# Gravitational path integral The gravitational path integral (GPI) is one of the most important and successful approaches to quantum gravity. As Jim Hartle once told Don Marolf, it is "very smart". GPI is easier to define in AdS than in flat space or dS. Many modern understandings of AdS/CFT use GPI at their core. ## Applications - [[0207 Euclidean state preparation|Euclidean state preparation]] such as [[0162 No-boundary wavefunction|Hartle-Hawking state]] - derivation of the [[0004 Black hole entropy|black hole entropy]] and more generally proof of the [[0007 RT surface|RT formula]] (along with other [[0127 Black hole thermodynamics|thermodynamical quantities]]) - [[0012 Hawking-Page transition|Hawking-Page transition]] - resolution of the [[0131 Information paradox|information paradox]] - [[0248 Black hole microstates|black hole microstate]] counting ## Contour and what to integrate over - [[0335 Complex metrics|complex metrics]]: complex metrics should be allowed, but not all of them - lapse contour - [[2024#Banihashemi, Jacobson]] ## Refs - original: [[1977#Gibbons, Hawking]]