Intersubjective Agreement about Measurement Outcomes Is Unnecessary in QBism

Abstract

The thought experiment called ``Wigner's Friend" has experienced a renewal of interest for interrogating the meaning of intersubjectivity and objectivity in quantum mechanics. These new inquiries extend to investigations at the intersection of phenomenology and QBism. Philosopher of physics Steven French argues that QBism does not give assurances that Wigner and friend must agree on the same quantum state or measurement outcomes. In this article, we draw on Wigner's Friend to argue that an external guarantee for agreement on either quantum states or measurement outcomes is unnecessary. We defend the view that the quantum formalism is already inherently intersubjective in the way required to sustain objectivity. Here we explore the QBist notion of reciprocity, which treats Wigner and friend as physical systems taking mutual actions on each other. The QBist notion of reciprocity leads to a sharper characterization of what it means to objectify quantum systems with the formalism. Drawing on phenomenological resources, we argue that state assignments for quantum systems, including those for Wigner and friend, are a form of objectification. To assign a quantum state is to objectify a phenomenon as a quantum system, to treat something as the sort of object to which the formalism applies. Our argument accounts for why the quantum formalism does not radically change in application for different systems because the systems themselves exceed their formalization.