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Penrose-Hameroff Orch-OR Theory

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A mathematical physicist and an anesthesiologist proposed that consciousness arises from quantum processes inside brain proteins called microtubules. A physicist's 2000 calculation found those quantum states should collapse almost instantly in a warm brain, a critique its authors dispute.

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Worth knowing · 1 of 5
What does the theory actually propose?
It proposes that consciousness arises from quantum processes inside microtubules, hollow protein structures found in nearly every cell, ending in an 'objective reduction' event tied to gravity that produces discrete moments of conscious experience.
Worth knowing · 2 of 5
Who developed it, and why together?
A mathematical physicist proposed that consciousness might involve physics beyond ordinary computation, and an anesthesiologist, drawing on how anesthetic drugs reliably switch off awareness, proposed microtubules as the specific site where that physics could happen. They combined the ideas in the 1990s.
Worth knowing · 3 of 5
What was the main physics objection to it?
In a 2000 paper, a physicist calculated that quantum states in microtubules should lose coherence in about 10 to the negative 13 seconds, far too fast to influence neural processes operating on a much slower timescale.
Worth knowing · 4 of 5
How did the theory's authors respond?
Collaborators published a reply arguing the original calculation used unrealistic assumptions about microtubule structure and treated the brain as if it were in thermal equilibrium, and that correcting those assumptions lengthens the coherence time considerably.
Worth knowing · 5 of 5
Has anything been tested more recently?
Some experiments published in the early 2020s reported anesthetic drugs measurably disrupting energy transfer or oscillations within microtubules, which proponents read as support. Independent replication and a settled explanation for these results are still ongoing.
Words to know
MicrotubuleA hollow, cylindrical protein structure found inside cells, including neurons, where the theory locates its proposed quantum activity.
Objective reductionPenrose's proposal that a quantum superposition collapses on its own once it reaches a threshold tied to gravity, not only when observed.
Quantum coherenceA fragile state in which a system's quantum behavior persists before disturbance from its environment breaks it down, or 'decoheres.'
More words
DecoherenceThe rapid loss of a system's quantum coherence caused by interaction with a warm, complex environment like a living cell.
A voice · 1 of 4
Roger Penrose · Mathematical physicist who co-developed the theory

Building on his argument that some aspects of consciousness can't be produced by ordinary computation, he proposed that gravity-linked 'objective reduction' of quantum states could be the physical basis for moments of conscious experience.

A voice · 2 of 4
Stuart Hameroff · Anesthesiologist who co-developed the theory

Drawing on decades of research into how anesthetic gases switch off consciousness, he proposed that microtubules inside neurons could host the quantum processes Penrose described, and has argued that anesthetics act on consciousness there.

A voice · 3 of 4
Max Tegmark · Physicist who published the main decoherence critique

In a 2000 paper, he calculated that quantum superpositions in microtubules would decohere in roughly 10 to the negative 13 seconds, concluding the proposed quantum processes are far too brief to play any role in neural activity.

A voice · 4 of 4
Scott Hagan · Physicist who responded to Tegmark's critique

Co-authoring a reply with Hameroff and Jack Tuszynski, he argued Tegmark's calculation assumed an unrealistically large separation between quantum states and ignored possible biological shielding, and that correcting these assumptions extends the coherence time by several orders of magnitude.

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The math is still being argued over. So is what counts as evidence here.
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