Bub has recently claimed that the Frauchiger-Renner argument does not require an assumption that the state vector undergoes projection after measurement. It is shown that this claim is not true, and an alternative understanding of the argument is offered.
Recent extended formulations of the Wigner's friend thought experiment throw the measurement problem of quantum mechanics into sharper relief. Here I respond to an invitation by Renner to provide a consistent and concrete set of rules for quantum mechanics which can avoid the apparent paradox formulated by Frauchiger and Renner [Nat. Comm. 9, 3711 (2018)]. I propose a slight addition to standard textbook quantum mechanics, in the form of two rules, which avoids the paradox. The first specifies when a given quantum dynamics can be interpreted as a measurement. Potentially any dynamics can, but doing so depends on the context of other performed operations. The second requires that a joint context be used to determine whether several different dynamical evolutions can all be interpreted as measurement. The paradox is then avoided because not every participant in the Frauchiger-Renner thought experiment regards the stated dynamical descriptions of the experiment as measurements.
Frauchiger and Renner have recently claimed to prove that “Single-world interpretations of quantum theory cannot be self-consistent”. This is contradicted by a construction due to Bell, inspired by Bohmian mechanics, which shows that any quantum system can be modelled in such a way that there is only one “world” at any time, but the predictions of quantum theory are reproduced. This Bell–Bohmian theory is applied to the experiment proposed by Frauchiger and Renner, and their argument is critically examined. It is concluded that it is their version of “standard quantum theory”, incorporating state vector collapse upon measurement, that is not self-consistent.