Quantum Gravity in Flatland

[Preview] From Scientific American

April 06, 2012Imagine space were 2-D rather than 3-D. How would the force of gravity work? The surprising answers are guiding physicists to a unified theory of nature

By Steven Carlip  | April 5, 2012 | 11

 

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.

In Brief

• 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.