Von Neumann–Wigner interpretation
The von Neumann–Wigner interpretation, also described as "consciousness causes collapse [of the wave function]", is an interpretation of quantum mechanics in which consciousness is postulated to be necessary for the completion of the process of quantum measurement.
Background: Observation in quantum mechanics
In the orthodox Copenhagen interpretation, quantum mechanics predicts only 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 upon observation is completely different than its usual behavior: the wavefunction that describes a system spreads out into an ever larger superposition of different possible situations. However, 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.
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. The observer will have a wavefunction which describes all the possible outcomes. Still, in actual experience, an observer never senses a superposition, but always senses 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).
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. In 1939, Fritz London and Edmond 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. The non-physical mind is postulated to be the only true measurement apparatus. Rudolf Peierls was also a proponent of this interpretation.
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...
Objections to the interpretation
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 everyday life (specifically, he rejected his initial idea of treating macroscopic objects as isolated systems). See Consciousness and Superposition.
To many scientists this interpretation fails to compete with other interpretations of quantum mechanics because "consciousness causes collapse" relies upon an interactionist form of dualism that is inconsistent with the materialism 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.
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… 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 Irreducibility of Consciousness" in The Conscious Mind: In Search of a Fundamental Theory
The interpretation has also been criticized for not explaining which things have sufficient consciousness to collapse the wave function. Also, 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". For example, Roger Penrose remarked, "[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 panpsychism and panexperientialism). To most physicists, this merely pushes the problem back, which some see as a fatal unparsimonious move in a competition with other theories.
Recently, it has been argued that the results of delayed choice quantum eraser experiments preclude this interpretation. However the experiment has also been used to argue for support of this interpretation.  Other researchers have expressed similar objections to the introduction of any subjective element in the collapse of the wavefunction.
A poll was conducted at a quantum mechanics conference in 2011 using 33 participants (including physicists, mathematicians, and philosophers). Researchers found that 6% of participants (2 of the 33) indicated that they believed the observer "plays a distinguished physical role (e.g., wave-function collapse by consciousness)". They also mention that "Popular accounts have sometimes suggested that the Copenhagen interpretation attributes such a role to consciousness. In our view, this is to misunderstand the Copenhagen interpretation."
Views of the pioneers of quantum mechanics
Many of the originators of quantum mechanical theory held that humans can effectively interrogate nature through interacting with it, and that in this regard quantum mechanics is not different from classical mechanics. In addition, Werner Heisenberg maintained that wave function collapse, "The discontinuous change in the probability function", takes place when the result of a measurement is registered in the mind of an observer. Albert Einstein, who believed in realism, and did not accept the theoretical completeness of quantum mechanics, similarly appealed for the merely epistemic conception of the wave function:
“[I advocate] that one conceives of the psi-function [i.e., wavefunction] only as an incomplete description of a real state of affairs, where the incompleteness of the description is forced by the fact that observation of the state is only able to grasp part of the real factual situation. Then one can at least escape the singular conception that observation (conceived as an act of consciousness) influences the real physical state of things; the change in the psi-function through observation then does not correspond essentially to the change in a real matter of fact but rather to the alteration in our knowledge of this matter of fact.”
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. 
- Interpretation of quantum mechanics
- Measurement in quantum mechanics
- Quantum mind
- Quantum Zeno effect
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- PHYSICS TODAY: "Is the moon there when nobody looks? Reality and the quantum theory" (pdf)
- "Quantum Cosmology and the Hard Problem of the Conscious Brain" (pdf)
- Mindful Sensationalism: A Quantum Framework for Consciousness.
- Brian Josephson on QM and consciousness
- Quantum Enigma from Oxford University Press
- "Critique of Quantum Enigma Physics encounters Consciousness", by Michael Nauenberg