Quantum Physics
<p>The earliest versions of quantum mechanics were formulated in the first decade of the 20th century. At around the same time, the <a href="http://en.wikipedia.org/wiki/Atomic_theory" title="Atomic theory">atomic theory</a> and the <a href="http://en.wikipedia.org/wiki/Corpuscular_theory_of_light" title="Corpuscular theory of light">corpuscular theory of light</a> (as updated by Einstein) first came to be widely accepted as scientific fact; these latter theories can be viewed as quantum theories of <a href="http://en.wikipedia.org/wiki/Matter" title="Matter">matter</a> and <a href="http://en.wikipedia.org/wiki/Electromagnetic_radiation" title="Electromagnetic radiation">electromagnetic radiation</a>, respectively. <a href="http://en.wikipedia.org/wiki/Old_quantum_theory" title="Old quantum theory">Early quantum theory</a> was significantly reformulated in the mid-1920s by <a href="http://en.wikipedia.org/wiki/Werner_Heisenberg" title="Werner Heisenberg">Werner Heisenberg</a>, <a href="http://en.wikipedia.org/wiki/Max_Born" title="Max Born">Max Born</a>, <a href="http://en.wikipedia.org/wiki/Wolfgang_Pauli" title="Wolfgang Pauli">Wolfgang Pauli</a> and their collaborators, and the <a href="http://en.wikipedia.org/wiki/Copenhagen_interpretation" title="Copenhagen interpretation">Copenhagen interpretation</a> of <a href="http://en.wikipedia.org/wiki/Niels_Bohr" title="Niels Bohr">Niels Bohr</a> became widely accepted. By 1930, quantum mechanics had been further unified and formalized by the work of <a href="http://en.wikipedia.org/wiki/Paul_Dirac" title="Paul Dirac">Paul Dirac</a> and <a href="http://en.wikipedia.org/wiki/John_von_Neumann" title="John von Neumann">John von Neumann</a>, with a greater emphasis placed on <a href="http://en.wikipedia.org/wiki/Measurement_in_quantum_mechanics" title="Measurement in quantum mechanics">measurement in quantum mechanics</a>, the statistical nature of our knowledge of reality, and philosophical speculation about the <a href="http://en.wikipedia.org/wiki/Quantum_mind%E2%80%93body_problem" title="Quantum mind–body problem">role of the observer</a>. Quantum mechanics has since branched out into almost every aspect of 20th century physics and other disciplines, such as <a href="http://en.wikipedia.org/wiki/Quantum_chemistry" title="Quantum chemistry">quantum chemistry</a>, <a href="http://en.wikipedia.org/wiki/Quantum_electronics" title="Quantum electronics" class="mw-redirect">quantum electronics</a>, <a href="http://en.wikipedia.org/wiki/Quantum_optics" title="Quantum optics">quantum optics</a>, and <a href="http://en.wikipedia.org/wiki/Quantum_information_science" title="Quantum information science">quantum information science</a>. Much 19th century physics has been re-evaluated as the "classical limit" of quantum mechanics, and its more advanced developments in terms of <a href="http://en.wikipedia.org/wiki/Quantum_field_theory" title="Quantum field theory">quantum field theory</a>, <a href="http://en.wikipedia.org/wiki/String_theory" title="String theory">string theory</a>, and speculative <a href="http://en.wikipedia.org/wiki/Quantum_gravity" title="Quantum gravity">quantum gravity</a> theories.
