Bulk AdS₃ gravity dual to CFT on boundary; Ryu-Takayanagi S = Area/4G
Maldacena's AdS/CFT (1997): type IIB string theory on AdS₅×S⁵ = N=4 SYM on the boundary. Ryu-Takayanagi formula (2006): entanglement entropy of boundary region A equals minimal geodesic area in bulk, S_A = Area(γ_A)/4G_N. Depth in AdS ↔ energy scale of CFT (RG flow); near-boundary behavior = UV physics, deep bulk = IR. Poincaré metric: ds² = L²(dz²+dx²)/z².