For thousands of years, scientists thought atoms were the smallest building blocks of matter (in fact the word atom comes from the Greek atomos, meaning indivisible). Only in the past century have physicists realized that atoms are composed of still smaller particles: protons, electrons, and neutrons. And for most of that time, they thought that these subatomic particles were the fundamental units of matter.
In the 1960s, however, experiments began to show that protons and neutrons display internal structure and thus must be made of still smaller particles.
Dubbed “quarks” by the American physicist Murray Gell-Mann after a line from James Joyce’s FINNEGAN’S WAKE, these particles come in six “flavors,” called up, down, strange, charmed, top, and bottom. A proton consists of two up quarks and one down quark, while a neutron is made of two downs and one up. Combinations of quarks yield still more particles, although most are unstable and decay quickly to protons or neutrons. The very early universe was likely a dense soup of quarks and antiquarks.