Decoherence and biological feasibility
Quantum Computation in brain microtubules: Decoherence and biological feasibilty
S. Hagan, Dept. of Mathematics, British Columbia Institute of Technology
S.R. Hameroff, Dept. of Anesthesiology and Centre for Consciousness Studies, University of Arizons
J. A. Tuszynski, Dept. of Physics, University of Alberta
Published in: Physical Review: vol 65, 10th June 2002
This article is the authors reply to Tegmark’s claim that the speed of decoherence makes the Penrose/Hameroff or Orch OR model for consciousness implausible. Tegmark’s main criticism was that coherence would collapse in 10-13 seconds in the conditions of the brain, and this meant it could have no useful involvement in brain function.
The authors’ reply is that Tegmark did not look at the Penrose/Hameroff model involving protein superpositions, but at another model, apparently proposed by Sataric that involves a soliton in superposition along the whole length of the microtubule.
Tegmark also seems to have thought that the suggested superposition must cover the whole 24 nm of the microtubule, whereas Penrose/Hameroff are thinking in terms of separation at the level of atomic nuclei within the tubulins. Thus there is a seven orders of magnitude difference between the Tegmark model and the Penrose/Hameroff model.
The article sees the microtubules as mediating between the quantum computation of the tubulins and the classical behaviour of the rest of the neuron. The article sees the microtubule superposition as needing to survive for tens of milliseconds in order to usefully interact with brain functions. The Penrose/Hameroff model suggests that the cytoplasm around the microtubules alternates between a type of gel and a liquid. During the former stage the microtubule is screened from the environment and contains superpositions and quantum computing. During the latter there are classical events such as attachment of microtubule associated proteins, membrane activities and synaptic functions. On the inward route, synaptic activity is suggested as affecting the cytoskeleton. The arrangement of the MAPs following synaptic activity is suggested to have an impact on the subsequent microtubule states.
The article goes on to discuss the existence of quantum behaviour in protein. It quotes A. Roitberg et al in Science 268 (1), who reports substantial quantum effects. It also quotes J. Tejada in Science 272 (2), who criticises Gidia et Al. The latter’s work claims to detect macroscopic quantum coherence in the protein ferritin. Tejada criticises their procedures, but Gidia defends the original conclusion in a response to Tejada. They also refer to a series of experiments involving brain scanning, by W.S. Warren et al, (3), R.R. Rizi et al (4) and W. Richter et al (5), which showed that quantum coherence between proton spins up to a micrometer apart could be artificially induced for tens of milliseconds. The length of the coherence periods allows it to be seen as possibly connected to the so-called 40Hz oscillation between the thalamus and the cortex and between other regions of the brain.
These experiments are seen as mainly important in demonstrating the possibility of quantum coherence within the brain. The argument that the brain could not sustain quantum coherence for a useful period has always been the most cogent argument against theories of quantum consciousness, and that argument is weakened by these experiments. However, it is stressed that the experiments did not involve entanglement, and the particular processes induced are not thought likely to be useful in brain function.
(1) A. Roitberg et al, Science 268 1319 (1995)
(2) J. Tejeda et al, Science 272 424 (1996)
(3) W.S. Warren et al, Science 281 247 (1998)
(4) R.R. Rizi et al, Magn Reson Med 43 627 (2000)
(5) W. Richter et al, Magn Resonance Imaging 18 489 (2000)