Physics in the Classical Limit

Classical physics is physics based on principles developed before the rise of general theory of relativity and quantum theory, usually including the special theory of relativity.

Scope
Among the branches of theory included in classical physics are:


 * Classical mechanics
 * Newton's laws of motion
 * Classical Lagrangian and Hamiltonian formalisms
 * Classical electrodynamics (Maxwell's Equations)
 * Classical thermodynamics
 * Special theory of relativity and General theory of relativity
 * Classical chaos theory and nonlinear dynamics

Differences
In contrast to classical physics, modern physics is a slightly looser term which may refer to just quantum physics or to 20th and 21st century physics in general and so always includes quantum theory and may include relativity.

A physical system on the classical level is a physical system in which the laws of classical physics are valid. There are no restrictions on the application of classical principles, but, practically, the scale of classical physics is the level of isolated atoms and molecules on upwards, including the macroscopic and astronomical realm. Inside the atom and among atoms in a molecule, the laws of classical physics break down and generally do not provide a correct description.

Moreover, the classical theory of electromagnetic radiation is somewhat limited in its ability to provide correct descriptions, since light is inherently a quantum phenomenon. Unlike quantum physics, classical physics is generally characterized by the principle of complete determinism.

Mathematically, classical physics equations are ones in which Planck's constant does not appear. According to the correspondence principle and Ehrenfest's theorem as a system becomes larger or more massive (action >> Planck's constant) the classical dynamics tends to emerge, with some exceptions, such as superfluidity. This is why we can usually ignore quantum mechanics when dealing with everyday objects; instead the classical description will suffice. However, one of the most vigorous on-going fields of research in physics is classical-quantum correspondence. This field of research is concerned with the discovery of how the laws of quantum physics give rise to classical physics in the limit of the large scales of the classical level.