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Quantum Mechanics

Quantum mechanics (QM – also known as quantum physics, or quantum theory) is a

branch of physicswhich deals with physical phenomena at microscopic scales, where

the action is on the order of the Planck constant. Quantum mechanics departs from classical

mechanics primarily at the quantum realm of atomic andsubatomic length scales. Quantum

mechanics provides a mathematical description of much of the dual particle-likeand wave-

like behavior and interactions of energy andmatter.

In advanced topics of quantum mechanics, some of these behaviors are macroscopic and

emerge at only extreme (i.e., very low or very high) energies ortemperatures.[citation needed] The

name quantum mechanics derives from the observation that some physical quantities can

change only in discrete amounts (Latin quanta), and not in a continuous (cf. analog) way. For

example, the angular momentum of an electron bound to an atom or molecule is quantized.[1] 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 mechanicsare abstract. A mathematical function known

as thewavefunction 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

numbersand linear functionals. The wavefunction treats the object as a quantum harmonic oscillator,

and the mathematics is akin to that describing acoustic resonance. Many of the results of quantum

mechanics are not easily visualized in terms of classical mechanics—for instance, the ground

state in a quantum mechanical model is a non-zero energy state that is the lowest permitted

energy state of a system, as opposed to 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.

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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 thecorpuscular 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 andelectromagnetic radiation, respectively. Early quantum

theory was significantly reformulated in the mid-1920s byWerner Heisenberg, Max

Born and Pascual Jordan, who created matrix mechanics; Louis de Broglie and Erwin

Schrödinger (Wave Mechanics); and Wolfgang Pauli andSatyendra Nath Bose (statistics of subatomic

particles). And the Copenhagen interpretation of Niels Bohr became widely accepted. By 1930,

quantum mechanics had been further unified and formalized by the work of David Hilbert, Paul

Dirac and John von Neumann,[2] 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.

Scientific inquiry into the wave nature of light go back to the 17th and 18th centuries when

scientists such as Robert Hooke, Christian Huygens andLeonhard Euler proposed a wave theory of

light based on experimental observations.[3] In 1803,Thomas Young, an English polymath,

performed the famous double-slit experiment that he later described in a paper entitled ―On the

nature of light and colours‖. This experiment played a major role in the general acceptance of

the wave theory of light.

In 1838 with the discovery of cathode rays byMichael Faraday, these studies were followed by the

1859 statement of the black-body radiationproblem by Gustav Kirchhoff, the 1877 suggestion

by Ludwig Boltzmann that the energy states of a physical system can be discrete, and the 1900

quantum hypothesis of Max Planck.[4] Planck‘s hypothesis that energy is radiated and absorbed

in discrete ―quanta‖ (or ―energy elements‖) precisely matched the observed patterns of black-

body radiation.

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