Every time a cognitive phenomenon appears that neuroscience can't quite explain, someone raises a hand at the back of the room and says the word. Quantum. The room nods. Nobody asks what exactly it means. That's the whole operation. A word that no longer means anything concrete is offered as an explanation for something that isn't understood either, and the sum of the two opacities produces the illusion that progress has been made.
The Orch OR Hypothesis
Roger Penrose and Stuart Hameroff have been proposing it seriously since the early nineties. Penrose supplied the physics and the mathematics. Hameroff supplied the biology. The hypothesis is called Orch OR (from orchestrated objective reduction of the wave function) and holds that human consciousness isn't a computational phenomenon but a quantum one that happens in the microtubules of the neuronal cytoskeleton.
The microtubules (tubular protein structures present inside every cell, part of the scaffolding that holds its shape) would be, according to the theory, tiny cellular quantum processors. They would be held in superposition —the state in which a quantum system simultaneously occupies several incompatible configurations— long enough for an objective reduction to occur in the sense Penrose had proposed earlier in The Emperor's New Mind. That gravitational reduction of the wave function would be the place where the non-computable appears.
Consciousness would be the beat of that reduction, synchronized across trillions of microtubules by some mechanism the theory calls orchestration and that is almost never specified in enough detail to be criticized.
The proposal has the rhetorical advantage of being fascinating. It joins two popular mysteries and presents them as if they were the same one.
The Hot, Wet, Noisy Brain
It also has the scientific disadvantage of clashing with an objection Max Tegmark published with explicit math in Physical Review E in the year 2000. Tegmark calculated the decoherence time (the span during which a quantum system maintains its superposition before interaction with the environment breaks it) to be expected for a quantum system immersed in a warm, wet and electrically noisy medium like the inside of a neuron.
He got picoseconds. Ten to the minus twelve seconds. The relevant neural processes happen on millisecond scales. The difference is some ten orders of magnitude, which in physics means it isn't a matter of tuning the machinery, it's a matter of the machinery not being able to exist.
Maintaining quantum coherence in a warm brain would be like expecting a snowflake to survive intact in a lit oven. It's not hard. It's impossible under any reasonable assumption of how matter interacts with its environment.
Hameroff replied. He has been replying for twenty years. The most ambitious defense was signed by him and Penrose in Consciousness in the universe (Physics of Life Reviews, 2014), where they gather all the accumulated objections and propose insulation mechanisms that would protect coherence in the microtubules. There's always some new insulation mechanism that would explain why the microtubules might escape the general decoherence, some structure in the water around them, some arrangement of the tubulins that would protect coherence, some reinterpretation of Tegmark's calculation that would recover a few orders of magnitude. McKemmish and others (2009), from quantum biology, reviewed the real properties of the tubulins and concluded, with the math in hand, that the proposal is still not biologically feasible. The discussion is legitimate in the sense that every hypothesis deserves its defense. It's illegitimate in the sense that every defensive move is made to save the hypothesis, not to make the hypothesis predict something new. Every time the objection is sharpened, the theory adds an epicycle (in ancient astronomy, a circle added on top of another circle so that the planetary orbits would keep fitting the data despite the underlying model being false). That happened before too. The Ptolemaics did the same work for fifteen centuries.
The Gödelian Argument
Behind Hameroff's biological bet there's a philosophical bet of Penrose's that is older and more interesting.
Penrose believes, and has believed since the eighties, that the human brain does something no Turing machine can do. The argument rests on Gödel's incompleteness theorems. If within a sufficiently powerful formal system there are always true propositions the system itself can't prove, and a human mathematician is able, looking at them from outside, to recognize that they're true, then the human mathematician is doing something the formal system doesn't do. Therefore the human mathematician isn't a formal system. Therefore the brain isn't computable. Therefore beneath the brain there must be a non-computable physical process, and the only candidate available in known physics is the reduction of the wave function. From there to the microtubules there's a biological leap that Penrose himself never quite makes with full confidence. That part Hameroff supplies.
The argument has been criticized since the day after its publication. Hilary Putnam, Solomon Feferman, David Chalmers and many others have pointed out the same hole to it in turn. That a human recognizes a Gödel proposition as true doesn't prove the human is a non-formal system. At most it proves the human isn't exactly this particular formal system. Any broader formal system could recognize the propositions of the more restricted one. The mathematician doesn't escape the calculus. He changes calculus.
Penrose has answered, has redone the argument, has published a whole Shadows of the Mind to shore it up, and most of the philosophical community still thinks the hole is there. The discussion is interesting for what it says about the insistence, not for what it adds to the question.
