Black Hole Microstates and the Information Paradox

Einstein’s General Relativity applied to black holes appears to lead to Information loss, thus violating one of the fundamental tenets of Quantum Mechanics. Recent Quantum Information Theory based arguments imply that information loss can only be avoided if at the scale of the black hole horizon there exists a structure (commonly called fuzzball or firewall) that allows information to escape. I will discuss the highly-unusual properties that this structure must have, and its realization via smooth supergravity solutions that have the same charges and asymptotics as black holes, but do not have a horizon. I also will argue that these solutions describe the typical microstates of (at least) extremal supersymmetric black holes, and therefore the black hole horizon should be seen as an approximate concept, that emerges from the quantum superposition of horizonless configurations.