The discovery of the neutron by James Chadwick in 1932 resolved an important problem in atomic physics: the discrepancy between atomic weight and atomic number. But it raised a new problem. If the nucleus consists only of positively-charged protons and neutral neutrons, how can it stay together? It would seem that the force of repulsion among protons would tear the nucleus apart. The resolution to this problem was suggested by the Japanese physicist, Hideki Yukawa, in 1935.

Yukawa reasoned that some very strong force must exist within the nucleus to hold protons and neutrons together. The force must act over only very short distances, the size of the nucleus (about 10-15 m), but dissipate rapidly over distances greater than that. To describe the nature of this force, Yukawa looked to the electromagnetic force for an analogy.

Scientists knew that charged particles interact with each other through the interchange of a...