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Quantum mechanics
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Quantum mechanics
Introduction
Glossary · History
Bra-ket notation · Classical mechanics
Hamiltonian · Interference
Old quantum theory
Complementarity · Decoherence
Duality · Ehrenfest theorem
Entanglement · Exclusion
Measurement · Probability amplitude
Nonlocality · Quantum state
Superposition · Tunnelling
Uncertainty · Wave function
Bell's inequality · Davisson–Germer
Delayed choice quantum eraser
Double-slit · Elitzur–Vaidman
Popper · Quantum eraser
Schrödinger's cat · Stern–Gerlach
Wheeler's delayed choice
Formulations
Heisenberg · Interaction
Matrix mechanics · Schrödinger
Sum over histories
Dirac · Klein–Gordon
Pauli · Rydberg
Schrödinger
Interpretations (overview)
Consciousness-caused
Consistent histories
Copenhagen · de Broglie–Bohm
Ensemble · Hidden variables
Many-worlds · Objective collapse
Pondicherry · Quantum logic
Relational · Stochastic
Transactional
Quantum chaos · Quantum field theory
Quantum information science
Scattering theory
Bell · Bohm · Bohr · Born · Bose
de Broglie · Dirac · Ehrenfest
Everett · Feynman · Heisenberg
Jordan · Kramers · von Neumann
Pauli · Planck · Schrödinger
Sommerfeld · Wien · Wigner
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Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics dealing with physical phenomena where the action is of the order of Planck constant; quantum mechanics departs from classical mechanics primarily at the atomic and subatomic scales, the so-called quantum realm. It provides a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. In advanced topics of quantum mechanics, some of these behaviors are macroscopic and only emerge at very low or very high energies or temperatures. The name "quantum mechanics" derives from the observation that some physical quantities can change only by discrete amounts, or quanta in Latin. For example, the angular momentum of an electron bound to an atom or molecule is quantized. In the context of quantum mechanics, the wave–particle duality of energy and matter and the uncertainty principle provide a unified view of the behavior of photons, electrons and other atomic-scale objects. The mathematical formulations of quantum mechanics are abstract. A mathematical function called the wavefunction provides information about the probability amplitude of position, momentum, and other physical properties of a particle. Mathematical manipulations of the wavefunction usually involve the bra-ket notation, which requires an understanding of complex numbers and linear functionals. The wavefunction treats the object as a quantum harmonic oscillator and the mathematics is akin to that of acoustic resonance. Many of the results of quantum mechanics are not easily visualized in terms of classical mechanics; for instance, the ground state in the quantum mechanical model is a non-zero energy state that is the lowest permitted energy state of a system, rather than a more traditional system that is thought of as simply being at rest with zero kinetic energy. Instead of a traditional static, unchanging zero state, quantum mechanics allows for far more dynamic, chaotic possibilities, according to John Wheeler.The earliest versions of quantum mechanics were formulated in the first decade of the 20th century. At around the same time, the atomic theory and the corpuscular theory of light (as updated by Einstein) first came to be widely accepted as scientific fact; these latter theories can be viewed as quantum theories of matter and electromagnetic radiation. The early quantum theory was significantly reformulated in the mid-1920s by Werner Heisenberg, Max Born, Wolfgang Pauli and their associates, and the Copenhagen interpretation of Niels Bohr became widely accepted. By 1930, quantum mechanics had been further unified and formalized by the work of Paul Dirac and John von Neumann, with a greater emphasis placed on measurement in quantum mechanics, the statistical nature of our knowledge of reality and philosophical speculation about the role of the observer. Quantum mechanics has since branched out into almost every aspect of 20th century physics and other disciplines such as quantum chemistry, quantum electronics, quantum optics and quantum information science. Much 19th century physics has been re-evaluated as the classical limit of quantum mechanics, and its more advanced developments in terms of quantum field theory, string theory, and speculative quantum gravity theories.
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