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The importance of quantum decoherence in brain processes

Max Tegmark

1999 · arXiv

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From the abstract

Based on a calculation of neural decoherence rates, we argue that that the degrees of freedom of the human brain that relate to cognitive processes should be thought of as a classical rather than quantum system, i.e., that there is nothing fundamentally wrong with the current classical approach to neural network simulations. We find that the decoherence timescales ~10^{-13}-10^{-20} seconds are typically much shorter than the relevant dynamical timescales (~0.001-0.1 seconds), both for regular neuron firing and for kink-like polarization excitations in microtubules. This conclusion disagrees with suggestions by Penrose and others that the brain acts as a quantum computer, and that quantum coherence is related to consciousness in a fundamental way.

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5 selected excerpts · 5 total mentions

bascule
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There have been some poorly received hypotheses to this effect, most notably Roger Penrose's Orch-OR: https://en.wikipedia.org/wiki/Orchestrated_objective_reducti... That said, most attempts at quantifying whether or not distinctly quantum mechanical processes in the brain related to things like microtubules and NMDA receptors are significant to cognition (i.e. is the brain a quantum computer?) have generally concluded the answer is no: See: https://arxiv.org/abs/quant-ph/9907009 https://onlinelibrary.wiley.com/doi/pdf/10.1207/s15516709cog...
duaneb
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Penrose's The Emperor's New Mind addresses this idea. Basically, Penrose assumes that: 1. Gödel's Incompleteness Theorems are equivalent to the halting problem (provable) 2. If the human mind is deterministic, it can be modeled with a deterministic algorithm (provable). 3. The human mind seems to be capable of proving arbitrary things about all algorithms (debatable; I'm extremely skeptical). 4. Therefore, the human mind is not bound by the halting theorem, and therefore the human mind is not deterministic. He then draws several possible speculations. 1, the human mind is driven by quantum mechanics. He explains this more in Shadow of the Mind, the "sequel" to this book,…
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and goes on to postulate specifically that "microtubules" allow quantum mechanics to have far-reaching effects that are indistinguishable from consciousness. Other people, most notably and substantially Max Tegmark, disagree[1], arguing, "we find that the decoherence timescales (∼10^−13 to 10^−20 seconds) are typically much shorter than the relevant dynamical timescales (∼10^−3 to 10^−1 seconds), both for regular neuron firing and for kink-like polarization excitations in microtubules. This conclusion disagrees with suggestions by Penrose and others that the brain acts as a quantum computer, and that quantum coherence is related to consciousness in a fundamental way." Since then, quantum effects, especially quantum teleportation, appear to be crucial at the molecular level in processes like photosynthesis[2], suggesting that, perhaps, quantum mechanics may play a role. However, even if quantum mechanics do play a role in human consciousness, I don't see how trading a deterministic brain for a random one is an improvement. Personally, I don't think that quantum mechanics do provide a crucial level to the extent that our brains are somehow not bound by logical axioms; I still believe there's a fundamental "algorithm" that drives the biological brain, though it may be difficult to conceive of, and I tend to agree with Tegmark in ideas of consciousness. Still, I'm open to any evidence on either side of the table, but I think we're a few decades away from key discoveries about the way our brains work that will shed any serious light on the subject. ------------------------------------------------------------- [1]: http://arxiv.org/abs/quant-ph/9907009 [2]: http://www.nature.com/nature/journal/v446/n7137/abs/nature05... Extra reading: see [wikipedia](http://en.wikipedia.org/wiki/Orch-OR)
bascule
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According to Max Tegmark's calculations, the importance of quantum mechanical effects on brain processes is negligible: http://arxiv.org/abs/quant-ph/9907009 This paper posits the brain can be modeled as a classical (e.g. Newtonian, "billiard ball"-style) physical system

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