nLab classical physics

The term ‘classical physics’ usually denotes the part of physics which is based on exploring fundamental laws of nature which do not involve quantum physics, or from another point of view, currently accepted areas of physics whose theoretical foundations in an almost canonical form existed before the discovery of quantum mechanics. This includes classical mechanics, continuum mechanics, classical electrodynamics, classical fluid mechanics (with its main part hydrodynamics (and enriched versions like magnetohydrodynamics), dynamics of classical plasma, dynamics of gaseous media…), aerodynamics (dynamics of gases and motion of bodies moving within fluids), thermodynamics, classical statistical mechanics, geometrical and wave optics and the areas of phenomena which can be explained using these. For example, due to high speeds quantum phenomena are surpressed in the study of plasma (e.g. research on fusion).

Depending on context, relativity theory (both special and general) may or may not be included in classical physics. In the Lagrangean and Hamiltonian approaches, special relativity can be easily introduced into a classical mechanical system; general relativity, however, brings up the problem of time?. Classical electrodynamics (with the Maxwell equation) is implicitly also a relativistic theory, even if it was originally described in a way hiding this fact.

Early (pre-quantum) theories which are generally rejected even in the non-quantum regime, for example the theory of light based on the concept of the luminiferous ether, do not usually count as classical physics today. (In the case of the ether, the deciding role was played by the appearance of relativity theory and the experiments supporting it.)