Classical physics

Classical physics consists of scientific theories in the field of physics that are non-quantum or both non-quantum and non-relativistic, depending on the context. In historical discussions, classical physics refers to pre-1900 physics, while modern physics refers to post-1900 physics, which incorporates elements of quantum mechanics and the theory of relativity. However, relativity is based on classical field theory rather than quantum field theory, and is often categorized as a part of "classical physics".

Overview

Classical theory has at least two distinct meanings in physics. It can include all those areas of physics that do not make use of quantum mechanics, which includes classical mechanics (using any of the Newtonian, Lagrangian, or Hamiltonian formulations), as well as classical electrodynamics and relativity. Alternatively, the term can refer to theories that are neither quantum or relativistic.

Depending on point of view, among the branches of theory sometimes included in classical physics are:

Classical mechanics

Newton's laws of motion

Classical Lagrangian and Hamiltonian formalisms

Classical electrodynamics (Maxwell's equations)

Classical thermodynamics

Comparison with modern physics

In contrast to classical physics, "modern physics" is usually used to focus on those revolutionary changes created by quantum physics and the theory of relativity.

A physical system can be described by classical physics when it satisfies conditions such that the laws of classical physics are approximately valid.

In practice, physical objects ranging from those larger than atoms and molecules to macroscopic and astronomical objects, can be well-described (understood) with classical mechanics. Beginning at the atomic level and lower, the laws of classical physics break down and generally do not provide a correct description of nature.