Caslav BRUKNER

« The discussion about the quantum mechanical thought experiment « Wigner’s friend » has recently gained renewed intensity. The original thought experiment involved the « observation of an observer », in which the friend not only observes a physical system, but is simultaneously the object of another observation by Wigner. Recent theoretical results and experimental tests of extended Wigner-friend scenarios suggest that treating observational knowledge of other agents as if it were one’s own can be problematic in quantum mechanics. In short, « facts are only relative to observers ». In my talk, I will review these findings before arguing that a single agent’s knowledge over time can have even more counterintuitive consequences: when making predictions about one’s observations at two different points in time, it may be untenable in quantum mechanics to treat a single agent’s knowledge acquired in the past as if it were still present.»