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In physics
Physics

Physics is the natural science which examines basic concepts such as energy, force, and spacetime and all that derives from these, such as mass, charge, matter and its Motion ....
, the adjective semiclassical has different precise meanings depending on the context. All these meanings usually refer to some approximation, limit or situation that combines quantum
Quantum mechanics

Quantum mechanics is a set of principles underlying the most fundamental known description of all physical systems at the microscopic scale . Notable amongst these principles are both a dual wave-like and particle-like behavior of matter and radiation, and prediction of probabilities in situations where classical physics predicts certaintie...
 and classical
Classical mechanics

Classical mechanics is used for describing the motion of macroscopic objects, from projectiles to parts of machinery, as well as astronomical objects, such as spacecraft, planets, stars, and galaxies....
 aspects in a given problem. Some of the possible significations are the following:

First, semiclassical approximation may refer to quantum-mechanical calculations that are obtained by considering a small perturbation of a classical calculation, for example the WKB approximation
WKB approximation

In physics, the WKB approximation, also known as WKBJ approximation, is the most familiar example of a semiclassical calculation in quantum mechanics in which the wavefunction is recast as an exponential function, semiclassically expanded, and then either the amplitude or the phase is taken to be slowly changing....
 in non-relativistic quantum mechanics
Quantum mechanics

Quantum mechanics is a set of principles underlying the most fundamental known description of all physical systems at the microscopic scale . Notable amongst these principles are both a dual wave-like and particle-like behavior of matter and radiation, and prediction of probabilities in situations where classical physics predicts certaintie...
 or the loop expansion or the instanton
Instanton

An instanton or pseudoparticle is a notion appearing in theoretical and mathematical physics. Mathematically, a Yang-Mills instanton is a self-dual or anti-self-dual connection in a principal bundle over a four-dimensional Riemannian manifold that plays the role of physical space-time in nonabelian gauge theory....
 methods in quantum field theory
Quantum field theory

Quantum field theory or QFT provides a theoretical framework for constructing quantum mechanics models of systems classically described by field or of Many-body problem....
.






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In physics
Physics

Physics is the natural science which examines basic concepts such as energy, force, and spacetime and all that derives from these, such as mass, charge, matter and its Motion ....
, the adjective semiclassical has different precise meanings depending on the context. All these meanings usually refer to some approximation, limit or situation that combines quantum
Quantum mechanics

Quantum mechanics is a set of principles underlying the most fundamental known description of all physical systems at the microscopic scale . Notable amongst these principles are both a dual wave-like and particle-like behavior of matter and radiation, and prediction of probabilities in situations where classical physics predicts certaintie...
 and classical
Classical mechanics

Classical mechanics is used for describing the motion of macroscopic objects, from projectiles to parts of machinery, as well as astronomical objects, such as spacecraft, planets, stars, and galaxies....
 aspects in a given problem. Some of the possible significations are the following:

First, semiclassical approximation may refer to quantum-mechanical calculations that are obtained by considering a small perturbation of a classical calculation, for example the WKB approximation
WKB approximation

In physics, the WKB approximation, also known as WKBJ approximation, is the most familiar example of a semiclassical calculation in quantum mechanics in which the wavefunction is recast as an exponential function, semiclassically expanded, and then either the amplitude or the phase is taken to be slowly changing....
 in non-relativistic quantum mechanics
Quantum mechanics

Quantum mechanics is a set of principles underlying the most fundamental known description of all physical systems at the microscopic scale . Notable amongst these principles are both a dual wave-like and particle-like behavior of matter and radiation, and prediction of probabilities in situations where classical physics predicts certaintie...
 or the loop expansion or the instanton
Instanton

An instanton or pseudoparticle is a notion appearing in theoretical and mathematical physics. Mathematically, a Yang-Mills instanton is a self-dual or anti-self-dual connection in a principal bundle over a four-dimensional Riemannian manifold that plays the role of physical space-time in nonabelian gauge theory....
 methods in quantum field theory
Quantum field theory

Quantum field theory or QFT provides a theoretical framework for constructing quantum mechanics models of systems classically described by field or of Many-body problem....
. In quantum field theory, a semiclassical correction arises from one-loop Feynman diagram
One-loop Feynman diagram

In physics, a one-loop Feynman diagram is a connected component Feynman diagram with only one cycle . Such a diagram can be obtained from a connected tree diagram by taking two external lines of the same type and joining them together into an edge....
s. The semiclassical effective action
Effective action

In quantum field theory, the effective action is a modified expression for the action , which takes into account quantum-mechanical corrections, in the following sense:...
 is

Second, in the context of open quantum system
Open quantum system

In physics, an open quantum system is a quantum mechanics system which is found to be in interaction with an external quantum system, the environment....
s and measurement theory, where one considers the dynamics of a given quantum system in interaction with an environment, the semiclassical regime may refer to the situation in which the wavefunction of the system is approximately peaked around the solution of the corresponding classical equations of motion. Corrections to the classical trajectory and the dispersion of the solution around the mean value are usually considered.

Third, semiclassical gravity
Semiclassical gravity

Semiclassical gravity is the approximation to the theory of quantum gravity in which one treats matter fields as being quantum and the Gravitation as being classical....
 is the approximation to the yet unknown theory of quantum gravity
Quantum gravity

Quantum gravity is the field of theoretical physics attempting to unify quantum mechanics, which describes three of the Fundamental interaction , with general relativity, the theory of the fourth fundamental force: Gravitation....
 in which one treats matter fields as being quantum and the gravitational field as being classical. The classical Einstein equations are computed with the expectation value of the quantum matter fields in the classical background. Semiclassical gravity has applications in black hole
Black hole

In general relativity, a black hole is a region of space in which the gravitational field is so powerful that nothing, including electromagnetic radiation , can escape its pull after having fallen past its event horizon....
 physics and physical cosmology
Physical cosmology

Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of our universe and is concerned with fundamental questions about its formation and evolution....
.

A semiclassical approximation is any high frequency approximation
High frequency approximation

A high frequency approximation for scattering or other wave propagation problems, in physics or engineering, is an approximation whose accuracy increases with the size of features on the scatterer or medium relative to the wavelength of the scattered particles....
 (or "high energy approximation"), less extreme than classical mechanics, that is used to approximate quantum mechanics.

Fourthly, certain early theories in quantum mechanics, notably the Bohr model
Bohr model

In atomic physics, the Bohr model created by Niels Bohr depicts the atom as a small, positively charged atomic nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar in structure to the solar system, but with electrostatic forces providing attraction, rather than gravity....
 of the hydrogen atom, are sometimes called semi-classical, as they incorporate the newer quantum ideas into an essentially classical framework.