Hideki Yukawa and the Pion

Once quantum electrodyamics had produced the picture of the electromagnetic force as a process of exchanging photons, the question of whether or not the other forces were also exchange forces was a natural one. In 1935, Hideki Yukawa reasoned that the electromagnetic force was infinite in range because the exchange particle was massless. He proposed that the short range strong force came about from the exchange of a massive particle which he called a meson. By observing that the effective range of the nuclear force was on the order of a fermi, a mass for the exchange particle could be predicted using the uncertainty principle. The predicted particle mass was about 100 MeV. It did not receive immediate attention since no one knew of a particle which fit that description.

In 1937 a particle of mass close to Yukawa's prediction was discovered in cosmic rays by Anderson & Neddermeyer and by Street & Stevenson in independent experiments. This particle, the muon, turned out not to interact by the strong interaction. Hans Bethe and Robert Marshak predicted that the muon could be a decay product of the particle sought. In 1947, Lattes, Muirhead, Occhialini and Powell conducted a high altitude experiment, flying photographic emulsions at 3000 meters. These emulsions revealed the pion, which met all the requirements of the Yukawa particle.


We now know that the pion is a meson, a composite particle, and the current view is that the strong interaction is an interaction between quarks, but the Yukawa theory stimulated a major advance in the understanding of the strong interaction.

Einstein talks with Hideki Yukawa, 1949 Nobel Laureate in Physics and John A. Wheeler, a Hopkins alum

Posted in: Astronomic, Hideki Yukawa, Quark