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Title: Observe entanglement created only by gravity between two masses
Plain: 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: 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 $1/t$.
Settled by: A positive entanglement witness between two masses whose only significant interaction is gravity.
Kind: well-posed
Answer type: yes-no
References: (empty)
Literature status: open
Status 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).
Posed since: 2017