A Microscopic Account of Black Hole Entropy

In the 1970’s, theorists determined that black holes have entropy [1], a remarkable finding that points at analogies between these spacetime singularities and systems of particles, such as classical gases. The crucial proof was provided by Stephen Hawking, who demonstrated, using a quantum-mechanical framework, that black holes radiate as if they were black bodies with a specific temperature [2]. The analogy was completed by extending all four laws of thermodynamics to black holes [3]. In thermodynamics, entropy is an important bridge between the macroscopic and the microscopic world: In a gas, for instance, entropy relates macroscopic heat transfer to the number of available microscopic states of the gas molecules. Providing a similar microscopic explanation of black hole entropy is an important test for theories that aim to unify gravity and quantum mechanics. Now, three independent studies have shown that string theory can offer a microscopic accounting of entropy for certain theoretical black holes [4–6]. The result sets up these black holes as ideal toy models for running thought experiments that tackle subtle questions of quantum gravity.