I write to you from Agra, India, where I've spent a week in a conference on the possible relationship between the foundations of quantum physics and the workings of the mind. The main focus of the conference was this: does quantum physics play a role in how the brain works? Or, more profoundly, is the mind, viewed as a collection of possible brain states, sustained by quantum effects? Or can it all be treated using classical physics?

My job was to give an inaugural lecture summarizing the history of the subject and different points of view. There's nothing better than mixing two great mysteries to produce an even bigger one!

For the truth is that, despite the tremendous success of quantum physics, when it comes to its applications — for example, all of our digital and nuclear technologies — its interpretation remains uncertain, a target of heated debate among physicists. (At least among those who care about deeper foundational questions, certainly not the majority.)

As for the workings of the brain, in particular how it sustains our mind and consciousness, we still know precious little, even if advances in imaging techniques in the past decade or so have revealed, to a certain extent, how clusters of neurons, often at different regions in the brain, ignite under different stimuli like lights in a Christmas tree.

Leaving the Greek philosophers aside, everything started when Descartes, in the 17th century, proposed a split between mind and matter: while matter has spatial extension (in fact, filling space completely, according to Descartes), mind does not, existing in some sort of ethereal fashion without occupying space. Mind is not matter but, in ways that stumped even Descartes, can influence matter. Descartes also postulated that mind is prior to matter, the essence of his famous "I think therefore I am" dictum.

This mind-body dualism caused — and causes — much confusion, especially for those who use it to defend the existence of some kind of soul or spirit that is independent of matter and that can survive the body's inexorable decay.

On the other hand, the vast majority of scientists and philosophers defend that only matter exists. The fact that the workings of the brain remain mysterious is not due to some immaterial entity but to our own difficulty of understanding its complexity. The philosophers Thomas Nagel, Colin McGinn and David Chalmers, sometimes known as the "mysterians," defend that we are cognitively incapable (or, as McGinn puts it, "cognitively closed") to understanding consciousness — the subjective experience we have when feeling something, be it the tone of a color or the emotion of love.

The bizarre behavior of quantum systems allows for speculative ideas on how they may play a role in the workings of the brain. After all, if we take a bottom up approach, the brain is made of neurons; and neurons, like any other cell, need proteins and a host of biomolecules to function. If quantum effects take place at the molecular level, it is possible that they may do something important.

There are two main quantum effects of note here: the first is superposition, the fact that from subatomic to molecular scales, systems can exist in many states at once. For example, before an electron is detected, it can be at many places at once; or at least that's how we interpret the data. The mathematical machinery of quantum mechanics allows us to compute the probability that the electron will be found here or there. The data are actually the measurements of its location within the accuracy of the measuring device.

Could thoughts exist in some sort of quantum superposition in an unconscious level only to become conscious when there is a specific selection akin to a measurement of the electron's position?

This is what the physicist Roger Penrose and anesthesiologist Stuart Hameroff, both present at the conference, have proposed. The active entity that promotes the selection is a protein called tubulin, which interacts with the microtubules that provide the neuron's skeletal support. In a sense, the microtubules are a sort of quantum highway network supporting the superposition and entangled states of tubulin along neurons. They supposedly act as a quantum computer to optimize the neuronal and inter-neuronal performance.

Entanglement is the second bizarre quantum property that enters here, the ability of two or more quantum systems to establish links between them that are sustained across long spatial distances. We say that entangled states behave as a single entity, losing their individual identities. The idea here is to use the spatial extent of entangled states to "spread out" quantum effects with a given signature across long distances within the neuronal networks.

There has been strong criticism of the Penrose-Hameroff ideas from experimental and theoretical arguments. Theoretical arguments, as for example, presented by physicist Max Tegmark, currently at MIT, suggest that the brain is too busy and warm an environment to sustain coherent quantum states. The point is that coherent quantum states are very fragile: influences from the surrounding environment (for example, colliding molecules, heat vibrations) can easily destroy the superposition of states, selecting only one of them. In effect, the environment can turn quantum into classical. In this case, quantum effects would be negligible.

It is certainly hard to avoid bafflement when one probes the impact of quantum physics on our understanding of physical reality. It is also certainly true that, at least at the synaptic level, where a host of neurotransmitters flow through narrow acceptance gates, quantum effects may indeed play a role. However, the majority opinion points toward a more classical explanation for the workings of the brain, through the myriad couplings of neuronal clusters and their incessant firings. Yet, given the complex nature of inter-neuronal connectivity, there is room for exploration and speculation.

As is often the case, the solution may be not either-or but both: there may be cooperation between quantum and classical effects that jointly determine the functioning of the brain at different levels. Whatever the resolution may be, we still don't know how to avoid the arguments from the mysterians, where the nature of consciousness may well be one of these unknowables that are so hard to live with.

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