The often‐quoted remark of I. I. Rabi, made when the muon was discovered—“Who ordered that?”—is again an apt commentary about the third charged lepton to be found, the tau. A recent experiment, by Gary Feldman, George Trilling and their collaborators at the PEP electron‐positron collider at SLAC has determined the lifetime of the tau. They find it to be (4.6±1.9)×10−13 sec, a value entirely consistent with the universality of the weak interaction. That is, the weak interaction between the tau and other particles appears, within experimental limits, to be the same as that of the electron and muon. They're all spin‐1/2, charged, pointlike particles, differing only in mass: the electron with 0.511003 MeV, the muon with 105.6595 MeV and the tau with 1782 MeV.

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