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Yet it happens in every experiment.","posed_since":"","precise":"For the first-order A-B transition, homogeneous nucleation with the measured interface tension gives a critical radius of order 1e-4 cm and a barrier of order $1e6 k_B T$, so rates of order $\\exp(-1e6)$. Identify the actual nucleation mechanism (local heating by cosmic rays in the 'Baked Alaska' scenario, resonant tunneling between phases, nucleation at surfaces, textures or impurities) and predict supercooling statistics versus pressure, field and cell geometry. An answer is a mechanism quantitatively reproducing measured supercooling distributions.","problem_ref":null,"references":"","settled_by":"Supercooling statistics measured with controlled radiation shielding and controlled wall roughness, compared with each mechanism's predicted rate.","status_note":"Path-dependent supercooling in confined cells (Lotnyk et al. 2021) and supercooling of the A phase in an isolated chamber (Tian et al., Nature Communications 2023) were not explained by any proposed mechanism.","title":"Why the 3He A-to-B transition nucleates far faster than 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