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Gravity in our world is curved spacetime. That's how Einstein's general theory of relativity describes it. But what happens to gravity at the level of atoms and particles, where quantum mechanics rules? This question has remained unanswered for nearly a century.

One of the pillars of Einsteinian gravity is the equivalence principle. It says that gravity is indistinguishable from acceleration. A part of it, local position invariance, states that the outcome of any local experiment does not depend on where and when it is performed. In the classical world, this works. But does it work in the quantum world?

The authors of the new study have proposed an elegant test. In their thought experiment, a pair of quantum clocks (devices that 'tick' with quantum precision) interact through gravity. If the equivalence principle is violated at the quantum level, the clocks become entangled: their states become inextricably linked. This is like two dancers who start moving in sync without even seeing each other, simply because the floor beneath them bends differently.

The researchers considered two scenarios for the initial state of the clocks. In the first case, entanglement arises only if the violation is truly quantum. In the second, a classical violation would show up in the oscillation frequency of the entanglement. This approach turns quantum clocks into a detector for fundamental properties of gravity.