Does consciousness emerge from quantum processes?

Stuart Hameroff and Roger Penrose think it does, while Patricia Churchland is less convinced

How can we comprehend the nature of our conscious experience? This question provokes four types of explanation. "Reductive materialists" believe that conscious experience simply emerges from computer-like excitations among the brain's neurons. "Dualists" view consciousness as separate from the brain, but able to influence brain activities. "Idealists" argue that consciousness is primary and itself creates reality: consciousness is all there is. "Panpsychists" say that conscious experience is intrinsic to physical reality, that a "protoconsciousness" (a "funda-mentality") is present even in inanimate structures.

Consider this fourth view. Could the raw components of conscious experience actually be "built-in" to the universe? Philosopher Alfred North Whitehead proposed that at a deeper level than atoms or electrons are fundamental units, which Whitehead termed "occasions of experience". Some modern thinkers argue that what makes up the universe is fundamental information with experiential properties. Perhaps neurobiological systems somehow access and organise precursors of conscious experience that are embedded in the physics of reality.

Present-day understanding of physical reality rests upon "space-time geometry", as described by both Einstein's general relativity and quantum theories. General relativity shows that our perceived reality of three spatial dimensions "moving through time" is, more appropriately, a four-dimensional space-time continuum. The presence of a physical object induces a curvature of this underlying space-time. Whereas large objects (eg planets, stars) produce measurable space-time curvatures, those produced by small ones (eg atoms, proteins) are tiny.

More relevant at this small scale is quantum theory, which has shown that individual particles (or even large collections of them) can coexist in a "superposition" of separate locations at once. As strange as this seems, simultaneous existence of a quantum object in two locations at once has been experimentally verified repeatedly. The major unresolved issue is that quantum superpositions seem to perish when the systems get "too large", and mysteriously "collapse" into definite locations. Because very small superposed systems do not spontaneously collapse in this way, conventional theory holds that quantum systems remain in superposition until consciously observed. Accordingly, the mythical Schrodinger's cat would remain both dead and alive in its closed box.

However, many physicists now believe that at some point between very small quantum-scale systems and large cat-size systems, an "objective" factor disturbs the superposition and causes an actual physical collapse (or reduction) to definite states and locations. This putative process is called "objective reduction" (OR). Moreover, some scientists believe that the measure of an object's "largeness" which elicits such "OR" is the degree of overall curvature it induces in space-time.

What about the space-time curvature induced by an object in superposition? An object in quantum superposition existing simultaneously in separated locations will evolve separate and distinct space-time curvatures. This leads to a "blistering" of space-time. If such blisters were to continue to enlarge indefinitely, the entire space-time geometry would separate (leading to "multiple universes"). According to the principles of OR, however, collapse abruptly occurs when the space-time separation reaches a critical degree. The instantaneous OR event selects a particular mass distribution and a particular configuration of space-time geometry. If "funda-mentality" is indeed embedded in the universe, here is a natural place for it: a self-organising OR process could select the individual space-time geometry of experience.

Could self-organising OR events of this nature be occurring in our brains? Why is neurobiology better than rocks or tables at selecting fundamental experience? A requirement for brain OR events would seem to be structures in nerve cells which could support quantum superposition of sufficient "largeness" to elicit OR at appropriate time-scales. Such structures should be highly prevalent, functionally important, coupled to quantum-level events and have crystal-like order.

Most important, they should be capable of information processing and have the ability to be isolated from external interaction. Membranes, membrane proteins, synapses, cell water, DNA, clathrins, myelin, centrioles and other neurobiological structures have been suggested, but in our view "cytoskeletal microtubules" are best suited.

Microtubules are crystal-like protein cylinders that perform varieties of cellular chores within nerve cells including forming and regulating synaptic connections. The individual protein subunits ("tubulins") which make up the microtubules can switch between different configurations, known as "conformations", governed by quantum-level events. Conceded to be the cell's structural support, microtubules, according to accumulating evidence can also process information by means of cooperative interaction of their tubulin subunits.

In a series of recent articles, we have proposed that microtubules process information while in quantum-coherent superposition. In this form of "quantum computing", multiple computations may be performed simultaneously, in parallel. In our view, the quantum computing phase in microtubules corresponds to preconscious processing lasting up to one second (and involves microtubules arrayed in thousands of neurons). The climactic and instantaneous ORs are conscious events. Sequences of such "occasions of experience" create a flow of time, giving rise to a stream of conscious thought.

Although dependent upon some unproven assumptions, our model has significant advantages. It is specific, and attempts to deal directly with the nature of experience. Moreover, like consciousness, quantum coherence has an essential global unity.

Consciousness undoubtedly has an important place in the universe. We take a new scientific approach towards understanding how conscious experience might deeply integrate with the workings of the physical universe.

Stuart Hameroff, MD, is a practising clinical anesthetist and professor at the University of Arizona. Sir Roger Penrose is Rouse Ball professor of mathematics, University of Oxford.