In the lab, Austrian physicists have demonstrated that the Wigner’s friend paradox does not require quantum theory. It emerges even in classical scenarios involving the duplication of observers. The paper by Karoline L. Jones and Markus P. Müller, published on June 30, 2026, in the journal Quantum, shifts the discussion from the narrow niche of quantum foundations into the broader realm of fundamental physics and philosophy.
Authors from the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences in Vienna, alongside colleagues from the University of Vienna and the Perimeter Institute for Theoretical Physics in Canada, analyzed extended "Wigner’s friend" scenarios. In the original paradox, Wigner describes a friend measuring a particle's spin while Wigner himself perceives a superposition. Newer versions introduce multiple agents, leading to contradictions with intuitive assumptions regarding the objectivity of facts. The researchers demonstrated that similar contradictions can be reproduced without quantum mechanics—classical probability theory and the possibility of perfectly duplicating sentient agents are sufficient.
The key element in all such scenarios is "Constraint A": a theory cannot provide a single, unified probabilistic picture of all agents' observations simultaneously. Imagine identical twins who, after being perfectly cloned, bet on the outcome of a coin toss, yet each sees only their own result. Their personal predictions do not coalesce into a single, consistent probability. This same structural conflict underpins the "Boltzmann brains" problem in cosmology and the Sleeping Beauty paradox in epistemology.
The study shows that the paradox relates not just to quantum measurement, but to a fundamental difficulty in describing reality when agents' observations are private and cannot be fully merged. Classical versions are even technologically easier to implement than quantum experiments involving entangled photons or ions. This changes our perspective on which assumptions we can consider universal in any physical theory.
The results highlight the need to study such constraints within a wider context—ranging from quantum computers to cosmological models. They help clarify where our theories inevitably leave room for private predictions that cannot be intersubjectively verified.
