Abandoning Galileo’s Ship: The Quest for Non-relational Empirical Significance

The recent debate about whether gauge symmetries can be empirically significant has focused on the possibility of ‘Galileo’s ship’ types of scenarios, where the symmetries effect relational differences between a subsystem and the environment. However, it has gone largely unremarked that apart from such Galileo’s ship scenarios, Greaves and Wallace proposed that gauge transformations can also be empirically significant in a ‘non-relational’ manner that is analogous to a Faraday-cage scenario, where the subsystem symmetry is related to a change in a charged boundary state. In this article, we investigate the question of whether such non-relational scenarios are possible for gauge theories. Remarkably, the answer to this question turns out to be closely related to a foundational puzzle that has driven a host of recent developments at the frontiers of theoretical physics. By drawing on these recent developments, we show that one way of elaborating on the idea of non-relational empirical significance for gauge symmetry is incoherent. However, we also show that it is still possible to construct non-relational models, albeit ones where the empirical significance is not witnessed by a gauge symmetry but instead by a superficially similar boundary symmetry. Furthermore, the latter casts doubt on whether one really abandons Galileo’s ship in such scenarios.