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Title: What limits the refractive index of a dense atomic gas?
Plain: Each atom bends light strongly, yet no known material made of atoms has a refractive index far above a few. Packing atoms denser does not help, and the reason is not fully settled.
Precise: Randomly positioned two-level atoms at density n with $n \lambda^3 >> 1$, including near-field dipole-dipole interactions and multiple scattering. Determine the maximum $\operatorname{Re}(n_{\mathrm{ref}})$ over detuning and density, whether its saturation near 1.7 is universal for disordered ensembles, and which mechanism sets it. Answer: value and mechanism, with conditions under which ordered or multilevel atoms exceed it.
Settled by: Large-scale coupled-dipole and multilevel simulations with a closed-form explanation, tested by dense-cloud or lattice measurements.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: partially-resolved
Status note: Coupled-dipole numerics found a maximum near 1.7 for disordered gases (Andreoli et al., PRX 2021).
Posed since: (empty)