Editing Many-worlds interpretation (section)

== 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.{{efn|Everett recounted his meeting with Bohr as "that was a hell&nbsp;... 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[sic] stupid and could not understand the simplest things in quantum mechanics."<ref name=Heresy/>{{rp|113}}}} Everett had already left academia in 1957, never to return, and in 1980, Wheeler disavowed the theory.<ref name="SchrodingersCatWheeler">[[John Gribbin]], ''[[In Search of Schrödinger's Cat]]'', {{ISBN|978-0552125550}}, p. 246.</ref>

=== Support ===

One of the strongest longtime advocates of MWI is David Deutsch.<ref name="deutsch98">[[David Deutsch]], ''The Fabric of Reality: The Science of Parallel Universes And Its Implications'', Penguin Books (1998), {{ISBN|0-14-027541-X}}.</ref> According to him, 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,<ref name="deutsch85">{{cite journal | last1 = Deutsch | first1 = David | author-link = David Deutsch | year = 1985 | title = Quantum theory, the Church–Turing principle and the universal quantum computer | journal = Proceedings of the Royal Society of London A | volume = 400 | issue = 1818| pages = 97–117 | doi=10.1098/rspa.1985.0070|bibcode = 1985RSPSA.400...97D | citeseerx = 10.1.1.144.7936 | s2cid = 1438116 }}</ref> Deutsch 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". He also proposed that MWI will be testable (at least against "naive" Copenhagenism) when [[Reversible computing|reversible computers]] become conscious via the reversible observation of spin.<ref name="davis86">[[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.</ref>

=== Equivocal ===
Philosophers of science James Ladyman and Don Ross say that MWI could be true, but 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 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.<ref name="ladymanross">{{cite book|last1=Ladyman|first1=James|url=https://books.google.com/books?id=YC91DgAAQBAJ|title=Every Thing Must Go: Metaphysics Naturalized|last2=Ross|first2=Don|date=2007|publisher=Clarendon Press|isbn=978-0-19-927619-6|pages=179–183|language=en}}</ref>

=== Rejection ===

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.<ref name=Bunge>{{cite book |author1=Bunge, M. |year=2012 |title=Evaluating Philosophies |chapter=Parallel Universes? Digital Physics? |series=Boston Studies in the Philosophy and History of Science |volume=295 |pages=152–153 |publisher=Springer |location=New York |doi=10.1007/978-94-007-4408-0|isbn=978-94-007-4407-3 }}</ref><ref name=Ellis>{{cite journal |author1=Ellis, G. |author2=Silk, J. |year=2014 |title=Scientific method: Defend the integrity of physics |journal=Nature |volume=516 |issue=7531 |pages= 321–323 |doi=10.1038/516321a |doi-access=free |bibcode=2014Natur.516..321E |pmid=25519115 }}</ref>

[[Victor J. Stenger]] remarked that [[Murray Gell-Mann]]'s published work explicitly rejects the existence of simultaneous parallel universes.<ref name="stenger1995">{{cite book |last=Stenger |first=V. J. |title=The Unconscious Quantum: Metaphysics in Modern Physics and Cosmology |publisher=Prometheus Books |year=1995 |isbn=978-1-57392-022-3 |lccn=lc95032599}}</ref> Collaborating with [[James Hartle]], Gell-Mann worked toward the development of a more "palatable" ''post-Everett quantum mechanics''. Stenger thought it fair to say that most physicists find MWI too extreme, though 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".{{efn|"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."<ref name="stenger1995"/>{{rp|176}}}}

[[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. 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.<ref name="penrose">{{cite web|last=Penrose |first=Roger |author-link=Roger Penrose |title=Roger Penrose Looks Beyond the Classic-Quantum Dichotomy |publisher=Sciencewatch |date=August 1991 |url=http://www.sciencewatch.com/interviews/roger_penrose2.htm |access-date=2007-10-21 |url-status=dead |archive-url=https://web.archive.org/web/20071023063403/http://www.sciencewatch.com/interviews/roger_penrose2.htm |archive-date=2007-10-23 }}</ref> 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".<ref name="ball">{{Cite web|first=Philip |last=Ball |author-link=Philip Ball |date=2015-02-17 |access-date=2021-09-23 |title=Too many worlds|url=https://aeon.co/essays/is-the-many-worlds-hypothesis-just-a-fantasy|website=[[Aeon.co]]}}</ref><ref>{{Cite web|title=If an Electron Can Be in Two Places at Once, Why Can't You?|url=https://www.discovermagazine.com/the-sciences/if-an-electron-can-be-in-two-places-at-once-why-cant-you|website=[[Discover Magazine]]}}</ref>

