Phys. Rev. E. (1999)
This paper is often quoted as the definitive refutation of the Penrose/Hameroff model. Less frequently quoted is the response of Hameroff et al pointing out a number of deficiencies in its arguments.
Tegmark stresses that the crucial factor for quantum theories of consciousness is the ability to sustain quantum coherence in the conditions of the brain. This much is accepted by Hameroff. However, Tegmark says that the purpose of his paper is to calculate the rate of decoherence in the brain, which he boldly states as something that will settle the whole matter.
An unexpectedly long section in the paper is devoted to discussing the speed of decoherence at the level of neurons, which is not the basis for any of the main theories of quantum consciousness. However, Tegmark finally goes on to examine the more rellevant matter of decoherence on the scale of microtubules, which is central to the Penrose/Hameroff model. Tegmark arrived at a decoherence time of 10-13 seconds, which would be too short to be of use in neural processes. However, Tegmark assumed a model involving a superposition of solitons 24 nm apart, whereas Penrose/Hameroff are working on the basis of the much smaller separation of nuclei within the tubulin protein subunits of the microtubules. It remains a mystery as to why Tegmark selected a model that is not only different from the Penrose-Hameroff model, but does not resemble any of the principal modern quantum consciousness models. Whatever the reason, it certainly makes his particular calculation irrelevant, although it remains true that decoherence would obliterate quantum coherence, unless such coherence is shielded from the environment in some way.
In fact, the other main complaint against Tegmark’s paper is that he does not adequately discuss Hameroff’s proposals for how microtubules might be shielded from decoherence, or the proposals of other theorists for how their models might also evade decoherence. Hameroff accepts that even without Tegmark’s soliton model, decoherence would happen too quickly to be neurally useful. He suggests that shielding for microtubules could be provided by ordered water, that is water molecule dipoles aligned with the biomolecule dipoles of the microtubule tubulin protein, particularly during the gel part of the cytoplasmic cycle. This might be supplemented by energy pumping given the lack of thermal equilibrium in biological tissue and also by quantum error correction facilitated by the design of the microtubule lattice.
In the discussion section towards the end of his paper, Tegmark is drawn back to the idea of decoherence at the neuron level, an irrelevance in terms of modern quantum consciousness. Here he tries to argue that even if it turns out that there is quantum coherence in the brain, it must be irrelevant to consciousness, because decoherence would occur as soon as there was communication at the scale of a neuron. However, this fails to discuss suggestion that structures such as microtubules or ions in ion channels could carry out quantum processing, collapse their wave functions, and afterwards communicate classically with the synapses.
At the distance of nearly a decade, some aspects of Tegmark’s paper appear slightly old fashioned. Considerable faith is attached to work on computational neural networks, which boomed in the 1990s as a way of modelling brain processes on classical computers. The same faith is apparent in some of Patricia Churchland’s work. However, in the intervening years the neural network story appears to have fallen silent. To judge by the lack of striking progress on the artificial intelligence front, nothing has emerged from the neural network activities to allow any close artificial simulation of the brain. Bear in mind that in the late 1990s, some serious writers were forecasting a robot takeover of our planet in the early years of the present decade.
Quantum computation in brain microtubules: Decoherence and Biological Feasibility
Hagan, S., Hameroff, S., Tuszynski, J.
Physical Review, vol. 65, 10 June 2002
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.