The extended Wigner's friend, many- and single-worlds and reasoning from observation

Abstract: The concept of an isolated system, and Frauchiger and Renner's extended `Wigner's friend' scenario are discussed. It is argued that: (i) it is questionable whether the approximation of the isolated system is valid when measurement-like processes are involved; (ii) one may infer, from Frauchiger and Renner's thought-experiment, and similar thought-experiments, that any interpretation of quantum theory involving subjective collapse fails; (iii) this does not distinguish single-world from many-world (relative-state) interpretations of quantum theory; (iv) reasoning from observations has to take into account the possible quantum-erasure of those observations if it is to be valid reasoning; (v) a single-world interpretation is valid if certain kinds of outcome are not quantum-erased in the future.