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QUANTUM MECHANICS

Facts are relative

Nature Physics volume 16, pages 1172–1174 (2020)Cite this article

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The discussion of the quantum mechanical Wigner’s friend thought experiment has regained intensity. Recent theoretical results and experimental tests restrict the possibility of maintaining an observer-independent notion of measurement outcomes.

In 1983, Daniel Patrick Moynihan, an American politician, sociologist and diplomat, wrote that “Everyone is entitled to his own opinion, but not his own facts”. As much as this may be applied to politics, in quantum mechanics, the objectivity of facts or ‘observed events’ in the sense that they exist absolutely — and not relative to a particular observer — has recently been challenged in a series of works1,2,3,4 that build on the Wigner’s friend thought experiment5. First conceived by physicist Eugene Wigner in 1961, the scenario results in a situation where two observers seem to experience different facts. Writing in Nature Physics, Kok-Wei Bong and co-workers6 have now rigorously proved and experimentally demonstrated that a set of plausible assumptions, together with the assumption that an observed event is objective and not relative to anything or anyone, contradicts quantum mechanical predictions and experimental data. The result can be interpreted to imply that in quantum physics, observers are indeed entitled to their own facts.

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Fig. 1: The extended Wigner’s friend thought experiment.

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Authors and Affiliations

  1. Vienna Center for Quantum Science and Technology (VCQ), Faculty of Physics, University of Vienna, Vienna, Austria

    Časlav Brukner

  2. Institute for Quantum Optics and Quantum Information (IQOQI-Vienna), Austrian Academy of Sciences, 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. Facts are relative. Nat. Phys. 16, 1172–1174 (2020). https://doi.org/10.1038/s41567-020-0984-8

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