What is consciousness? Some philosophers have contended that "qualia," or an experiential medium from which consciousness is derived, exists as a fundamental component of reality. Whitehead, for example, described the universe as being comprised of "occasions of experience." To examine this possibility scientifically, the very nature of physical reality must be re-examined. We must come to terms with the physics of space-time--as is described by Einstein's general theory of relativity--and its relation to the fundamental theory of matter--as described by quantum theory. This leads us to employ a new physics of objective reduction: " OR" which appeals to a form of quantum gravity' to provide a useful description of fundamental processes at the quantum/classical borderline (Penrose, 1994; 1996). Within the OR scheme, we consider that consciousness occurs if an appropriately organized system is able to develop and maintain quantum coherent superposition until a specific "objective" criterion (a threshold related to quantum gravity) is reached; the coherent system then self-reduces (objective reduction: OR). We contend that this type of objective self-collapse introduced non-computability, an essential feature of consciousness. OR is taken as an instantaneous event--the climax of a self-organizing process in fundamental space-time--and a candidate for a conscious Whitehead "occasion" of experience. How could an OR process occur in the brain, be coupled to neural activities, and account for other features of consciousness? We nominate an OR process with the requisite characteristics to be occurring in cytoskeletal microtubules within the brain's neurons (Penrose and Hameroff, 1995; Hameroff and Penrose, 1995; 1996).
|In this model, quantum-superposed states develop in
microtubule subunit proteins ("tubulins"), remain coherent
and recruit more superposed tubulins until a mass-time-energy threshold
(related to quantum gravity) is reached. At that point, self-collapse,
or objective reduction (OR) abruptly occurs. We equate the
pre-reduction, coherent superposition ("quantum computing")
phase with pre-conscious processes, and each instantaneous (and non-computable)
OR, or self-collapse, with a discrete conscious event. Sequences
of OR events give rise to a "stream" of consciousness.
Microtubule-associated-proteins can "tune" the quantum oscillations
of the coherent superposed states; the OR is thus self-organized,
or "orchestrated" ("Orch OR"). Each Orch
OR event selects (non-computably) microtubule subunit states which
regulate synaptic/neural functions using classical signaling.
The quantum gravity threshold for self-collapse is relevant to consciousness, according to our arguments, because macroscopic superposed quantum states each have their own space-time geometries (Penrose, 1994; 1996). These geometries are also superposed, and in some way "separated," but when sufficiently separated, the superposition of space-time geometries becomes significantly unstable, and reduce to a single universe state. Quantum gravity determines the limits of the instability; we contend that the actual choice of state made by Nature is non-computable. Thus each Orch OR event is a self-selection of space-time geometry, coupled to the brain through microtubules and other biomolecules.
If conscious experience is intimately connected with the very physics
underlying space-time structure, then Orch OR in microtubules
indeed provides us with a completely new and uniquely promising
perspective on the hard problem of consciousness.
One set of philosophical positions, addressing the hard problem, views consciousness as a fundamental component of physical reality. For example an extreme view - "panpsychism" - is that consciousness is a quality of all matter: atoms and their subatomic components having elements of consciousness (e.g. Spinoza, 1677; Rensch, 1960). "Mentalists" such as Leibniz and Whitehead (e.g. 1929) contended that systems ordinarily considered to be physical are constructed in some sense from mental entities. Bertrand Russell (1954) described "neutral monism" in which a common underlying entity, neither physical nor mental, gave rise to both. Recently Stubenberg (1996) has claimed that qualia are that common entity. In monistic idealism, matter and mind arise from consciousness - the fundamental constituent of reality (e.g. Goswami, 1993). Wheeler (1990) has suggested that information is fundamental to the physics of the universe. From this, Chalmers (1995;1996) proposes a double-aspect theory in which information has both physical and experiential aspects.
Among these positions, the philosophy of Alfred North Whitehead (1929; 1933) may be most directly applicable. Whitehead describes the ultimate concrete entities in the cosmos as being actual "occasions of experience," each bearing a quality akin to "feeling." Whitehead construes "experience" broadly - in a manner consistent with panpsychism - so that even "temporal events in the career of an electron have a kind of 'protomentality'." Whitehead's view may be considered to differ from panpsychism, however, in that his discrete 'occasions of experience' can be taken to be related to "quantum events" (Shimony, 1993). In the standard descriptions of quantum mechanics, randomness occurs in the events described as quantum state reductions--these being events which appear to take place when a quantum-level process gets magnified to a macroscopic scale.
