The idea in one paragraph

Orch OR stands for orchestrated objective reduction. In quantum mechanics a system can exist in a superposition of states until it “reduces” to one outcome. Roger Penrose proposed that this reduction is a real physical event, triggered when a superposition involves enough mass displacement, rather than something that happens only when we measure. Stuart Hameroff proposed where this might happen in the brain: in microtubules, protein polymers inside neurons. Together they suggested that each such “objective reduction”, orchestrated by the biology of the cell, corresponds to a moment of conscious experience.

Penrose’s half: non-computability and objective reduction

Penrose came to the problem through mathematics. In The Emperor’s New Mind (1989) and Shadows of the Mind (1994) he argued, drawing on Gödel’s incompleteness theorems, that human mathematical understanding cannot be fully captured by any algorithm. If that is right, the brain must exploit some physics that is not computable. He located it in a proposed link between quantum superposition and gravity: superposed states that differ too much in their distribution of mass would become unstable and collapse on their own, on a timescale set by the energy involved.

Both steps are contested. Many logicians and philosophers reject the Gödelian argument, and the gravity-induced collapse is a hypothesis about physics, not established physics.

Hameroff’s half: why microtubules?

Microtubules are hollow cylinders built from the protein tubulin. They give cells their shape and act as tracks for transport, and they are especially abundant in neurons. Hameroff, an anaesthesiologist, was struck by the fact that anaesthetic gases switch consciousness off while leaving much of the brain’s other activity intact, and he suspected their target lay inside neurons. He proposed that tubulin subunits could hold quantum states, coordinated across many microtubules and connected between neurons. More on the biology: microtubules and consciousness.

The decoherence objection

Quantum superpositions are fragile. Interactions with surrounding molecules cause decoherence, which in practice makes the system behave classically. In 2000 the physicist Max Tegmark estimated that relevant superpositions in the brain would decohere in something like 10⁻¹³ seconds or less, many orders of magnitude faster than the milliseconds over which neural processes unfold. Hameroff and colleagues replied that Tegmark modelled the wrong structures and that shielding and ordered water inside microtubules could extend coherence times. The disagreement turns on assumptions that have not yet been measured directly.

Experiments so far

• Collapse physics. Underground experiments looking for the faint radiation predicted by the simplest gravity-related collapse models (Donadi et al., Nature Physics, 2020) found none, ruling out the most basic version of that model. Penrose has argued his own proposal is not tied to that version.
• Quantum effects in biology. Quantum phenomena clearly matter in some biological processes (for example in enzyme reactions and probably in bird navigation), which makes “the brain is too warm” less of an automatic objection, but does not show anything about consciousness.
• Anaesthesia and microtubules. Some laboratory studies report that anaesthetics affect microtubule properties, and a 2024 study in rats reported that a microtubule-stabilising drug delayed loss of consciousness under isoflurane. These findings are suggestive and await independent replication; they are also compatible with non-quantum explanations.

What would falsify it

Supporters and critics broadly agree on what would count: showing that microtubules cannot sustain coherence on relevant timescales under physiological conditions; ruling out Penrose-style objective reduction in physics experiments; or showing that consciousness persists when microtubule function is selectively disrupted. To date, none of these has been demonstrated cleanly in either direction.

Evidence Meter

Note: The Conscious Field is an independent publication and is not affiliated with Stuart Hameroff, Roger Penrose or the University of Arizona. For their own presentations of the theory, see the Center for Consciousness Studies and their published papers.