The Many-minds interpretation of quantum mechanics extends the many-worlds interpretation by proposing that the distinction between worlds should be made at the level of the mind of an individual observer. The concept was first introduced in 1970 by H. Dieter Zeh as a variant of the Hugh Everett interpretation in connection with quantum decoherence, and later (in 1981) explicitly called a many or multi-consciousness interpretation.The name many-minds interpretation was first used by David Albert and Barry Loewer in 1988.
- 1 History
- 2 Quantum Non-Locality in the Many Minds Interpretation
- 3 Support
- 4 Objections
- 5 See also
- 6 References
- 7 External links
Interpretations of Quantum Mechanics
The various interpretations of quantum mechanics typically involve explaining the mathematical formalism of quantum mechanics, or to create a physical picture of the theory. While the mathematical structure has a strong foundation, there is still much debate about the physical and philosophical interpretation of the theory. These interpretations aim to tackle various concepts such as:
- Evolution of the state of a quantum system (given by the wavefunction), typically through the use of the Schrödinger equation. This concept is almost universally accepted, and is rarely put up to debate.
- The measurement problem, which relates to what we call wavefunction collapse – the collapse of a quantum state into a definite measurement (i.e. a specific eigenstate of the wavefunction). The debate on whether this collapse actually occurs is a central problem in interpreting quantum mechanics.
The standard solution to the measurement problem is the "Orthodox" or "Cophenhagen" interpretation, which claims that the wave function collapses as the result of a measurement by an observer or apparatus external to the quantum system. An alternative interpretation, the Many-worlds Interpretation, was first described by Hugh Everett in 1957  (where it was called the relative state interpretation, the name Many-worlds was coined by Bryce Seligman DeWitt starting in the 1960s and finalized in the 70s). His formalism of quantum mechanics denied that a measurement requires a wave collapse, instead suggesting that all that is truly necessary of a measurement is that a quantum connection is formed between the particle, the measuring device, and the observer.
The Many-worlds interpretation
In the original relative state formulation, Everett proposed that there is one universal wavefunction that describes the objective reality of the whole universe. He stated that when multiple subsystems interact, the total system becomes a superposition of these subsystems. This includes observers and measurement systems, which become part of one universal state (the wavefunction) that is always described via the Schrödinger Equation (or its relativistic alternative). That is, they become entangled such that one cannot define one without the other. Thus, each subsystems state can only be described relative to each subsystem with which it interacts (hence the name relative state).
This has some interesting implications. For starters, Everett suggested that the universe is actually indeterminate as a whole. To see this, consider an observer measuring the spin of some particle, which previously is in an undetermined state (a superposition of possible spins). When you measure the spin, you may determine it to be up or down. But in fact, there has been a “splitting” of the universe into two states, one where you measure up, and the other where you measure down. Thus, the indeterminacy of the system simply grows and you become a part of it.
The “worlds” in the many worlds theory is then just the complete measurement history up until and during the measurement in question, where splitting happens. These “worlds” each describe a different state of the universal wave function and cannot communicate. There is no collapse of the wavefunction into one state or another, but rather you just find yourself in the world leading up to what measurement you have made and are unaware of the other possibilities that are equally real.
The Many-minds Interpretation
The many-minds interpretation of quantum theory is many-worlds with the distinction between worlds constructed at the level of the individual observer. Rather than the worlds that branch, it is the observer’s mind.
The purpose of this interpretation is to overcome the fundamentally strange concept of observers being in a superposition with themselves. In their 1988 paper, Albert and Loewer argue that it simply makes no sense for one to think of the mind of an observer to be in a indefinite state. Rather, when someone answers the question about which state of a system they have observed, they must answer with complete certainty. If they are in a superposition of states, then this certainty is not possible and we arrive at a contradiction. To overcome this, they then suggest that it is merely the “bodies” of the minds that are in a superposition, and that the minds must have definite states that are never in superposition
When an observer measures a quantum system and becomes entangled with it, it now constitutes a larger quantum system. In regards to each possibility within the wave function, a mental state of the brain corresponds. And ultimately, only one mind is experienced, leading the others to branch off and become inaccessible, albeit real. In this way, every sentient being is attributed with an infinity of minds, whose prevalence correspond to the amplitude of the wavefunction. As an observer checks a measurement, the probability of realizing a specific measurement directly correlates to the number of minds they have where they see that measurement. It is in this way that the probabilistic nature of quantum measurements are obtained by the Many-minds Interpretation.
