Following the discovery of radioactivity by A. H. Becquerel in 1896, Ernest Rutherford identified two types of radiation given off by natural radioactive substances and named them alpha and beta; a third, gamma, was later identified. In 1911 he bombarded a thin target of gold foil with alpha rays (subsequently identified as helium nuclei) and found that, although most of the alpha particles passed directly through the foil, a few were deflected by large amounts. By a quantitative analysis of his experimental results, he was able to propose the existence of the nucleus and estimate its size and charge.
After the discovery of the neutron in 1932, physicists turned their attention to the understanding of the strong interactions, or strong nuclear force, that bind protons and neutrons together in nuclei. This force must be great enough to overcome the considerable repulsive force existing between several protons because of their electrical charge. It must exist between nucleons without regard to their charge, since it acts equally on protons and neutrons, and it must not extend very far away from the nucleons (i.e., it must be a short-range force), since it has negligible effect on protons or neutrons outside the nucleus.
In 1935 Hideki Yukawa proposed a theory that this nuclear "glue" was produced by the exchange of a particle between nucleons, just as the electromagnetic force is produced by the exchange of a photon between charged particles. The range of a force is dependent on the mass of the particle carrying the force; the greater the mass of the particle, the shorter the range of the force. The range of the electromagnetic force is infinite because the mass of the photon is zero. From the known range of the nuclear force, Yukawa estimated the mass of the hypothetical carrier of the nuclear force to be about 200 times that of the electron. Given the name meson because its mass is between that of the electron and those of the nucleons, this particle was finally observed in 1947 and is now called the pi meson, or pion, to distinguish it from other mesons that have been discovered (see elementary particles).
Both the proton and the neutron are surrounded by a cloud of pions given off and reabsorbed again within an incredibly short interval of time. Certain other mesons are assumed to be created and destroyed in this way as well, all such particles being termed "virtual" because they exist in violation of the law of conservation of energy (see conservation laws) for a very short span of time allowed by the uncertainty principle. It is now known, however, that at a more fundamental level the actual carrier of the strong force is a particle called the gluon.