title: Semiclassical physics
text: # Semiclassical physics
  
  In physics, 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
  
  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.
  