Title

Logic Meets Wigner’s Friend(s): the epistemology of quantum observers

Abstract

This presentation is about Wigner's Friend thought-experiment [1], and its more recent variations [2] and extensions such as the Frauchiger-Renner (FR) Paradox [3], that have recently reignited the debates in the foundations of quantum theory. Such thought experiments seem to indicate that, if quantum theory is assumed to be universally valid (and hence can be applied to multi-partite systems that may include classical observers), then different agents are rationally entitled to ascribe different (mutually inconsistent) states to the same system, and as a result they cannot share their information in a consistent manner. More precisely, the result in [3] is stated in the format of a no-go theorem.

To analyze this problem, we focus on a few questions: what is the correct epistemic interpretation of the multiplicity of state assignments in these scenarios?; under which conditions can one include classical observers into the quantum state descriptions, in a way that is still compatible with Quantum Mechanics?; under which conditions can one system be admitted as an additional ‘observer’ from the perspective of another (background) observer?; when can the standard axioms of multi-agent Epistemic Logic (that allow for “knowledge transfer” between agents) be applied to quantum-physical observers? After discussing some of the various answers to these questions proposed in the literature, we propose a new such answer, sketch a particular formal implementation of it, and apply it to obtain a principled solution to Wigner Friend-type paradoxes. The presentation is based on recent joint work [4].

References

1. E.P. Wigner, Remarks on the mind-body question, in I.J. Good, The Scientist Speculates, London Heinemann,1961.
2. D. Deutsch, Quantum theory as a universal physical theory, International Journal of Theoretical Physics, 24, I, 1985.
3. D. Frauchiger and R. Renner, Quantum theory cannot consistently describe the use of itself, Nature Communications, 9(1):3711. Preprint, arXiv:1604.07422, 2018.
4. A. Baltag and S. Smets, Logic meets Wigner’s Friend (and their Friends), preprint, arXiv:2307.01713 [quant-ph], July 2023.

If you wish to to attend this seminar online, please send an email to w.merbis@uva.nl to receive the zoom-link.