Reply to Dr. Wigner's Objections

Eugene Paul Wigner

S. M. Shafroth

Siamak Khademi, Parvin Sadeghi, S. Nasiri

Sadeghi et al. [Phys. Rev. A 82, 012102 (2010)] presented a new nonclassicality indicator which works properly for Husimi and Rivier distribution functions as well as the Wigner distribution function. Some objections on the Sadeghi nonclassicality indicator are presented by Kenfack (preceding Comment, ``Nonclassicality indicator for the real phase-space distribution functions'', Phys. Rev. A 93, 036101 (2016). In this Reply, some misconceptions on Sadeghi's nonclassicality indicator are discussed and clarified.

Igor Salom

A recent paper "Single-world interpretations of quantum theory cannot be self-consistent" [arXiv:1604.07422] by D. Frauchiger and R. Renner has attracted a considerable interest of a broader physics audience and shortly elicited a number of replies. In spite of the objections that ensued, we find that significant part of the controversial initial claim has not been refuted - on the contrary, arguments presented both in the initial paper and in the replies seem to leave no longer room for the basic Copenhagen interpretation of quantum mechanics. We revisit the controversy trying to pinpoint the exact phenomenon that lies in its core, pointing out to some aspects of the problem that seem to have been mostly overlooked. Taking all conclusions into account, we here propose novel ways to approach the ostensible paradox, in which the Copenhagen interpretation is naturally preserved. Whereas arguments presented in Frauchiger and Renner paper and the replies that followed revolve over whether Wigner's friend would perceive only one or both of the possible outcomes, we offer the third, pretty obvious solution - namely, that Wigner's friend, in the well known setup, would subjectively perceive {\it none} of the potentially conflicting outcomes. As a side note, we also argue that, contrary to the often stated beliefs, hypothetical real experimental realization of Wigner's friend thought experiment actually would not provide us with any new insight into the quantum mechanics.

Hilary Putnam

The Margenau and Wigner ‘Comments’ (1962) on my ‘Comments on the Paper of David Sharp’ (Putnam, 1961; Sharp, 1961) is a strange document. First the authors say, in effect, ‘had anything been wrong (with the fundamentals of quantum mechanics) we should certainly have heard’. Then they issue various obiter dicta (e.g. the ‘cut between observer and object’ is unavoidable in quantum mechanics; the – highly subjectivistic – London-Bauer treatment of quantum mechanics is described, along with von Neumann's book, as ‘the most compact and explicit formulation of the conceptual structure of quantum mechanics’). My assumption 2 (that the whole universe is a system) is described as ‘not supportable’, because ‘the measurement is an interaction between the object and the observer’. The ‘object’ (the closed system) cannot include the observer. The issues involved in this discussion are fundamental ones. I believe that the conceptual structure of quantum mechanics today is as unhealthy as the conceptual structure of the calculus was at the time Berkeley's famous criticism was issued. For this reason – as much to emphasize the seriousness of the present situation in the foundations of quantum mechanics as to remove confusions that may be left in the mind of the general reader upon reading the Margenau and Wigner ‘Comments’ – I intend to restate the main points of my previous ‘Comments’, and to show in detail why the Margenau and Wigner remarks fail completely to meet them.

Antonín Vašíček