Quantum state reduction (here denoted by the letter R; cf. Penrose, 1989, 1994) is the random procedure that is adopted by physicists in their descriptions of the quantum measurement process. It is still a highly controversial matter whether R is to be taken as a "real" physical process, or whether it is some kind of illusion and not to be regarded as a fundamental ingredient of the behavior of Nature. Our position is to take R to be indeed real--or, rather to regard it as a close approximation to an objectively real process OR (objective reduction), which is to be a non-computable process instead of merely a random one (see Penrose 1989; 1994). In almost all physical situations, OR would come about in situations in which the random effects of the environment dominate, so OR would be virtually indistinguishable form the random R procedure that is normally adopted by quantum theorists. However, when the quantum system under consideration remains coherent and well isolated from its environment, then it becomes possible for its state to collapse spontaneously, in accordance with the OR scheme we adopt, and to behave in non-computable rather than random ways. Moreover, this OR scheme intimately involves the geometry of the physical universe at its deepest levels.
Our viewpoint is to regard experiential phenomena as also inseparable from the physical universe, and in fact to be deeply connected with the very laws which govern the physical universe. The connection is so deep, however, that we perceive only glimmerings of it in our present day physics. One of these glimmerings, we contend, is a necessary non-computability in conscious thought processes; and we argue that this non-computability must also be inherent in the phenomenon of quantum state self-reduction--the "objective reduction" (OR) referred to above. This is the main thread of argument in Shadows of the Mind (Penrose, 1994). The argument that conscious thought, whatever other attributes it may also have, is non-computable (as follows most powerfully from certain deductions from Gödel's incompleteness theorem) grabs hold of one tiny but extremely valuable point. This means that at least some conscious states cannot be derived from previous states by an algorithmic process - a property which distinguishes human and other animal minds from computers. Non-computability per se does not directly address the 'hard problem' of the nature of experience, but it is a clue to the kind of physical activity that lies behind it. This points to OR, an underlying physical action of a completely different character from that which seems to underlie non-conscious activity. Following this clue with sensitivity and patience should ultimately lead to real progress towards understanding mental phenomena in their inward manifestations as well as outward.
In the OR description, consciousness occurs if an organized quantum system is able to isolate and sustain coherent superposition until its quantum gravity threshold for space-time separation is met; it then self-reduces (non-computably). For consciousness to occur, self-reduction is essential, as opposed to reduction being triggered by the system's random environment. (In the latter case, the reduction would itself be effectively random and would lack useful non-computability, being unsuitable for direct involvement in consciousness.) We take the self-reduction to be an instantaneous event -- the climax of a self-organizing process fundamental to the structure of space-time - and apparently consistent with a Whitehead "occasion of experience."
As OR could, in principle, occur ubiquitously within many types of inanimate media, it may seem to imply a form of 'panpsychism' (in which individual electrons, for example, possess an experiential quality). However according to the principles of OR (as expounded in Penrose 1994; 1996), a single superposed electron would spontaneously reduce its state (assuming it could maintain isolation) only once in a period much longer than the present age of the universe. Only large collections of particles acting coherently in a single macroscopic quantum state could possibly sustain isolation and support coherent superposition in a time frame brief enough to be relevant to our consciousness. Thus only very special circumstances could support consciousness:
|By reaching quantum gravity threshold, eachOR
event has a fundamental bearing on space-time geometry. One could
say that a cascade of OR events charts an actual course of
physical space-time geometry selections.
It may seem surprising that quantum gravity effects could plausibly have relevance at the physical scales relevant to brain processes. For quantum gravity is normally viewed as having only absurdly tiny influences at ordinary dimensions. However, we shall show later that this is not the case, and the scales determined by basic quantum gravity principles are indeed those that are relevant for conscious brain processes.
