Refuting Observer-Independence in Quantum Theory

When Eugene Wigner conceived his Gedanken experiment in 1961 [1], he argued that in quantum theory two observers, Wigner and his friend, can experience two fundamentally different descriptions of reality. Yet, only six decades later, this question has been rigorously tackled independently by Brukner [2] and Frauchiger and Renner [3], leveraging on the Bell's theorem and on the Hardy's Paradox respectively. They exploit a new extended Wigner's friend scenario, whereby two superobservers (Alice and Bob) and two observers (Alice and Bob's friends) are now entangled, and record their own facts shown to be subjective to the observer who established them.