This year we give thanks for one of the very few clues we have to the quantum nature of spacetime: black hole entropy. (We’ve previously given thanks for the Standard Model Lagrangian, Hubble’s Law, the Spin-Statistics Theorem, conservation of momentum, effective field theory, the error bar, gauge symmetry, Landauer’s Principle, the Fourier Transform, Riemannian Geometry, the speed of light, the Jarzynski equality, the moons of Jupiter, and space.)

Black holes are regions of spacetime where, ...