To use all functions of this page, please activate cookies in your browser.
My watch list
my.bionity.com
my.bionity.com
With an accout for my.bionity.com you can always see everything at a glance – and you can configure your own website and individual newsletter.
- My watch list
- My saved searches
- My saved topics
- My newsletter
Orch-OROrch OR ('Orchestrated Objective Reduction') is a theory of consciousness, which is the joint work of theoretical physicist Sir Roger Penrose and anesthesiologist Stuart Hameroff. Whereas some theories assume consciousness emerges from the brain, and among these some assume that mind emerges from complex computation at the level of synapses among brain neurons, Orch OR involves a specific form of quantum computation which underlies these neuronal synaptic activities. Quantum computation is proposed to occur in structures inside the brain’s neurons called microtubules [1-3]. The microtubules are self-assembling cylindrical polymers of the protein tubulin, and their known functions are involved with the structural support, shape and movement of cells and the transport of molecules within neurons, notably molecules that are transported to the synapses. Hameroff has attempted to demonstrate that computation within the microtubules could support consciousness[4].
In his first book 'The Emperor's New Mind' Penrose used Gödel’s theorem to argue that human consciousness and understanding required a factor outside algorithmic computation, and that the missing 'non-computable' factor was related to a specific type of quantum computation involving what he termed 'objective reduction' (OR), his solution to the measurement problem in quantum mechanics. Penrose postulated that quantum superpositions were separations in underlying reality at its most basic level, the Planck scale. Tying quantum superposition to general relativity, he identified superpositions as spacetime curvatures in opposite directions, hence a separation in fundamental spacetime geometry. According to Penrose, such separations would be unstable above the Planck scale and would reduce above this threshold. Penrose's OR differs markedly both from the traditional Copenhagen interpretation of quantum theory propounded by Neils Bohr and from modern divergences from Copenhagen, many of which do away with the idea of wave function collapse altogether, as in many worlds and some forms of decoherence theory. The threshold for Penrose OR is given by the indeterminacy principle E=ħ/t, where E is the gravitational self-energy, i.e. the degree of spacetime separation given by the superpositioned mass, ħ is the reduced Planck constant, and 't' is the time until OR occurs. Thus the larger the superposition, the faster it will undergo OR, and visa versa. Small superpositions, e.g. an electron separated from itself, if isolated from environment, would require 10 million years to reach OR threshold. An isolated one kilogram object (e.g. Schrodinger’s cat) would reach OR threshold in only 10-37 seconds. An essential feature of Penrose OR is that the choice of states when OR occurs is selected neither randomly, as are choices following measurement or decoherence, nor completely algorithmically. Rather, states are proposed to be selected by a 'non-computable' influence involving information embedded in the fundamental level of spacetime geometry at the Planck scale. Penrose claimed that such information is Platonic, representing pure mathematical truth, aesthetic and ethical values. Plato had proposed such pure values and forms, but in an abstract realm. Penrose placed the Platonic realm at the Planck scale. This relates to Penrose's ideas concerning the three worlds, physical, mental and the Platonic mathematical world. In his theory, the physical world can be seen as the external reality, which is actually an oscillation of waves/particles, the mental world as information processing in the brain and the Platonic world as the encryption, measurement or geometry of fundamental spacetime that is claimed to support non-computational understanding. Additional recommended knowledge
The role of quantum computationIn his first book,'The Emperor’s New Mind' Penrose suggested that consciousness required a form of quantum computation in the brain. Quantum computation had been suggested by Richard Feynman and David Deutsch in the 1980s. The basic concept is that classical information, e.g. bit states of either 1 or 0, could also be quantum superpositions of both 1 and 0, known as quantum bits or qubits. Such qubits interact and compute by non-local quantum entanglement, eventually being measured/observed and reducing to definite states as the solution. Quantum computations were shown to have enormous capacity if they could be constructed, possibly using qubits of ion states, electron spin, photon polarization, current in Josephson junction, quantum dots etc. They would be able to solve problems such as 'the travelling salesman problem', which with classical computers are only solvable in non-polynomial time, that is they are intractable. During quantum computation, qubits must be isolated from environmental interaction to avoid loss of superposition, i.e. decoherence. Some research suggests that the brain requires quantum computing for perception. T. Kanade[5&6] says that while a perception algorithm working bottom up from the identification of contours could achieve a result in polynomial time, it would not, however, achieve a unique solution. This means that the brain has to work top down, so it is effectively a search engine trawling through all the possible things that it might be perceiving. W. Bialek[7&8] claims that classical computers cannot solve such top down perception problems in polynomial time. As presently envisioned, quantum computers will utilize superpositions of electron spin or current, photon polarization or atomic location. None of these have significant superpositioned/separated mass and thus E would be very, very small, and 't' very, very long. Collapse would therefore come as a result of measurement and would not involve OR, which is only suggested to happen after the spacetime separation between superpositions reaches the Planck length. The processing of manmade quantum computers would therefore not be conscious. Hameroff proposes that microtubules support quantum computation leading to OR and thence consciousness, but according to the theory, this is only a possibility because the time to collapse is much longer than for an artificial quantum computer. The creation of the Orch-OR modelWhen he first advanced these ideas, Penrose lacked a detailed proposal for how such quantum processes could be implemented in the brain. Subsequently, Hameroff read 'The Emperor’s New