Making sense of relativistic Wigner friend scenarios: a problem for unitary accounts of quantum measurements ?

Wigner-friend scenarios – in which external agents describe a closed laboratory containing a friend making a measurement – highlight the difficulties inherent to quantum theory when accounting for measurements. In non-relativistic scenarios, the difficulty is to accommodate unitary evolution for a closed system with a definite outcome obtained by the friend. In relativistic scenarios the tensions between quantum theory and relativity induce additional constraints. A generic property of relativistic scenarios is the frame-dependence of state update upon a measurement. Based on a definite example, we will show that this property leads to inconsistent accounts for outcomes obtained in different reference frames. We will further argue that these results point to some fundamental inadequacy when attempting to model actions taken by a complex agent as unitary operations made on simple wavefunctions.