Editing Copenhagen interpretation (section)

==Criticism==
===Incompleteness and indeterminism===
[[Image:Niels Bohr Albert Einstein4 by Ehrenfest cr.jpg|thumb|[[Niels Bohr]] and [[Albert Einstein]], pictured here at [[Paul Ehrenfest]]'s home in Leiden (December 1925), had a [[Bohr–Einstein debates|long-running collegial dispute]] about what quantum mechanics implied for the nature of reality.]]
Einstein was an early and persistent supporter of objective reality. Bohr and Heisenberg advanced the position that no physical property could be understood without an act of measurement, while Einstein refused to accept this. [[Abraham Pais]] recalled a walk with Einstein when the two discussed quantum mechanics: "Einstein suddenly stopped, turned to me and asked whether I really believed that the moon exists only when I look at it."<ref>{{cite journal|first=Abraham |last=Pais |author-link=Abraham Pais |title=Einstein and the quantum theory |journal=[[Reviews of Modern Physics]] |volume=51 |pages=863–914 |year=1979 |issue=4 |doi=10.1103/RevModPhys.51.863 |bibcode=1979RvMP...51..863P}}</ref> While Einstein did not doubt that quantum mechanics was a correct physical theory in that it gave correct predictions, he maintained that it could not be a ''complete'' theory. The most famous product of his efforts to argue the incompleteness of quantum theory is the [[EPR paradox|Einstein–Podolsky–Rosen thought experiment]], which was intended to show that physical properties like position and momentum have values even if not measured.{{refn|group=note|The published form of the EPR argument was due to Podolsky, and Einstein himself was not satisfied with it. In his own publications and correspondence, Einstein used a different argument to insist that quantum mechanics is an incomplete theory.<ref name="spekkens">{{cite journal|author2-link=Robert Spekkens|first1=Nicholas |last1=Harrigan |first2=Robert W. |last2=Spekkens |title=Einstein, incompleteness, and the epistemic view of quantum states |journal=[[Foundations of Physics]] |volume=40 |issue=2 |pages=125 |year=2010 |doi=10.1007/s10701-009-9347-0 |arxiv=0706.2661|bibcode=2010FoPh...40..125H |s2cid=32755624 }}</ref><ref name="howard">{{cite journal |last1=Howard |first1=D. |title=Einstein on locality and separability |journal=[[Studies in History and Philosophy of Science Part A]] |date=1985 |volume=16 |issue=3 |pages=171–201 |doi=10.1016/0039-3681(85)90001-9|bibcode=1985SHPSA..16..171H }}</ref><ref>{{Cite journal|last=Sauer|first=Tilman|date=2007-12-01|title=An Einstein manuscript on the EPR paradox for spin observables|url=http://philsci-archive.pitt.edu/3222/|journal=[[Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics]] |language=en |volume=38 |issue=4 |pages=879–887 |doi=10.1016/j.shpsb.2007.03.002 |issn=1355-2198|bibcode=2007SHPMP..38..879S|citeseerx=10.1.1.571.6089}}</ref><ref>{{cite encyclopedia |last=Einstein |first=Albert |title=Autobiographical Notes |encyclopedia=Albert Einstein: Philosopher-Scientist |year=1949 |publisher=Open Court Publishing Company |editor-last=Schilpp |editor-first=Paul Arthur}}</ref>}} The argument of EPR was not generally persuasive to other physicists.<ref name=":0" />{{rp|189–251}}

[[Carl Friedrich von Weizsäcker]], while participating in a colloquium at Cambridge, denied that the Copenhagen interpretation asserted "What cannot be observed does not exist". Instead, he suggested that the Copenhagen interpretation follows the principle "What is observed certainly exists; about what is not observed we are still free to make suitable assumptions. We use that freedom to avoid paradoxes."<ref name="Cramer 649" />

Einstein was likewise dissatisfied with the indeterminism of quantum theory. Regarding the possibility of randomness in nature, Einstein said that he was "convinced that He [God] does not throw dice."<ref>Letter to Max Born (4 December 1926); {{cite book|title=The Born-Einstein Letters |translator-first=Irene |translator-last=Born |publisher=Walker and Company |location=New York |year=1971 |isbn=0-8027-0326-7 |oclc=439521601}}</ref> Bohr, in response, reputedly said that "it cannot be for us to tell God, how he is to run the world".{{refn|group=note|Bohr recollected his reply to Einstein at the 1927 [[Solvay Congress]] in his essay "Discussion with Einstein on Epistemological Problems in Atomic Physics", in ''Albert Einstein, Philosopher–Scientist'', ed. Paul Arthur Shilpp, Harper, 1949, p. 211: "...in spite of all divergencies of approach and opinion, a most humorous spirit animated the discussions.  On his side, Einstein mockingly asked us whether we could really believe that the providential authorities took recourse to dice-playing ("''ob der liebe Gott würfelt''"), to which I replied by pointing at the great caution, already called for by ancient thinkers, in ascribing attributes to Providence in everyday language."  Werner Heisenberg, who also attended the congress, recalled the exchange in ''Encounters with Einstein'', Princeton University Press, 1983, p. 117: "But he [Einstein] still stood by his watchword, which he clothed in the words: 'God does not play at dice.'  To which Bohr could only answer: 'But still, it cannot be for us to tell God, how he is to run the world.'"}}

