Max Tegmark's quantum mechanics library: brain

I've added a rejoinder to a temperature critique below.

This paper was covered in Science, Cern Courier + critique, Physics Web and Dagens Nyheter (Swedish).

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The importance of quantum decoherence in brain processes


Max Tegmark


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, ie, 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. Our result is also difficult to reconcile with the Stapp model where thought processes correspond to top-level multi-neuron quantum events.

To place this problem in context, we also discuss a decomposition of the global density matrix into three subsystems as in the figure on the right, where the three interaction Hamiltonians cause qualitatively different effects.

Reference info:

quant-ph/9907009, Phys. Rev. E, 61, 4194-4206

Related papers:

In this paper, Hagan, Hameroff and Tuszynski argued that my calculation must be flawed because the decoherence timescales that I derived decrease as you lower the temperature of the brain, whereas you might intuitively expect the opposite. The point they overlooked is that as soon as you drop the absolute temperature by about 10%, below 0 Celsius, your brain freezes and the decoherence time grows dramatically. The slight decrease in decoherence time for tiny temperature reductions simply reflects the fact that the scattering cross section grows as you lower the temperature, just as slow neutrons have larger cross section than fast ones in a nuclear reactor.

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Here's an article about my paper that appeared in the the February 4, 2000 issue of Science:

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This page was last modified October 16, 2005.