Brian Greene book The Fabric of the Cosmos
The Fabric of the Cosmos : Space, Time, and the Texture of Reality (2004), p. 17
Context: Superstring theory starts off by proposing a new answer to an old question: what are the smallest, indivisible constituents of matter? For many decades, the conventional answer has been that matter is composed of particles... that can be modeled as dots that are indivisible and that have no size and no internal structure. Conventional theory claims, and experiments confirm, that these particles combine in various ways to produce protons, neutrons, and a wide variety of atoms and molecules... Superstring theory tells a different story.... it does claim that these particles are not dots. Instead... every particle is composed of a tiny filament of energy, some hundred billion billion times smaller than a single atomic nucleus, which is shaped like a string. And just as a violin string can vibrate in different patterns, each of which produces a different musical tone, the filaments of superstring theory can also vibrate in different patterns. But these vibrations... produce different particle properties.... All species of particles are unified in superstring theory since each arises from a different vibrational pattern executed by the same underlying entity.