QI In the literature: open

Measure the gravity of a mass that is in superposition

In plain words

Gravity has been measured from masses of about 90 milligrams, but never from an object that is itself in a superposition of two places. Doing so would show whether a quantum source pulls on a probe through its average position or through each branch separately.

Precise statement

Prepare a source mass $M$ in a spatial superposition of separation $\Delta x$ and measure a probe's response that distinguishes a field sourced by the expectation value $\langle T_{00}\rangle$ (Moller-Rosenfeld semiclassical gravity) from a branch-correlated field, at force scale $\sim G M m_{\mathrm{probe}} \Delta x / d^3$. Required: source masses small enough to superpose ($M <\sim 1e-11\,\mathrm{g}$) and probe force sensitivity at that scale.

What would settle it

A probe measurement whose outcome statistics are correlated with the source branch (quantum) or follow the mean field (semiclassical).

Status in the literature

Page and Geilker (1981) excluded the simplest mean-field model using a classically randomized source; no experiment has used a source in a coherent spatial superposition.