Editing Copenhagen interpretation (section)

=== Completion by hidden variables? ===
{{main|Hidden-variable theory}}
In metaphysical terms, the Copenhagen interpretation views [[quantum mechanics]] as providing knowledge of phenomena, but not as pointing to 'really existing objects', which it regards as residues of ordinary intuition. This makes it an [[epistemic]] theory. This may be contrasted with Einstein's view, that physics should look for 'really existing objects', making itself an [[ontic]] theory.<ref>[[Max Jammer|Jammer, M.]] (1982). 'Einstein and quantum physics', pp. 59–76 in ''Albert Einstein: Historical and Cultural Perspectives; the Centennial Symposium in Jerusalem'', edited by G. Holton, Y. Elkana, Princeton University Press, Princeton NJ, {{ISBN|0-691-08299-5}}. On pp. 73–74, Jammer quotes a 1952 letter from Einstein to Besso: "The present quantum theory is unable to provide the description of a real state of physical facts, but only of an (incomplete) knowledge of such. Moreover, the very concept of a real factual state is debarred by the orthodox theoreticians. The situation arrived at corresponds almost exactly to that of the good old Bishop Berkeley."</ref>

The metaphysical question is sometimes asked: "Could quantum mechanics be extended by adding so-called "hidden variables" to the mathematical formalism, to convert it from an epistemic to an ontic theory?" The Copenhagen interpretation answers this with a strong 'No'.<ref>[[Werner Heisenberg|Heisenberg, W.]] (1927). Über den anschaulichen Inhalt der quantentheoretischen Kinematik und Mechanik, ''Z. Phys.'' '''43''': 172–198. Translation as 'The actual content of quantum theoretical kinematics and mechanics' [https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19840008978.pdf here]: "Since the statistical nature of quantum theory is so closely [linked] to the uncertainty in all observations or perceptions, one could be tempted to conclude that behind the observed, statistical world a "real" world is hidden, in which the law of causality is applicable. We want to state explicitly that we believe such speculations to be both fruitless and pointless. The only task of physics is to describe the relation between observations."</ref> It is sometimes alleged, for example by [[John Stewart Bell|J.S. Bell]], that Einstein opposed the Copenhagen interpretation because he believed that the answer to that question of "hidden variables" was "yes". By contrast, [[Max Jammer]] writes "Einstein never proposed a hidden variable theory."<ref>[[Max Jammer|Jammer, M.]] (1982). 'Einstein and quantum physics', pp. 59–76 in ''Albert Einstein: Historical and Cultural Perspectives; the Centennial Symposium in Jerusalem'', edited by G. Holton, Y. Elkana, Princeton University Press, Princeton NJ, {{ISBN|0-691-08299-5}}, p. 72.</ref> Einstein explored the possibility of a hidden variable theory, and wrote a paper describing his exploration, but withdrew it from publication because he felt it was faulty.<ref>{{cite journal |last1=Belousek |first1=D.W. |year=1996 |title=Einstein's 1927 unpublished hidden-variable theory: its background, context and significance |journal=[[Studies in History and Philosophy of Modern Physics]] |volume=21 |issue=4 |pages=431–461 |bibcode=1996SHPMP..27..437B |doi=10.1016/S1355-2198(96)00015-9 }}</ref><ref>{{cite journal |last1=Holland |first1=P |year=2005 |title=What's wrong with Einstein's 1927 hidden-variable interpretation of quantum mechanics? |journal=[[Foundations of Physics]] |volume=35 |issue=2 |pages=177–196 |doi=10.1007/s10701-004-1940-7 |arxiv=quant-ph/0401017|bibcode=2005FoPh...35..177H |s2cid=119426936 }}</ref>