Quantum Non-Locality in the Many Minds Interpretation
Now, consider an experiment where we are measuring the polarization of two photons. When the photon is created it has an indeterminate polarization. If a stream of these photons is passed through a polarization filter, 50% of the light is passed through. This corresponds to each photon having a 50% chance of aligning perfectly with the filter and thus passing, or being misaligned (by 90 degrees relative to the polarization filter) and being absorbed. Quantum mechanically, this means the photon is in a superposition of states where it is either passed or observed. Now, consider the inclusion of another photon and polarization detector. Now, the photons are created in such a way that they are entangled. That is, when one photon takes on a polarization state, the other photon will always behave if it has the same polarization. For simplicity, take the second filter to either be perfectly aligned with the first, or to be perfectly misaligned ( 90 degree difference in angle, such that it is absorbed). If the detectors are aligned, both photons are passed (i.e. we say they agree). If they are misaligned, only the first passes and the second is absorbed (now they disagree). Thus, the entanglement causes perfect correlations between the two measurements - regardless of separation distance, making the interaction non-local. This sort of experiment is further explained in Tim Maudlin's Quantum Non-Locality and Relativity, and can be related to Bell test experiments. Now, consider the analysis of this experiment from the many minds point of view:
No Sentient Observer
Consider the case where there is no sentient observer, i.e. no mind around to observe the experiment. In this case, the detector will be in an indefinite state. The photon is both passed and absorbed, and will remain in this state. The correlations are withheld in that none of the possible "minds", or wave function states, correspond to non correlated results.
One Sentient Observer
Now expand the situation to have one sentient being observing the device. Now, they too enter the indefinite state. Their eyes, body, and brain are seeing both spins at the same time. The mind however, stochastically chooses one of the directions, and that is what the mind sees. When this observer goes over to the second detector, their body will see both results. Their mind will choose the result that agrees with the first detector, and the observer will see the expected results. However, the observers mind seeing one result does not directly affect the distant state - there is just no wave function in which the expected correlations do not exist. The true correlation only happens when they actually go over to the second detector.
Two Sentient Observers
When two people look at two different detectors that scan entangled particles, both observers will enter an indefinite state, as with one observer. These results need not agree – the second observer's mind does not have to have results that correlate with the first's. When one observer tells the results to the second observer, their two minds cannot communicate and thus will only interact with the other's body, which is still indefinite. When the second observer responds, its body will respond with whatever result agrees with the first observer's mind. This means that both observer's minds will be in a state of the wavefunction that always get the expected results, but individually their results could be different.
Locality of the Many-Minds Interpretation
As we have thus seen, any correlations seen in the wavefunction of each observer's minds are only concrete after interaction between the different polarizers. Even though the correlations on the level of individual minds correspond to the appearance of non-locality (or equivalently, violation of Bell's inequality). However, since the interactions only take place in individual minds they are local, since there is no real interaction between space-like separated events that could influence the minds of observers at two distant points. This, like the many worlds theory, makes the many-minds theory completely local.
There is currently no empirical evidence for the many-minds interpretation. However, there are theoretical theories that do not discredit the many-minds interpretation. In light of Bell’s analysis of the consequences of quantum non-locality, empirical evidence is needed to avoid inventing novel fundamental concepts (hidden variables). Two different solutions of the measurement problem then appear conceivable: von Neumann’s collapse or Everett’s relative state interpretation. In both cases a (suitably modiﬁed) psycho-physical parallelism can be re-established.
Since conscious awareness has to be coupled with local physical systems, the observer’s physical environment has to interact and can influence the brain. The brain itself must have some physico-chemical processes that affect the states of awareness. If these neural processes can be described and analyzed then some experiments could potentially be created to test whether affecting neural processes can have an effect on a quantum system. Speculation about the details of this awareness-local physical system coupling on a purely theoretical basis could occur, however experimentally searching for them through neurological and psychological studies would be ideal.
When considering psycho-physical parallelism, superpositions appear rich enough to represent primitive conscious awareness. It seems that quantum superpositions have never been considered, for example, in neuronal models, since only classical states of deﬁnite neuronal excitation are usually taken into account. These quasi-classical states are also measured by external neurobiologists. Quantum theory would admit their superpositions, too, thus giving rise to a far greater variety of physical states which may be experienced by the subjective observer. When used for information processing, such superpositions would now be called “quantumbits” or qubits. As demonstrated by M. Tegmark, they can not be relevant for neuronal and similar processes in the brain.
