This article argues that quantum observables can include not just self-adjoint operators but any member of the class of normal operators, including those with nonreal eigenvalues. Concrete experiments, statistics, and symmetries are all expressed in this more general context. However, this more general class of observables also requires a new restriction on sets of operators that can be interpreted as observables at once, called ‘sharp sets’.
Thanks to Jeremy Butterfield for many helpful suggestions about these ideas.