Quantum computation in brain microtubules: Decoherence and biological feasibility

S. Hagan, S. R. Hameroff, and J. A. Tuszyński
Phys. Rev. E 65, 061901 – Published 10 June 2002
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Abstract

The Penrose-Hameroff orchestrated objective reduction (orch. OR) model assigns a cognitive role to quantum computations in microtubules within the neurons of the brain. Despite an apparently “warm, wet, and noisy” intracellular milieu, the proposal suggests that microtubules avoid environmental decoherence long enough to reach threshold for “self-collapse” (objective reduction) by a quantum gravity mechanism put forth by Penrose. The model has been criticized as regards the issue of environmental decoherence, and a recent report by Tegmark finds that microtubules can maintain quantum coherence for only 1013s, far too short to be neurophysiologically relevant. Here, we critically examine the decoherence mechanisms likely to dominate in a biological setting and find that (1) Tegmark’s commentary is not aimed at an existing model in the literature but rather at a hybrid that replaces the superposed protein conformations of the orch. OR theory with a soliton in superposition along the microtubule; (2) recalculation after correcting for differences between the model on which Tegmark bases his calculations and the orch. OR model (superposition separation, charge vs dipole, dielectric constant) lengthens the decoherence time to 105104s; (3) decoherence times on this order invalidate the assumptions of the derivation and determine the approximation regime considered by Tegmark to be inappropriate to the orch. OR superposition; (4) Tegmark’s formulation yields decoherence times that increase with temperature contrary to well-established physical intuitions and the observed behavior of quantum coherent states; (5) incoherent metabolic energy supplied to the collective dynamics ordering water in the vicinity of microtubules at a rate exceeding that of decoherence can counter decoherence effects (in the same way that lasers avoid decoherence at room temperature); (6) microtubules are surrounded by a Debye layer of counterions, which can screen thermal fluctuations, and by an actin gel that might enhance the ordering of water in bundles of microtubules, further increasing the decoherence-free zone by an order of magnitude and, if the dependence on the distance between environmental ion and superposed state is accurately reflected in Tegmark’s calculation, extending decoherence times by three orders of magnitude; (7) topological quantum computation in microtubules may be error correcting, resistant to decoherence; and (8) the decohering effect of radiative scatterers on microtubule quantum states is negligible. These considerations bring microtubule decoherence into a regime in which quantum gravity could interact with neurophysiology.

  • Received 2 May 2000

DOI:https://doi.org/10.1103/PhysRevE.65.061901

©2002 American Physical Society

Authors & Affiliations

S. Hagan1, S. R. Hameroff2, and J. A. Tuszyński3

  • 1Department of Mathematics, British Columbia Institute of Technology, Burnaby, British Columbia, Canada V5G 3H2
  • 2Departments of Anesthesiology and Psychology and Center for Consciousness Studies, University of Arizona, Tucson, Arizona 85724
  • 3Department of Physics, University of Alberta, Edmonton, Alberta, Canada T6G 2J1

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Vol. 65, Iss. 6 — June 2002

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