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Experiments suggest a floor of about $\\hbar/m$, but two kinds of measurement disagree.","posed_since":"2011","precise":"Unitary Fermi gas in 3D (and 2D). Determine the minimum longitudinal spin diffusivity $D_s$ and transverse diffusivity $D_{\\perp}$ in units of $\\hbar/m$, and whether a universal bound $D \\ge c \\hbar/m$ holds. Answer: the value of $c$ for each, and a theory explaining the difference.","problem_ref":"a87240394c001c90c7ea93a8c21346d60ca346b17fe614b6c98ba94f349b862e","references":"","settled_by":"Homogeneous measurements of both diffusivities in the same apparatus near $T_{c}$, with a theory that reproduces both.","status_note":"Longitudinal (Sommer et al. 2011, about $6.3\\,\\hbar/\\mathrm{m}$) and transverse (Bardon et al. 2014, about $1.3\\,\\hbar/\\mathrm{m}$) minima differ by about a factor of 5, and spin-hydrodynamics work continues in 2026 (arXiv:2607.29647).","title":"Universal lower bound on spin diffusion in strongly interacting 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