The quantum mind–body problem refers to the philosophical discussions of the mind–body problem in the context of quantum mechanics. Some interpretations of quantum mechanics posit a special role for consciousness in the process of quantum measurement.
In many philosophies[which?], the conscious mind is considered to be a separate entity, existing in a parallel realm not described by physical law. Some people[who?] claim that this idea gains support from the description of the physical world provided by quantum mechanics. Parallels between quantum mechanics and mind/body dualism were first drawn by the founders of quantum mechanics.
The reason is that quantum mechanics requires interpretation before it describes the experience of an observer. While particles and fields are described by a wavefunction, the results of observations are described by classical information which tells you the result. The information about observations is not in the wavefunction, but is additional random data. The wavefunction gives only the probability of getting different outcomes, and it turns into a classical probability only during the act of measurement, when its magnitude squared gives a probability for different outcomes.
The nature of observation has often been a point of contention in quantum mechanics, because quantum mechanics describes the experiences of observers with different numbers than it uses to describe material objects. With the notable exceptions of Louis DeBroglie, Max von Laue, Erwin Schrödinger and Albert Einstein, who believed that quantum mechanics was a statistical approximation to a deeper reality which is deterministic, most of the founders of quantum mechanics believed that this problem is purely philosophical. Eugene Wigner went further, and explicitly identified it as a quantum version of the mind-body problem.
In classical mechanics the world is measurable, the measurements reveal the true state of the world, and the behavior is deterministic. Given the initial positions and momentum of a collection of the basic particles, the future of those particles can be predicted. When these assumptions are applied to an observer the conclusion is that with enough information about the present, the entire future behavior of the observer will be determined. This led many scientists to reject pre-scientific notions of dualism, and to identify the mind of the observer with the classical state of the observer's atoms.
Yet even from a classical perspective many philosophers doubt that the material description of a hypothetical Newtonian observer is all that is necessary to understand internal experience. That is, they suggest that there may be a mind-body problem. Even though the atoms of the brain are constantly replaced, the information gets copied into new atoms, and perception continues into the new brain. In certain thought experiments, this type of copying leads to strange outcomes. For example, Daniel Dennett talks about the situation where a conscious Newtonian observer is duplicated, by having a second system store all the information in the brain. Once the second system is built, the two systems make two separate observers which contain the same information. The two observers start out exactly the same and receive the same sensory input, but eventually diverge. The divergence could be due to randomness, or glitches, or because the sensory input is slightly different; the reason is not important. The important thing is that one observer has been copied into two systems, and in such a situation it is not clear to this observer into which of the copies their experiences will continue.
Dennett notes this by assuming that he himself is copied. Before the copies diverge, there is no way for him to know which of the two copies he is. This bit of information becomes apparent to Dennett only after the two copies become different. He cannot know this information before the divergence, even if he is given full information about the material state of both copies.
The introduction of quantum mechanics substantially changed the status of the observer and measurements. The measurement problem studies how a classical observer can exist in a quantum world. The quantum world describes superpositions of very different states, but our perception is that of "classical" states in the macroscopic world, that is, a comparatively small subset of the states allowed by the quantum-mechanical superposition principle, having only a few, but determinate and robust, properties, such as position, momentum, etc. The question of why and how our experience of a "classical" world emerges from quantum mechanics thus lies at the heart of the foundational problems of quantum theory.
The determinism and materialism of classical mechanics divorced or at least distanced science from many pre-scientific philosophies that held various dualist perspectives towards the mind. Some scientists (like Wigner) believe that quantum mechanics makes certain dualist ideas about the mind/body problem acceptable again within mainstream science, while others think there is little to gain from science entertaining those possibilities further (as described in the criticism section below).
In the Copenhagen interpretation, quantum mechanics can only be used to predict the probabilities for different outcomes of pre-specified observations. What constitutes an "observer" or an "observation" is not directly specified by the theory, and the behavior of a system after observation is completely different than the usual behavior. During observation, the wavefunction describing the system collapses to one of several options. If there is no observation, this collapse does not occur, and none of the options ever become less likely.
