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Wigner’s friend and relational objectivity

Nature Reviews Physics volume 4, pages 628–630 (2022)Cite this article

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The ‘Wigner’s friend’ thought experiment illustrates the puzzling nature of quantum measurement. Časlav Brukner discusses how recent results suggest that in quantum theory the objectivity of measurement outcomes is relative to observation and observer.

The Wigner-friend paradox continues to fascinate physicists and philosophers alike 60 years after its discovery. This thought experiment1 conceived in 1961 by Eugene Wigner describes a physical situation in which two observers experience different facts. Since then, various attempts have been made to reconcile the observers’ different views, but a series of recent studies shows that they are fundamentally incompatible. The results can be interpreted to imply that in quantum physics the objectivity of facts is not absolute, but only relative to the observation and the observer.

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Fig. 1: Illustration of the two ‘bubbles’: the laboratory of Wigner’s friend and that of Wigner.

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  1. Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences and Faculty of Physics, University of Vienna, Vienna, Austria

    Časlav Brukner

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  1. Časlav Brukner
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Correspondence to Časlav Brukner.

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Brukner, Č. Wigner’s friend and relational objectivity. Nat Rev Phys 4, 628–630 (2022). https://doi.org/10.1038/s42254-022-00505-8

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