Editing Wigner's friend (section)

===Many-worlds interpretations===
The various versions of the [[many worlds interpretation]] avoid the need to postulate that [[consciousness causes collapse]] – indeed, that collapse occurs at all.

[[Hugh Everett III]]'s doctoral thesis {{"'}}Relative state' formulation of quantum mechanics"<ref>{{Cite journal |last=Everett |first=Hugh III |year=1957 |title='Relative State' Formulation of Quantum Mechanics |journal=[[Reviews of Modern Physics]] |volume=29 |issue=3 |pages=454–462 |bibcode=1957RvMP...29..454E |doi=10.1103/RevModPhys.29.454}}</ref> serves as the foundation for today's many versions of many-worlds interpretations. In the introductory part of his work, Everett discusses the "amusing, but ''extremely hypothetical'' drama" of the Wigner's friend paradox. Note that there is evidence of a drawing of the scenario in an early draft of Everett's thesis.<ref name=":1">Barrett, J. A., and Byrne, P. (eds.). (2012). ''The Everett interpretation of quantum mechanics: Collected works 1955–1980 with commentary''. Princeton University Press.</ref> It was therefore Everett who provided the first written discussion of the problem four or five years before it was discussed in "Remarks on the mind-body question"<ref name=":0" /> by Wigner, of whom it received the name and fame thereafter. However, Everett being a student of Wigner, it is clear that they must have discussed it together at some point.<ref name=":1" />

In contrast to his teacher Wigner, who held the consciousness of an observer to be responsible for a collapse, Everett understands the Wigner's friend scenario in a different way: Insisting that quantum states' assignments should be objective and non-perspectival, Everett derives a straightforward logical contradiction when letting <math>F</math> and <math>W</math> reason about the laboratory's state of <math>S</math> together with <math>F</math>. Then, the Wigner's Friend scenario shows to Everett an incompatibility of the collapse postulate for describing measurements with the deterministic evolution of closed systems.<ref name="barrett">{{cite journal |last=Barrett |first=Jeffrey |author-link=Jeffrey A. Barrett |date=2016-10-10 |title=Everett's Relative-State Formulation of Quantum Mechanics |journal=[[Stanford Encyclopedia of Philosophy]] |url=https://plato.stanford.edu/entries/qm-everett/}}</ref> In the context of his new theory, Everett claims to solve the Wigner's friend paradox by only allowing a continuous unitary time evolution of the wave function of the universe. However, there is no evidence of any written argument of [[Hugh Everett III|Everett]]'s on the topic.<ref>{{Cite journal |last=Lehner |first=Christoph |date=2015 |title=Hugh Everett III. The Everett Interpretation of Quantum Mechanics: Collected Works, 1955–1980, with Commentary. Edited by, Jeffrey A. Barrett and Peter Byrne. xii + 392 pp., illus., apps., index. Princeton, N.J.: Princeton University Press, 2012. $75 (cloth) |journal=Isis |volume=106 |issue=1 |pages=220–221 |doi=10.1086/681886 |issn=0021-1753}}</ref>

In [[many-worlds interpretation]]s, [[Quantum measurement|measurements]] are modelled as interactions between subsystems of the universe and manifest themselves as a branching of the universal state. The different branches account for the different possible measurement outcomes and are seen to exist as subjective experiences of the corresponding observers. In this view, the friend's measurement of the spin results in a branching of the world into two parallel worlds, one, in which the friend has measured the spin to be 1, and another, in which the friend has received the measurement outcome 0. If then Wigner measures at a later time the combined system of friend and spin system, the world again splits into two parallel parts.