By 1932, scientists had identified the basic structure of an atom: a nucleus consisting of protons and neutrons, surrounded by one or more electrons. This model, however, presented a difficult puzzle. How was it that the nucleus, whose only charged particles (protons) all carried the same charge, could stay together? It would appear that the force of electrostatic repulsion among the protons would tend to drive a body with this composition apart.

A number of physicists attempted to solve this problem. Werner Heisenberg suggested in 1932, for example, that protons and neutrons might exchange their identities very rapidly. Two protons might not have time to repel each other, he said, if one or both instantaneously became a neutron.

Another solution was suggested by the Japanese physicist Hideki Yukawa in 1935. Yukawa recognized that any force holding two like-charged particles together must be a very strong force. He suggested that...