Unlike classical mechanics, in quantum mechanics, there is no naive way of identifying the true state of the world. The wavefunction that describes a system spreads out into an ever larger superposition of different possible situations. Schrödinger's cat is an illustration of this: after interacting with a quantum system, the von Neumann/Wigner interpretation holds that the wavefunction of the cat describes it as a superposition of dead and alive. The standard interpretation, given by the Copenhagen interpretation is that the Geiger counter has already triggered the collapse of the wavefunction.
It can be predicted using quantum mechanics, absent a collapse postulate, that an observer observing a quantum superposition will turn into a superposition of different observers seeing different things. Just like Schrödinger's cat, the observer will have a wavefunction which describes all the possible outcomes. Still, in actual experience, an observer never feels a superposition, but always feels that one of the outcomes has occurred with certainty. This apparent conflict between a wavefunction description and classical experience is called the problem of observation (see: Measurement problem). The founders of quantum mechanics were aware of this problem, and had varying opinions about its resolution. These views reflect different stances on an argument which is anything but resolved today:
Albert Einstein, and with him Louis De Broglie and later David Bohm, believed that quantum mechanics was incomplete, that the wavefunction was only a statistical description of a deeper causal structure. Einstein saw quantum mechanics as analogous to statistical mechanics, and the wavefunction as just a peculiar statistical device for observers who are ignorant of the values of the hidden variables underneath. This point of view makes the extra information not at all mysterious – the results of observations are simply revealing the values of the hidden variables. David Bohm was able to explicitly formulate a nonlocal theory which reproduces the predictions of quantum mechanics. Although no error in Bohm's approach could be found, his theory did not find acceptance, and it was (incorrectly) believed that his theory was ruled out by an argument of John von Neumann. In 1964, John Bell realized that local hidden variables set a limit on the degree to which the results of distant experiments can be correlated, a limit which is violated in quantum mechanics. The experimental observation of violations of Bell's inequality showed that the original local hidden variables of Einstein, Podolsky, and Rosen could not be correct. Bell also criticized von Neumann's argument, showing that von Neumann's proof is not universally valid (i.e., applicable to all of the possible types of hidden variable theories), and does not rule out Bohm's theory. Most physicists do not accept hidden variable interpretations as compelling.
The mainstream of the scientific community adopted an approach attributed to Niels Bohr. Bohr believed that quantum mechanics was a complete description of nature, but that it was simply a language ill suited to describing the world of everyday experience, and that in the human realm experiences were described by classical mechanics and by probability. Later an amalgamated, Copenhagen interpretation, similar to the views of Max Born, Werner Heisenberg and others, became the standard view. It requires a demarcation line, a boundary, above which an object would cease to be quantum and would start to be classical. Bohr never specified this line precisely, since he believed that it was not a question of physics, but of pure philosophy or even convenience. Von Neumann, in his analysis of measurements, interpreted the demarcation line as the point where wave-function collapse occurs, and he showed that within quantum mechanics, the point of collapse is largely arbitrary, and may be placed anywhere from the first incoherent interaction with a complex enough object, to the interface of the brain with consciousness.
Hugh Everett proposed an entirely mechanistic interpretation of quantum mechanics that has come to be known as the many-worlds interpretation. In Everett's view, the whole universe is a wavefunction (the universal wavefunction), describing a dizzying multiplicity of worlds. In this interpretation, observers are to be treated like computers, or as any other measuring device, as if their memories could be written out on magnetic tape. To understand the subjectively probabilistic nature of their experiences, one correlates the answer given by an observer with questions asked by a so-called external agency, who is likewise an observer and thus internal to the combined quantum system. Everett believed that this line of reasoning shows there is no conflict between the objective deterministic evolution of the wavefunction and the subjective indeterminate experiences of an observer.
Since the physical description in Everett's realist account is the deterministic wavefunction, the issue of interpretation is only relevant when analyzing the experience of an observer. The answer to the question "what does this observer see?" is only ambiguous to the extent that the specification of the observer is imprecise. An observer's state is a particular high dimensional projection of the universal wavefunction, but not all parts of the wavefunction describe a single observer – only those parts which describe a consistent past. In Everett's picture, the interpretation is a clarification, it tells you which observer you are examining.
