Quantum mechanics
<p ><b>Quantum mechanics</b> (<b>QM</b>; also known as <b>quantum physics</b>, or <b>quantum theory</b>), including quantum field theory, is a fundamental branch of <a href="https://en.wikipedia.org/wiki/Physics" title="Physics" >physics</a>concerned with processes involving, for example, <a href="https://en.wikipedia.org/wiki/Atom" title="Atom" >atoms</a> and <a href="https://en.wikipedia.org/wiki/Photons" title="Photons" class="mw-redirect" >photons</a>. In such processes, said to be quantized, the <a href="https://en.wikipedia.org/wiki/Action_(physics)" title="Action (physics)" >action</a> has been observed to be only in integer multiples of the <a href="https://en.wikipedia.org/wiki/Planck_constant" title="Planck constant" >Planck constant</a>, a physical quantity that is exceedingly, indeed perhaps ultimately, small. This is utterly inexplicable in <a href="https://en.wikipedia.org/wiki/Classical_physics" title="Classical physics" >classical physics</a>.
Quantum mechanics <a href="https://en.wikipedia.org/wiki/History_of_quantum_mechanics" title="History of quantum mechanics" >gradually arose</a> from <a href="https://en.wikipedia.org/wiki/Max_Planck" title="Max Planck" >Max Planck</a>'s solution in 1900 to the <a href="https://en.wikipedia.org/wiki/Black-body_radiation" title="Black-body radiation" >black-body radiation</a> problem (reported 1859) and <a href="https://en.wikipedia.org/wiki/Albert_Einstein" title="Albert Einstein" >Albert Einstein</a>'s 1905 paper which offered a quantum-based theory to explain the <a href="https://en.wikipedia.org/wiki/Photoelectric_effect" title="Photoelectric effect" >photoelectric effect</a> (reported 1887). <a href="https://en.wikipedia.org/wiki/Old_quantum_theory" title="Old quantum theory" >Early quantum theory</a> was significantly reformulated in the mid-1920s.
The <a href="https://en.wikipedia.org/wiki/Mathematical_formulations_of_quantum_mechanics" title="Mathematical formulations of quantum mechanics" class="mw-redirect" >mathematical formulations of quantum mechanics</a> are abstract. A mathematical function, the <a href="https://en.wikipedia.org/wiki/Wave_function" title="Wave function" >wave function</a>, provides information about the <a href="https://en.wikipedia.org/wiki/Probability_amplitude" title="Probability amplitude" >probability amplitude</a> of position, momentum, and other physical properties of a particle.
Important applications of quantum mechanical theory include <a href="https://en.wikipedia.org/wiki/Superconducting_magnet" title="Superconducting magnet" >superconducting magnets</a>, <a href="https://en.wikipedia.org/wiki/LED" title="LED" class="mw-redirect" >LEDs</a> and the <a href="https://en.wikipedia.org/wiki/Laser" title="Laser" >laser</a>, the <a href="https://en.wikipedia.org/wiki/Transistor" title="Transistor" >transistor</a> and <a href="https://en.wikipedia.org/wiki/Semiconductor" title="Semiconductor" >semiconductors</a> such as the<a href="https://en.wikipedia.org/wiki/Microprocessor" title="Microprocessor" >microprocessor</a>, <a href="https://en.wikipedia.org/wiki/Medical_imaging" title="Medical imaging" >medical and research imaging</a> such as <a href="https://en.wikipedia.org/wiki/MRI" title="MRI" class="mw-redirect" >MRI</a> and <a href="https://en.wikipedia.org/wiki/Electron_microscope" title="Electron microscope" >electron microscopy</a>, and explanations for many biological and physical phenomena.</p>