Title: The Atoms of Spacetime
Abstract: What did Einstein do that was so remarkable? What are black holes and do we need quantum gravity to understand them? Do black holes have thermodynamics and what does that mean? Is spacetime emergent? From what?
In 1915, Einstein proposed a new theory of gravity. Soon, it was discovered that this theory of general relativity has special kinds of solutions that came to be known as black holes. They are distinguished by having singularities (places where gravity becomes so strong that our classical notion of space and time breaks down) as well as event horizons (regions from which even light cannot escape).
Using the rules of quantum theory leads us to conclude that black holes obey laws analogous to the principles of thermodynamics. Thermodynamics, originally developed to understand steam engines, is the study of things like heat, work, energy, efficiency, and entropy. Entropy in normal matter (like a gas in a room) counts the number of different ways a large number of microscopic atoms can be arranged. Macroscopic properties of matter can be understood by averaging over all of the possible microscopic configurations.
The fact that black holes carry entropy suggests that spacetime can also be thought of as being composed of some sort of “atoms” at a fundamental level. What would the implications of that be?
In some simplified cases we can (theoretically) construct the “microscopic” configurations of black holes. Features of black hole geometries such as the event horizon and singularity arise as a consequence of averaging over many states. By studying these theoretical models of black holes, we get hints about how spacetime can be emergent. Circumstantial evidence suggest that these lessons transfer to astrophysical settings as well.