a Skepchick comment thread.
For various reasons, I am extremely skeptical of the notion that consciousness could be rooted in quantum phenomena. Of course, the entire world is quantum, in a sense: itâ€™s the principles of quantum mechanics which determine the properties of materials out of which the world is made. Like Democritus of Abdera said twenty-five hundred years ago, â€œNothing exists save atoms and the voidâ€, and quantum physics constitutes the rules by which atoms play.
The challenge, then, is not to say â€œall is quantumâ€ (a statement with no more content, by itself, than saying â€œall is loveâ€). In what way do the strange and esoteric mathematical descriptions of the atomic and sub-atomic world build up the everyday stuff with which we are so familiar? This is a deep problem, one with many mysteries left to resolve, and physicists spend lots of time worrying about it. One thing which we do know is that when you put a lot of quantum particles together, at a certain point they stop acting in the quantum way and become better approximated by Newtonâ€™s laws of classical mechanics. This is odd, because if you put a pile of classical pieces together, you get a bigger classical object! Newtonâ€™s laws reproduce themselves at higher scales, but the quantum laws do not.
Itâ€™s a bit like discovering that all the ordinary houses on your ordinary street are made of bricks from Faerie.
So, in order to test the idea that consciousness, Mind, Spirit or any such vaguely defined phenomenon has a quantum flavor, we need to know if essential aspects of Mind depend upon objects which are small and placid enough for quantum oddities to apply. Lengths, time scales and temperatures need to be sufficiently small to avoid the problem of decoherence, the tendency of quantum objects to collapse into classical behavior.
On the one hand, we have fairly specific models of how brain cells might contain tiny switching elements to which quantum mechanics might apply. The most notable by far is Penrose and Hameroffâ€™s proposal that â€œmicrotubulesâ€ â€” protein rods which form a kind of cellular skeleton, used for transporting molecules around and giving the cell mechanical rigidity â€” can transmit quantum pulses. Unfortunately, this model doesnâ€™t stand up to close scrutiny very well: decoherence steps in and ruins everything. Hameroff, an anaesthesiologist, came up with a (moderately wacky) way in which this â€œmodelâ€ explained the action of anaesthetics. Supporters made a big fooferaw about how scientists had no other model to explain aneasthetics, but oops, never tell a scientist that something canâ€™t be explained; we now have biochemical theories which handle anaesthetics without needing to invoke quantum consciousness.
The previous hypothesis of hydrophobic action, that anesthetics interact with cell membrane lipid, has been abandoned in favor of mechanisms involving protein ion channels in the brain, particularly ligand-gated channels such as receptors for GABA, NMDA receptors, and nicotinic acetylcholine receptors (Flood, 2002). These hypotheses have substantial empirical support (e.g., Garrett & Gan, 1998; Siegwart, KrÃ¤henbÃ¼hl, Lambert, & Rudolph, 2003; Williams & Akabas, 2002). As Flood stated, â€œEvery general anesthetic in use today acts on at least one type and in some cases several types of ligand gated ion channelsâ€ (p. 153). None of the proposed explanatory mechanisms involve quantum mechanical properties or quantum computation.
The neuroscientists have also done a very good job finding places in brains where neurons can be probed directly. In the barn owl, for example, there is a brain part called the inferior colliculus, which the owl uses to process sound. We can identify places in the inferior colliculus where neurons act like AND gates: they have two inputs and only produce an output when both inputs fire simultaneously. Models exist to explain this in terms of ion fluxes through the neuronâ€™s cell membrane (“dendritic computationâ€ is one term, referring to the dendrites which carry the input signals). These models do not invoke quantum mechanics.
One of the better general resources I have found on this subject is a paper by Litt et al., in the journal Cognitive Science (2006). They lay out the evidence that
explaining brain function by appeal to quantum mechanics is akin to explaining bird flight by appeal to atomic bonding characteristics. The structures of all bird wings do involve atomic bonding properties that are correlated with the kinds of materials in bird wings: most wing feathers are made of keratin, which has specific bonding properties. Nevertheless, everything we might want to explain about wing function can be stated independently of this atomic structure. Geometry, stiffness, and strength are much more relevant to the explanatory target of flight, even though atomic bonding properties may give rise to specific geometric and tensile properties. Explaining how birds fly simply does not require specifying how atoms bond in feathers.
In essence, we can enclose all the quantum weirdness within â€œblack boxesâ€ and discuss the interaction of the boxes using classical science. Thereâ€™s legitimate science in figuring out what goes on inside those black boxes, but itâ€™s equally legitimate (and perhaps more useful) to understand what happens when they hook up together.
Indisputably, phenomena requiring quantum mechanical explanation exist throughout the brain, and are fundamental to any complete understanding of its structure and physical mechanics. [â€¦] However, none of these effects contribute essentially to explaining the overall functionality of the associated system, which can be fully described without explicit appeal to quantum-level phenomena. In our wing analogy, it is unnecessary to refer to atomic bonding properties to explain flight. We contend that information processing in the brain can similarly be described without reference to quantum theory. Mechanisms for brain function need not appeal to quantum theory for a full account of the higher level explanatory targets.
My favorite bit might be the quotation they give from P. S. Churchland, who said, â€œThe want of directly relevant data is frustrating enough, but the explanatory vacuum is catastrophic. Pixie dust in the synapses is about as explanatorily powerful as quantum coherence in the microtubules.â€
Penrose has also argued that classical computers cannot perform some of the tasks which humans do quite readily. This argument from computational complexity, however, also falls flat. Solomon Feferman dissected it fairly neatly in a review entitled â€œPenroseâ€™s GÃ¶delian Argumentâ€; more recently, Mark C. Chu-Carroll has written relevant posts about â€œquantum complexity classesâ€ at his blog Good Math, Bad Math. (The computer scientist Scott Aaronson has also advanced plausible arguments that a quantum computer canâ€™t solve NP-complete problems in polynomial time.) It is not widely appreciated that Penrose’s hypothesis requires much more than your plain-vanilla quantum computing: in his view, the brain must be able to solve uncomputable problems. Somehow, in this view, the brain exploits subtleties in an as-yet-unknown theory of quantum gravity in order to tweak wavefunction collapse and beat the GÃ¶del limit.
That’s quite a lot to ask for! Needing quantum mechanics is one thing, but needing a hyperquantum theory nobody has discovered yet is a less attractive proposition.
Then we have the people who claim that quantum entanglement can explain telepathy or telekinesis. Using esoteric jargon and vague pseudo-math to lend credence to a phenomenon which nobody has experimentally observed, and which experiments have quite frequently ruled out â€” itâ€™s a bit like a ghost leading a blind man.
This is the realm of Deepak Chopra and the charlatans who made What the Bleep Do We Know!? To them, quantum is just a handy term, on a par with â€œenergy fieldâ€ or â€œGood Side of the Forceâ€. None of the specifics of modern physics relate in any way to their specious psychobabble. Thereâ€™s more good science in the Beach Boysâ€™ song â€œGood Vibrationsâ€ than in Chopraâ€™s whole catalog. They want the credibility of modern science, the trust people place in the competence of white-coated Einsteins, but theyâ€™re not willing to pay the price. They want to provide the appearance of reconciling science and faith, but what they have truly reconciled is jargon with gullibility.