Editing Copenhagen interpretation (section)

=== Wigner's friend ===
{{main|Wigner's friend}}
"Wigner's friend" is a thought experiment intended to make that of Schrödinger's cat more striking by involving two conscious beings, traditionally known as Wigner and his friend.<ref name="omnes" />{{Rp|pages=91-92}} (In more recent literature, they may also be known as [[Alice and Bob]], per the convention of describing protocols in [[information theory]].<ref>{{Cite journal |last1=Fuchs |first1=Christopher A. |last2=Mermin |first2=N. David |author-link2=N. David Mermin |last3=Schack |first3=Rüdiger |date=August 2014 |title=An introduction to QBism with an application to the locality of quantum mechanics |url=http://aapt.scitation.org/doi/10.1119/1.4874855 |journal=[[American Journal of Physics]] |language=en |volume=82 |issue=8 |pages=749–754 |arxiv=1311.5253 |bibcode=2014AmJPh..82..749F |doi=10.1119/1.4874855 |s2cid=56387090 |issn=0002-9505}}</ref>) Wigner puts his friend in with the cat. The external observer believes the system is in state <math>(|\text{dead}\rangle + |\text{alive}\rangle)/\sqrt 2</math>. However, his friend is convinced that the cat is alive, i.e. for him, the cat is in the state <math>|\text{alive}\rangle</math>. ''How can Wigner and his friend see different wave functions?''

In a Heisenbergian view, the answer depends on the positioning of [[Heisenberg cut]], which can be placed arbitrarily (at least according to Heisenberg, though not to Bohr<ref name="camilleri2015" />). If Wigner's friend is positioned on the same side of the cut as the external observer, his measurements collapse the wave function for both observers. If he is positioned on the cat's side, his interaction with the cat is not considered a measurement.<ref name=":6">{{Cite journal |last1=Nurgalieva |first1=Nuriya |last2=Renner |first2=Renato |date=2020-07-02 |title=Testing quantum theory with thought experiments |url=https://www.tandfonline.com/doi/full/10.1080/00107514.2021.1880075 |journal=[[Contemporary Physics]] |language=en |volume=61 |issue=3 |pages=193–216 |arxiv=2106.05314 |bibcode=2020ConPh..61..193N |doi=10.1080/00107514.2021.1880075 |s2cid=232313237 |issn=0010-7514}}</ref> Different Copenhagen-type interpretations take different positions as to whether observers can be placed on the quantum side of the cut.<ref name=":6" />