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Which atoms move, and how many, has never been identified in any glass.","posed_since":"","precise":"For amorphous $\\mathrm{SiO_2}$ (and a-Si, a-Al2O3), identify the atomic configurations that form tunneling two-level systems with energies below $\\sim 10^{-4}\\,\\mathrm{eV}$ ($\\sim 1\\,\\mathrm{K}$), and reproduce their measured density of states $P$, deformation potential $\\gamma \\sim 0.5-1\\,\\mathrm{eV}$, electric dipole moments and isotope dependence.","problem_ref":"a66cd1c635e55e1c596798193e37a822aa65f54c099d89d28d965bebc120f930","references":"","settled_by":"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.","status_note":"","title":"What atoms tunnel in the two-level systems of amorphous 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