First introduced by Wigner[1] in a curious book,[2] this is a thought experiment on the role of the conscious observer in quantum mechanics. It became to experimental metaphysics what EPR is to the foundations of quantum mechanics, especially following K. Bong et al.'s experimental violations of so-called local friendliness.[3]
Brukner[4] introduced an extended Wigner's friend scenario (EWFS) with two spatially separated observers, (Charlie and Debbie, since Alice and Bob are selected to be the superobservers), measuring their share of an EPR-entangled pair. Wigner would be the one "superobserver" observing all this, with the possibility of applying unitary transforms on the system (Charlie & Debbie included), except in extended versions where two superobservers are called for (A&B).
Proietti et al.[5] performed a six-photon experiment, with pioneering endorsing of observing status to simple degrees of freedom.
This brought forward the notion of observer-independent facts (OIF).
Bong et al. went further by formulating strong no-go theorems involving absoluteness of observed events (AOE).
If the super-observer (Wigner) can unitary reverse his friend’s evolution, there is no joint probability distribution for events observed by the friend and Wigner, from which Wigner’s observation can be recovered as marginals. This makes reality relative to the observers, and open some sort of Wigner bubbles.[6]