Objections that apply to the Many-worlds Interpretation also apply to the Many-minds Interpretation. On the surface both of these theories arguably violate Occam's Razor; proponents counter that in fact these solutions minimize entities by simplifying the rules that would be required to describe the universe.
Nothing within quantum theory itself requires each possibility within a wave function to complement a mental state. As all physical states (i.e. brain states) are quantum states, their associated mental states should be also. Nonetheless, it is not what we experience within physical reality. Albert and Loewer argue that the mind must be intrinsically different than the physical reality as described by quantum theory. Thereby, they reject type-identity physicalism in favour of a non-reductive stance. However, Lockwood saves materialism through the notion of supervenience of the mental on the physical.
Nonetheless, the Many-minds Interpretation does not solve the mindless hulks problem as a problem of supervenience. Mental states do not supervene on brain states as a given brain state is compatible with different configurations of mental states.
Another serious objection is that workers in No Collapse interpretations have produced no more than elementary models based on the definite existence of specific measuring devices. They have assumed, for example, that the Hilbert space of the universe splits naturally into a tensor product structure compatible with the measurement under consideration. They have also assumed, even when describing the behaviour of macroscopic objects, that it is appropriate to employ models in which only a few dimensions of Hilbert space are used to describe all the relevant behaviour.
Furthermore, as the Many-minds Interpretation is corroborated by our experience of physical reality, a notion of many unseen worlds and its compatibility with other physical theories (i.e. the principle of the conservation of mass) is difficult to reconcile. According to Schrödinger's equation, the mass-energy of the combined observed system and measurement apparatus is the same before and after. However, with every measurement process (i.e. splitting), the total mass-energy would seemingly increase 
Peter J. Lewis argues that the Many-minds Interpretation of quantum mechanics has absurd implications for agents facing life-or-death decisions.
In general, the Many-minds theory holds that a conscious being who observes the outcome of a random zero-sum experiment will evolve into two successors in different observer states, each of whom observes one of the possible outcomes. Moreover, the theory advises you to favour choices in such situations in proportion to the probability that they will bring good results to your various successors. But in a life-or-death case like getting into the box with Schrödinger's cat, you will only have one successor, since one of the outcomes will ensure your death. So it seems that the Many-minds Interpretation advises you to get in the box with the cat, since it is certain that your only successor will emerge unharmed. See also quantum suicide and immortality.
Finally, it supposes that there is some physical distinction between a conscious observer and a non-conscious measuring device, so it seems to require eliminating the strong Church–Turing hypothesis or postulating a physical model for consciousness.
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- Albert, David; Loewer, Barry (1988-01-01). "Interpreting the Many-Worlds Interpretation". Synthese. 77 (November): 195–213.
- Everett, Hugh (1957-07-01). ""Relative State" Formulation of Quantum Mechanics". Reviews of Modern Physics. 29 (3): 454–462. Bibcode:1957RvMP...29..454E. doi:10.1103/RevModPhys.29.454.
- Everett, Hugh (1973-01-01). DeWitt, B.; Graham, N., eds. The Theory of the Universal Wavefunction. Princeton UP.
- Dewitt, Bryce S. (1973-01-01). "Quantum Mechanics and Reality": 155. Bibcode:1973mwiq.conf..155D.
- Wendt, Alexander (2015-04-23). Quantum Mind and Social Science. Cambridge University Press. ISBN 9781107082540.
- Lockwood, Michael (1996-01-01). "Many-Minds Interpretations of Quantum Mechanics". British Journal for the Philosophy of Science. 47 (2): 159–88. doi:10.1093/bjps/47.2.159.
- Maudlin, Tim (2011-05-06). Quantum Non-Locality and Relativity: Metaphysical Intimations of Modern Physics. John Wiley & Sons. ISBN 9781444331264.
- Bell, John (1964). "On the Einstein Podolsky Rosen Paradox" (PDF). Physics. 1 (3): 195–200.
- Zeh, H. D. (1999-08-28). "The Problem of Conscious Observation in Quantum Mechanical Description". arXiv: .
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- Tegmark, Max (2000-04-01). "The importance of quantum decoherence in brain processes". Physical Review E. 61 (4): 4194–4206. doi:10.1103/PhysRevE.61.4194. ISSN 1063-651X.
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- "Locality and Mentality in Everett Interpretations: Albert and Loewer's Many Minds". www.academia.edu. Retrieved 2016-03-14.
- Lewis, Peter J. (2000-01-01). "What Is It like to Be Schrödinger's Cat?". Analysis. 60 (1): 22–29. doi:10.1093/analys/60.1.22. JSTOR 3329285.