In physics, 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 and classical aspects in a given problem. The plurality of meanings comes from the fact that the passage from quantum to classical mechanics is generally a very difficult task. Some of the possible significations are the following: The willingness to question previously held truths and search for new answers resulted in a period of major scientific advancements, now known as the Scientific Revolution. ... Fig. ... In physics, Classical mechanics is one of the two major sub-fields of study in the science of mechanics, which is concerned with the motions of bodies, and the forces that cause them. ...

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 in non-relativistic quantum mechanics or the loop expansion or the instanton methods in quantum field theory. In quantum field theory, a semiclassical correction arises from one-loop Feynman diagrams. 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 approximated by The phase is simply determined by the condition that the rate of its change is fully determined by the energy E... Fig. ... In quantum field theory, an instanton is a topologically nontrivial field configuration in four-dimensional Euclidean space (considered as the Wick rotation of Minkowski spacetime). ... Quantum field theory (QFT) is the application of quantum mechanics to fields. ... In physics, a one-loop Feynman diagram is a connected Feynman diagram with only one cycle (unicyclic). ...

Second, in the context of open quantum systems 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. In physics, an open quantum system is a quantum system which is found to be in interaction with an external quantum system, the environment. ...

Third, semiclassical gravity is the approximation to the yet unknown theory of quantum gravity 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 physics and cosmology. Semiclassical gravity is the approximation to the theory of quantum gravity in which one treats matter fields as being quantum and the gravitational field as being classical. ... Quantum gravity is the field of theoretical physics attempting to unify the theory of quantum mechanics, which describes three of the fundamental forces of nature, with general relativity, the theory of the fourth fundamental force: gravity. ... An artists impression of a black hole with a closely orbiting companion star that exceeds its Roche limit. ... Wiktionary has a definition of: Cosmology For the jazz band, see: Cosmology (band) Cosmology, from the Greek: κοσμολογία (κόσμος world + λογια discourse) is the study of the universe in its totality and by extension mans place in it. ...