A few theoretical physicists have argued that classical physics is intrinsically incapable of explaining the holistic aspects of consciousness, but that quantum theory provides the missing aspects. However, some physicists and philosophers consider the arguments for an important role of quantum phenomena to be unconvincing. Physicist Victor Stenger characterized quantum consciousness as a "myth" having "no scientific basis" that "should take its place along with gods, unicorns and dragons." 
The main argument against the quantum mind proposition is that quantum states in the brain would decohere before they reached a spatial or temporal scale at which they could be useful for neural processing. This argument was elaborated by the physicist, Max Tegmark. Based on his calculations, Tegmark concluded that quantum systems in the brain decohere quickly and cannot control brain function.
The philosopher David Chalmers has also argued against quantum consciousness. He speculated on a number of ways in which quantum mechanics might relate to consciousness. However, Chalmers is also skeptical about the ability of any kind of New Physics to resolve the Hard Problem of Consciousness:  
Quantum consciousness is not a single theory, but several quite distinct theories, and these are described in the sections below.
 Description of main quantum mind approaches
 David BohmThe late David Bohm took the view that quantum theory and relativity contradicted one another, and that this contradiction implied that there existed a more fundamental level in the physical universe. He claimed that both quantum theory and relativity pointed towards this deeper theory, which is now understood [according to whom?] to be quantum field theory. This more fundamental level was supposed to represent an undivided wholeness and an implicate order, from which arose the explicate order of the universe as we experience it.
Bohm's proposed implicate order applies both to matter and consciousness, and he suggests that it could explain the relationship between them. Mind and matter are here seen as projections into our explicate order from the underlying reality of the implicate order. Bohm claims that when we look at the matter in space, we can see nothing in these concepts that helps us to understand consciousness.
In trying to describe the nature of consciousness, Bohm discusses the experience of listening to music. He thinks that the feeling of movement and change that make up our experience of music derives from both the immediate past and the present both being held in the brain together, with the notes from the past seen as transformations rather than memories. The notes that were implicate in the immediate past are seen as becoming explicate in the present. Bohm views this as consciousness emerging from the implicate order.
Bohm sees the movement, change or flow and also the coherence of experiences, such as listening to music as a manifestation of the implicate order. He claims to derive evidence for this from the work of Jean Piaget in studying infants. He states that these studies show that young children have to learn about time and space, because they are part of the explicate order, but have a "hard-wired" understanding of movement, because it is part of the implicate order. He compares this "hard-wiring" to Chomsky's theory that grammar is "hard-wired" into young human brains.
In his writings, Bohm never proposed any specific brain mechanism by which his implicate order could emerge in a way that was relevant to consciousness, nor any means by which the propositions could be tested or falsified.
 Roger Penrose and Stuart Hameroff
|This section relies on references to primary sources. (February 2012)|
Theoretical physicist Roger Penrose and anaesthesiologist Stuart Hameroff collaborated to produce the theory known as Orchestrated Objective Reduction (Orch-OR). Penrose and Hameroff initially developed their ideas separately, and only later cooperated to produce Orch-OR. Penrose came to the problem from the point of view of mathematics and in particular Gödel's theorem, while Hameroff came from a career in cancer research and anaesthesia.
Gödel's theorem is central to this theory. In 1931, Gödel proved that any theory capable of expressing elementary arithmetic cannot be both consistent and complete. Further to that, for any consistent formal theory that proves certain basic arithmetic truths there is an arithmetical statement that is true, but not provable in the theory.
The theorem is not in itself controversial, but what Penrose developed from it is. In his first book on consciousness, The Emperor's New Mind (1989), Penrose argued that the theorem showed that the brain had the ability to go beyond what could be achieved by axioms or formal systems. He argued that this meant that the brain had some additional function that was not based on algorithms (a system of calculations), whereas a computer is driven solely by algorithms. Penrose asserted that the brain could perform functions that no computer could perform. He called this type of processing non-computable.
Penrose went on to consider what it was in the human brain that was not driven by algorithms. Given the algorithm-based nature of most of physics, he decided that the random choice of position etc. that occurs when a quantum wave collapses into a particle was the only possibility for a non-computable process. However, Penrose admitted that the randomness of the wave function collapse, although free from algorithms, is not a basis for any useful form of human understanding.
Penrose now proposed a second form of wave function collapse that could apply where quanta did not interact with the environment, but might collapse on their own accord. He suggests that each quantum superposition has its own piece of spacetime curvature, and when these become separated by more than the Planck length of 10−35 metres, they become unstable and collapse. Penrose called this form of collapse objective reduction.
Penrose suggested that objective reduction represented neither randomness nor the algorithm based processing of most physics, but instead a non-computable influence embedded in the fundamental level of spacetime geometry from which mathematical understanding and, by later extension of the theory, consciousness derived.
When he wrote his first book on consciousness, The Emperor's New Mind in 1989, Penrose lacked a detailed proposal for how quantum processing could be implemented in the brain. Subsequently, Hameroff read Penrose's work, and suggested that microtubules could be suitable candidates for quantum processing, an hypothesis which remains intensely controversial. The Orch-OR theory arose from the collaboration of Penrose and Hameroff in the early 1990s.
