Introduction: the quantum theory

The quantum theory was born in 1900, with the twentieth century, and future centuries will list it among our own's most remarkable achievements. Designed to account for the puzzling behavior of matter at the submicroscopic scale of individual atoms, the theory has enjoyed phenomenal success. It has accounted in a quantitative way for atomic phenomena with a numerical precision never before achieved in any field of science. When it was subsequently applied within the atom to the atomic nucleus – an object some hundred thousand times smaller – no further modification in the theory was needed. More recently, when applied within the particles making up the nucleus to describe their own internal structure – an investigation on a scale as much smaller than atoms as atoms are smaller than us – the quantum theory still shows no signs of requiring extension or revision.