The God of the Gaps in a Physics Edition
What's worth looking at closely isn't Orch OR. Orch OR is just the most visible clinical case.
What's worth looking at is the general move the hypothesis represents. There's a cognitive phenomenon that computational neuroscience can't yet explain. Someone appears and proposes that the explanation lies at a physical level computational neuroscience can't reach. The proposal isn't falsifiable (it doesn't allow you to design an experiment whose result, if it came out a certain way, would refute it) with present means, because the quantum processes at the relevant biological level can't be measured directly. The proposal doesn't predict anything new that could be checked in a classical experiment either. All it does is move the mystery from one level to another and, along the way, confer on whoever holds it the prestige of handling two technical vocabularies at once.
That's exactly the structure of the god of the gaps argument, only the gap is filled by Planck instead of Yahweh.
The Vis Vitalis Hasn't Left, It's Changed Its Name
The move has its history. The nineteenth century spent the whole thing answering Darwinism with appeals to a vis vitalis (a special vital force that would separate living matter from inert matter and explain the properties of living beings) that organic chemistry kept dislodging from the laboratory step by step. The vitalists had their arguments, their journals and their academic societies. What they didn't have was any prediction that distinguished a world with a vital force from a world without one. Organic chemistry kept occupying the ground reaction by reaction, and the vis vitalis retreated toward the zones chemistry couldn't yet reach, until it ran out of places to retreat to.
The structure of the argument is identical. Where science can't reach, there's something special. When science reaches it, that something special was a bit further on. The name of the special thing changes. The structure doesn't.
What There Can Be
A caveat is in order, because without it the critique becomes cruder than it needs to be.
Biology does use quantum effects. There's solid experimental literature on the role of quantum coherence in plant photosynthesis, on radical pairs in the magnetoreception system (the ability of some animals to orient themselves by sensing the Earth's magnetic field) of migratory birds, on tunneling (the passage of a particle through an energy barrier it classically couldn't cross) in some enzymatic reactions, possibly on smell.
Living organisms have spent billions of years squeezing what physics offers them, and quantum mechanics is what there is. To say there may be occasional quantum effects in the brain isn't a radical claim, it's almost a reasonable expectation. What doesn't follow from there is that consciousness is a quantum effect. An enzymatic process in a neuron may exploit tunneling without that turning the neuron into a qubit (the unit of information in quantum computing, equivalent to the classical bit but able to remain in superposition) or the mind into a quantum computation.
The distinction is visible if you want to see it. Some don't want to.
The Word Quantum Outside the Laboratory
There's also another vector, less academic and more insidious, which is where the real damage of the vocabulary accumulates.
The word quantum has become popular outside the laboratory as a synonym for mysterious, indeterminate and therefore compatible with anything the person using it wants to defend. Quantum is quantum healing, the quantum consciousness of spiritual awakening, quantum coaching, quantum leadership, quantum love. In all those uses the word means exactly the same thing. That the usual rules don't apply and therefore anything one wants to slip in fits.
That colonization of the vocabulary isn't innocent. The cultural authority quantum physics acquired during the twentieth century, associated with Bohr, Heisenberg, Schrödinger and company, is now being spent to sell products of a much more dubious quality. Every time a serious book on consciousness mentions the microtubules as if the matter were more settled than it is, it gives a little more fuel to the quantum self-help section of the bookshops.
The Artificial-Intelligence Variant
The phenomenon has a specific variant in the debate over artificial intelligence.
When it's argued that no current machine can be truly intelligent because machines are classical and intelligence is quantum, exactly the same move is being used. Human intelligence is reserved at an inaccessible level to guarantee that nothing engineering builds can reach it.
It's an understandable strategy. What machines do each year resembles ever more what only the human brain was supposed to do, and the border has to be put somewhere. Putting it at the quantum level has the advantage that they can't reach it there yet. It has the disadvantage that it isn't even known whether there's any border at all at that level, or whether the quantum is relevant to cognition in any way. The border is drawn in nebulous territory so that it's unfalsifiable. That protects the position. It doesn't justify it.
The Price of Elegance
The uncomfortable thing about all this isn't that Penrose is wrong, if he's wrong. To be wrong at the level he's wrong, with that technical precision and that amount of mathematics put on the table, is something few researchers can do and that science is grateful for, because it forces those who answer him to sharpen their arguments too.
