| Abstract: |
The diffeomorphism symmetry of gravitational theories challenges the construction of gauge-invariant observables and notions of locality familiar from quantum field theory. In revisiting these challenges, I will introduce the concept of dynamical reference frames, i.e. frame fields for the diffeomorphism group built out of the field content of the theory. I will explain how they can be used to build gravitational observables that describe the remaining degrees of freedom relative to such a frame and which unify previous classical approaches to gravitational observables. This manifests that a diffeomorphism-invariant notion of locality is relational and gives rise to a novel form of covariance under changes of dynamical frames. I will then describe steps to translate the story into the quantum realm, where these frame fields become what are known as quantum reference frames. This will connect with recent discussions of observers, gravitational algebras and generalized entropy in perturbative quantum gravity.
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