The quantum-mechanical "Schrödinger's cat" paradox according to the many-worlds interpretation. In this interpretation, every quantum event is a branch point; the cat is both alive and dead, even before the box is opened, but the "alive" and "dead" cats are in different branches of the multiverse, both of which are equally real, but which do not interact with each other.[a]

The many-worlds interpretation (MWI) is an interpretation of quantum mechanics that asserts that the universal wavefunction is objectively real, and that there is no wave function collapse.[2] This implies that all possible outcomes of quantum measurements are physically realized in some "world" or universe.[3] In contrast to some other interpretations, such as the Copenhagen interpretation, the evolution of reality as a whole in MWI is rigidly deterministic.[2]: 8–9  Many-worlds is also called the relative state formulation or the Everett interpretation, after physicist Hugh Everett, who first proposed it in 1957.[4][5] Bryce DeWitt popularized the formulation and named it many-worlds in the 1970s.[1][2][6][7]

In many-worlds, the subjective appearance of wavefunction collapse is explained by the mechanism of quantum decoherence. Decoherence approaches to interpreting quantum theory have been widely explored and developed since the 1970s,[8][9][10] and have become quite popular. MWI is now considered a mainstream interpretation along with the other decoherence interpretations, collapse theories (including the Copenhagen interpretation), and hidden variable theories such as Bohmian mechanics.

The many-worlds interpretation implies that there are most likely an uncountably infinite number of universes.[11] It is one of many multiverse hypotheses in physics and philosophy. MWI views time as a many-branched tree, wherein every possible quantum outcome is realised. This is intended to resolve some paradoxes of quantum theory, such as the EPR paradox[5]: 462 [2]: 118  and Schrödinger's cat,[1] since every possible outcome of a quantum event exists in its own universe.

## Overview of the interpretation

The key idea of the many-worlds interpretation is that unitary quantum mechanics describes the whole universe. In particular, it describes a measurement as a unitary transformation, a correlation-inducing interaction, without using a collapse postulate, and describes observers as ordinary quantum-mechanical systems.[12]: 35–38  This stands in sharp contrast to the Copenhagen interpretation, in which a measurement is a "primitive" concept, not describable by unitary quantum mechanics; in Copenhagen the universe is divided into a quantum and a classical domain, and the collapse postulate is central.[12]: 29–30  MWI's main conclusion is that the universe (or multiverse in this context) is composed of a quantum superposition of an infinite[11] or undefinable[13]: 14–17  amount or number of increasingly divergent, non-communicating parallel universes or quantum worlds.[2] Sometimes dubbed Everett worlds,[2]: 234  each is a consistent and actualized alternative history or timeline.

The many-worlds interpretation makes use of decoherence to explain the measurement process and the emergence of a quasi-classical world.[13][14] Wojciech H. Zurek, one of decoherence theory's pioneers, stated: "Under scrutiny of the environment, only pointer states remain unchanged. Other states decohere into mixtures of stable pointer states that can persist, and, in this sense, exist: They are einselected."[15] Zurek emphasizes that his work does not depend on a particular interpretation.[b]

The many-worlds interpretation shares many similarities with the decoherent histories interpretation, which also uses decoherence to explain the process of measurement or wavefunction collapse.[14]: 9–11  MWI treats the other histories or worlds as real since it regards the universal wavefunction as the "basic physical entity"[5]: 455  or "the fundamental entity, obeying at all times a deterministic wave equation".[4]: 115  Decoherent histories, on the other hand, needs only one of the histories (or worlds) to be real.[14]: 10

Several authors, including Wheeler, Everett and Deutsch, call many-worlds a theory or metatheory, rather than just an interpretation.[11][16]: 328  Everett argued that it was the "only completely coherent approach to explaining both the contents of quantum mechanics and the appearance of the world."[17] Deutsch dismissed the idea that many-worlds is an "interpretation", saying that to call it an interpretation "is like talking about dinosaurs as an 'interpretation' of fossil records."[18]

### Formulation

In his 1957 doctoral dissertation, Everett proposed that, rather than relying on external observation for analysis of isolated quantum systems, one could mathematically model an object, as well as its observers, as purely physical systems within the mathematical framework developed by Paul Dirac, John von Neumann and others, discarding altogether the ad hoc mechanism of wave function collapse.[4][2]

### Relative state

Everett's original work introduced the concept of a relative state. Two (or more) subsystems, after a general interaction, become entangled. Everett noted that such entangled systems can be expressed as the sum of products of states, where the two or more subsystems are each in a state relative to each other. After a measurement or observation one of the pair (or triple) is the measured, object or observed system, and the other member is the measuring apparatus (which may include an observer) having recorded the state of the measured system.

In the example of Schrödinger's cat, after the box is opened, the entangled system is the cat, the poison vial and the observer. One relative triple of states would be the alive cat, the unbroken vial and the observer seeing an alive cat. Another relative triple of states would be the dead cat, the broken vial and the observer seeing a dead cat.

The process of measurement or observation splits the system up into sets of relative states, where each set of relative states, forming a branch of the universal wavefunction, is consistent within itself, and all future measurements (including by multiple observers) will confirm this consistency.

The many-worlds interpretation is DeWitt's popularisation of Everett, who had referred to the combined observer–object system as split by an observation, each split corresponding to the different or multiple possible outcomes of an observation. These splits generate a branching tree, where each branch is a set of all the states relative to each other. Subsequently, DeWitt introduced the term "world" to describe a single branch of that tree, which is a consistent history. All observations or measurements in any branch are consistent with each other.[4][2]

Under the many-worlds interpretation, the Schrödinger equation, or relativistic analog, holds all the time everywhere. An observation or measurement is modeled by applying the wave equation to the entire system, comprising the observer and the object. One consequence is that every observation can be thought of as causing the combined observer–object's wavefunction to change into a quantum superposition of two or more non-interacting branches, or split into many "worlds". Since many observation-like events have happened and are constantly happening, there are an enormous and growing number of simultaneously existing states.

