The quantum mind–body problem refers to the philosophical discussions of the mind–body problem in the context of quantum mechanics. Since quantum mechanics involves quantum superpositions, which are not perceived by observers, some interpretations of quantum mechanics place conscious observers in a special position.
The founders of quantum mechanics debated the role of the observer, and of them, Wolfgang Pauli and Werner Heisenberg believed that it was the observer that produced collapse. This point of view, which was never fully endorsed by Niels Bohr, was denounced as mystical and anti-scientific by Albert Einstein. Pauli accepted the term, and described quantum mechanics as lucid mysticism.[1].
Heisenberg and Bohr always 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.[2] 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.
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. Commonly known as "consciousness causes collapse", this interpretation of quantum mechanics states that observation by a conscious observer is what makes the wave function collapse.
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In many philosophies, the conscious mind is seen as a separate entity, existing in a realm not described by physical law. Some people 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 including Erwin Schrödinger,[3] Werner Heisenberg,[4] Wolfgang Pauli,[5] Niels Bohr,[6] and Eugene Wigner[7]
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 only gives the probability of getting different outcomes, and it only turns into a classical probability during the act of measurement, when its magnitude squared gives a probability for different outcomes.[8]
The nature of observation has often been a point of contention in quantum mechanics,[9] 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,[10] 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.[11]
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 velocities 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.[12][13]
Yet even from a classical perspective many philosophers doubt the material description of a hypothetical Newtonian observer is the only thing you need in order to understand internal experience. That is, they suggest that there may be a mind-body problem.[14][15][16] 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 only becomes apparent to Dennett 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.[17]
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[18] 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 collapsed 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 each had a different opinion 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 structure which was deterministic. 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.[19] Bell also pointed out the flaw in von Neumann's argument and that it had been incorrectly held against Bohm's theory: he showed that van Neumann's argument holds against local theories only. Most physicists do not accept hidden variable interpretations as compelling.[20]
The mainstream of the scientific community adopted the approach of 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. This point of view, the Copenhagen interpretation, was shared by Max Born and Werner Heisenberg and 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. 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.[21]
Hugh Everett proposed an entirely mechanistic interpretation of quantum mechanics that has come to be known as the many-worlds interpretation. In Everett's description, the whole universe is an enormous wavefunction (the universal wavefunction), describing a dizzying multiplying possibility of worlds. In this formalism, observers were to be treated as computers or as any other measuring device, their memories written out on magnetic tape.[22] To understand their experiences, you would focus on the answer which these observers would give to questions asked by an external observer. Everett believed that this line of reasoning showed that any interpretational problems in quantum mechanics were entirely philosophical, because he could show that there was no conflict between deterministic evolution of the wavefunction with the subjective randomness experienced by the observers, when analyzed using the theory itself.[23]
Since the physical description in Everett's picture 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 wavefunction, but not all parts of the wavefunction describe a single observer – only those parts which describe a consistent past of memories. In Everett's picture, the interpretation is a clarification, it tells you which observer you are examining. But the description of the observer is now a major chunk of the description of the world--- it includes a lot of extra information not present in the original wavefunction.[24]
This extra information includes most observable parameters in our universe. For example, if the universe started out perfectly homogeneous and isotropic, the universal wavefunction would still be homogeneous and isotropic. But for any observer, the description would be irregular describing a different pattern of galaxies, stars and planets. The information which specifies the observer specifies the positions of all those stars, the distance to Jupiter, the location of the moon in its orbit, the contents of today's newspaper, etc. None of this is in the universal wavefunction, that object is only a quantum superposition of all possible worlds. Most of the nontrivial information is in the history of past random events.
Everett's approach has been elaborated into a field of study called decoherence, which attempts to identify the way in which classical behaviour emerges from quantum mechanics when the systems become large.[25]
The description of the observer in decoherence approaches, 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.
The presence of large amounts of additional information has been interpreted as a component associated with the consciousness of the observer, because it is data which is associated with the observer, not with the matter from which the observer is built. Since this includes most information about the universe, considering the quantum mechanical description to be complete leads to a reevaluation of the nature of the observer.[26]
First introduced by John von Neumann, in his 1932 book The Mathematical Foundations of Quantum Mechanics, the argument for the involvement of consciousness in the collapse of the wave function 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.[18]
This interpretation attributes the process of wave function collapse (directly, indirectly, or even partially) to consciousness itself. Specifically, a non-physical mind is postulated to be the only true measurement apparatus.[18]
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 unifed way provides a conceptually simple and logically coherent theoretical foundation...[27]
The consciousness causes collapse interpretation was Wigner's motivation for introducing the "Wigner's friend" thought experiment by asserting that collapse occurs at the first "conscious" observer. Wigner believed that consciousness is necessary for the quantum mechanical process. See Consciousness and measurement. There are other possible solutions to the Wigner's friend thought experiment, however, which do not require consciousness to be different from other physical processes. See, Consciousness and Superposition.
Very technically, the interpretation identifies 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. In other words, the idea is that creatures' minds somehow cause microscopic-scale probabilities to become reality.
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:
Fritjof Capra popularizes the subject with The Tao of Physics.[29] 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.
The view is also presented in various aspect of the New Thought Movement such as the popular and pseudoscientific 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 and Dan Brown's novel The Lost Symbol as well as playing a significant role in Charlie Stross's novel The Atrocity Archives.
To many scientists this interpretation fails a priori to compete with other interpretations of quantum mechanics because "consciousness causes collapse" relies upon a dualistic philosophy of mind (particularly interactionism), which contradicts the materialist monism presupposed by many physicists.[18] They point to the main philosophical argument against Cartesian dualism, namely the problem of how consciousness and matter might interact. Some physicists conclude that science's success at modeling the world materialistically—without reference to the existence of mental substances or mental properties—vindicates the idea that consciousness and subjectivity are wholly reducible to objective brain matter, and justifies the widespread belief among physical scientists in a strictly materialist account of metaphysics.[18] Wigner accuses other physicists of "exalting the problem" (of the study of physical law) by ignoring this possibility that minds are indeed special and immaterial components of the universe.[7] Some people have raised other issues with this interpretation of quantum mechanics.
Deepak Chopra, a supporter of some of the ideas of consciousness causes collapse, appeals to the work of an accomplished mathematical physicist Roger Penrose.[30] Penrose pursued various lines of argument to suggest that human consciousness cannot be explained by existing principles in physics.[31][32][33]
Consciousness causes collapse theory does not explain which things have sufficient consciousness to collapse the wave function. Hence there is an open question that this theory may need to test. 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.[18] 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!"[34]
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.
The hypothesis of consciousness causing collapse has been criticized also on the basis of empirical evidence. Yu and Nikolić claim that the existing empirical evidence can be used to falsify the predictions derived from the collapse-by-consciousness hypothesis.[35]
Quantum mechanics has provided a great deal of data that is not well understood (e.g., the phenomenon of quantum entanglement) and so there are still very many interpretations in competition with each other. Many of the alternative interpretations do not postulate that measurement in quantum mechanics requires consciousness in order to cause collapse (i.e., even a Geiger counter would cause wave function collapse[36]). In fact, the many worlds interpretation avoids the postulate that collapse occurs at all. In this interpretation, observation of a system results in a superposition, each element of which sees a (different) single outcome to the experiment. See Wigner's friend in Many Worlds. According to objective collapse theories, wave function collapse occurs when a superposed system reaches a certain objective threshold of size, complexity etc. Thus, wave function collapse occurs in the absence of observers, and the fact that the observers see an un-superposed classical world is a side-effect of the fact that they are themselves macroscopic.