Gravity as an imitation game

Original: "The quantum-gravitational imitation game"

arXiv:2606.03688 · 2026-06-02 · CC BY 4.0 · 1 min

The quantum channel of gravity can be told apart from the classical one if you make it teleport unknown states.

Two pendulums hang in a vacuum chamber, and one of them sends an invisible signal to the other. Scientists have come up with a way to check whether gravity is quantum or classical by making it teleport unknown states.

When two weights oscillate slowly enough and sit close together, their mutual attraction starts exchanging quanta of vibration. It is a bit like two connected vessels: liquid flows from one into the other until the levels equalize. Only here it is not water that flows, but a quantum state. If gravity is quantum, it carries the state perfectly, with fidelity 1.

If gravity is classical, however, it can only use ordinary signals, not quantum states. Then the maximum transfer fidelity cannot exceed 1/2. The gap between 1 and 1/2 is enormous, and it can be measured in the lab.

This test could be run within the next few years, because the technique for cooling mechanical oscillators to their ground state is advancing fast. So two pendulums in a vacuum chamber may become the judges in a dispute that has lasted a century.

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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."

\hat{H}_{\text{int}} = \hbar \gamma_g (\hat{a}\hat{b}^{\dagger} + \hat{a}^{\dagger}\hat{b}), \quad \gamma_g = \frac{G m}{\omega d^3}

The interaction exchanges quanta of oscillation between the oscillators at a rate γ_g that depends on mass, distance, and the natural frequency.

F_{c\ell} = \frac{1+\lambda}{2+\lambda}

The maximum average fidelity achievable without quantum resources depends on the parameter λ, which characterizes the prior distribution of states.

F_{c\ell}(t) = \frac{1+\lambda}{1+\lambda+\sin^2(\gamma_g t)}

For t < t_s the bound is higher than for full teleportation, but still below one, which makes the test possible at intermediate times.

Scientists

Albert Einstein Robert H. Dicke John Stewart Bell Robert Hooke Jacob Bekenstein Karl Schwarzschild

Concepts

quantum nature of gravity quantum gravity probes gravitational superposition experiments gravitational entanglement tabletop experiment equivalence principle test Law of Universal Gravitation game theory geophysics equivalence principle virtual particle interaction Water vacuum spacetime foam magnetic coupling probability thought experiment impact force three body problem

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Gravity as an imitation game