Editing Reality (section)

=== Realism and locality in physics ===
{{main | Principle of locality}}
''Realism'' in the sense used by physicists does not equate to [[Philosophical realism|realism]] in metaphysics.<ref>{{cite journal|title=Against 'Realism'|first=Travis|last=Norsen|date=26 February 2007|journal=Foundations of Physics|volume=37|issue=3|pages=311–340|doi=10.1007/s10701-007-9104-1|arxiv=quant-ph/0607057|bibcode=2007FoPh...37..311N|s2cid=15072850}}</ref> The latter is the claim that the world is mind-independent: that even if the results of a measurement do not pre-exist the act of measurement, that does not require that they are the creation of the observer. Furthermore, a mind-independent property does not have to be the value of some physical variable such as position or [[momentum]]. A property can be ''[[disposition]]al'' (or potential), i.e. it can be a tendency: in the way that glass objects tend to break, or are disposed to break, even if they do not ''actually'' break. Likewise, the mind-independent properties of quantum systems could consist of a tendency to respond to particular measurements with particular values with ascertainable probability. Such an ontology would be metaphysically realistic, without being realistic in the physicist's sense of "local realism" (which would require that a single value be produced with certainty).

A closely related term is [[counterfactual definiteness]] (CFD), used to refer to the claim that one can meaningfully speak of the definiteness of results of measurements that have not been performed (i.e. the ability to assume the existence of objects, and properties of objects, even when they have not been measured).

[[Local realism]] is a significant feature of classical mechanics, of [[general relativity]], and of classical [[electrodynamics]]; but not [[quantum mechanics]]. In a work now called the [[EPR paradox]], Einstein relied on local realism to suggest that [[hidden variable theory| hidden variables]] were missing in quantum mechanics. However, [[John S. Bell]] subsequently showed that the predictions of quantum mechanics are inconsistent with hidden variables, a result known as [[Bell's theorem]]. The predictions of quantum mechanics have been verified: Bell's inequalities are violated. This means either particles have no definite positions independent of observation (no realism) ''or'' distant measurements can affect each other (no locality) or both. Different [[interpretation of quantum mechanics|interpretations of quantum mechanics]] violate different parts of local realism.<ref>{{cite book| last1=Nielsen |first1=Michael A. |last2=Chuang |first2=Isaac L. |year=2000 |title=Quantum Computation and Quantum Information |publisher=[[Cambridge University Press]] |pages=112–113| isbn=978-0-521-63503-5}}</ref>{{rp|117}}

The transition from "possible" to "actual" is a major topic of [[quantum physics]], with related theories including [[quantum darwinism]].

==== Role of "observation" in quantum mechanics ====
{{See also|Quantum decoherence}}
The [[quantum mind]]–body problem refers to the philosophical discussions of the [[mind–body problem]] in the context of quantum mechanics. Since quantum mechanics involves [[quantum superposition]]s, which [[Measurement in quantum mechanics|are not perceived by observers]], some [[interpretations of quantum mechanics]] place conscious observers in a special position.

The founders of quantum mechanics debated the role of the observer, and of them, [[Wolfgang Pauli]] and [[Werner Heisenberg]] believed that quantum mechanics expressed the observers knowledge and when an experiment was completed the additional knowledge should be incorporated in the wave function, an effect that came to be called state reduction or [[wave function collapse|collapse]]. This point of view, which was never fully endorsed by [[Niels Bohr]], was denounced as mystical and anti-scientific by [[Albert Einstein]]. Pauli accepted the term, and described quantum mechanics as ''lucid mysticism''.<ref>{{cite journal |author=Marin |first=Juan Miguel |year=2009 |title='Mysticism' in quantum mechanics: the forgotten controversy |journal=European Journal of Physics |volume=30 |issue=4 |pages=807–822 |bibcode=2009EJPh...30..807M |doi=10.1088/0143-0807/30/4/014 |s2cid=122757714}} [http://www.iop.org/EJ/article/0143-0807/30/4/014/ejp9_4_014.pdf?request-id=9350419a-e5ea-42e2-b5f3-7878a09dfe42 link], summarized here [http://www.physorg.com/news163670588.html%7care/]. {{webarchive|url=https://web.archive.org/web/20110606092933/http://www.physorg.com/news163670588.html%7Care/|date=2011-06-06}}.</ref>

Heisenberg and Bohr always described quantum mechanics in [[logical positivism|logical positivist]] terms. Bohr also took an active interest in the philosophical implications of quantum theories such as his [[Complementarity (physics)|complementarity]], for example.<ref>{{cite journal |author=Honner |first=John |year=2005 |title=Niels Bohr and the Mysticism of Nature |journal=Zygon: Journal of Religion & Science |volume=17-3 |pages=243–253}}</ref> He believed quantum theory offers a complete description of nature, albeit one that is simply ill-suited for everyday experiences&nbsp;– which are better described by classical mechanics and probability. Bohr famously avoided any characterization of "reality".<ref>Symposium On The Foundations Of Modern Physics 1987 - The Copenhagen Interpretation 60 Years After The Como Lecture. (1988). Singapore: World Scientific Publishing Company.</ref>{{rp|163}}

[[Eugene Wigner]] reformulated the "[[Schrödinger's cat]]" [[thought experiment]] as "[[Wigner's friend]]" and proposed that the consciousness of an observer is the demarcation line which precipitates collapse of the wave function, independent of any realist interpretation. Commonly known as "[[consciousness causes collapse]]", this controversial [[interpretation of quantum mechanics]] states that [[observation]] by a [[conscious]] observer is what makes the wave function collapse. However, this is a minority view among quantum philosophers, considering it a misunderstanding.<ref>{{cite journal |last1=Schlosshauer |first1=M. |last2=Koer |first2=J. |last3=Zeilinger |first3=A. |year=2013 |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 |arxiv=1301.1069 |bibcode=2013SHPMP..44..222S |doi=10.1016/j.shpsb.2013.04.004 |s2cid=55537196}}</ref> There are other possible solutions to the "[[Wigner's friend]]" thought experiment, which do not require consciousness to be different from other physical processes. Moreover, Wigner shifted to those interpretations in his later years.<ref name="Esfeld">Michael Esfeld, (1999), [http://www.unil.ch/webdav/site/philo/shared/DocsPerso/EsfeldMichael/1999/SHPMP99.pdf Essay Review: Wigner's View of Physical Reality] {{Webarchive|url=https://web.archive.org/web/20140201151438/http://www.unil.ch/webdav/site/philo/shared/DocsPerso/EsfeldMichael/1999/SHPMP99.pdf |date=2014-02-01 }}, published in Studies in History and Philosophy of Modern Physics, 30B, pp. 145–154, Elsevier Science Limited.</ref>