Mind' and suggested to Penrose that microtubules within neurons were suitable for quantum computing and OR. The Orch OR theory arose from the cooperation of Penrose and Hameroff. The biological qubits were seen as being the conformational states of the tubulin subunit proteins in microtubules. Tubulin qubits would interact and compute by entanglement with other tubulin qubits in microtubules in the same and different neurons. Penrose and Hameroff considered three possible types of tubulin superpositions: separation at the level of the entire protein, separation at the level of the atomic nuclei of the individual atoms within the proteins, and separation at the level of the protons and neutrons (nucleons) within the protein. Calculating the gravitational self-energy E of the three types, separation at the level of atomic nuclei was found to have the highest energy, and would be expected to be the dominant factor. In 1998, Hameroff further proposed that quantum activity could be widespread in the brain, and could be the ultimate source of the gamma (40Hz) synchronisation observed in the brain. The gamma synchronisation has often been identified as a neural correlate of consciousness. The link to the gamma synchronisation was suggested to occur via electrotonic gap junctions, windows between adjacent neurons and glia, which have in recent years been shown to mediate the gamma EEG synchronisation that is often proposed as a neural correlate of consciousness[9-19]. The suggested extension of quantum states via gap junctions allowed Hameroff to suggest that quantum coherence could extend over large regions of the brain. It was further postulated that when this coherence collapsed, there was an instant of consciousness (an OR event), and the brain had access to a 'non-computational' process embedded in the fundamental level of space time geometry. The result of each such OR event would proceed to organize intraneuronal activities including axonal firing and synaptic modulation/learning, while synaptic activity would also act on the microtubules. This was referred to as 'orchestration' giving the theory its name of 'Orchestrated Objective Reduction or more commonly Orch OR. The decoherence problemQuantum coherent states would normally be expected to decohere extremely rapidly in the environment of the brain. Hameroff's theory requires coherence to survive for 25ms in order to match the 40Hz gamma synchrony. It has been questioned how microtubule quantum superpositions would avoid environmental decoherence. Where researchers are currently attempting to build quantum computers, they are constructed in isolation at extremely cold temperatures to avoid decoherence – loss of quantum superposition by heat and environmental interactions. It is asked how could microtubule quantum states persist within neurons at brain temperature of 37.6 degrees Celsius for 25 ms or longer. A response to it is given as follows. Cell interiors are known to alternate between liquid phases (solution: “sol”) and quasi-solid (gelatinous: “gel”) phases due to polymerization states of the ubiquitous protein actin. In the actin-polymerized gel phase, cell water and ions are ordered on actin surfaces, so microtubules are embedded in a highly structured medium, and Hameroff proposes that this shields the microtubules from decoherence. Tubulins are also known to have C termini “tails”, negatively charged peptide sequences extending string-like from the tubulin body into the cytoplasm, attracting positive ions and forming a plasma-like Debye layer which is also suggested as possible shielding for microtubule quantum states. Finally, it is suggested that tubulins in microtubules could be coherently pumped into quantum states by biochemical energy. Tegmark and his criticsThe physicist, Max Tegmark[20] published a refutation of the Orch OR model. Tegmark developed a formula for decoherence time and calculated that microtubule quantum states would persist for only 10-13 seconds at brain temperatures, far too brief for neurophysiological effects. In their reply to his paper, physicists Scott Hagan and Jack Tuszynski along with Hameroff [21] claimed that Tegmark did not address the Orch OR model, but instead his own model. This was based on superpositions of solitons separated from themselves by 24 nanometers along the microtubules, rather than the Orch OR stipulations of superposition separations of femtometers at the level of atomic nuclei in proteins. In their paper, Hameroff et al claimed that changing the model lengthened the calculated decoherence time by seven orders of magnitude to microseconds (i.e. 10-6 s). This is still a long way short of the 25ms required for Orch OR. However, Hagan, Tuszyński and Hameroff, went on to make further calculations, in which they assumed the existence of other Orch OR proposals for the screening of the microtubules from the environment, including shielding by actin gelation, Debye layer screening, metabolic Frohlich coherent pumping and topological quantum error correction due to the particular geometry of the microtubule lattice. On this basis, decoherence times of tens to hundreds of milliseconds or longer were calculated. As yet there is no experimental confirmation that the methods of shielding proposed by Hameroff et al are effective as a means of preventing decoherence. There is some sporadic evidence for instances of quantum coherence in biological tissue but none of this refers to the structures or processes postulated in Orch OR or other theories of quantum consciousness. Some of the reports deal with artificially induced states and some with very different biological systems such as plants. In 1996, 'Science' published a debate between Gider et al, who claimed to have observed macroscopic quantum coherence in the protein, ferritin, and Tejeda and Garg, who criticised their procedures[22]. Work by the Warren group at Princeton is claimed to have shown quantum coherence between nuclear spins in separate molecules. These effects were artificially induced as part of the groups research into MRI methods[23-5]. Prokhorenko, working on retinal molecules claimed results that showed the wave properties of matter can be observed and manipulated in protein[26]. Binhi & Savin indicate the existence of unpaired electron spins, which are shielded from the environment, and lead to functional quantum interaction at physiological temperatures[27]. Finally, in 2007 Nature published a paper by Engel et al claiming evidence for quantum coherence being involved in energy transfer in the photosynthetic systems of plants[28]. See also
References
|
|||||||||||||||||
| This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Orch-OR". A list of authors is available in Wikipedia. |
