Semiclassical gravity

Semiclassical gravity couples a classical spacetime metric to quantum matter. In its simplest expectation-value form, the classical gravitational field is sourced by the quantum expectation of the matter stress-energy tensor.

The semiclassical Einstein equation isin units with $c=1$. A single classical metric responds to the expectation value over all matter branches.

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.