We must ask how such an OR process could actually occur in the brain. How could it be coupled to neural activities at a high rate of information exchange; how could it account for preconscious to conscious transitions, have spatial and temporal binding, and both simultaniety and time flow?
We here nominate an OR process with the requisite characteristics occurring in cytoskeletal microtubules within the brain's neurons. In our model, microtubule-associated proteins "tune" the quantum oscillations leading to OR; we thus term the process "orchestrated objective reduction" (Orch OR).
Space-Time: Quantum Theory and Einstein's Gravity
Quantum theory describes the extraordinary behavior of the matter and energy which comprise our universe at a fundamental level. At the root of quantum theory is the wave/particle duality of atoms, molecules and their constituent particles. A quantum system such as an atom or sub-atomic particle which remains isolated from its environment behaves as a "wave of possibilities" and exists in a coherent complex-number valued "superposition" of many possible states. The behavior of such wave-like, quantum-level objects can be satisfactorily described in terms of a state vector which evolves deterministically according to the Schrödinger equation (unitary evolution), denoted by U.
Somehow, quantum microlevel superpositions lead to unsuperposed stable structures in our macro-world. In a transition known as wave function collapse, or reduction (R), the quantum wave to alternative possibilities reduces to a single macroscopic reality, an "eigenstate" of some appropriate operator. (This would be just one out of many possible alternative eigenstates relevant to the quantum operator.) This process is invokes in the description of a macroscopic measurement, when effects are magnified from the small, quantum scale to the large, classical scale.
According to conventional quantum theory (as part of the standard "Copenhagen interpretation"), each choice of eigenstate is entirely random, weighted according to a probability value that can be calculated from the previous state according to the precise procedures of quantum formalism. This probabilistic ingredient was a feature with which Einstein, among others, expressed displeasure: "You believe in a God who plays dice and I in complete law and order"(from a letter to Max Born). Penrose (1989; 1994) has contended that, at a deeper level of description, the choices may more accurately arise as a result of some presently unknown "non-computational" mathematical/physical (i.e., "Platonic realm") theory, that is they cannot be deduced algorithmically. Penrose argues that such non-computability is essential to consciousness, because (at least some) conscious mental activity is unattainable by computers.
It can be argued that present-day physics has no clear explanation for the cause and occurrence of wave function collapse R. Experimental and theoretical evidence through the 1930's led quantum phycisists (such as Schrödinger, Heisenberg, Dirac, von Neumann and others) to postulate that quantum-coherent superpositions persist indefinitely in time, and would, in principle be maintained from the micro to macro levels. Or perhaps they would persist until conscious observation collapses, or reduces, the wave function (subjective reduction, or "SR"). Accordingly, even macroscopic objects, if unobserved, could remain superposed. To illustrate the apparent absurdity of this notion, Erwin Schrödinger (e.g. 1935) described his now-famous "cat in a box" being simultaneously both dead and alive until the box was opened and the cat observed.
As a counter this unsettling prospect, various new physical schemes for collapse according to objective criteria (objective reduction - "OR") have recently been proposed. According to such a scheme, the growth and persistence of superposed states could reach a critical threshold, at which collapse, or OR rapidly occurs (e.g. Pearle, 1989 ; Ghirardi et al, 1986). Some such schemes are based specifically on gravitational effects mediating OR (e.g.Károlyházy, 1986; Diósi, 1989; Ghirardi et al., 1990; Penrose, 1989;1994; Pearle and Squires, 1994; Percival, 1995). Table 1 categorizes types of reduction.
|Context||Cause of Collapse (Reduction)||Description||Acronym|
|Quantum coherent superposition||No collapse||Evolution of the wave function (Schrödinger equation)||U|
|Conventional quantum theory (Copenhagen interpretation)||Environmental entanglement, Measurement, Conscious observation||Reduction; Subjective reduction||R
|New physics (Penrose, 1994)||Self-collapse -quantum gravity induced (Penrose, Diósi, etc)||Objective reduction||OR|
|Consciousness (present paper)||Self-collapse, quantum gravity threshold in microtubules orchestrated by MAPs etc||Orchestrated objective reduction||Orch OR|
Table 1 Descriptions of wave function collapse.