Observe entanglement created only by gravity between two masses
In plain words
Two tiny crystals, each in a superposition of two places, are let fall side by side; if gravity is quantum, their attraction should entangle them within seconds. This needs masses of roughly a hundred-billionth of a gram kept coherent for seconds with almost no other force between them.
Precise statement
Two masses $m \sim 1e-11\,\mathrm{g}$, each in a spatial superposition of size $\Delta x \sim 2.5e-2\,\mathrm{cm}$ at separation $d \sim 4.5e-2\,\mathrm{cm}$, acquire branch-dependent phases $\phi = G m^{2} t / (\hbar d)$ over $t \sim 2.5\,\mathrm{s}$ (illustrative parameters of Bose et al. 2017, approximate). Demonstrate a positive entanglement witness (e.g. embedded-spin correlations) with Casimir-Polder and electromagnetic couplings suppressed below the gravitational one and decoherence rates below $1/t$.
What would settle it
A positive entanglement witness between two masses whose only significant interaction is gravity.
Status in the literature
Unverified note
As of 2026 no spatial superposition of a mass near $1e-11\,\mathrm{g}$ has been made; 2026 analyses give witness values of order $-1e-2$ for $m = 1e-11\,\mathrm{g}$ over 0.1 s with decoherence rates of $0.1 \text{ to } 1\,\mathrm{s}^{-1}$ (arXiv:2609.10697).
Related problems
- Special case of Must the gravitational field obey quantum superposition?
See also
- Related Measure the gravity of a mass that is in superposition
- Related Spatial superposition of a levitated nanoparticle larger than its size
- Related Can causal inequalities be violated when spacetime itself is superposed?
- Related Bell test for temporal order using a mass in superposition
- Related Does gravity-mediated entanglement prove the gravitational field is quantum?