Description
Good books on GR agree on certain aspects of the theory that we can consider to be its core features. Beyond the core, however, we do not know exactly what GR — and semiclassical gravity more generally — should allow or not allow physically. For example, should we consider spacetimes with closed causal loops as part of physical GR or not? Are spacetimes with certain types of boundaries or singularities part of GR? In this fertile arena Matt has played a leading rôle in pushing back the Veil of Unknowing, elucidating the physics of spacetime beyond mere "axioms". Such questions can also be addressed from the direction of quantum gravity in which Lorentzian spacetime is not assumed but must be derived. Within the framework of the gravitational path integral and a Feynmanian heuristic for the recovery of the classical approximation I will use the example of causal set quantum gravity to sketch out how predictions about the properties of physically allowed spacetimes might arise.