Philosopher of science [[Robert P. Crease]] says that MWI is "one of the most implausible and unrealistic ideas in the history of science" because it means that everything conceivable happens.<ref name="ball"/> 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{{'"}}.<ref name="ball"/>

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."<ref>{{Cite news|last=Melinda|first=Baldwin|date=2017-07-11|title=Q&A: Gerard 't Hooft on the future of quantum mechanics|journal=Physics Today|issue=7 |url=https://physicstoday.scitation.org/do/10.1063/PT.6.4.20170711a/abs/|language=EN|doi=10.1063/PT.6.4.20170711a}}</ref>

[[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.<ref>{{Cite book|last=Peres|first=Asher|title=[[Quantum Theory: Concepts and Methods]]|publisher=Kluwer Academic Publishers|year=1995|isbn=0-7923-2549-4|pages=374|author-link=Asher Peres}}</ref>

=== Polls ===
A poll of 72 "leading quantum [[cosmologist]]s and other quantum field theorists" conducted before 1991 by L. David Raub showed 58% agreement with "Yes, I think MWI is true".<ref name=":1">{{cite book |pages=170–171 |title=The Physics of Immortality:Modern Cosmology, God and the Resurrection of the Dead |first= Frank |last= Tipler| author-link=Frank J. Tipler |date=1994|quote=In the "yes" column were Stephen Hawking, Richard Feynman, and Murray Gell-Mann}}</ref>

[[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 interpretation]]s."<ref>{{cite web | url = http://space.mit.edu/home/tegmark/quantum.html | title = Max Tegmark on many-worlds (contains MWI poll) }}</ref>

In response to [[Sean M. Carroll]]'s statement "As crazy as it sounds, most working physicists buy into the many-worlds theory",<ref name=carrollcrazy>{{cite web |url=http://preposterousuniverse.blogspot.com/2004_04_01_preposterousuniverse_archive.html#108087902367974365 |archive-url=https://web.archive.org/web/20040908014703/http://preposterousuniverse.blogspot.com/2004_04_01_preposterousuniverse_archive.html#108087902367974365 |first=Sean |last=Caroll |title=Preposterous Universe |date=1 April 2004 |archive-date=8 September 2004 }}</ref> [[Michael Nielsen]] counters: "at a quantum computing conference at Cambridge in 1998, a many-worlder surveyed the audience of approximately 200 people&nbsp;... 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?{{'"}}<ref>{{cite web |url=http://www.qinfo.org/people/nielsen/blog/archive/000060.html |archive-url=https://web.archive.org/web/20040520222807/http://www.qinfo.org/people/nielsen/blog/archive/000060.html |first=Michael |last=Nielsen |title=Michael Nielsen: The Interpretation of Quantum Mechanics | archive-date=20 May 2004 |date=3 April 2004 }}</ref>

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.<ref>[http://www.iqc.ca/~qipcourse/interpret/survey.html Survey Results] {{webarchive |url=https://web.archive.org/web/20101104152717/http://www.iqc.ca/~qipcourse/interpret/survey.html |date=2010-11-04 }}</ref>

A 2011 poll of 33 participants at an Austrian conference on [[quantum foundations]] found 6 endorsed MWI, 8 "Information-based/information-theoretical", and 14 Copenhagen;<ref name=poll2011>{{cite journal |arxiv=1301.1069 |last1=Schlosshauer |first1=Maximilian |title=A Snapshot of Foundational Attitudes Toward Quantum Mechanics |journal=Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics |volume=44 |issue=3 |pages=222–230 |last2=Kofler |first2=Johannes |last3=Zeilinger |first3=Anton |year=2013 |doi=10.1016/j.shpsb.2013.04.004 |bibcode=2013SHPMP..44..222S |s2cid=55537196 }}</ref> the authors remark that MWI received a similar percentage of votes as in Tegmark's 1997 poll.<ref name=poll2011/>