"At first sight, the problem of constructing a quantum theory of gravity sounds easy since there are no experimental constraints! The task is simply to find any theory which unifies general relativity and quantum theory. However, on second thought, the problem sounds extremely difficult. General relativity teaches us that gravity is just a manifestation of the curvature of space and time. So quantum gravity must involve the quantization of space and time, something we have no previous experience with. Surprisingly, even though there are no experimental constraints, this is a constraint on quantum gravity which was found in the early 1970’s by studying black holes. Motivated by the close analogy between the laws of black hole mechanics and ordinary thermodynamics, Bekenstein proposed that ..."