Microtubules have a well established position in conventional biology and neuroscience. Microtubules are the main component of a supportive structure within neurons known as the cytoskeleton. In addition to providing a supportive structure, the known functions of microtubules include transport of molecules including neurotransmitters bound for synapses and control of the development of the cell. Microtubules are composed of tubulin protein dimer subunits. The tubulin dimers each have hydrophobic pockets that are 8 nm apart, and which may contain delocalised pi electrons. Tubulins have other smaller non-polar regions that contain pi electron-rich indole rings separated by only about 2 nm.
Hameroff proposes that these electrons are close enough to become quantum entangled. In the original version of his proposals, Hameroff went on to hypothesise that these electrons could become locked in phase, forming a state known as a Bose-Einstein condensate. In his most recent paper, he has amended this to suggest that electrons within the tubulin subunits are part of a Frohlich condensate, which is a coherent oscillation of dipolar molecules.
Furthermore, he proposes that condensates in one neuron could extend to many others via gap junctions between neurons, thus forming a macroscopic quantum feature across an extended area of the brain. When the wave function of this extended condensate collapsed, it was suggested that this could give access to non-computational influences related to mathematical understanding and ultimately conscious experience that are embedded in the geometry of spacetime.
Finally, Hameroff postulated that the activity of these condensates is the source of gamma wave synchronisation in the brain. This synchronisation has also been viewed as a likely correlate of consciousness in conventional neuroscience, and it has been shown to be linked to the functioning of gap junctions.
Another neuroscientist, Danko Georgiev, has provided a footnote to the Orch-OR theory. He accepts much of Penrose's ideas, but criticises a good part of Hameroff's scheme. He proposes that quantum coherence on the surface of the microtubules extends via presynaptic scaffold proteins to the synapses, where it both influences synaptic firing, and is transmitted across the synaptic cleft to other neurons.
Recently the debate about Hameroff's proposals has focused round papers by Reimers et al. and McKemmish et al. and Hameroff's replies to these, which is not regarded as being independently reviewed. The Reimer's paper claimed that microtubules could only support 'weak' 8 MHz coherence, but that the Orch-OR proposals required a higher rate of coherence. Hameroff, however, claims that 8 MHz coherence is sufficient to support the Orch-OR proposal. McKemmish et al. makes two claims; firstly that aromatic molecules cannot switch states because they are delocalised. Hameroff, however, claims that he is referring to the behaviour of two or more electron clouds; secondly McKemmish shows that changes in tubulin conformation driven by GTP conversion would result in a prohibitive energy requirement. Against this, Hameroff claims that all that is required is switching in electron cloud dipole states produced by London forces.
 Max TegmarkThe main argument against the quantum mind proposition is that quantum states in the brain would decohere before they reached a spatial or temporal scale, at which they could be useful for neural processing. This argument was elaborated by the physicist, Max Tegmark. Based on his calculations, Tegmark concluded that quantum systems in the brain decohere quickly and cannot control brain function. He states that "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".
 See also
- Bohm interpretation of quantum mechanics
- Quantum mind–body problem
- Quantum cognition
- Electromagnetic theories of consciousness
- Evolutionary neuroscience
- Hard problem of consciousness
- Holonomic brain theory
- Mechanism (philosophy)
- Hameroff-Penrose Orchestrated Objective Reduction
- Theory of mind
- John Searle (1997). The Mystery of Consciousness. The New York Review of Books. pp. 53–88. ISBN 978-0-940322-06-6.
- Stenger, Victor, "The Myth of Quantum Consciousness", The Humanist Vol 53 No 3 (May–June 1992) pp. 13-15 
- Tegmark, M. (2000). "Importance of quantum decoherence in brain processes". Physical Review E 61 (4): 4194–4206. arXiv:quant-ph/9907009. Bibcode 2000PhRvE..61.4194T. doi:10.1103/PhysRevE.61.4194.
- Charles Seife (4 February 2000). "Cold Numbers Unmake the Quantum Mind". Science 287 (5454): 791. doi:10.1126/science.287.5454.791. PMID 10691548. http://www.sciencemag.org/cgi/content/full/287/5454/791.
- David J. Chalmers (2003). "Consciousness and its Place in Nature". In Stephen P. Stich & Ted A. Warfield. Blackwell Guide to Philosophy of Mind. Blackwell. ISBN 978-0-631-21774-9. http://consc.net/papers/nature.pdf.
- David J. Chalmers (1995). "Facing Up to the Problem of Consciousness". Journal of Consciousness Studies 2 (3): 200–219. http://consc.net/papers/facing.html.
- David J. Chalmers (1997). The Conscious Mind: In Search of a Fundamental Theory. Philosophy of Mind Series. Oxford University Press. ISBN 978-0-19-511789-9.
- David Bohm (1980). Wholeness and the Implicate Order. London: Routledge. ISBN 0-203-99515-5.
