The gravitational path integral from an observer's point of view

          @misc{ pirsa_PIRSA:25060016,
            doi = {10.48660/25060016},
            url = {https://pirsa.org/25060016},
            author = {Iliesiu, Luca},
            keywords = {Quantum Gravity, Quantum Information},
            language = {en},
            title = {The gravitational path integral from an observer{\textquoteright}s point of view},
            publisher = {Perimeter Institute},
            year = {2025},
            month = {jun},
            note = {PIRSA:25060016 see, \url{https://pirsa.org}}
          }
          

Abstract

One of the fundamental problems in quantum gravity is to describe the experience of a gravitating observer in generic spacetimes. In this talk, I will describe a framework within which we can analyze non-perturbative physics relative to an observer using the gravitational path integral. We apply our proposal to an observer that lives in a closed universe and one that falls behind a black hole horizon. We find that the Hilbert space that describes the experience of the observer is much larger than the Hilbert space in the absence of an observer. In the case of closed universes, the Hilbert space is not one-dimensional, as calculations in the absence of the observer suggest. Rather, its dimension scales exponentially in 1/G_N. Similarly, from an observer's perspective, the dimension of the Hilbert space in a two-sided black hole is increased and this drastically changes what an observer sees when falling past the horizon of a black hole at late times.