If a system is composed of two or more subsystems, the system's state will be a superposition of products of the subsystems' states. Each product of subsystem states in the overall superposition evolves over time independently of other products. Once the subsystems interact, their states have become correlated or entangled and can no longer be considered independent. In Everett's terminology, each subsystem state was now correlated with its relative state, since each subsystem must now be considered relative to the other subsystems with which it has interacted.

### Properties

MWI removes the observer-dependent role in the quantum measurement process by replacing wavefunction collapse with quantum decoherence.[citation needed] Since the observer's role lies at the heart of most if not all "quantum paradoxes", this automatically resolves a number of problems, such as Schrödinger's cat thought experiment, the EPR paradox, von Neumann's "boundary problem", and others.[5]

Since the Copenhagen interpretation requires the existence of a classical domain beyond the one described by quantum mechanics, it has been criticized as inadequate for the study of cosmology.[19] MWI was developed with the explicit goal of allowing quantum mechanics to be applied to the universe as a whole, making quantum cosmology possible.[5]

MWI is a realist, deterministic and local theory. It achieves this by removing wave function collapse, which is indeterministic and nonlocal, from the deterministic and local equations of quantum theory.[20]

MWI (like other, broader multiverse theories) provides a context for the anthropic principle, which may provide an explanation for the fine-tuned universe.[21][22]

MWI depends crucially on the linearity of quantum mechanics. If the final theory of everything is non-linear with respect to wavefunctions, then many-worlds is invalid.[1][2][5][6][7] While quantum gravity or string theory may be non-linear in this respect,[23] there is as yet no evidence of this.[24][25]

### Interpreting wavefunction collapse

This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.Find sources: "Many-worlds interpretation" – news · newspapers · books · scholar · JSTOR (February 2020) (Learn how and when to remove this template message)

As with the other interpretations of quantum mechanics, the many-worlds interpretation is motivated by behavior that can be illustrated by the double-slit experiment. When particles of light (or anything else) pass through the double slit, a calculation assuming wavelike behavior of light can be used to identify where the particles are likely to be observed. Yet when the particles are observed in this experiment, they appear as particles (i.e., at definite places) and not as non-localized waves.

Some versions of the Copenhagen interpretation of quantum mechanics proposed a process of "collapse" in which an indeterminate quantum system would probabilistically collapse down onto, or select, just one determinate outcome to "explain" this phenomenon of observation. Wavefunction collapse was widely regarded as artificial and ad hoc,[26] so an alternative interpretation in which the behavior of measurement could be understood from more fundamental physical principles was considered desirable.

Everett's Ph.D. work provided such an interpretation. He argued that for a composite system—such as a subject (the "observer" or measuring apparatus) observing an object (the "observed" system, such as a particle)—the claim that either the observer or the observed has a well-defined state is meaningless; in modern parlance, the observer and the observed have become entangled: we can only specify the state of one relative to the other, i.e., the state of the observer and the observed are correlated after the observation is made. This led Everett to derive from the unitary, deterministic dynamics alone (i.e., without assuming wavefunction collapse) the notion of a relativity of states.

Everett noticed that the unitary, deterministic dynamics alone entailed that after an observation is made each element of the quantum superposition of the combined subject–object wavefunction contains two "relative states": a "collapsed" object state and an associated observer who has observed the same collapsed outcome; what the observer sees and the state of the object have become correlated by the act of measurement or observation. The subsequent evolution of each pair of relative subject–object states proceeds with complete indifference as to the presence or absence of the other elements, as if wavefunction collapse has occurred, which has the consequence that later observations are always consistent with the earlier observations. Thus the appearance of the object's wavefunction's collapse has emerged from the unitary, deterministic theory itself. (This answered Einstein's early criticism of quantum theory, that the theory should define what is observed, not for the observables to define the theory.[c]) Since the wavefunction merely appears to have collapsed then, Everett reasoned, there was no need to actually assume that it had collapsed. And so, invoking Occam's razor, he removed the postulate of wavefunction collapse from the theory.

### Testability

In 1985, David Deutsch proposed a variant of the Wigner's friend thought experiment as a test of many-worlds versus the Copenhagen interpretation.[28] It consists of an experimenter (Wigner's friend) making a measurement on a quantum system in an isolated laboratory, and another experimenter (Wigner) who would make a measurement on the first one. According to the many-worlds theory, the first experimenter would end up in a macroscopic superposition of seeing one result of the measurement in one branch, and another result in another branch. The second experimenter could then interfere these two branches in order to test whether it is in fact in a macroscopic superposition or has collapsed into a single branch, as predicted by the Copenhagen interpretation. Since then Lockwood (1989), Vaidman and others have made similar proposals.[29] These proposals require placing macroscopic objects in a coherent superposition and interfering them, a task now beyond experimental capability.

## Probability and the Born rule

Since the many-worlds interpretation's inception, physicists have been puzzled about the role of probability in it. As put by Wallace, there are two facets to the question:[30] the incoherence problem, which asks why we should assign probabilities at all to outcomes that are certain to occur in some worlds, and the quantitative problem, which asks why the probabilities should be given by the Born rule.

Everett tried to answer these questions in the paper that introduced many-worlds. To address the incoherence problem, he argued that an observer who makes a sequence of measurements on a quantum system will in general have an apparently random sequence of results in their memory, which justifies the use of probabilities to describe the measurement process.[4]: 69–70  To address the quantitative problem, Everett proposed a derivation of the Born rule based on the properties that a measure on the branches of the wavefunction should have.[4]: 70–72  His derivation has been criticized as relying on unmotivated assumptions.[31] Since then several other derivations of the Born rule in the many-worlds framework have been proposed. There is no consensus on whether this has been successful.[32][33][34]

### Frequentism

DeWitt and Graham[2] and Farhi et al.,[35] among others, have proposed derivations of the Born rule based on a frequentist interpretation of probability. They try to show that in the limit of infinitely many measurements no worlds would have relative frequencies that didn't match the probabilities given by the Born rule, but these derivations have been shown to be mathematically incorrect.[36][37]

