Many-worlds interpretation
[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.
The many-worlds interpretation's key idea is that the linear and unitary dynamics of quantum mechanics applies everywhere and at all times and so describes the whole universe.
[16]: 10 Several authors, including Everett, John Archibald Wheeler and David Deutsch, call many-worlds a theory or metatheory, rather than just an interpretation.
"[20]: 382 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.
In Everett's scheme, there is no collapse; instead, the Schrödinger equation, or its quantum field theory, relativistic analog, holds all the time, everywhere.
Focusing on the splitting process, DeWitt introduced the term "world" to describe a single branch of that tree, which is a consistent history.
[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.
[23] While there is no evidence that Everett was inspired by issues of cosmology,[14]: 7 he developed his theory with the explicit goal of allowing quantum mechanics to be applied to the universe as a whole, hoping to stimulate the discovery of new phenomena.
It achieves this by removing wave function collapse, which is indeterministic and nonlocal, from the deterministic and local equations of quantum theory.
Wave function collapse was widely regarded as artificial and ad hoc,[30] so an alternative interpretation in which the behavior of measurement could be understood from more fundamental physical principles was considered desirable.
This led Everett to derive from the unitary, deterministic dynamics alone (i.e., without assuming wave function collapse) the notion of a relativity of states.
[1]: 8 In 1985, David Deutsch proposed a variant of the Wigner's friend thought experiment as a test of many-worlds versus the Copenhagen interpretation.
Since then Lockwood, Vaidman, and others have made similar proposals,[33] which require placing macroscopic objects in a coherent superposition and interfering them, a task currently beyond experimental capability.
[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 wave function should have.
They try to show that in the limit of uncountably 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.
Some reviews have been positive, although these arguments remain highly controversial; some theoretical physicists have taken them as supporting the case for parallel universes.
[48] For example, a New Scientist story on a 2007 conference about Everettian interpretations[49] quoted physicist Andy Albrecht as saying, "This work will go down as one of the most important developments in the history of science.
[52] In 2016, Charles Sebens and Sean M. Carroll, building on work by Lev Vaidman,[53] proposed a similar approach based on self-locating uncertainty.
[57] 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.
[13]: 253–254 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.
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.
[77] Some scientists consider some aspects of MWI to be unfalsifiable and hence unscientific because the multiple parallel universes are non-communicating, in the sense that no information can be passed between them.
[78][79] Victor J. Stenger remarked that Murray Gell-Mann's published work explicitly rejects the existence of simultaneous parallel universes.
[e] Roger Penrose argues that the idea is flawed because it is based on an oversimplified version of quantum mechanics that does not account for gravity.
[81] Science writer Philip Ball calls MWI's implications fantasies, since "beneath their apparel of scientific equations or symbolic logic, they are acts of imagination, of 'just supposing'".
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?
"[91] David Deutsch speculates in his book The Beginning of Infinity that some fiction, such as alternate history, could occur somewhere in the multiverse, as long as it is consistent with the laws of physics.
[92][93] According to Ladyman and Ross, many seemingly physically plausible but unrealized possibilities, such as those discussed in other scientific fields, generally have no counterparts in other branches, because they are in fact incompatible with the universal wave function.
[77] According to Carroll, human decision-making, contrary to common misconceptions, is best thought of as a classical process, not a quantum one, because it works on the level of neurochemistry rather than fundamental particles.
