It is now widely believed that black holes should be described by ordinary (though complicated) quantum systems. This can be made precise for supersymmetric (BPS) black holes in Anti de-Sitter space, where the AdS/CFT correspondence may be used to reliably count black hole microstates. We will review this proposal for 4d superconformal field theories dual to AdS5 black holes and explain the challenges in characterizing these microstates directly in terms of gravitational variables. Surprisingly, a gravitational path integral calculation predicts certain universal features of the spectrum, including a large number of exactly degenerate states and a “mass gap” between the BPS and non-BPS states.
If BPS black holes are described by ordinary quantum systems, we should be able to act with operators which probe the microstates. We find one such probe is a certain generalization of the supersymmetric Wilson line in 4d N=4 SYM; holographically dual to a D-brane which wraps the horizon, and further demonstrate a matching of these descriptions when the spacetime description is valid. In addition to detecting the familiar deconfinement transition in conformal gauge theories, this provides an example of a system in which a black hole interacts with other degrees of freedom but has an exact microscopic description.