A little over 50 years ago, Hideki Yukawa, a young Japanese theoretical physicist at the University of Osaka, proposed a fundamental theory of nuclear forces involving the exchange of massive charged particles between neutrons and protons. He called the exchanged particles “heavy quanta” to distinguish them from the light quanta of the electromagnetic field, but now they are known as pions, the lightest members of a large family of particles called mesons. These days, the meson theory seems to be a straightforward application of quantum field theory to the nuclear forces, but it could not have appeared so 50 years ago. Otherwise it would not have been invented by an obscure, unpublished Japanese physicist who had never even traveled abroad, but instead by one of the many Western physicists, some of them of great reputation, who were working on the theory of nuclear forces. Furthermore, Yukawa's paper was totally neglected for more than two years, although it was written in lucid English and published in a respected journal of rather wide circulation. The meson theory has turned out to be an important paradigm for the theory of elementary particles, as seminal as Ernest O. Lawrence's cyclotron has been for its experimental practice. In this article I will trace the intellectual trail Yukawa followed to arrive at his theory, using unpublished documents that have recently been discovered among Yukawa's papers at Kyoto University. (These documents have been organized and cataloged in the Yukawa Hall Archival Library, and I am grateful to Rokuo Kawabe and Michiji Konuma for translations and comments.)

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