A post-Everettian approach has been developed into a field of study called Quantum decoherence, which analyses the way in which classical behaviour emerges from quantum mechanics when the systems become large. Decoherence can be viewed as the loss of information from a system into the environment (often modeled as a heat bath), since every system is loosely coupled with the energetic state of its surroundings. Viewed in isolation, the system's dynamics are non-unitary (although the combined system plus environment evolves in a unitary fashion). Thus the dynamics of the system alone are irreversible. As with any coupling, entanglements are generated between the system and environment, which have the effect of sharing quantum information with – or transferring it to – the surroundings.
Decoherence does not generate literal wave function collapse. Rather, it only provides an explanation for the appearance of wavefunction collapse, as the quantum nature of the system "leaks" into the environment. That is, components of the wavefunction are decoupled from a coherent system, and acquire phases from their immediate surroundings. A total superposition of the universal wavefunction still exists (and remains coherent at the global level), but its fundamentality remains an interpretational issue. "Post-Everett" decoherence also answers the measurement problem, holding that literal wavefunction collapse simply doesn't exist. Rather, decoherence provides an explanation for the transition of the system to a mixture of states that seem to correspond to those states observers perceive. Moreover, our observation tells us that this mixture looks like a proper quantum ensemble in a measurement situation, as we observe that measurements lead to the "realization" of precisely one state in the "ensemble".
According to E.J. Squires, the description of the observer in a decoherence approach, as in the Copenhagen approach, always involves extra information, the information which specifies the outcome of all the random events in the past. This information answers the question "which observer?" in many-worlds, and correspondingly answers the question "what outcomes of past measurements?" in the Copenhagen approach.
Squires associates this with the consciousness of the observer, because it is purportedly associated with the observer, not with the matter from which the observer is built. This includes most information about the universe.
The only form of interactionist dualism that has seemed even remotely tenable in the contemporary picture is one that exploits certain properties of quantum mechanics. There are two ways this might go. First, some [e.g., Eccles 1986] have appealed to the existence of quantum indeterminacy, and have suggested that a nonphysical consciousness might be responsible for filling the resultant causal gaps, determining which values some physical magnitudes might take within an apparently "probabilistic" distribution… Although these decisions would have only a tiny proximate effect, perhaps nonlinear dynamics could amplify these tiny fluctuations into significant macroscopic effects on behavior.
This is an audacious and interesting suggestion, but it has a number of problems… A second way in which quantum mechanics bears on the issue of causal closure lies with the fact that in some interpretations of the quantum formalism, consciousness itself plays a vital causal role, being required to bring about the so-called "collapse of the wave-function." This collapse is supposed to occur upon any act of measurement; and in one interpretation, the only way to distinguish a measurement from a nonmeasurement is via the presence of consciousness. This theory is certainly not universally accepted (for a start, it presupposes that consciousness is not itself physical, surely contrary to the views of most physicists), and I do not accept it myself, but in any case it seems that the kind of causal work consciousness performs here is quite different from the kind required for consciousness to play a role in directing behavior… In any case, all versions of interactionist dualism have a conceptual problem that suggests that they are less successful in avoiding epiphenomenalism than they might seem; or at least they are no better off than [naturalistic dualism]. Even on these views, there is a sense in which the phenomenal is irrelevant. We can always subtract the phenomenal component from any explanatory account, yielding a purely causal component.
— David Chalmers, The Conscious Mind: In Search of a Fundamental Theory, "The Irreducibility of Consciousness"
In his 1932 book The Mathematical Foundations of Quantum Mechanics, John von Neumann argued that the mathematics of quantum mechanics allows for the collapse of the wave function to be placed at any position in the causal chain from the measurement device to the "subjective perception" of the human observer – the notion of such a chain, more specifically a chain of interacting systems in which the values of one system is correlated with that of the immediately following system, has since become known as the von Neumann chain. In 1939, F. London and E. Bauer argued for the latter boundary (consciousness). In the 1960s, Eugene Wigner reformulated the "Schrödinger's cat" thought experiment as "Wigner's friend" and proposed that the consciousness of an observer is the demarcation line which precipitates collapse of the wave function, independent of any realist interpretation. See Consciousness and measurement. Very technically, Wigner identified the non-linear probabilistic projection transformation which occurs during measurement with the selection of a definite state by a mind from the different possibilities which it could have in a quantum mechanical superposition. Thus, the non-physical mind is postulated to be the only true measurement apparatus. This interpretation has been summarized thus:
The rules of quantum mechanics are correct but there is only one system which may be treated with quantum mechanics, namely the entire material world. There exist external observers which cannot be treated within quantum mechanics, namely human (and perhaps animal) minds, which perform measurements on the brain causing wave function collapse.
