View source for Quantum mechanics

'''Quantum mechanics''' ('''QM'''; also known as '''quantum physics''', or '''quantum theory'''), including quantum field theory, is a fundamental branch of [[physics]] concerned with processes involving, for example, [[atom]]s and [[photons]].

== Description ==

In such processes, the action has been observed to be only in integer multiples of the [[Planck constant]], a physical quantity that is exceedingly, indeed perhaps ultimately, small. It is said to be [[Quantum|quantized]]. Other physical quantities change in corresponding discrete amounts. In the light of the laws of classical physics, this discreteness has no explanation, and seems perhaps mysterious. Moreover, it entails that the finest possible experimental preparation of a physical system is defined by quanta, and that the finest possible physical observation is of a single quantum. Physical measurements ''a priori'', in classical terms, assume that continuous measurements are possible. Consequently, quantum phenomena can be accurately described only if the elementary processes are conceived as [[Stochastic process|stochastic]] or [[Probability|probabilistic]], averaging to provide continuous measurements. The vast numbers of elementary quantum effects involved in everyday macroscopic observations mean that discrete quantum behaviors are usually hidden by much larger statistical effects, as [[averaging|averages]]. Quantum mechanics often leads to classical mechanics in macroscopic situations.

Important applications of quantum mechanical theory include [[superconducting magnet]]s, [[LED]]s and the [[laser]], the [[transistor]] and [[semiconductor]]s such as the [[microprocessor]], [[medical imaging|medical and research imaging]] such as [[MRI]] and [[electron microscope|electron microscopy]], and explanations for many biological and physical phenomena.

Two fundamental quantum mechanical principles are ''[[wave-particle duality]]'' (quanta exhibit both 'wave-like' behaviors such as [[refraction]] and 'particle-like' behavior), the ''[[uncertainty principle]]'' (attempting to measure one attribute such as [[velocity]] or [[position (vector)|position]] may cause another attribute to become ''less'' measurable), and ''[[Quantum superposition|superposition]]'' and the status of the ''[[Observer (quantum physics)|observer]]'' (a wave function superimposes multiple co-existing states that have different [[probability|probabilities]]; observation causes [[wave function collapse|collapse of the wave function]] to some specific state, in [[Interpretations of quantum mechanics|several interpretations]], as in the famous example of [[Schrödinger's cat|Schrödinger's Cat]]).

Quantum mechanics [[History of quantum mechanics|gradually arose]] from [[Max Planck]]'s solution in 1900 to the [[black-body radiation]] problem (reported 1859) and [[Albert Einstein]]'s 1905 paper which offered a quantum-based theory to explain the [[photoelectric effect]] (reported 1887).  Around 1900-1910, the [[atomic theory]] and the [[corpuscular theory of light]]<ref>{{cite book |title=Historical Encyclopedia of Natural and Mathematical Sciences, Volume 1 |first1=Ari  |last1=Ben-Menahem |publisher=Springer |year=2009 |isbn=3540688315 |page=3678 |url=http://books.google.com/books?id=9tUrarQYhKMC}}, [http://books.google.com/books?id=9tUrarQYhKMC&pg=PA3678 Extract of page 3678]</ref> first came to be widely accepted as scientific fact; these latter theories can be viewed as quantum theories of [[matter]] and [[electromagnetic radiation]], respectively. [[old quantum theory|Early quantum theory]] was significantly reformulated in the mid-1920s by [[Werner Heisenberg]], [[Max Born]] and [[Pascual Jordan]] ([[matrix mechanics]]); [[Louis de Broglie]] and [[Erwin Schrödinger]] ([[Schrödinger equation|wave mechanics]]); and [[Wolfgang Pauli]] and [[Satyendra Nath Bose]] (statistics of subatomic particles). Moreover, 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]]<ref>{{cite journal|last=van Hove|first=Leon|title=Von Neumann's contributions to quantum mechanics|journal=Bulletin of the American Mathematical Society|year=1958|volume=64|pages=Part2:95–99 |url=http://www.ams.org/journals/bull/1958-64-03/S0002-9904-1958-10206-2/S0002-9904-1958-10206-2.pdf |format=PDF|doi=10.1090/s0002-9904-1958-10206-2}}</ref> with greater emphasis on [[measurement in quantum mechanics|measurement]], the statistical nature of our knowledge of reality, and [[Interpretations of quantum mechanics|philosophical speculation about the 'observer']]. It has since permeated many disciplines including [[quantum chemistry]], [[quantum electronics]], [[quantum optics]], and [[quantum information science]]. Its speculative modern developments include [[string theory]] and [[quantum gravity]] theories. It also provides a useful framework for many features of the modern [[periodic table|periodic table of elements]], and describes the behaviors of [[atoms]] during [[chemical bond]]ing and the flow of [[electron]]s in [[computer]] [[semiconductor]]s, and therefore plays a crucial role in many modern technologies.

The [[mathematical formulations of quantum mechanics]] are abstract. A mathematical function, the [[wave function]], provides information about the [[probability amplitude]] of position, momentum, and other physical properties of a particle. Mathematical manipulations of the wave function usually involve [[bra–ket notation]], which requires an understanding of [[complex number]]s and [[Linear form|linear functional]]s. The wavefunction formulation treats the particle 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, in a quantum mechanical model, the lowest energy state of a system, the [[ground state]], is non-zero as opposed to a more "traditional" ground state with zero [[kinetic energy]] (all particles at rest). Instead of a traditional static, unchanging zero energy state, quantum mechanics allows for far more dynamic, chaotic possibilities, according to [[John Archibald Wheeler|John Wheeler]].

The term "[[Quantum# Etymology and discovery|quantum]]" itself (plural: ''quanta'') comes from the [[Latin]] word ''[[wikt:quantum|quantus]]'' meaning ''how much'' or ''as much as'', referring to discrete units of change.

== See also ==

* [[Physics]]

== External links == 

* [https://en.wikipedia.org/wiki/Quantum_mechanics Quantum mechanic] @ Wikipedia

[[Category:Physics]]