Many-worlds interpretation
The many-worlds interpretation (MWI) is a philosophical position about how the mathematics used in quantum mechanics relates to physical reality. It asserts that the universal wavefunction is objectively real, and that there is no wave function collapse.[1] This implies that all possible outcomes of quantum measurements are physically realized in some "world" or universe.[2] In contrast to some other interpretations of quantum mechanics, the evolution of reality as a whole in MWI is rigidly deterministic[1]: 9 and local.[3] 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.[6][1][7][8]
In modern versions of many-worlds, the subjective appearance of wave function collapse is explained by the mechanism of quantum decoherence.[2] Decoherence approaches to interpreting quantum theory have been widely explored and developed since the 1970s.[9][10][11] MWI is considered a mainstream interpretation of quantum mechanics, along with the other decoherence interpretations, the Copenhagen interpretation, and hidden variable theories such as Bohmian mechanics.[12][2]
The many-worlds interpretation implies that there are most likely an uncountable number of universes.[13] It is one of a number of multiverse hypotheses in physics and philosophy. MWI views time as a many-branched tree, wherein every possible quantum outcome is realized. This is intended to resolve the measurement problem and thus some paradoxes of quantum theory, such as Wigner's friend,[4]: 4–6 the EPR paradox[5]: 462 [1]: 118 and Schrödinger's cat,[6] since every possible outcome of a quantum event exists in its own universe.
The preferred basis problem[edit]
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 delocalized states. The assumption is that the preferred basis to use is the one which assigns a unique measurement outcome to each world. 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.[18][35] This is known today as the preferred basis problem.
The preferred basis problem has been solved, according to Saunders and Wallace, among others,[16] by incorporating decoherence into the many-worlds theory.[23][58][59][60] 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.[61] 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.[62][15] 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.[63]
This approach to deriving the preferred basis has been criticized as creating 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.[37][51][64] Wallace contends that decoherence theory depends not on probability but only on the notion that one is allowed to do approximations in physics.[14]: 253–254
History[edit]
MWI originated in Everett's Princeton University PhD thesis "The Theory of the Universal Wave Function",[1] 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";[65] Everett originally called his approach the "Correlation Interpretation", where "correlation" refers to quantum entanglement). The phrase "many-worlds" is due to Bryce DeWitt,[1] who was responsible for the wider popularization of Everett's theory, which had been largely ignored for a decade after publication in 1957.[13]
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.[66][67][68] 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 wave function as physical and treating it as information became interchangeable.[69]
Leon Cooper and Deborah Van Vechten developed a very similar approach before reading Everett's work.[70] Zeh also came to the same conclusions as Everett before reading his work, then built a new theory of quantum decoherence based on these ideas.[71]
According to people who knew him, Everett believed in the literal reality of the other quantum worlds.[20] His son and wife reported that he "never wavered in his belief over his many-worlds theory".[72] In their detailed review of Everett's work, Osnaghi, Freitas, and Freire Jr. note that Everett consistently used quotes around "real" to indicate a meaning within scientific practice.[13]