- Stymied by the difficulty of unifying quantum mechanics with Einstein’s general theory of relativity, physicists have turned to a simplified version of the problem: imagining space to be just two-dimensional and asking how gravity would then operate.
- At first, they expected 2-D gravity to be trivial. Shoehorned into one fewer dimension, gravity would become so tightly circumscribed that gravitational waves could not propagate, in which case quantum gravity should be a nonstarter.
- Physicists have found it is not so trivial after all. Waves might not ripple through the continuum, but the universe as a whole could morph. The resulting quantum theory of gravity solves various puzzles of unification, such as how time may emerge from timeless physics.
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.
This article was originally published with the title Quantum Gravity in Flatland.