From its earliest days as a science, physics has searched for unity in nature. Isaac Newton showed that the same force responsible for the fall of an apple also holds the planets in their orbits. James Clerk Maxwell combined electricity, magnetism and light into a single theory of electromagnetism; a century later physicists added the weak nuclear force to form a unified “electroweak” theory. Albert Einstein joined space and time themselves into a single spacetime continuum.
Today the biggest missing link in this quest is the unification of gravity and quantum mechanics. Einstein’s theory of gravity, his general theory of relativity, describes the birth of the universe, the orbits of planets and the fall of Newton’s apple. Quantum mechanics describes atoms and molecules, electrons and quarks, the fundamental subatomic forces, and much besides. Yet in the places where both theories should apply—where both gravity and quantum effects are strong, such as black holes—they also seem incompatible. Physicists’ best efforts to combine them into a quantum theory of gravity have failed miserably, giving answers that make no sense or no answers at all. Despite 80 years of work by generations of physicists, including a dozen or so Nobel laureates, a quantum theory of gravity remains elusive.