In that case, protons in virtual contact, as within the nucleus, would attract each other, but if the distance between them was increased sufficiently to place one outside the nucleus, the nuclear interaction would decrease in intensity to less than the electromagnetic repulsion. The proton would now be repelled by the positive charge of the nucleus and would go flying away. That is why atomic nuclei have to be so small; it is only when they are so tiny that the nuclear interaction can hold them together. In 1932 Heisenberg tried to work out how these interactions might come into being. He suggested that attractions and repulsions were the result of particles being constantly and rapidly exchanged by the bodies experiencing the attractions and repulsions. Under some conditions, these “exchange particles” moving back and forth very rapidly between 2 bodies might force those bodies apart; under other conditions they might pull those bodies together. In the case of the electromagnetic interaction, the exchange particles seemed to be “photons”, wave packets that made up gamma rays, X rays, or even ordinary light (all of which are examples of “electromagnetic radiation”). The gravitational interaction would be the result of exchange particles called “gravitons”. (In 1969, there were reports that gravitons had actually been detected.) Both the photon and the graviton have zero mass and there is a connection between that and the fact that electromagnetic interaction and gravitational interaction decline only slowly with distance. For a nuclear interaction, which declines very rapidly with distance, the exchange particle (if any) would have to have mass. In 1935 the Japanese physicist Hideki Yukawa (1907- ) worked out in considerable detail the theory of such exchange particles in order to decide what kind of properties the one involved in the nuclear interaction would have. He decided it ought to have a mass about 250 times that of an electron, which would make it about ¹/₇ as massive as a proton. Since this mass is intermediate between that of an electron and proton, such particles eventually came to be called “mesons” from a Greek word meaning “intermediate”. Once Yukawa published his theory, the search was on for the hypothetical mesons. Ideally, if they existed within the nucleus, shooting back and forth between protons and neutrons, there ought to be some way of knocking them out of the nucleus and studying them in isolation. Unfortunately, the bombarding particles at the disposal of physicists in the 1930s possessed far too little energy to knock mesons out of nuclei, assuming they were there in the first place.