Quantum Gravity: Strings or Loops? A Human Quest to Understand the Universe One of the greatest unfinished stories in modern science is our attempt to understand gravity at its deepest level. We have two incredibly successful theories that describe the universe—but they don’t get along. Quantum mechanics explains the strange, probabilistic behavior of particles at the smallest scales, while Einstein’s general relativity describes gravity as the smooth curvature of spacetime on cosmic scales. Eac