### Decision theory

A decision-theoretic derivation of the Born rule was produced by David Deutsch (1999)[38] and refined by Wallace (2002–2009)[30][39][40][41] and Saunders (2004).[42][43] They consider an agent who takes part in a quantum gamble: the agent makes a measurement on a quantum system, branches as a consequence, and each of the agent's future selves receives a reward that depends on the measurement result. The agent uses decision theory to evaluate the price they would pay to take part in such a gamble, and concludes that the price is given by the utility of the rewards weighted according to the Born rule. Some reviews have been positive, although these arguments remain highly controversial; some theoretical physicists have taken them as supporting the case for parallel universes.[44] For example, a New Scientist story on a 2007 conference about Everettian interpretations[45] quoted physicist Andy Albrecht as saying, "This work will go down as one of the most important developments in the history of science."[44] In contrast, the philosopher Huw Price, also attending the conference, found the Deutsch–Wallace–Saunders approach fundamentally flawed.[46]

### Symmetries and invariance

Zurek (2005)[47] has produced a derivation of the Born rule based on the symmetries of entangled states; Schlosshauer and Fine argue that Zurek's derivation is not rigorous, as it does not define what probability is and has several unstated assumptions about how it should behave.[48]

Charles Sebens and Sean M. Carroll, building on work by Lev Vaidman,[49] proposed a similar approach based on self-locating uncertainty.[50] In this approach, decoherence creates multiple identical copies of observers, who can assign credences to being on different branches using the Born rule. The Sebens–Carroll approach has been criticized by Adrian Kent,[51] and Vaidman himself does not find it satisfactory.[52]

## The preferred basis problem

As originally formulated by Everett and DeWitt, the many-worlds interpretation had a privileged role for measurements: they determined which basis of a quantum system would give rise to the eponymous worlds. Without this the theory was ambiguous, as a quantum state can equally well be described (e.g.) as having a well-defined position or as being a superposition of two delocalised states. The assumption that the preferred basis to use is the one from a measurement of position results in worlds having objects in well-defined positions, instead of worlds with delocalised objects (which would be grossly incompatible with experiment). This special role for measurements is problematic for the theory, as it contradicts Everett and DeWitt's goal of having a reductionist theory and undermines their criticism of the ill-defined measurement postulate of the Copenhagen interpretation.[16][31] This is known today as the preferred basis problem.

The preferred basis problem has been solved, according to Saunders and Wallace, among others,[14] by incorporating decoherence into the many-worlds theory.[19][53][54][55] In this approach, the preferred basis does not have to be postulated, but rather is identified as the basis stable under environmental decoherence. In this way measurements no longer play a special role; rather, any interaction that causes decoherence causes the world to split. Since decoherence is never complete, there will always remain some infinitesimal overlap between two worlds, making it arbitrary whether a pair of worlds has split or not.[56] Wallace argues that this is not problematic: it only shows that worlds are not a part of the fundamental ontology, but rather of the emergent ontology, where these approximate, effective descriptions are routine in the physical sciences.[57][13] Since in this approach the worlds are derived, it follows that they must be present in any other interpretation of quantum mechanics that does not have a collapse mechanism, such as Bohmian mechanics.[58]

This approach to deriving the preferred basis has been criticized as creating a circularity with derivations of probability in the many-worlds interpretation, as decoherence theory depends on probability, and probability depends on the ontology derived from decoherence.[33][47][59] Wallace contends that decoherence theory depends not on probability but only on the notion that one is allowed to do approximations in physics.[12]: 253–254

## History

MWI originated in Everett's Princeton Ph.D. thesis "The Theory of the Universal Wavefunction",[2] developed under his thesis advisor John Archibald Wheeler, a shorter summary of which was published in 1957 under the title "Relative State Formulation of Quantum Mechanics" (Wheeler contributed the title "relative state";[60] Everett originally called his approach the "Correlation Interpretation", where "correlation" refers to quantum entanglement). The phrase "many-worlds" is due to Bryce DeWitt,[2] who was responsible for the wider popularisation of Everett's theory, which had been largely ignored for a decade after publication in 1957.[11]

Everett's proposal was not without precedent. In 1952, Erwin Schrödinger gave a lecture in Dublin in which at one point he jocularly warned his audience that what he was about to say might "seem lunatic". He went on to assert that while the Schrödinger equation seemed to be describing several different histories, they were "not alternatives but all really happen simultaneously". According to David Deutsch, this is the earliest known reference to many-worlds; Jeffrey A. Barrett describes it as indicating the similarity of "general views" between Everett and Schrödinger.[61][62][63] Schrödinger's writings from the period also contain elements resembling the modal interpretation originated by Bas van Fraassen. Because Schrödinger subscribed to a kind of post-Machian neutral monism, in which "matter" and "mind" are only different aspects or arrangements of the same common elements, treating the wavefunction as physical and treating it as information became interchangeable.[64]

## Reception

MWI's initial reception was overwhelmingly negative, in the sense that it was ignored, with the notable exception of DeWitt. Wheeler made considerable efforts to formulate the theory in a way that would be palatable to Bohr, visited Copenhagen in 1956 to discuss it with him, and convinced Everett to visit as well, which happened in 1959. Nevertheless, Bohr and his collaborators completely rejected the theory.[d] Everett left academia in 1956, never to return, and Wheeler eventually disavowed the theory.[11]

### Support

One of MWI's strongest advocates is David Deutsch.[65] According to Deutsch, the single photon interference pattern observed in the double slit experiment can be explained by interference of photons in multiple universes. Viewed this way, the single photon interference experiment is indistinguishable from the multiple photon interference experiment. In a more practical vein, in one of the earliest papers on quantum computing,[66] he suggested that parallelism that results from MWI could lead to "a method by which certain probabilistic tasks can be performed faster by a universal quantum computer than by any classical restriction of it". Deutsch has also proposed that MWI will be testable (at least against "naive" Copenhagenism) when reversible computers become conscious via the reversible observation of spin.[67]

### Equivocal

Philosophers of science James Ladyman and Don Ross say that the MWI could be true, but that they do not embrace it. They note that no quantum theory is yet empirically adequate for describing all of reality, given its lack of unification with general relativity, and so they do not see a reason to regard any interpretation of quantum mechanics as the final word in metaphysics. They also suggest that the multiple branches may be an artifact of incomplete descriptions and of using quantum mechanics to represent the states of macroscopic objects. They argue that macroscopic objects are significantly different from microscopic objects in not being isolated from the environment, and that using quantum formalism to describe them lacks explanatory and descriptive power and accuracy.[68]