Henry Stapp has argued for the concept as follows:
From the point of view of the mathematics of quantum theory it makes no sense to treat a measuring device as intrinsically different from the collection of atomic constituents that make it up. A device is just another part of the physical universe... Moreover, the conscious thoughts of a human observer ought to be causally connected most directly and immediately to what is happening in his brain, not to what is happening out at some measuring device... Our bodies and brains thus become...parts of the quantum mechanically described physical universe. Treating the entire physical universe in this unified way provides a conceptually simple and logically coherent theoretical foundation...
Hilary Putnam, a philosopher who has written on quantum mechanics a number of times, states that Consciousness Causes Collapse is no longer a popular view among physicists:-
[..]one might say --- von Neumann hints at this in his book, and Eugene Wigner famously advocated it -- "No, the collapse occurs when the result of a measurement is registered by a consciousness."
I do not know of anyone who currently advocates this "psychic" view, [..]
There are other possible solutions to the "Wigner's friend" thought experiment, which do not require consciousness to be different from other physical processes. Moreover, Wigner actually shifted to those interpretations (and away from "consciousness causes collapse") in his later years. This was partly because he was embarrassed that "consciousness causes collapse" can lead to a kind of solipsism, but also because he decided that he had been wrong to try to apply quantum physics at the scale of every day life (specifically, he rejected his initial idea of treating macroscopic objects as isolated systems—as one might microscopic objects). See, Consciousness and Superposition.
Recently, it has been argued that the results of delayed choice quantum eraser experiments effectively preclude the dualist or "consciousness" interpretation. Other researchers have expressed similar objections to the introduction of any subjective element in the collapse of the wavefunction.
To many scientists the dualist interpretation fails a priori to compete with other interpretations of quantum mechanics because "consciousness causes collapse" relies upon a dualistic philosophy of mind (in particular, a radical interactionism), which is inconsistent with the materialist monism presupposed by many physicists. The measurement problem not withstanding, they point to a causal closure of physics, suggesting a problem with how consciousness and matter might interact, reminiscent of objections to Descartes' substance dualism. Some physicists conclude that science's success at modeling the world materialistically—without reference to mental properties—vindicates that neglect. Psychology, on the other hand, has benefited from "an intellectual stampede" following Francis Crick and Christof Koch's challenge in 1990, that the time is ripe to tackle consciousness. Wigner accused materialist scientists of "exalting the problem [of the study of physical phenomena]". See also, Orch-OR.
Consciousness causes collapse theory does not explain which things have sufficient consciousness to collapse the wave function. A more fundamental issue is that it posits an important role for the conscious mind, and it has been questioned how this could be the case for the earlier universe, before consciousness had evolved or emerged. It has been argued that "[consciousness causes collapse] does not allow sensible discussion of Big Bang cosmology or biological evolution, at least on the assumption of an atheistic universe. For example, as Roger Penrose put it, "[T]he evolution of conscious life on this planet is due to appropriate mutations having taken place at various times. These, presumably, are quantum events, so they would exist only in linearly superposed form until they finally led to the evolution of a conscious being—whose very existence depends on all the right mutations having 'actually' taken place!"
Others further suppose a universal mind (see also pantheism and panentheism). To most physicists, including David Bohm and Basil Hiley, this merely pushes the problem back, which some see as a fatal unparsimonious move in a competition with other theories. Physicist Victor Stenger says that the "myth" of quantum consciousness has no scientific basis, nor does "the related belief that the human mind commands special powers—psychic forces—that transcend the material universe".
