[QUAST Seminar] The gravitational path integral from an observer's point of view

One of the fundamental problems in quantum gravity is to describe the experience of a gravitating observer in generic spacetimes. In particular, several results indicate that the quantum gravity Hilbert space of closed universes is one-dimensional, which is incompatible with semiclassical EFT. In this talk, I will discuss a framework for describing non-perturbative physics relative to an observer using the gravitational path integral, which was recently introduced in https://arxiv.org/pdf/2501.02632. I will describe our proposal for an observer that lives in a closed universe and one that falls behind a black hole horizon. In the case of closed universes, we find that the resulting Hilbert space is not one-dimensional once an observer is included. Rather, its dimension scales exponentially with $1/G$. Similarly, from an observer’s perspective, the dimension of the Hilbert space in a two-sided black hole is increased. I will then discuss various observables probing the experience of a gravitating observer in this Hilbert space. In the closed universe, an observer experiences non-trivial physics, in contrast to what it would see in a one-dimensional Hilbert space. In the two-sided black hole setting, our proposal implies that non-perturbative corrections to effective field theory for an infalling observer are suppressed until times exponential in the black hole entropy, resolving a recently-raised puzzle in black hole physics. While the framework that we develop is exemplified in the toy-model of JT gravity, most of our analysis can be extended to higher dimensions and to generic spacetimes not admitting a conventional holographic description.