Victor J. Stenger remarked that Murray Gell-Mann's published work explicitly rejects the existence of simultaneous parallel universes.[69] Collaborating with James Hartle, Gell-Mann worked toward the development a more "palatable" post-Everett quantum mechanics. Stenger thought it fair to say that most physicists find the MWI too extreme, while noting it "has merit in finding a place for the observer inside the system being analyzed and doing away with the troublesome notion of wave function collapse".[e]

Richard Feynman, described as an Everettian in some sources,[70] said of the MWI in 1982, "It's possible, but I'm not very happy with it."[71]

### Rejection

Some scientists consider MWI unfalsifiable and hence unscientific because the multiple parallel universes are non-communicating, in the sense that no information can be passed between them.[72][73] Others claim MWI is directly testable.[67]

Roger Penrose argues that the idea is flawed because it is based on an oversimple version of quantum mechanics that does not account for gravity. In his view, applying conventional quantum mechanics to the universe implies the MWI, but the lack of a successful theory of quantum gravity negates the claimed universality of conventional quantum mechanics.[23] According to Penrose, "the rules must change when gravity is involved". He further asserts that gravity helps anchor reality and "blurry" events have only one allowable outcome: "electrons, atoms, molecules, etc., are so minute that they require almost no amount of energy to maintain their gravity, and therefore their overlapping states. They can stay in that state forever, as described in standard quantum theory". On the other hand, "in the case of large objects, the duplicate states disappear in an instant due to the fact that these objects create a large gravitational field".[74][75]

Philosopher of science Robert P. Crease says that the MWI is "one of the most implausible and unrealistic ideas in the history of science" because it means that everything conceivable happens.[74] Science writer Philip Ball describes the MWI's implications as fantasies, since "beneath their apparel of scientific equations or symbolic logic, they are acts of imagination, of 'just supposing'".[74]

Theoretical physicist Gerard 't Hooft also dismisses the idea: "I do not believe that we have to live with the many-worlds interpretation. Indeed, it would be a stupendous number of parallel worlds, which are only there because physicists couldn't decide which of them is real."[76]

Asher Peres was an outspoken critic of MWI. A section of his 1993 textbook had the title Everett's interpretation and other bizarre theories. Peres argued that the various many-worlds interpretations merely shift the arbitrariness or vagueness of the collapse postulate to the question of when "worlds" can be regarded as separate, and that no objective criterion for that separation can actually be formulated.[77]

### Polls

A poll of 72 "leading quantum cosmologists and other quantum field theorists" conducted before 1991 by L. David Raub showed 58% agreement with "Yes, I think MWI is true".[70]

Max Tegmark reports the result of a "highly unscientific" poll taken at a 1997 quantum mechanics workshop. According to Tegmark, "The many worlds interpretation (MWI) scored second, comfortably ahead of the consistent histories and Bohm interpretations."[78]

In response to Sean M. Carroll's statement "As crazy as it sounds, most working physicists buy into the many-worlds theory",[79] Michael Nielsen counters: "at a quantum computing conference at Cambridge in 1998, a many-worlder surveyed the audience of approximately 200 people... Many-worlds did just fine, garnering support on a level comparable to, but somewhat below, Copenhagen and decoherence." But Nielsen notes that it seemed most attendees found it to be a waste of time: Peres "got a huge and sustained round of applause…when he got up at the end of the polling and asked 'And who here believes the laws of physics are decided by a democratic vote?'"[80]

A 2005 poll of fewer than 40 students and researchers taken after a course on the Interpretation of Quantum Mechanics at the Institute for Quantum Computing University of Waterloo found "Many Worlds (and decoherence)" to be the least favored.[81]

A 2011 poll of 33 participants at an Austrian conference found 6 endorsed MWI, 8 "Information-based/information-theoretical", and 14 Copenhagen;[82] the authors remark that MWI received a similar percentage of votes as in Tegmark's 1997 poll.[82]

## Debate whether the other worlds are real

Everett believed in the literal reality of the other quantum worlds.[18] His son reported that he "never wavered in his belief over his many-worlds theory".[83]

According to Martin Gardner, the "other" worlds of MWI have two different interpretations: real or unreal; he claimed that Stephen Hawking and Steven Weinberg both favour the unreal interpretation.[84] Gardner also claimed that most physicists favour the unreal interpretation, whereas the "realist" view is supported only by MWI experts such as Deutsch and DeWitt. Gardner reports Hawking saying that MWI is "trivially true".[84] In a 1983 interview, Hawking also said he regarded MWI as "self-evidently correct" but was dismissive of questions about the interpretation of quantum mechanics, saying, "When I hear of Schrödinger's cat, I reach for my gun." In the same interview, he also said, "But, look: All that one does, really, is to calculate conditional probabilities—in other words, the probability of A happening, given B. I think that that's all the many-worlds interpretation is. Some people overlay it with a lot of mysticism about the wave function splitting into different parts. But all that you're calculating is conditional probabilities."[85] Elsewhere Hawking contrasted his attitude towards the "reality" of physical theories with that of his colleague Roger Penrose, saying, "He's a Platonist and I'm a positivist. He's worried that Schrödinger's cat is in a quantum state, where it is half alive and half dead. He feels that can't correspond to reality. But that doesn't bother me. I don't demand that a theory correspond to reality because I don't know what it is. Reality is not a quality you can test with litmus paper. All I'm concerned with is that the theory should predict the results of measurements. Quantum theory does this very successfully."[86]

Gell-Mann described himself as a "post-Everett investigator"[87] and wrote, "it is not necessary to become queasy trying to conceive of many 'parallel universes,' all equally real". Instead, he advocated the language of "many histories, all treated alike by the theory except for their different probabilities."[88]

## Speculative implications

### Quantum suicide thought experiment

 Main article: Quantum suicide and immortality

Quantum suicide is a thought experiment in quantum mechanics and the philosophy of physics. Purportedly, it can distinguish between the Copenhagen interpretation of quantum mechanics and the many-worlds interpretation by means of a variation of the Schrödinger's cat thought experiment, from the cat's point of view. Quantum immortality refers to the subjective experience of surviving quantum suicide.[89]

