Two pendulums hang in a vacuum chamber, and one of them sends the other an invisible signal. That is the gist of a new test: make gravity play the role of a quantum teleporter and see whether it handles the job better than classical physics allows.
The idea is simple. If two small masses oscillate slowly enough and sit close enough to each other, their mutual attraction starts exchanging quanta of vibration. Mathematically it looks like two connected vessels: liquid flows from one into the other until the levels equalize. Only here what flows is not water but a quantum state. After a time that depends on the masses, the distance and the frequency, the states swap places completely.
But what if gravity is actually classical, and we simply fail to see the hidden mechanisms? To check this, the author proposes an imitation game. One participant, Alice, randomly picks a state from a set and sends it through the gravitational channel. A second participant, Bob, prepares his device in an "empty" state. A third, Charlie, measures how faithfully the state arrived. If gravity is classical, it can use only local operations and classical communication, that is, it can send ordinary bits rather than quantum states. For that case the maximum possible fidelity has been computed: it equals (1+λ)/(2+λ), where λ is a parameter setting the width of the set of states. In the limit of a very narrow set, this bound tends to 1/2.
This means quantum gravity would give a fidelity of 1, while a classical one cannot rise above 1/2. The gap is enormous. And it can be measured in the lab within the next few years, because the technique of cooling mechanical oscillators to their ground state is advancing fast.
The imitation game opens the door to a whole hierarchy of tests. One can loosen the constraints by allowing more general classical channels, or tighten them by testing specific semiclassical theories. Instead of a single yes-or-no answer, we get a quantitative measure of how far gravity deviates from classical behavior. Two pendulums in a vacuum chamber could become the judges in a dispute that has lasted for a century.
A poll at the "Black Holes Inside and Out" conference in Copenhagen in the summer of 2024 showed that 67% of physicists believe in the quantum nature of gravity, while 9% consider it classical and 24% have no opinion. The paper quotes ChatGPT's answer to the question "Is gravity quantized?": "Short answer: we strongly suspect gravity is quantized, but we haven't conclusively proven it yet."