Can science explain the mystery of consciousness?

Nobel physics prize winner Roger Penrose believes the answer lies in quantum mechanics

In the second part of a series on the science of consciousness, Seán Duke features those who believe the human brain works more like a quantum computer.

The mystery of consciousness, according to Roger Penrose, the 89-year-old winner of the 2020 Nobel Prize in physics, will only be solved when an understanding is found for how brain structures can harness the properties of quantum mechanics to make it possible.

Penrose, emeritus professor of mathematics at the University of Oxford – a collaborator of the late Stephen Hawking – who won the Nobel for his work on the nature of black holes, has been interested in consciousness since he was a Cambridge graduate student. He has authored many books on consciousness, most notably The Emperor's New Mind (1989), and believes it to be so complex that it cannot be explained by our current understanding of physics and biology.

As a young mathematician, Penrose believed, and still does today, that something is true, not because it is derived from the rules or axioms, but because it’s possible to see that it’s true. The ultimate truth in mathematics, he reasoned, cannot, therefore, be proven by following algorithms; a set of calculations performed to instruction.


It followed, Penrose deduced, that the truth of how consciousness operates in the brain may not be provable by algorithms or thinking of the brain as a computer. This idea set off a life-long quest to understand the mysterious processes governing consciousness going on in our heads, which, Penrose says, remain beyond our existing understanding of physics, mathematics, biology or computers.

Quantum world

After The Emperor's New Mind was published, Penrose received a letter from Stuart Hameroff, professor of anaesthesiology at the University of Arizona, who also had a long interest in understanding consciousness. In the letter, Hameroff described tiny structures in the brain called microtubules, which he believed were capable of generating consciousness by tapping into the quantum world.

Hameroff, who has worked as an anaesthesiologist for 45 years, believes anaesthesia may work through specifically targeting consciousness through its action on the neural microtubules. After writing the letter, he met Penrose in 1992, and over the next two years they developed radical ideas about consciousness which ran counter to the thinking of most neuroscientists, and still do.

Penrose and Hameroff believe that the human brain works more like a quantum computer than any classical computers. This is because future quantum computers will be designed to harness the ability of quantum particles to exist in multiple locations, states and positions – all at once. These quantum effects arise in the microtubules, they suggest, which then act as the brain’s link to the quantum world.

The microtubules were structures that Hameroff had studied in since his graduate student days. They interested him initially, he recalls, because of their role in cancer. The microtubules were crucial to cell division, by splitting chromosomes perfectly in two. If microtubules did not function then chromosomes could be divided unevenly in three or four, not two, he says, thus triggering cancer.

The central role that the microtubules played in cell division, led Hameroff to speculate that they were controlled by some form of natural computing. In his book Ultimate Computing (1987), he argues that microtubules have sufficient computation power to produce thought. He also argues that the microtubules – the tiny structures which give the cell its shape and act like a scaffold – are the most basic units of information processing in the brain, not the neurons.

The fact that microtubules are found in animals, plants and even single-celled amoeba, says Hameroff means that consciousness is probably widespread and exists at many levels. The way microtubules work to produce consciousness, he says, can be thought of as being similar to how a conductor directs the sounds produced by individual musicians and orchestrates it into a coherent functioning orchestra.

Consciousness will be a different experience in humans compared to amoeba, says Hameroff. “A single-celled organism might have proto-consciousness; that is consciousness without no memory, without context, isolated, not connected with anything else, and occurring at low intensity. There wouldn’t be any sense of self memory or meaning, but there would be some glimmer of feeling or awareness.”

Penrose agreed with Hameroff that the microtubules could possibly maintain the “quantum coherence” needed for complex thought and a collaboration began that continues today. Consciousness, the two believed, was a non-logarithmic, quantum process that could only be understood by a theory that linked the brain to quantum mechanics.

This led Penrose and Hameroff to develop a theory called orchestrated reduction, or OR. This proposed that areas of the brain where consciousness occurs must be structured so that they can hold innumerable quantum possibilities all at once – per the rules of quantum mechanics – while permitting the controlled reduction of such endless possibilities, without destroying the quantum system.

The microtubules were, both agreed, the best currently known structures in the brain where quantum processes could take place in a stable way and be harnessed to generate our conscious experience. They agreed that consciousness might ultimately be found in many locations across the brain, not just confined to the microtubules.

According to Hameroff, the presence of pyramid-shaped cells containing microtubules organised to run in two directions, rather than in parallel, which is more usual, was the difference between the parts of the brain where consciousness happens and the unconscious brain. It’s notable, he says, that these pyramidal cells are not present in the cerebellum; an area considered to be unconscious.

One of the main criticisms of the Penrose-Hameroff quantum-based theory of consciousness is that there is no way to measure whether quantum processes are happening in the microtubules or any other parts of the brain. Penrose accepts such criticism but believes such measurements will become possible over the long term.

Conscious thought

Hameroff already has plans to test whether quantum states exist inside microtubules. If he can prove this, his next step will be to see if such states disappear under anaesthesia. If they do then he says it strengthens the theory that microtubules host conscious thought.

Brain scanning techniques like PET and MRI, have become very powerful but are of little or no use in consciousness studies, says Penrose. They can, he notes, monitor blood flow and where activity is happening in the brain but they can’t say whether that activity involves conscious thought. For that something else is required.

One way to measure thought, some scientists believe, is by observing brainwaves. For example, some evidence suggests that brainwaves, oscillating at about 40 Hertz, can be correlated with consciousness.

Penrose and Hameroff would like to find evidence for quantum brain oscillations in the microtubules but have no tools yet to achieve this.

“This is a long-term project, which I don’t see resolving for many years,” says Penrose who, given his age, would like to see things moving faster. “I feel pretty sure that we haven’t really understood fully how biological systems are organised and how they may be taking advantage of the subtle effects of [quantum] physics.”

The big difficulty with trying to measure quantum processes in the brain, Penrose points out, is that such effects are destroyed when they are observed or brought into contact with the outside world. “It is going to be very hard to have direct access to consciousness, as to observe it, currently, would be to destroy it.”

Seán Duke

Seán Duke, a contributor to The Irish Times, is a science journalist