Most experts believe that the experiment would not work in the real world, because the world with the surviving experimenter has a lower "measure" than the world before the experiment, making it less likely that the experimenter will experience their survival.[12]: 371 [29][90][91]

### Absurdly improbable timelines

DeWitt has stated that "[Everett, Wheeler and Graham] do not in the end exclude any element of the superposition. All the worlds are there, even those in which everything goes wrong and all the statistical laws break down."[92]

Max Tegmark has affirmed that absurd or highly unlikely events are inevitable but rare under MWI. To quote Tegmark, "Things inconsistent with the laws of physics will never happen—everything else will... it's important to keep track of the statistics, since even if everything conceivable happens somewhere, really freak events happen only exponentially rarely."[93]

Ladyman and Ross state that, in general, many of the unrealized possibilities that are discussed in other scientific fields will not have counterparts in other branches, because they are in fact incompatible with the universal wavefunction.[68]

## Notes

1. ^ "every quantum transition taking place on every star, in every galaxy, in every remote corner of the universe is splitting our local world on earth into myriads of copies of itself."[1]
2. ^ Relative states of Everett come to mind. One could speculate about reality of branches with other outcomes. We abstain from this; our discussion is interpretation-free, and this is a virtue.[15]
3. ^ "Whether you can observe a thing or not depends on the theory which you use. It is the theory which decides what can be observed."—Albert Einstein to Werner Heisenberg, objecting to placing observables at the heart of the new quantum mechanics, during Heisenberg's 1926 lecture at Berlin; related by Heisenberg in 1968.[27]
4. ^ Everett recounted his meeting with Bohr as "that was a hell... doomed from the beginning". Léon Rosenfeld, a close collaborator of Bohr, said "With regard to Everett neither I nor even Niels Bohr could have any patience with him, when he visited us in Copenhagen more than 12 years ago in order to sell the hopelessly wrong ideas he had been encouraged, most unwisely, by Wheeler to develop. He was undescribably stupid and could not understand the simplest things in quantum mechanics."[11]: 113
5. ^ Gell-Mann and Hartle, along with a score of others, have been working to develop a more palatable interpretation of quantum mechanics that is free of the problems that plague all the interpretations we have considered so far. This new interpretation is called, in its various incarnations, post-Everett quantum mechanics, alternate histories, consistent histories, or decoherent histories. I will not be overly concerned with the detailed differences between these characterizations and will use the terms more or less interchangeably.[69]: 176