- Piaget, J. (1956). The Origin of Intelligence in the Child. Routledge.
- Hameroff, Stuart (2008). "That's life! The geometry of π electron resonance clouds". In Abbott, D; Davies, P; Pati, A. Quantum aspects of life. World Scientific. pp. 403–434. http://www.quantumconsciousness.org/documents/Hameroff_received-1-05-07.pdf. Retrieved Jan 21, 2010.
- Hameroff, S.R. (2006). "The entwined mysteries of anesthesia and consciousness". Anesthesiology 105 (2): 400–412. doi:10.1097/00000542-200608000-00024. PMID 16871075.
- Hameroff, S. (2009). "The "conscious pilot"—dendritic synchrony moves through the brain to mediate consciousness". Journal of Biological Physics 36 (1): 71–93. doi:10.1007/s10867-009-9148-x. PMC 2791805. PMID 19669425. //www.ncbi.nlm.nih.gov/pmc/articles/PMC2791805/.
- Roger Penrose & Stuart Hameroff (2011). "Consciousness in the Universe: Neuroscience, Quantum Space-Time Geometry and Orch OR Theory". Journal of Cosmology 14. http://journalofcosmology.com/Consciousness160.html.
- Bennett, M.V.L., and Zukin, R.S. (2004). "Electrical Coupling and Neuronal Synchronization in the Mammalian Brain". Neuron 41 (4): 495–511. doi:10.1016/S0896-6273(04)00043-1. PMID 14980200.
- Dermietzel, R. (1998). "Gap junction wiring: a 'new' principle in cell-to-cell communication in the nervous system?". Brain Research Reviews 26 (2–3): 176–183. doi:10.1016/S0165-0173(97)00031-3. PMID 9651521.
- Draguhn, A., Traub, R.D., Schmitz, D., and Jefferys, J.G.R. (1998). "Electrical coupling underlies high-frequency oscillations in the hippocampus in vitro". Nature 394 (6689): 189–192. Bibcode 1998Natur.394..189D. doi:10.1038/28184. PMID 9671303.
- Fries, P., Schroder, J.-H., Roelfsema, P.R., Singer, W., and Engel, A.K. (2002). "Oscillatory Neuronal Synchronization in Primary Visual Cortex as a Correlate of Stimulus Selection". Journal of Neuroscience 22 (9): 3739–3754. PMID 11978850.
- De Zeeuw, C.I., Hertzberg, E.L., Mugnaini, E. (1995). "The dendritic lamellar body: A new neuronal organelle putatively associated with dendrodentritic gap junctions". Journal of Neuroscience 15 (2): 1587–1604. PMID 7869120.
- Reimers, J.R., McKemmish, L.K., McKenzie, R.H., Mark, A.E., and Hush, N.S. (2009). "Weak, strong, and coherent regimes of Fröhlich condensation and their applications to terahertz medicine and quantum consciousness". Proceedings of the National Academy of Sciences 106 (11): 4219–4224. Bibcode 2009PNAS..106.4219R. doi:10.1073/pnas.0806273106. PMC 2657444. PMID 19251667. //www.ncbi.nlm.nih.gov/pmc/articles/PMC2657444/.
- McKemmish, L.K., Reimers, J.R., McKenzie, R.H., Mark, A.E., and Hush, N.S. (2009). "Penrose-Hameroff orchestrated objective-reduction proposal for human consciousness is not biologically feasible". Physical Review E 80 (2): 021912–021916. Bibcode 2009PhRvE..80b1912M. doi:10.1103/PhysRevE.80.021912. http://link.aps.org/doi/10.1103/PhysRevE.80.021912.
 Further reading
- Flanagan, Brian. Are Perceptual Fields Quantum Fields?
- Hodgson, David. The Mind Matters. Oxford University Press, 1993.
- Koch, C. and Hepp, K., "Quantum mechanics in the brain", Nature 440, 611 (30 March 2006)
- Litt et al., "Is the Brain a Quantum Computer?", Cognitive Science (2006)
- Lockwood, Michael. Mind, Brain and the Quantum. Cambridge, MA: Basil Blackwell Ltd., 1989.
- McFadden, Johnjoe (2000) Quantum Evolution HarperCollins. ISBN 0-00-255948-X; ISBN 0-00-655128-9 . Final chapter on the quantum mind.
- Pharaoh, M.C. (online). Looking to quantum mechanics for a reductive explanation of the noumenon of consciousness Retrieved June.21 2008.
- Schrödinger, Erwin. Mind and Matter. Cambridge University Press, 1959.
- Weyl, Hermann. Mind and Nature, University of Pennsylvania Press, 1934.
- Wigner, Eugene. "Physics and the Explanation of Life", in Foundations of Physics, vol. 1, 1970, pp. 34–45.
- Center for Consciousness Studies, directed by Stuart Hameroff
- Online papers on consciousness
- Stanford Univ. Encyclopedia of Philosophy critical survey article Harald Altmanspacher (2006) Quantum Approaches to Consciousness
- "What is an Essentially Quantum Mechanical Effect?", Osvaldo Pessoa Jr.