The atom long was thought to be the most fundamental building block of all known matter; indeed, its name derives from the Greek word for indivisible (“atomos”). In the 1900s, however, scientists discovered that even smaller particles, known as protons and neutrons, composed the atomic nucleus. These nucleons then were believed to be “elementary” – that is, to have no known smaller parts. But experiments conducted in the 1960s began to suggest that protons and neutrons possessed an internal structure and therefore were “composite,” not elementary, particles.

In 1964, physicists Murray Gell-Mann and George Zweig, working independently at the California Institute of Technology, postulated the existence of subatomic particles even more basic than protons and neutrons. Borrowing an obscure line from the James Joyce novel Finnegan’s Wake (“Three quarks for Muster Mark”), Gell-Mann gave them the whimsical name “quarks.” In the late 1960s, three physicists at the Stanford Linear Accelerator Center demonstrated the existence of quarks; for this work, the Stanford scientists received the 1990 Nobel Prize in physics.

Quarks (pronounced “kworks”) appear to be truly elementary and are considered the ultimate building blocks, constituting more than 99.5% of every atom. Indeed, famed physicist Stephen Hawking believes that the very early universe likely was a “dense soup” of quarks and antiquarks.

PostScript: There are six types or “flavors” of quarks, differentiated by mass and electrical charge. The flavors arbitrarily are named up, down, strange, charmed, bottom, and top. Though the flavors often are given visual representations, quarks themselves are described as “point-like” and have yet to be observed in isolation.