Persistent gravitational wave observables

The gravitational wave memory effect is the permanent relative displacement of a pair of initially comoving test particles caused by the passage of a burst of gravitational waves. Recent research has clarified the physical origin and the interpretation of this effect in terms of conserved charges at null infinity and "soft theorems". In this talk, we describe a more general class of effects, not necessarily associated with those charges and soft theorems, that are, in principle, measurable. We shall refer to these effects as persistent gravitational wave observables. Like geodesic deviation in gravitational wave memory, these observables vanish in non-radiative regions of a spacetime, and their effects "persist" after a region of spacetime which is radiating. We give three examples of such persistent observables: (i) a generalization of geodesic deviation that allows for acceleration, (ii) a holonomy observable defined in terms of a closed loop that includes the relative velocity, proper time and accumulated relative rotation effects previously identified, as well as new effects, and (iii) an observable defined using using a spinning test particle. We briefly discuss the ability of gravitational wave detectors (such as LIGO) to measure these observables.