===The Heisenberg cut===
Much criticism of Copenhagen-type interpretations has focused on the need for a classical domain where observers or measuring devices can reside, and the imprecision of how the boundary between quantum and classical might be defined. This boundary came to be termed the [[Heisenberg cut]] (while [[John Stewart Bell|John Bell]] derisively called it the "shifty split"<ref name="Bell-Against-Measurement" />). As typically portrayed, Copenhagen-type interpretations involve two different kinds of time evolution for wave functions, the deterministic flow according to the [[Schrödinger equation]] and the probabilistic jump during measurement, without a clear criterion for when each kind applies. Why should these two different processes exist, when physicists and laboratory equipment are made of the same matter as the rest of the universe?<ref>{{cite journal|title=Einstein's Mistakes |first=Steven |last=Weinberg |author-link=Steven Weinberg |journal=[[Physics Today]] |date=November 2005 |volume=58 |issue=11 |page=31 |doi=10.1063/1.2155755|bibcode=2005PhT....58k..31W |s2cid=120594692 |doi-access=free }}</ref> And if there is somehow a split, where should it be placed? [[Steven Weinberg]] writes that the traditional presentation gives "no way to locate the boundary between the realms in which [...] quantum mechanics does or does not apply."<ref>{{cite news | first=Steven |last=Weinberg|author-link=Steven Weinberg |title=The Trouble with Quantum Mechanics | url=https://www.nybooks.com/articles/2017/01/19/trouble-with-quantum-mechanics/ | work=New York Review of Books | date=19 January 2017 | access-date=8 January 2017}}</ref>

The problem of thinking in terms of classical measurements of a quantum system becomes particularly acute in the field of [[quantum cosmology]], where the quantum system is the universe.<ref>'Since the Universe naturally contains all of its observers, the problem arises to come up with an interpretation of quantum theory that contains no classical realms on the fundamental level.', {{cite magazine|arxiv=quant-ph/0210152|author1=Claus Kiefer|title=On the interpretation of quantum theory – from Copenhagen to the present day|url=https://archive.org/details/arxiv-quant-ph0210152|magazine=Time|pages=291|year=2002|bibcode=2003tqi..conf..291K}}</ref><ref name=":4">{{cite journal|last=Haag|first=Rudolf|author-link=Rudolf Haag|year=2010|title=Some people and some problems met in half a century of commitment to mathematical physics|journal=[[The European Physical Journal H]]|volume=35|pages=263–307|bibcode=2010EPJH...35..263H|doi=10.1140/epjh/e2010-10032-4|number=3|s2cid=59320730}}</ref> How does an observer stand outside the universe in order to measure it, and who was there to observe the universe in its earliest stages? Advocates of Copenhagen-type interpretations have disputed the seriousness of these objections. [[Rudolf Peierls]] noted that "the observer does not have to be contemporaneous with the event"; for example, we study the early universe through the [[cosmic microwave background]], and we can apply quantum mechanics to that just as well as to any electromagnetic field.<ref name="Peierls" /> Likewise, [[Asher Peres]] argued that physicists ''are'', conceptually, outside those degrees of freedom that cosmology studies, and applying quantum mechanics to the radius of the universe while neglecting the physicists in it is no different from quantizing the electric current in a [[superconductor]] while neglecting the atomic-level details.<ref name="peres1998">{{Cite journal|last=Peres|first=Asher|author-link=Asher Peres|date=1998-12-01|title=Interpreting the Quantum World|url=|journal=[[Studies in History and Philosophy of Modern Physics]]|language=en|volume=29|issue=4|pages=611–620|arxiv=quant-ph/9711003|bibcode=1997quant.ph.11003P|doi=10.1016/S1355-2198(98)00017-3|issn=1355-2198}}</ref>

{{Blockquote|text=You may object that there is only one universe, but likewise there is only one [[SQUID]] in my laboratory.<ref name="peres1998"/>}}