In 1935 the Japanese physicist Hideki Yukawa made the daring assumption that even the "strong" nuclear force, with a range of a femtometer, was mediated by particles, just as the electron is bound to the proton in a hydrogen atom by exchange of photons. The short range can be explained by assuming that these mediator particles, different from the photon, are massive Þ have a rest mass of 140 MeV/c2

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