## References

1. ^ a b c d Bryce S. DeWitt (1970). "Quantum mechanics and reality". Physics Today. 23 (9): 30–35. Bibcode:1970PhT....23i..30D. doi:10.1063/1.3022331. See also Leslie E. Ballentine; Philip Pearle; Evan Harris Walker; Mendel Sachs; Toyoki Koga; Joseph Gerver; Bryce DeWitt (1971). "Quantum‐mechanics debate". Physics Today. 24 (4): 36–44. Bibcode:1971PhT....24d..36.. doi:10.1063/1.3022676.
2. Everett, Hugh; Wheeler, J. A.; DeWitt, B. S.; Cooper, L. N.; Van Vechten, D.; Graham, N. (1973). DeWitt, Bryce; Graham, R. Neill (eds.). The Many-Worlds Interpretation of Quantum Mechanics. Princeton Series in Physics. Princeton, NJ: Princeton University Press. p. v. ISBN 0-691-08131-X.
3. ^ Tegmark, Max (1998). "The Interpretation of Quantum Mechanics: Many Worlds or Many Words?". Fortschritte der Physik. 46 (6–8): 855–862. arXiv:quant-ph/9709032. Bibcode:1998ForPh..46..855T. doi:10.1002/(SICI)1521-3978(199811)46:6/8<855::AID-PROP855>3.0.CO;2-Q.
4. Hugh Everett Theory of the Universal Wavefunction, Thesis, Princeton University, (1956, 1973), pp 1–140
5. Everett, Hugh (1957). "Relative State Formulation of Quantum Mechanics". Reviews of Modern Physics. 29 (3): 454–462. Bibcode:1957RvMP...29..454E. doi:10.1103/RevModPhys.29.454. Archived from the original on 2011-10-27. Retrieved 2011-10-24.
6. ^ a b Cecile M. DeWitt, John A. Wheeler eds, The Everett–Wheeler Interpretation of Quantum Mechanics, Battelle Rencontres: 1967 Lectures in Mathematics and Physics (1968)
7. ^ a b Bryce Seligman DeWitt, The Many-Universes Interpretation of Quantum Mechanics, Proceedings of the International School of Physics "Enrico Fermi" Course IL: Foundations of Quantum Mechanics, Academic Press (1972)
8. ^ H. Dieter Zeh, On the Interpretation of Measurement in Quantum Theory, Foundations of Physics, vol. 1, pp. 69–76, (1970).
9. ^ Wojciech Hubert Zurek, Decoherence and the transition from quantum to classical, Physics Today, vol. 44, issue 10, pp. 36–44, (1991).
10. ^ Wojciech Hubert Zurek, Decoherence, einselection, and the quantum origins of the classical, Reviews of Modern Physics, 75, pp 715–775, (2003)
11. Osnaghi, Stefano; Freitas, Fabio; Olival Freire, Jr (2009). "The Origin of the Everettian Heresy". Studies in History and Philosophy of Modern Physics. 40 (2): 97–123. Bibcode:2009SHPMP..40...97O. CiteSeerX 10.1.1.397.3933. doi:10.1016/j.shpsb.2008.10.002.
12. ^ a b c d Wallace, David (2012). The Emergent Multiverse: Quantum Theory According to the Everett Interpretation. Oxford University Press. ISBN 978-0-19-954696-1.
13. ^ a b c David Wallace (2010). "Decoherence and Ontology, or: How I Learned To Stop Worrying And Love FAPP". In S. Saunders; J. Barrett; A. Kent; D. Wallace (eds.). Many Worlds? Everett, Quantum Theory and Reality. Oxford University Press. arXiv:1111.2189.
14. ^ a b c d Saunders, Simon (2010). "Many Worlds? An Introduction". In S. Saunders; J. Barrett; A. Kent; D. Wallace (eds.). Many Worlds? Everett, Quantum Theory and Reality (PDF). Oxford University Press.
15. ^ a b Zurek, Wojciech (March 2009). "Quantum Darwinism". Nature Physics. 5 (3): 181–188. arXiv:0903.5082. Bibcode:2009NatPh...5..181Z. doi:10.1038/nphys1202. S2CID 119205282.
16. ^ a b Brian Skyrms (1976). "Possible Worlds, Physics and Metaphysics". Philosophical Studies. 30 (5): 323–332. doi:10.1007/BF00357930. S2CID 170852547.
17. ^ Letter from Everett to David Raub, 1980-04-07, UCI. Accessed 12 April 2020.
18. ^ a b Peter Byrne (2010). The Many Worlds of Hugh Everett III: Multiple Universes, Mutual Assured Destruction, and the Meltdown of a Nuclear Family. Oxford University Press. ISBN 978-0-19-955227-6.
19. ^ a b M Gell-Mann; J.B. Hartle (1990). "Quantum mechanics in the light of quantum cosmology". In W.H. Zurek (ed.). Complexity, Entropy, and the Physics of Information. Addison-Wesley. arXiv:1803.04605.
20. ^ Harvey R. Brown; Christopher G. Timpson (2016). "Bell on Bell's Theorem: The Changing Face of Nonlocality". In Mary Bell; Shan Gao (eds.). Quantum Nonlocality and Reality: 50 years of Bell's theorem. Cambridge University Press. pp. 91–123. arXiv:1501.03521. doi:10.1017/CBO9781316219393.008. ISBN 9781316219393. S2CID 118686956. On locality:"Amongst those who have taken Everett’s approach to quantum theory at all seriously as an option, it is a commonplace that—given an Everettian interpretation—quantum theory is (dynamically) local-there is no action-at-a-distance" on determinism:"But zooming-out (in a God’s-eye view) from a particular branch will be seen all the other branches, each with a different result of measurement being recorded and observed, all coexisting equally; and all underpinned by (supervenient on) the deterministically, unitarily, evolving universal wavefunction"
21. ^ Paul C.W. Davies, Other Worlds, chapters 8 & 9 The Anthropic Principle & Is the Universe an accident?, (1980) ISBN 0-460-04400-1
22. ^ Paul C.W. Davies, The Accidental Universe, (1982) ISBN 0-521-28692-1
23. ^ a b Penrose, Roger (August 1991). "Roger Penrose Looks Beyond the Classic-Quantum Dichotomy". Sciencewatch. Archived from the original on 2007-10-23. Retrieved 2007-10-21.
24. ^ Steven Weinberg, Dreams of a Final Theory: The Search for the Fundamental Laws of Nature (1993), ISBN 0-09-922391-0, pg 68–69
25. ^ Steven Weinberg Testing Quantum Mechanics, Annals of Physics Vol 194 #2 (1989), pg 336–386
26. ^ Wimmel Hermann. Quantum Physics And Observed Reality: A Critical Interpretation Of Quantum Mechanics, p.45, World Scientific, May 26, 1992
27. ^ Abdus Salam, Unification of Fundamental Forces, Cambridge University Press (1990) ISBN 0-521-37140-6, pp 98–101
28. ^ Deutsch, D. (1985). "Quantum theory as a universal physical theory". International Journal of Theoretical Physics. 24 (1): 1–41. Bibcode:1985IJTP...24....1D. doi:10.1007/BF00670071. S2CID 17530632.
