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The Page–Geilker experiment used a quantum random decision to choose a macroscopic source-mass configuration and measured its field with a torsion balance. The observed field followed the configuration in the observed branch rather than the nearly cancelling expectation-value average predicted by the simplest no-collapse semiclassical Einstein equation.

In the Page–Geilker experiment, radioactive-decay counts supplied a quantum random decision that determined which of two macroscopically distinct configurations of lead masses the experimenters placed around a Cavendish torsion balance. In each observed branch, the balance deflected in the direction predicted by the mass configuration recorded in that branch, with a strong correlation between the decision and the measured gravitational torque.

The unobserved alternative was the prediction of the simplest semiclassical Einstein equation, in which one classical metric is sourced by the expectation value $⟨T_{\mu \nu }⟩$ of the matter stress-energy tensor. If the universal wavefunction does not collapse, the two nearly equally weighted mass configurations both contribute to that expectation value. Their opposite torques should then largely average away, and the balance should show little branch-correlated deflection. That behavior was not seen.

The experiment was designed to go beyond earlier observations of ordinary gravity and quantum matter. It deliberately created macroscopically different mass distributions in different quantum branches and tested whether the gravitational field followed the branch actually observed or the expectation-value average over all branches. It ruled out that simplest no-collapse semiclassical coupling under the experiment's assumptions. It supplied indirect evidence for quantum gravity, while leaving more elaborate classical-quantum couplings logically possible.