# Firewall The firewall paradox can be understood in three simple logical steps: (1) requiring a smooth horizon implies that ingoing and outgoing particles are entangled near the horizon; (2) the black hole is already close to being maximally entangled with early radiation, so late radiation must be entangled with early radiation to preserve unitarity; (3) but [[0218 Strong subadditivity|strong subadditivity]] forbids a single system be strongly entangled with two different systems at once, hence a paradox. ## Refs - originals - [[2012#Almheiri, Marolf, Polchinski, Sully]] - resolved by later developments - [[2013#Almheiri, Marolf, Polchinski, Stanford, Sully]] - further arguments and discussions - [[2013#Marolf, Polchinski]] - the real paradox - firewalls from tunnelling by emitting [[0051 Baby universes|baby universes]] - [[2022#Stanford, Yang]]: genus-1 (emitting one baby universe) - [[2024#Zolfi]]: higher genus (emitting more than one baby universe) - [[2024#Blommaert, Chen, Nomura]]: exponentially late time; resum the expansion in the number of wormholes; appearance of [[0117 Shockwave|shockwaves]] - [[2024#Cui, Rozali]]: JT with matter - [[2025#Zolfi, Alishahiha]]: higher genus and multiple geodesic boundaries - final state proposal - [[2025#Almheiri]]