There are numerous philosophical interpretations of quantum mechanics competing with one another. Although measurement in quantum mechanics remains controversial, mainstream interpretations have never required a conscious observer to perform the wave function collapse, (by stipulation, a Geiger counter will do). Even less endearing to some is the many worlds interpretation, which spares no ontological expense to avoid it. In this interpretation, measurement results in a superposition, each outcome of an experiment persisting orthogonality (see Wigner's friend in Many Worlds). Decoherence alleviates the purported epistemic necessity of stipulating that wave function collapse occurs at some threshold of size, complexity, convenience or participation by providing a realist account without the superluminal and non-local requirements of objective collapse theories. Quantum effect rapidly decohere and become negligible during an interaction with the scientific instrument performing a measurement, absent any literal observer… Previously, scientists had not observed macroscopic quantum effects and assumed they never would.
The originators of quantum mechanical theory held diverse opinions on this subject. Many of them held that humans can effectively interrogate nature through interacting with it, and that in this regard quantum mechanics is not different from classical mechanics. Werner Heisenberg maintained that wave function collapse—the destruction of quantum superposition—occurs when the result of a measurement is registered in the mind of an observer. Albert Einstein, who believed in determinism, and did not accept the theoretical completeness of quantum mechanics, considered the belief that consciousness has any effect on physics to be mystical and non-scientific.
Heisenberg and Bohr described quantum mechanics in logical positivist terms. Bohr also took an active interest in the philosophical implications of quantum theories such as his complementarity, for example. He believed quantum theory offers a complete description of nature, albeit one that is simply ill suited for everyday experiences—which are better described by classical mechanics and probability. Bohr never specified a demarcation line above which objects cease to be quantum and become classical. He believed that it was not a question of physics, but one of philosophy or convenience. 
Wolfgang Pauli interpreted the laws of quantum mechanics as leading to a lucid Platonic mysticism, a position intermediate between the skepticism of Western science centered on objective observer-independent facts, and the philosophies of ancient Eastern mysticism which put primary emphasis on conscious experience. Werner Heisenberg reported on Pauli's position, and his own, as follows:
...Pauli once spoke of two limiting conceptions, both of which have been extraordinarily fruitful in the history of human thought, although no genuine reality corresponds to them. At one extreme is the idea of an objective world, pursuing its regular course in space and time, independently of any kind of observing subject; this has been the guiding image of modern science. At the other extreme is the idea of a subject, mystically experiencing the unity of the world and no longer confronted by an object or by any objective world; this has been the guiding image of Asian mysticism. Our thinking moves somewhere in the middle, between these two limiting conceptions; we should maintain the tension resulting from these two opposites.
Fritjof Capra popularized the subject with The Tao of Physics. In this book, he notes that many of the founders of quantum mechanics believed that the theory meshes well with ancient Eastern mysticism and philosophy, including that of Hinduism, Taoism, and Buddhism which includes a belief in the transitory, interconnected nature of all things and the illusion of separation of thought and existence.
Deepak Chopra, a supporter of some of the ideas of consciousness causes collapse, appeals to the work of physicist Roger Penrose. Penrose pursued various lines of argument to suggest that human consciousness cannot be explained by existing principles in physics, but his arguments were rejected by experts in the relevant fields.
The view is also presented in various aspects of the New Thought Movement, the film What the Bleep Do We Know!?, and is a major plot point in Greg Egan's novel Quarantine and Dan Brown's novel The Lost Symbol.
"Consciousness causes collapse" is the name given to the claim that observation by a conscious observer is responsible for the wavefunction collapse in quantum mechanics. It has been summarised as:
The rules of quantum mechanics are correct but there is only one system which may be treated with quantum mechanics, namely the entire material world. There exist external observers which cannot be treated within quantum mechanics, namely human (and perhaps animal) minds, which perform measurements on the brain causing wave function collapse 
It can also be seen as an attempt to solve the Wigner's friend paradox by asserting that collapse occurs at the first "conscious" observer. Supporters assert this is not a revival of substance dualism, since (in a ramification of this view) consciousness and objects are "entangled"  and cannot be considered separate. Opponents assert that it is unfalsifiable, and also does not simplify our physical understanding of the universe, and is therefore scientifically uninteresting.
