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At least one experiment has an error nobody has found.","posed_since":"2020","precise":"Atom-interferometric photon-recoil determinations give $\\alpha^{-1}(\\mathrm{Cs},\\ \\mathrm{Berkeley}\\ 2018) = 137.035999046(27)$ and $\\alpha^{-1}(\\mathrm{Rb},\\ \\mathrm{LKB}\\ 2020) = 137.035999206(11)$, a $5.4\\,\\sigma$ difference. Identify the systematic effect (or new physics distinguishing the two species) responsible, or produce new $h/m_{\\mathrm{Cs}}$ and $h/m_{\\mathrm{Rb}}$ values that agree at the 0.1 ppb level.","problem_ref":null,"references":"","settled_by":"An independent remeasurement of h/m for Cs or Rb (or both) at below 0.1 ppb with a fully re-evaluated error budget that either reproduces one value or exposes a specific missing systematic.","status_note":"As of 2025 the gap is unexplained and no new h/m result at the required level has been published.","title":"Origin of the $5\\,\\sigma$ gap between cesium and rubidium 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