# On Formalisms and Interpretations

Veronika Baumann1,2 and Stefan Wolf1

1Faculty of Informatics, Università della Svizzera italiana, Via G. Buffi 13, CH-6900 Lugano, Switzerland
2Faculty of Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria

### Abstract

One of the reasons for the heated debates around the interpretations of quantum theory is a simple confusion between the notions of formalism $\textit{versus}$ interpretation. In this note, we make a clear distinction between them and show that there are actually two $\textit{inequivalent}$ quantum formalisms, namely the relative-state formalism and the standard formalism with the Born and measurement-update rules. We further propose a different probability rule for the relative-state formalism and discuss how Wigner's-friend-type experiments could show the inequivalence with the standard formalism. The feasibility in principle of such experiments, however, remains an open question.

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### Cited by

[1] Cyril Elouard, Philippe Lewalle, Sreenath K. Manikandan, Spencer Rogers, Adam Frank, and Andrew N. Jordan, "Quantum erasing the memory of Wigner's friend", Quantum 5, 498 (2021).

[2] Reinhold A. Bertlmann, "Real or not real that is the question...", The European Physical Journal H 45 2-3, 205 (2020).

[3] Kyrylo Simonov, "Particle mixing and the emergence of classicality: A spontaneous-collapse-model view", Physical Review A 102 2, 022226 (2020).

[4] R. E. Kastner, "Unitary Interactions Do Not Yield Outcomes: Attempting to Model “Wigner’s Friend”", Foundations of Physics 51 4, 89 (2021).

[5] Veronika Baumann and Časlav Brukner, Jerusalem Studies in Philosophy and History of Science 91 (2020) ISBN:978-3-030-34315-6.

[6] Veronika Baumann, Flavio Del Santo, Alexander R. H. Smith, Flaminia Giacomini, Esteban Castro-Ruiz, and Caslav Brukner, "Generalized probability rules from a timeless formulation of Wigner's friend scenarios", Quantum 5, 524 (2021).

[7] Kok-Wei Bong, Aníbal Utreras-Alarcón, Farzad Ghafari, Yeong-Cherng Liang, Nora Tischler, Eric G. Cavalcanti, Geoff J. Pryde, and Howard M. Wiseman, "A strong no-go theorem on the Wigner’s friend paradox", Nature Physics 16 12, 1199 (2020).

[8] Nuriya Nurgalieva and Renato Renner, "Testing quantum theory with thought experiments", Contemporary Physics 61 3, 193 (2020).

[9] Armando Relaño, "Decoherence framework for Wigner's-friend experiments", Physical Review A 101 3, 032107 (2020).

[10] Jacques Pienaar, "A Quintet of Quandaries: Five No-Go Theorems for Relational Quantum Mechanics", Foundations of Physics 51 5, 97 (2021).

[11] Michael Dascal, "What's left for the neo-Copenhagen theorist", Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics 72, 310 (2020).

[12] Philippe Allard Guérin, Veronika Baumann, Flavio Del Santo, and Časlav Brukner, "A no-go theorem for the persistent reality of Wigner’s friend’s perception", Communications Physics 4 1, 93 (2021).

[13] Flavio Del Santo and Nicolas Gisin, "Physics without determinism: Alternative interpretations of classical physics", Physical Review A 100 6, 062107 (2019).

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[17] Jacques Pienaar, "QBism and Relational Quantum Mechanics compared", Foundations of Physics 51 5, 96 (2021).

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The above citations are from Crossref's cited-by service (last updated successfully 2022-05-17 13:08:02) and SAO/NASA ADS (last updated successfully 2022-05-17 13:08:03). The list may be incomplete as not all publishers provide suitable and complete citation data.