The Wigner’s friend paradox is a thought experiment, a what-if in theoretical quantum physics. It was presented by Eugene Wigner, a theoretical physicist and mathematician who won the Nobel Prize in 1963.
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 according to basic quantum theory, the outcome is not just unknown to Wigner, for him it has to be physically indeterminate.
The idea that quantum systems can be in an indeterminate state is fundamental to quantum theory. Wigner uses this idea to show is that not only is the result of the experiment indeterminate, so is the state of his friend. So now there is a conscious human being in this bizarre quantum condition. That is the meaning of the Wigner’s friend paradox. The further surreal implication is that Wigner and his friend live in different versions of the physical world. So this is the basis of the relative world concept.
In the standard viewpoint, this sounds crazy. But it real. 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. (2022)
This experiment has recently been carried out in real life, and the idea has been confirmed. As physicist Alexander Poltorac writes, when 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.
Superposition
The explanation is inherent in Vaidman’s description of the relative world. The whole situation that is not under Wigner’s observation is in a superposition of different classical states. This includes his friend. But at the same time, of course, it is the other way round for the friend. His state is determinate, whereas the state of Wigner is in a superposition of different classical states. Wigner and his friend live in different versions of the real world, different relative worlds.
Even though the Wigner’s Friend Paradox is 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 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. The superworld explains exactly how the relative world works. And it fits the facts of experiment.
There is further evidence in The Holographic Universe.