Classical physics
Classical physics is a term used to describe physics outside of quantum theory. Occassionally the term is taken to imply physics prior to relativity as well, although this definition is lessed used. Roughly taken, 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. Moroever, the classical theory of electromagnetic radiation is somewhat limited in its ability to provide correct descriptions, since light is inherently a quantum phenomenon.
Physicists use the term "on the classical level" to refer to physical systems in which the laws of classical physics are valid. Unlike quantum physics, classical physics is generally characterized by the principle of complete determinism.
Among the branches of theory included in classical physics are:
Mathematically, classical physics equations are generally ones in which Planck's constant does not appear.
The current paradigm of physics is that the fundamental laws of nature are the laws of quantum physics, and that classical theory is the by-product of quantum rules applies to the macroscopic realm. At present, this conjecture is more easily assumed than proven, however, and one of the most vigorous on-going fields of research in physics is classical-quantum correspondence. This field of research is concerned with which 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.
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