Revision 1

Accepted · first revision · record

Title: Origin of the $5\,\sigma$ gap between cesium and rubidium $\alpha$
Plain: Two labs measured $\alpha$ by timing how fast an atom moves after absorbing a photon, one with cesium and one with rubidium, and their answers differ by more than five times their stated errors. At least one experiment has an error nobody has found.
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.
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.
Kind: well-posed
Answer type: identification
References: (empty)
Literature status: open
Status note: As of 2025 the gap is unexplained and no new h/m result at the required level has been published.
Posed since: 2020