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Although there is no record of Einstein responding to Born and Heisenberg during the technical sessions of the Fifth Solvay Congress, he did challenge the completeness of quantum mechanics at various times. In his tribute article for Born's retirement he discussed the quantum representation of a macroscopic ball bouncing elastically between rigid barriers. He argues that such a quantum representation does not represent a specific ball, but "time ensemble of systems". As such the representation is correct, but incomplete because it does not represent the real individual macroscopic case.<ref>{{Cite arXiv |last=Einstein |first=Albert |title=Elementary Considerations on the Interpretation of the Foundations of Quantum Mechanics |date=2011 |class=physics.hist-ph |eprint=1107.3701 |quote=This paper, whose original title was “Elementare Uberlegungen zur Interpretation ¨ der Grundlagen der Quanten-Mechanik”, has been translated from the German by Dileep Karanth, Department of Physics, University of Wisconsin-Parkside, Kenosha, USA}}</ref> Einstein considered quantum mechanics incomplete "because the state function, in general, does not even describe the individual event/system".<ref>{{Cite journal |last=Ballentine |first=L. E. |date=1972-12-01 |title=Einstein's Interpretation of Quantum Mechanics |url=https://pubs.aip.org/ajp/article/40/12/1763/527506/Einstein-s-Interpretation-of-Quantum-Mechanics |journal=American Journal of Physics |language=en |volume=40 |issue=12 |pages=1763–1771 |doi=10.1119/1.1987060 |bibcode=1972AmJPh..40.1763B |issn=0002-9505|doi-access=free }}</ref>
=== Von Neumann's
[[John von Neumann]] in his 1932 book [[Mathematical Foundations of Quantum Mechanics|''Mathematical Foundations of Quantum Mechanics'']] had presented a proof that there could be no "hidden parameters" in quantum mechanics. The validity of von Neumann's proof was questioned by [[Grete Hermann]] in 1935, who found a flaw in the proof. The critical issue concerned averages over ensembles. Von Neumann assumed that a relation between the [[expected value]]s of different observable quantities holds for each possible value of the "hidden parameters", rather than only for a statistical average over them.<ref>{{cite book|first=Max |last=Jammer |author-link=Max Jammer |title=The Philosophy of Quantum Mechanics |pages=265–274 |year=1974 |publisher=John Wiley and Sons |isbn=0-471-43958-4}}</ref><ref>{{Cite journal |last1=Mermin |first1=N. David |author-link1=N. David Mermin |last2=Schack |first2=Rüdiger |date=September 2018 |title=Homer Nodded: Von Neumann's Surprising Oversight |journal=Foundations of Physics |language=en |volume=48 |issue=9 |pages=1007–1020 |arxiv=1805.10311 |bibcode=2018FoPh...48.1007M |doi=10.1007/s10701-018-0197-5 |issn=0015-9018 |doi-access=free}}</ref> However Hermann's work went mostly unnoticed until its rediscovery by [[John Stewart Bell]] more than 30 years later.<ref>Hermann, G.: Die naturphilosophischen Grundlagen der Quantenmechanik (Auszug). Abhandlungen
der Fries’schen Schule 6, 75–152 (1935). English translation: Chapter 15 of “Grete Hermann —
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