The uncomfortable thing is the ease with which his hypothesis has been adopted outside the circle where it could be discussed rigorously, turned into ambient certainty, taken for granted in popular-science conversations as if it were the current state of the consensus. The quantum has to do with the mind because Penrose has said so, and Penrose is a Nobel laureate, and therefore it's more or less settled. None of the steps is valid. Each is taken with the lightness of someone who doesn't need to justify it.
The question of how consciousness emerges from a physical system stays open, and possibly stays open for several more generations. It's legitimate not to know. What isn't legitimate is to buy the appearance of an answer. Saying quantum where one doesn't know is an elegant way of saying nothing. Elegance, when it comes to replace the missing answer, is an aggravating factor.
Definitions
Orch OR. Short for orchestrated objective reduction. A hypothesis formulated by Roger Penrose and Stuart Hameroff that locates human consciousness in coordinated quantum processes inside the neuronal microtubules.
Microtubules. Tubular protein (tubulin) structures present inside cells. They form part of the cytoskeleton. In the Orch OR hypothesis they're attributed the role of cellular quantum processors, an attribution that isn't the usual reading of cell biology.
Superposition. The quantum property by which a system simultaneously occupies several configurations that would classically be incompatible. Superposition is maintained as long as the system doesn't interact with its environment.
Decoherence. The loss of quantum superposition through interaction with the environment. The decoherence time is the span during which the system retains its quantum behavior before behaving classically.
Wave-function reduction. The transition from the superposition state to a concrete observable state. Penrose proposes that this step, in its gravitational version, harbors non-computable physical processes.
Formal system. In mathematical logic, a set of symbols and rules that allow propositions to be derived from axioms. Gödel's incompleteness theorems state that every sufficiently powerful formal system contains true propositions the system can't prove.
Falsifiable. The property of a scientific hypothesis by which it would be possible to design an experiment whose result, if it came out a certain way, would refute it. A non-falsifiable hypothesis is one that no experimental result can disprove.
Qubit. The basic unit of information in quantum computing. Unlike the classical bit, which is 0 or 1, the qubit can remain in a superposition of both states as long as it isn't measured.
Quantum tunneling. The phenomenon by which a particle crosses an energy barrier it classically couldn't surmount. It's experimentally documented in enzymatic reactions and other biological processes.
Vis vitalis. A concept of nineteenth-century vitalism that posited a vital force proper to living matter, irreducible to the laws of chemistry and physics. Organic chemistry progressively displaced the theoretical need for this force until it was left with no explanatory function.
God of the gaps. An expression used in the philosophy of science and of religion to name the argumentative move that places the posited entity (originally God, today the quantum, the vital or the emergent) in the gaps scientific knowledge hasn't yet filled, so that the entity keeps retreating as those gaps close.
References
Penrose, R. & Hameroff, S. (1996). Orchestrated reduction of quantum coherence in brain microtubules. The Orch OR model for consciousness. Mathematics and Computers in Simulation 40, 453–480. The initial joint formulation of the hypothesis discussed in the article.
Hameroff, S. & Penrose, R. (2014). Consciousness in the universe. A review of the Orch OR theory. Physics of Life Reviews 11, 39–78. The later defensive review, where the authors try to answer the objections accumulated over two decades. Available at https://www.sciencedirect.com/science/article/pii/S1571064513001188.
Tegmark, M. (2000). Importance of quantum decoherence in brain processes. Physical Review E 61, 4194–4206. The fundamental decoherence critique, cited in the body of the article with its orders of magnitude.
McKemmish, L. K. et al. (2009). Penrose-Hameroff orchestrated objective-reduction proposal for human consciousness is not biologically feasible. Physical Review E 80, 021912. An independent reinforcement of Tegmark's critique from quantum biology. Available at https://pubmed.ncbi.nlm.nih.gov/19792156/.
Penrose, R. (1989). The Emperor's New Mind (Oxford University Press). The starting point of the Gödelian argument against the computability of the mind.
Penrose, R. (1994). Shadows of the Mind (Oxford University Press). A reformulation of the same argument after the first criticisms.
Chalmers, D. (1996). The Conscious Mind (Oxford University Press). Contains the philosophical critique of Penrose's use of the Gödelian argument, referred to in the corresponding section.
Putnam, H. and Feferman, S. Technical critiques of Penrose's Gödelian argument published as responses and reviews of Shadows of the Mind, cited in the article alongside Chalmers's as examples of the philosophical community's consensus against Penrose's version.
Tegmark, M. (2014). Our Mathematical Universe (Knopf). The same Tegmark who signed the decoherence objection presents, in this popular book, a position on the mathematical nature of the cosmos that is cited as a general counterpoint to the Penrose-Hameroff program.
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