Facts are relative, Caslav Brukner, Nature Physics volume 16, pages 1172–1174 (2020),
 A no-go theorem for the persistent reality of Wigner’s friend’s perception (under review)
The quantum measurement problem is one of the most fundamental in quantum theory and its putative solutions are at the centre of controversy over their alternative interpretations. The measurement problem has gained renewed interest in recent years due to novel, extended versions of the famous Wigner’s friend (gedanken) experiment. The original Wigner’s friend experiment involves “observations of observers” in which the friend not only observes a physical system but is simultaneously the subject of another observation by Wigner. Several studies in this area suggested that treating observational knowledge of other agents as if it were one’s own could be problematic in quantum mechanics. In short, “there are no facts of the world per se, but only relative to observers” (see e.g. , for news & views on a recent experiment). Among others this view is in agreement with Rovelli’s relational interpretation of quantum theory.
In , we go beyond these findings and conclude that a single agent’s knowledge over time may have even more counterintuitive and drastic consequences: If one makes predictions about one’s observations at two different points in time, it may be unattainable in quantum mechanics to treat a single agent’s knowledge, which was acquired in the past, as if it were still presently existing.