The Wigner’s friend paradox is a thought experiment in theoretical quantum physics, presented by Eugene Wigner. Wigner was a theoretical physicist and mathematician who won the Nobel Prize in 1963. As stated by quantum physicist Časlav Brukner:
This thought experiment 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. (2022)
An observer, Wigner’s friend, carries out a quantum experiment, and observes the result. Wigner is not present, thus he does not know the outcome of the experiment. But quantum theory tells us that for Wigner the outcome is not just unknown but physically indeterminate. Once again, as in Vaidman’s description of the relative world, the situation that is not under his observation is in a superposition of different (classical) states. In Wigner’s world the state of his friend is a superposition. But of course for the friend the outcome of the experiment, and his observation of the result, is all determinate. In other words, Wigner and his friend live in different versions of the real world.
This has recently been confirmed in experimental physics. As physicist Alexander Poltorac writes, Massimiliano Proietti and his colleagues tested the Wigner’s Friend Paradox:
They proved that two contradictory realities could coexist. Eugene Wigner was right; the quantum reality is observer-dependent. (2019)
‘Observer-dependent’ means the world is a relative world. Coexisting, contradictory realities means that the world is different for different people. The experiment shows we live in personal worlds, parallel realities. This is described in the New Scientist article Quantum experiment suggests there really are ‘alternative facts’. A second experiment has produced the same result as reported in the Scientific American article This Twist on Schrödinger’s Cat Paradox Has Major Implications for Quantum Theory.
This should not surprise us. The relative world is meaning of the fundamental tenet of Everett’s theory. Physical reality is relative to the individual, and this means it must be different for different people.
Even though the Wigner’s Friend Paradox is simply the direct result of the fundamentals of quantum theory, the whole idea has been given very little attention until now. The reason, of course, is that it clashes directly with the current scientific worldview, the generally accepted understanding of the world. Naturally, the experiments have garnered quite a bit of attention. Acceptance of the relative world concept, however, is still just a debate. The key problem is that there has been no ontology, no basic explanation of how this could be. This is the new concept presented here. This is the superworld.