29. ^ a b Vaidman, Lev (2018). Many-Worlds Interpretation of Quantum Mechanics. The Stanford Encyclopedia of Philosophy.
30. ^ a b Wallace, David (2003). "Everettian Rationality: defending Deutsch's approach to probability in the Everett interpretation". Stud. Hist. Phil. Mod. Phys. 34 (3): 415–438. arXiv:quant-ph/0303050. Bibcode:2003SHPMP..34..415W. doi:10.1016/S1355-2198(03)00036-4. S2CID 1921913.
31. ^ a b L. E. Ballentine (1973). "Can the statistical postulate of quantum theory be derived?—A critique of the many-universes interpretation". Foundations of Physics. 3 (2): 229–240. Bibcode:1973FoPh....3..229B. doi:10.1007/BF00708440. S2CID 121747282.
32. ^ N.P. Landsman, "The conclusion seems to be that no generally accepted derivation of the Born rule has been given to date, but this does not imply that such a derivation is impossible in principle.", in Compendium of Quantum Physics (eds.) F. Weinert, K. Hentschel, D.Greenberger and B. Falkenburg (Springer, 2008), ISBN 3-540-70622-4
33. ^ a b Kent, Adrian (2010). "One world versus many: The inadequacy of Everettian accounts of evolution, probability, and scientific confirmation". In S. Saunders; J. Barrett; A. Kent; D. Wallace (eds.). Many Worlds? Everett, Quantum Theory and Reality. Oxford University Press. arXiv:0905.0624. Bibcode:2009arXiv0905.0624K.
34. ^ Kent, Adrian (1990). "Against Many-Worlds Interpretations". Int. J. Mod. Phys. A. 5 (9): 1745–1762. arXiv:gr-qc/9703089. Bibcode:1990IJMPA...5.1745K. doi:10.1142/S0217751X90000805. S2CID 14523184.
35. ^ Edward Farhi; Jeffrey Goldstone; Sam Gutmann (1989). "How probability arises in quantum mechanics". Annals of Physics. 192 (2): 368–382. Bibcode:1989AnPhy.192..368F. doi:10.1016/0003-4916(89)90141-3.
36. ^ Benioff, Paul (October 1978). "A note on the Everett interpretation of quantum mechanics". Foundations of Physics. 8 (9–10): 709–720. Bibcode:1978FoPh....8..709B. doi:10.1007/BF00717501. ISSN 0015-9018. S2CID 123279967.
37. ^ Carlton M. Caves; Rüdiger Schack (2005). "Properties of the frequency operator do not imply the quantum probability postulate". Annals of Physics. 315 (1): 123–146. arXiv:quant-ph/0409144. Bibcode:2005AnPhy.315..123C. doi:10.1016/j.aop.2004.09.009. S2CID 33263618.
38. ^ Deutsch, David (1999). "Quantum Theory of Probability and Decisions". Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences. 455 (1988): 3129–3137. arXiv:quant-ph/9906015. Bibcode:1999RSPSA.455.3129D. doi:10.1098/rspa.1999.0443. S2CID 5217034.
39. ^ Wallace, David (2002). "Quantum Probability and Decision Theory, Revisited". arXiv:quant-ph/0211104.
40. ^ Wallace, David (2003). "Quantum Probability from Subjective Likelihood: Improving on Deutsch's proof of the probability rule". arXiv:quant-ph/0312157.
41. ^ Wallace, David (2009). "A formal proof of the Born rule from decision-theoretic assumptions". arXiv:0906.2718 [quant-ph].
42. ^ Saunders, Simon (2004). "Derivation of the Born rule from operational assumptions". Proc. Roy. Soc. Lond. A. 460 (2046): 1771–1788. arXiv:quant-ph/0211138. Bibcode:2004RSPSA.460.1771S. doi:10.1098/rspa.2003.1230. S2CID 1459183.
43. ^ Saunders, Simon (2004). "What is Probability?". Quo Vadis Quantum Mechanics?. The Frontiers Collection. pp. 209–238. arXiv:quant-ph/0412194. doi:10.1007/3-540-26669-0_12. ISBN 978-3-540-22188-3. S2CID 117218061.
44. ^ a b Merali, Zeeya (2007-09-21). "Parallel universes make quantum sense". New Scientist. No. 2622. Retrieved 2013-11-22. (Summary only).
45. ^
46. ^ Price, Huw (2010). "Decisions, Decisions, Decisions: Can Savage Salvage Everettian Probability?". In S. Saunders; J. Barrett; A. Kent; D. Wallace (eds.). Many Worlds? Everett, Quantum Theory and Reality. Oxford University Press. arXiv:0802.1390.
47. ^ a b Zurek, Wojciech H. (2005). "Probabilities from entanglement, Born's rule from envariance". Phys. Rev. A. 71 (5): 052105. arXiv:quant-ph/0405161. Bibcode:2005PhRvA..71e2105Z. doi:10.1103/physreva.71.052105. S2CID 18210481.
48. ^ Schlosshauer, M.; Fine, A. (2005). "On Zurek's derivation of the Born rule". Found. Phys. 35 (2): 197–213. arXiv:quant-ph/0312058. Bibcode:2005FoPh...35..197S. doi:10.1007/s10701-004-1941-6. S2CID 119100306.
49. ^ Vaidman, L. "Probability in the Many-Worlds Interpretation of Quantum Mechanics." In: Ben-Menahem, Y., & Hemmo, M. (eds), The Probable and the Improbable: Understanding Probability in Physics, Essays in Memory of Itamar Pitowsky. Springer.
50. ^ Sebens, Charles T; Carroll, Sean M (2016). "Self-Locating Uncertainty and the Origin of Probability in Everettian Quantum Mechanics". The British Journal for the Philosophy of Science. 69 (1): 25–74. arXiv:1405.7577. doi:10.1093/bjps/axw004. S2CID 53648469.
51. ^ Kent, Adrian (February 2015). "Does it Make Sense to Speak of Self-Locating Uncertainty in the Universal Wave Function? Remarks on Sebens and Carroll". Foundations of Physics. 45 (2): 211–217. arXiv:1408.1944. Bibcode:2015FoPh...45..211K. doi:10.1007/s10701-014-9862-5. ISSN 0015-9018. S2CID 118471198.
52. ^ Vaidman, Lev (2020). "Derivations of the Born Rule". In Meir Hemmo; Orly Shenker (eds.). Quantum, Probability, Logic: Itamar Pitowsky's Work and Influence. Springer Nature Switzerland. PhilSci:15943.
53. ^ Simon Saunders (1993). "Decoherence, relative states, and evolutionary adaptation". Foundations of Physics. 23 (12): 1553–1585. Bibcode:1993FoPh...23.1553S. doi:10.1007/BF00732365. S2CID 119754481.
54. ^ Simon Saunders (1995). "Time, quantum mechanics, and decoherence" (PDF). Synthese. 102 (2): 235–266. doi:10.1007/BF01089802. S2CID 14550985.
55. ^ James B. Hartle (2011). "The quasiclassical realms of this quantum universe". Foundations of Physics. 41 (6): 982–1006. arXiv:0806.3776. Bibcode:2011FoPh...41..982H. doi:10.1007/s10701-010-9460-0. S2CID 118469123.