It has been claimed that the theory meshes well with ancient Eastern mysticism and philosophy, including that of Hinduism, Taoism, and Buddhism which includes a belief in the transitory, interconnected nature of all things and the illusion of separation of thought and existence. This is one of the major themes of the book The Dancing Wu Li Masters. It also meshes well with the views of the New Thought Movement.
The view is also presented in the popular and controversial documentaries What the Bleep Do We Know!? and The Secret, alongside some unrelated biological discussions, and is a major plot point in Greg Egan's novel Quarantine, as well as playing a significant role in Charlie Stross's novel The Atrocity Archives.
With the publication of Die Mathematischen Grundlagen der Quantenmechanik (The Mathematical Foundations of Quantum Mechanics), it was John von Neumann who became the first person to hint that quantum theory may imply an active role for consciousness in the process of reality creation. Others, such as Walter Heitler, Fritz London, Edmond Bauer, and Eugene Wigner further carried von Neumann's argument to a claimed logical conclusion that consciousness-created reality is the inevitable outcome of von Neumann's picture of quantum theory. Wigner concluded from his own arguments about symmetry in physics that the action of matter upon mind must give rise to, as he put it, a "direct action of mind upon matter".
Among the more recent followers of consciousness-causes-collapse, or other consciousness-based theories, one can find Fred Alan Wolf, William A. Tiller, John Hagelin, Stuart Hameroff, Bernard Baars, Amit Goswami, Russell Targ, Nick Herbert, Jeffrey M. Schwartz, Menas Kafatos, and the Princeton Engineering Anomalies Research Lab in New Jersey. The British Philosopher-Theologian Keith Ward is also a major proponent of this idea.
More than one notable scientist has hinted at a belief in the existence of some form of connection between contemporary physics and metaphysical concepts related to consciousness, mind, our role as the observer of reality, or a deeper meaning of reality by itself:
This sentiment was echoed by French physicist Bernard d'Espagnat:
This interpretation, attributes the process of wave function collapse (directly, indirectly, or even partially) to consciousness itself. However, it is not explained by this theory which animals, living creatures, or objects have sufficient consciousness to collapse of the wave function ("Was the wave function waiting to jump for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer for some highly qualified measurer - with a PhD?"). It is also not clear whether measuring devices might also be considered conscious, though generally measuring devices are considered by propnents of consciousness causes collapse to be in the same indeterminate state as what they measure until observed by a conscious entity. Some even suggest that some beings have a "higher consciousness" and therefore more capability to collapse the wavefunction, whereas others believe all conscious entities have an equal capability. Solipsists would assert that they are the only conscious entity, and therefore only they cause wave function collapse upon objects.
Most physicists regard this theory as a non-scientific concept, claiming that it is experimentally unfalsifiable, and that it introduces unnecessary elements into physics, rather than simplifying.
Wim De Muynck comments that
The human observer is as dispensable in quantum mechanics as he (short for `he or she') is in classical mechanics. He sees only the macroscopic parts of his measuring instruments. In present-day practice of the physical science of the microscopic domain human observation is largely restricted to the tables and graphs that have been printed on the basis of data obtained by the scientists computer from a measuring instrument of which the measurement results are sent to the computer without any human interference.
An influence like the reduction (collapse) of the wave packet, allegedly exerted by a human observer on a microscopic object by means of observation, would be equally miraculous as killing a fly by just looking at ones fly swatter. 
Some commentators question how the interpretation copes with the epochs of history before any conscious observer. One response to this, as given for example by Peter Russell, is that our definition of consciousness is too limited, and that we should instead make the philosophical posit that consciousness is fundamental.
Recent study of quantum decoherence casts new light onto the problem by reducing the importance of the "macroscopic observer" originally introduced in the language of the Copenhagen interpretation of quantum theory. Modern scientific discourse has evolved to try to quantify how quantum systems decohere due to their interactions with the surroundings. In this manner a unified view of all quantum interactions can be developed that treats neighboring quantum systems, thermal baths and the measurement apparatus on the same footing.
While the process of einselection can explain the appearance of distinct outcome states and the onset of classical reality, the apparent random selection of which outcome state remains one of greatest mysteries of science.