CM
In the literature: open
What atoms tunnel in the two-level systems of amorphous silica?
In plain words
The tunneling model assumes small groups of atoms that flip between two nearly equal positions. Which atoms move, and how many, has never been identified in any glass.
Precise statement
For amorphous SiO2 (and a-Si, a-Al2O3), identify the atomic configurations that form tunneling two-level systems with energies below $\sim 1e-4\,\mathrm{eV}$ ($\sim 1\,\mathrm{K}$), and reproduce their measured density of states $P$, deformation potential $\gamma \sim 0.5-1\,\mathrm{eV}$, electric dipole moments and isotope dependence.
What would settle it
Atomistic simulation of a realistically quenched glass that finds tunneling double wells with the measured $P$, $\gamma$ and dipole moments, confirmed by an isotope-substitution or strain-spectroscopy experiment.