Physicists have measured the smallest gravitational field ever recorded, in an experiment that could help in the search for a unified theory of physics.

Of the four fundamental forces known to physics — the weak and strong interactions, the electromagnetic force and the gravitational force — only gravity remains unintegrated into the playbook of physics called the Standard Model, which describes how the zoo of subatomic particles behaves. Gravity is instead described by Einstein’s general theory of relativity, but as this breaks down at the quantum scale, our best picture of the universe is left divided in two.