Quantum Clocks to Test the Equivalence Principle express
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.
Original abstract
as written by the authors arXiv:2607.19141
Whether the principles of general relativity extend to a quantum regime remains one of the open questions in modern physics. In classical general relativity, Einstein's equivalence principle underpins the interpretation of gravity as spacetime geometry. However, it is not known whether the principle remains valid when the source of gravity could be quantum. Here we show that gravity-induced entanglement (GIE) of quantum clocks provides a framework to characterize local position invariance (LPI) in such a regime, which constitutes one of the subprinciples of the equivalence principle. By adapting the existing formulation of quantum LPI to the context of GIE, we analyze two schemes that address complementary aspects of LPI and differ only in the choice of the initial clock state: one in which entanglement will be generated if and only if there is a genuinely quantum violation of LPI, and the other where classical-like LPI violation manifests in the frequency of entanglement oscillation. Our results suggest that GIE of quantum clocks offers an approach to investigating fundamental principles of general relativity in the quantum regime, shedding new light on the interplay between quantum mechanics and the theory of gravity.
The equivalence principle has even been tested on the Moon: in 1971, astronaut David Scott dropped a hammer and a feather, and they fell simultaneously in the airless environment.
In Liu Cixin's novel 'The Three-Body Problem', aliens use quantum entanglement for instantaneous communication, bypassing the speed of light — echoing the idea of an all-pervasive gravitational connection.