The concept of an elementary particle is not based on measurements but instead depends on a theoretical framework. Ordinary matter is composed of
atoms, themselves once thought to be indivisible elementary particles. The name
atom comes from the Ancient Greek word
ἄτομος (
atomos) which means
indivisible or
uncuttable. Despite the
theories about atoms that had existed for
thousands of years, their factual existence remained controversial until 1905. In that year,
Albert Einstein published
his paper on
Brownian motion, putting to rest theories that had regarded
molecules as mathematical illusions. Einstein subsequently identified matter as ultimately composed of various concentrations of
energy. Subatomic constituents of the atom were first identified toward the end of the
19th century, beginning with the
electron, followed by the
proton in 1919, the
photon in the 1920s, and the
neutron in 1932. Many theoretical elaborations upon, and
beyond, the Standard Model have been made since its
codification in the 1970s. These include notions of
supersymmetry, which double the number of elementary particles by hypothesizing that each known particle associates with a "shadow" partner far more massive. However, like an
additional elementary boson mediating gravitation, such
superpartners remain undiscovered as of 2026. == Overview ==