Editing Many-worlds interpretation (section)

=== Properties ===

MWI removes the observer-dependent role in the [[quantum measurement]] process by replacing [[wave function collapse]] with the established mechanism of [[quantum decoherence]].<ref>{{cite journal|doi=10.1016/j.physrep.2019.10.001 |first=Max |last=Schlosshauer |title=Quantum decoherence |journal=Physics Reports |volume=831 |date=2019-10-25 |pages=1–57 |arxiv=1911.06282|bibcode=2019PhR...831....1S |s2cid=208006050 }}</ref> As the observer's role lies at the heart of all "quantum paradoxes" such as the [[EPR paradox]] and von Neumann's "boundary problem", this provides a clearer and easier approach to their resolution.<ref name=everett57/>

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.<ref name="gell-man1990"/> While there is no evidence that Everett was inspired by issues of cosmology,<ref name="Heresy"/>{{rp|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.<ref name=everett57/> This hope has been realized in the later development of [[quantum cosmology]].<ref>{{Cite book |last1=Gell-Mann |first1=Murray |last2=Hartle |first2=James B. |date=January 1997 |title=Quantum Mechanics in the Light of Quantum Cosmology |url=http://www.worldscientific.com/doi/abs/10.1142/9789812819895_0036 |language=en |publisher=World Scientific |volume=4 |pages=347–369 |doi=10.1142/9789812819895_0036 |isbn=978-981-02-2844-6}}</ref>

MWI is a [[philosophical realism|realist]], [[deterministic]] and [[principle of locality|local]] theory. It achieves this by removing wave function collapse, which is indeterministic and nonlocal, from the deterministic and local equations of quantum theory.<ref name="DeterminismLocal"/>

MWI (like other, broader multiverse theories) provides a context for the [[anthropic principle]], which may provide an explanation for the [[fine-tuned universe]].<ref>[[Paul C.W. Davies|Paul C. W. Davies]], ''Other Worlds'', chapters 8 & 9 ''The Anthropic Principle'' & ''Is the Universe an accident?'', (1980) {{ISBN|0-460-04400-1}}.</ref><ref>[[Paul C.W. Davies|Paul C. W. Davies]], ''The Accidental Universe'', (1982) {{ISBN|0-521-28692-1}}.</ref>

MWI depends crucially on the [[linear combination|linearity]] of quantum mechanics, which underpins the [[quantum superposition|superposition principle]]. If the final [[theory of everything]] is non-linear with respect to wavefunctions, then many-worlds is invalid.<ref name="dewitt71"/><ref name="dewitt73"/><ref name="everett57"/><ref name="dewitt67"/><ref name="dewitt72"/> All [[quantum field theory|quantum field theories]] are linear and compatible with the MWI, a point Everett emphasized as a motivation for the MWI.<ref name="everett57"/> While quantum gravity or [[string theory]] may be non-linear in this respect,<ref name="penrose"/> there is as yet no evidence of this.<ref name="weinberglinearitydoft">[[Steven Weinberg]], ''Dreams of a Final Theory: The Search for the Fundamental Laws of Nature'' (1993), {{ISBN|0-09-922391-0}}, pp. 68–69.</ref><ref name="weinberglinearityannalsop">[[Steven Weinberg]]. ''Testing Quantum Mechanics'', Annals of Physics Vol. 194, #2 (1989), pp. 336–386.</ref>

Weingarten<ref>{{cite journal|first1=Don|last1=Weingarten|title=Macroscopic Reality from Quantum Complexity|url=https://link.springer.com/article/10.1007/s10701-022-00554-0|journal=Foundations of Physics|date=5 April 2022|issn=1572-9516|pages=45|volume=52|issue=2|doi=10.1007/s10701-022-00554-0|arxiv=2105.04545}}</ref> and Taylor & McCulloch<ref>{{cite journal|first1=Jordan|last1=Taylor|first2=Ian|last2=McCulloch|title=Wavefunction branching: when you can’t tell pure states from mixed states|url=https://quantum-journal.org/papers/q-2025-03-25-1670/|journal=Quantum|date=25 March 2025|issn=2521-327X|pages=1670|volume=9|doi=10.22331/q-2025-03-25-1670|arxiv=2308.04494}}</ref> have made separate proposals for how to define wavefunction branches in terms of quantum circuit [[Quantum_complexity_theory|complexity]].