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