# Semiclassical physics

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**Semiclassical physics**, or simply **semiclassical** refers to a theory in which one part of a system is described quantum-mechanically whereas the other is treated classically. For example, external fields will be constant, or when changing will be classically described. In general, it incorporates a development in powers of Planck's constant, resulting in the classical physics of power 0, and the first nontrivial approximation to the power of (−1). In this case, there is a clear link between the quantum-mechanical system and the associated semi-classical and classical approximations, as it is similar in appearance to the transition from physical optics to geometric optics.

## Instances[edit]

Four examples of a semiclassical approximation include:

- WKB approximation: electrons in classical external electromagnetic fields.
- semiclassical gravity: quantum field theory within a classical curved gravitational background (see general relativity).
- quantum chaos; quantization of classical chaotic systems.
- quantum field theory, only Feynman diagrams with at most a single closed loop (see for example one-loop Feynman diagram) are considered, which corresponds to the powers of Planck's constant.

## See also[edit]

- Bohr model
- Correspondence principle
- Eikonal approximation
- Einstein–Brillouin–Keller method
- Old quantum theory

## References[edit]

- R. Resnick; R. Eisberg (1985).
*Quantum Physics of Atoms, Molecules, Solids, Nuclei and Particles*(2nd ed.). John Wiley & Sons. ISBN 978-0-471-87373-0. - P.A.M. Dirac (1981).
*Principles of Quantum Mechanics*(4th ed.). Clarendon Press. ISBN 978-0-19-852011-5. - W. Pauli (1980).
*General Principles of Quantum Mechanics*. Springer. ISBN 3-540-09842-9. - R.P. Feynman; R.B. Leighton; M. Sands (1965).
*Feynman Lectures on Physics*.**3**. Addison-Wesley. ISBN 0-201-02118-8. - C.B. Parker (1994).
*McGraw-Hill Encyclopaedia of Physics*(2nd ed.). McGraw-Hill. ISBN 0-07-051400-3.