56. ^ Stapp, Henry (2002). "The basis problem in many-world theories" (PDF). Canadian Journal of Physics. 80 (9): 1043–1052. arXiv:quant-ph/0110148. Bibcode:2002CaJPh..80.1043S. doi:10.1139/p02-068. S2CID 18634782.
57. ^ David Wallace (2003). "Everett and structure". Studies in History and Philosophy of Science. 34 (1): 87–105. arXiv:quant-ph/0107144. Bibcode:2003SHPMP..34...87W. doi:10.1016/S1355-2198(02)00085-0. S2CID 15222560.
58. ^ Brown, Harvey R; Wallace, David (2005). "Solving the measurement problem: de Broglie–Bohm loses out to Everett" (PDF). Foundations of Physics. 35 (4): 517–540. arXiv:quant-ph/0403094. Bibcode:2005FoPh...35..517B. doi:10.1007/s10701-004-2009-3. S2CID 412240.
59. ^ David J Baker (2007). "Measurement outcomes and probability in Everettian quantum mechanics" (PDF). Studies in History and Philosophy of Science. 38 (1): 153–169. Bibcode:2007SHPMP..38..153B. doi:10.1016/j.shpsb.2006.05.003.
60. ^ Wheeler, John Archibald (2000). Geons, Black Holes and Quantum Foam. W. W. Norton & Company. pp. 268–270. ISBN 0-393-31991-1.
61. ^ Deutsch, David (2010). "Apart from Universes". In S. Saunders; J. Barrett; A. Kent; D. Wallace (eds.). Many Worlds? Everett, Quantum Theory and Reality. Oxford University Press.
62. ^ Schrödinger, Erwin (1996). Bitbol, Michel (ed.). The Interpretation of Quantum Mechanics: Dublin Seminars (1949–1955) and other unpublished essays. OxBow Press.
63. ^ Barrett, Jeffrey A. (1999). The Quantum Mechanics of Minds and Worlds. Oxford University Press. ISBN 9780191583254.
64. ^ Bitbol, Michel (1996). Schrödinger's Philosophy of Quantum Mechanics. Dordrecht: Springer Netherlands. ISBN 978-94-009-1772-9. OCLC 851376153.
65. ^ David Deutsch, The Fabric of Reality: The Science of Parallel Universes And Its Implications, Penguin Books (1998), ISBN 0-14-027541-X
66. ^ Deutsch, David (1985). "Quantum theory, the Church–Turing principle and the universal quantum computer". Proceedings of the Royal Society of London A. 400 (1818): 97–117. Bibcode:1985RSPSA.400...97D. CiteSeerX 10.1.1.144.7936. doi:10.1098/rspa.1985.0070. S2CID 1438116.
67. ^ a b Paul C.W. Davies, J.R. Brown, The Ghost in the Atom (1986) ISBN 0-521-31316-3, pp. 34–38: "The Many-Universes Interpretation", pp 83–105 for David Deutsch's test of MWI and reversible quantum memories
68. ^ a b Ladyman, James; Ross, Don (2007). Every Thing Must Go: Metaphysics Naturalized. Clarendon Press. pp. 179–183. ISBN 978-0-19-927619-6.
69. ^ a b Stenger, V.J. (1995). The Unconscious Quantum: Metaphysics in Modern Physics and Cosmology. Prometheus Books. ISBN 978-1-57392-022-3. LCCN lc95032599.
70. ^ a b Tipler, Frank (1994). The Physics of Immortality. pp. 170–171. In the "yes" column were Stephen Hawking, Richard Feynman, and Murray Gell-Mann
71. ^ Feynman, Richard P. (June 1982). "Simulating physics with computers". International Journal of Theoretical Physics. 21 (6–7): 467–488. Bibcode:1982IJTP...21..467F. doi:10.1007/BF02650179. ISSN 0020-7748. S2CID 124545445.
72. ^ Bunge, M. (2012). "Parallel Universes? Digital Physics?". Evaluating Philosophies. New York: Springer. pp. 152–153. doi:10.1007/978-94-007-4408-0. ISBN 978-94-007-4407-3.
73. ^ Ellis, G.; Silk, J. (2014). "Scientific method: Defend the integrity of physics". Nature. 516 (7531): 321–323. Bibcode:2014Natur.516..321E. doi:10.1038/516321a. PMID 25519115.
74. ^ a b c Ball, Philip (2015-02-17). "Too many worlds". Aeon.co. Retrieved 2021-09-23.((cite web)): CS1 maint: url-status (link)
75. ^ "If an Electron Can Be in Two Places at Once, Why Can't You?". Discover Magazine.((cite web)): CS1 maint: url-status (link)
76. ^ Melinda, Baldwin (2017-07-11). "Q&A: Gerard 't Hooft on the future of quantum mechanics". Physics Today. doi:10.1063/PT.6.4.20170711a.
77. ^ Peres, Asher (1995). Quantum Theory: Concepts and Methods. Kluwer Academic Publishers. p. 374. ISBN 0-7923-2549-4.
78. ^
79. ^ Caroll, Sean (1 April 2004). "Preposterous Universe". Archived from the original on 8 September 2004.
80. ^ Nielsen, Michael (3 April 2004). "Michael Nielsen: The Interpretation of Quantum Mechanics". Archived from the original on 20 May 2004.
81. ^ Survey Results Archived 2010-11-04 at the Wayback Machine
82. ^ a b Schlosshauer, Maximilian; Kofler, Johannes; Zeilinger, Anton (2013). "A Snapshot of Foundational Attitudes Toward Quantum Mechanics". Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics. 44 (3): 222–230. arXiv:1301.1069. Bibcode:2013SHPMP..44..222S. doi:10.1016/j.shpsb.2013.04.004. S2CID 55537196.
83. ^ Aldhous, Peter (2007-11-24). "Parallel lives can never touch". New Scientist. No. 2631. Retrieved 2007-11-21.
84. ^ a b Gardner, Martin (2003). Are universes thicker than blackberries?. W.W. Norton. p. 10. ISBN 978-0-393-05742-3.
85. ^ Ferris, Timothy (1997). The Whole Shebang. Simon & Schuster. pp. 345. ISBN 978-0-684-81020-1.
86. ^ Hawking, Stephen; Roger Penrose (1996). The Nature of Space and Time. Princeton University Press. pp. 121. ISBN 978-0-691-03791-2.
87. ^ Halliwell, J. J.; Pérez-Mercader, J.; Zurek, Wojciech Hubert, eds. (1996). Physical origins of time asymmetry (1st paperback ed.). Cambridge [England]: Cambridge University Press. p. 231. ISBN 0-521-56837-4. OCLC 36415828.
88. ^ Gell-Mann, Murray (1994). The Quark and the Jaguar: Adventures in the Simple and the Complex. New York: Owl Books. p. 138. ISBN 0-8050-7253-5. OCLC 56388449.
89. ^ Tegmark, Max (November 1998). "Quantum immortality". Retrieved 25 October 2010.
90. ^ Carroll, Sean (2019). "The Human Side - Living and Thinking in a Quantum Universe". Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime. Penguin. ISBN 9781524743024. At Google Books.
91. ^ Deutsch, David (2011). "The Beginning". The Beginning of Infinity. Penguin Group.
92. ^ DeWitt, Bryce S. (1970). "Quantum mechanics and reality". Physics Today. 23 (9): 30–35. Bibcode:1970PhT....23i..30D. doi:10.1063/1.3022331.
93. ^