# Replica wormholes Replica wormholes play an important role in recent developments that resolved the black hole [[0131 Information paradox|information paradox]]. They are saddles of the [[0555 Gravitational path integral|gravitational path integral]] with a boundary condition that involves multiples copies of the original boundary. The multiple copies arise because the [[0301 Entanglement entropy|entanglement entropy]] can be written as the $n\to 1$ limit of the [[0293 Renyi entropy|Renyi entropy]] with index $n$, a quantity whose definition involves $n$ copies of the original spacetime. Just like how Lewkowycz-Maldacena proved the [[0007 RT surface|RT/HRT]] formula, the replica wormholes were used to prove the [[0213 Islands|island formula]]. ## Refs - parent page: [[0131 Information paradox]] - originals - "east coast": [[2019#Almheiri, Hartman, Maldacena, Shaghoulian, Tajdini]] - "west coast": [[2019#Penington, Shenker, Stanford, Yang]] ## For non-gravitational systems - [[Ohmori2021]][](https://arxiv.org/abs/2101.07854) - replica instanton - [[0246 Spin glass]] - [[SuZhangZhai2021]]@[](https://arxiv.org/abs/2101.11238) - Page curve from non-Markovianity ## Things it can compute - entanglement entropy - negativity - [[2021#Dong, Qi, Walter]] - entanglement of purification? - unitarity issues - Zhenbin Yang and Stanford in progress (as of 20210120) - global symmetry violation - via computing scattering amplitude - [[2020#Hsin, Iliesiu, Yang]] ## Applications - to CFT OPE - [[2020#Belin, de Boer]] - to chaotic CFT via Renyi calculations - [[2021#Sasieta]]