Boundaries, frames and the issue of physical covariance

Abstract

We focus on three distinct lines of recent developments: edge modes and boundary charges in gravitational physics, relational dynamics in classical and quantum gravity, and quantum reference frames. We argue that these research directions are in fact linked in multiple ways, and can be seen as different aspects of the same research programme. This research programme has two main physical goals and one general focus, as well as broader conceptual implications. The physical goals are to move beyond the two idealizations/approximations of asymptotic or closed boundary conditions in gravitational physics and of ideal reference frames (coded in coordinate frames or gauge fixings), thus achieving a more realistic modelling of (quantum) gravitational physical phenomena. These two goals combine to identify a key open issue: a proper characterization of physical covariance, i.e. covariance across fully physical (as opposed to idealized) reference frames. The broader conceptual implications concern the influence of observers in physics and possible physical limits to objectivity.