II.

The Rise of Particle Physics

Particle physics is the latest stage in the study of smaller and smaller building blocks of matter. Before the 20th century, physicists studied the properties of bulk, or macroscopic, matter. In the late 19th century, however, the physics of atoms and molecules captured their attention. Atoms and molecules have diameters of about 10^-8 cm (about 4 × 10^-9 in), and the study of their structures resulted in the great achievements of quantum theory between 1925 and 1930. In the early 1930s physicists began investigating the structure of atomic nuclei, which have diameters of 10^-13 to 10^-12 cm (4 × 10^-14 to 4 × 10^-13 in). Enough was learned of nuclear structure to make practical use of nuclear energy, as in nuclear power generators and in nuclear weapons. In the years after World War II, however, physicists came to realize the necessity of studying the structure of elementary particles in order to understand the fundamental structure of atomic nuclei.