Quantum gravity attempts to combine Einstein’s General theory of Relativity with quantum mechanics. Quantum corrections to classical gravity are visualized as loop diagrams, as the one shown here in white. Alternatively, it’s possible that gravity is always classical and continuous, and that quantum field theory, not general relativity, needs to be modified. (Credit: SLAC National Accelerator Laboratory)

Ask Ethan: Is it possible that gravity isn’t quantum?

For generations, physicists have been searching for a quantum theory of gravity. But what if gravity isn’t actually quantum at all?

The two greatest leaps of 20th century physics still leave physicists grappling to understand how it’s possible, at a fundamental level, that they can coexist. On the one hand, we have Einstein’s general theory of relativity (GR), which treats space as a continuous, smooth background that’s deformed, distorted, and compelled to flow and evolve by the presence of all the matter and energy within it, while simultaneously determining the motion of all matter and energy within it via the curvature of that background. On the other hand, there’s quantum physics, governed at a fundamental level by quantum field theory (QFT). All the quantum “weirdness” is encoded in that description, including ideas like quantum uncertainty, the superposition of states, and quantum indeterminism: fundamentally anti-classical notions.

Traditionally, approaches to unify the two have focused on quantizing gravity, attempting to place it on the same footing as the other quantum forces. But a series of new papers, led by Jonathan Oppenheim, takes a very different approach: creating a “postquantum” theory of classical gravity. It’s led to…