Counting microstates of black holes

On the basis of general relativity and quantum mechanics in curved spacetimes, Bekenstein and Hawking proposed that black holes behave as thermodynamic objects carrying an entropy S = A / 4 G, where A is the area of the event horizon and G is Newton's constant. This remarkable formula is universal. It applies to any black hole regardless of its mass, charge, or angular momentum, and in any spacetime dimension. One of the most outstanding goals in theoretical physics in the intervening decades has been to explain the origin and universality of this formula. I will describe recent progress, where we give a universal statistical mechanical interpretation of black hole entropy by counting the underlying microstates. Our calculation applies to any consistent theory of quantum gravity that has general relativity as its low energy limit. The results imply dramatic, macroscopic manifestations of quantum mechanics in a cosmic setting, such that we can regard long Einstein-Rosen bridges between universes as quantum superpositions of short bridges.