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- Semiclassical physics

**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.

Some 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.
- magnetic properties of materials and astrophysical bodies under the effect of large magnetic fields (see for example De Haas–Van Alphen effect)
- 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.

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

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- Book: Principles of Quantum Mechanics. P.A.M. Dirac. Paul Dirac. 4th. Clarendon Press. 1981. 978-0-19-852011-5.
- Book: General Principles of Quantum Mechanics. W. Pauli. Wolfgang Pauli. 1980. Springer. 3-540-09842-9.
- Book: R.P. Feynman . R.B. Leighton . M. Sands . 1965 . Feynman Lectures on Physics. Addison-Wesley. 3. 0-201-02118-8.
- Book: C.B. Parker. McGraw-Hill Encyclopaedia of Physics. McGraw-Hill. 2nd. 1994. 0-07-051400-3. registration.