 
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<div class="rellink relarticle mainarticle">Main article: <a href="http://en.wikipedia.org/wiki/History_of_quantum_mechanics" title="History of quantum mechanics">History of quantum mechanics</a></div>
<p>The history of quantum mechanics dates back to the 1838 discovery of <a href="http://en.wikipedia.org/wiki/Cathode_rays" title="Cathode rays" class="mw-redirect">cathode rays</a> by <a href="http://en.wikipedia.org/wiki/Michael_Faraday" title="Michael Faraday">Michael Faraday</a>. This was followed by the 1859 statement of the <a href="http://en.wikipedia.org/wiki/Black_body_radiation" title="Black body radiation" class="mw-redirect">black body radiation</a> problem by <a href="http://en.wikipedia.org/wiki/Gustav_Kirchhoff" title="Gustav Kirchhoff">Gustav Kirchhoff</a>, the 1877 suggestion by <a href="http://en.wikipedia.org/wiki/Ludwig_Boltzmann" title="Ludwig Boltzmann">Ludwig Boltzmann</a> that the energy states of a physical system can be discrete, and the 1900 quantum hypothesis of <a href="http://en.wikipedia.org/wiki/Max_Planck" title="Max Planck">Max Planck</a>.<sup id="cite_ref-1" class="reference"><a href="http://en.wikipedia.org/wiki/Quantum_physics#cite_note-1">[2]</a></sup> Planck's hypothesis that energy is radiated and absorbed in discrete "quanta" (or "energy elements") precisely matched the observed patterns of blackbody radiation. According to Planck, each energy element <i>E</i> is proportional to its <a href="http://en.wikipedia.org/wiki/Frequency" title="Frequency">frequency</a> <i>ν</i>:</p>
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Planck is considered the father of the Quantum Theory</div>
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<p>where <i>h</i> is <a href="http://en.wikipedia.org/wiki/Planck_constant" title="Planck constant">Planck's constant</a>. Planck (cautiously) insisted that this was simply an aspect of the <i>processes</i> of absorption and emission of radiation and had nothing to do with the <i>physical reality</i> of the radiation itself.<sup id="cite_ref-2" class="reference"><a href="http://en.wikipedia.org/wiki/Quantum_physics#cite_note-2">[3]</a></sup> However, in 1905 <a href="http://en.wikipedia.org/wiki/Albert_Einstein" title="Albert Einstein">Albert Einstein</a> interpreted Planck's quantum hypothesis <a href="http://en.wikipedia.org/wiki/Local_realism" title="Local realism" class="mw-redirect">realistically</a> and used it to explain the <a href="http://en.wikipedia.org/wiki/Photoelectric_effect" title="Photoelectric effect">photoelectric effect</a>, in which shining light on certain materials can eject electrons from the material.</p>
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The 1927 <a href="http://en.wikipedia.org/wiki/Solvay_Conference" title="Solvay Conference">Solvay Conference</a> in <a href="http://en.wikipedia.org/wiki/Brussels" title="Brussels">Brussels</a>.</div>
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<p>The foundations of quantum mechanics were established during the first half of the 20th century by <a href="http://en.wikipedia.org/wiki/Max_Planck" title="Max Planck">Max Planck</a> , <a href="http://en.wikipedia.org/wiki/Niels_Bohr" title="Niels Bohr">Niels Bohr</a>, <a href="http://en.wikipedia.org/wiki/Werner_Heisenberg" title="Werner Heisenberg">Werner Heisenberg</a>, <a href="http://en.wikipedia.org/wiki/Louis_de_Broglie" title="Louis de Broglie">Louis de Broglie</a>, <a href="http://en.wikipedia.org/wiki/Arthur_Compton" title="Arthur Compton">Arthur Compton</a>, <a href="http://en.wikipedia.org/wiki/Albert_Einstein" title="Albert Einstein">Albert Einstein</a>, <a href="http://en.wikipedia.org/wiki/Erwin_Schr%C3%B6dinger" title="Erwin Schrödinger">Erwin Schrödinger</a>, <a href="http://en.wikipedia.org/wiki/Max_Born" title="Max Born">Max Born</a>, <a href="http://en.wikipedia.org/wiki/John_von_Neumann" title="John von Neumann">John von Neumann</a>, <a href="http://en.wikipedia.org/wiki/Paul_Dirac" title="Paul Dirac">Paul Dirac</a>, <a href="http://en.wikipedia.org/wiki/Enrico_Fermi" title="Enrico Fermi">Enrico Fermi</a>, <a href="http://en.wikipedia.org/wiki/Wolfgang_Pauli" title="Wolfgang Pauli">Wolfgang Pauli</a>, <a href="http://en.wikipedia.org/wiki/David_Hilbert" title="David Hilbert">David Hilbert</a>, <a href="http://en.wikipedia.org/wiki/Wilhelm_Wien" title="Wilhelm Wien">Wilhelm Wien</a> , <a href="http://en.wikipedia.org/wiki/Satyendra_Nath_Bose" title="Satyendra Nath Bose">Satyendra Nath Bose</a>, <a href="http://en.wikipedia.org/wiki/Arnold_Sommerfeld" title="Arnold Sommerfeld">Arnold Sommerfeld</a> and <a href="http://en.wikipedia.org/wiki/Category:Quantum_physicists" title="Category:Quantum physicists">others</a>. In the mid-1920s, developments in quantum mechanics led to its becoming the standard formulation for atomic physics. In the summer of 1925, Bohr and Heisenberg published results that closed the <a href="http://en.wikipedia.org/wiki/Old_quantum_theory" title="Old quantum theory">"Old Quantum Theory"</a>. Out of deference to their particle-like behavior in certain processes and measurements, light quanta came to be called <a href="http://en.wikipedia.org/wiki/Photons" title="Photons" class="mw-redirect">photons</a> (1926). From Einstein's simple postulation was born a flurry of debating, theorizing, and testing. Thus the entire field of quantum physics emerged, leading to its wider acceptance at the Fifth <a href="http://en.wikipedia.org/wiki/Solvay_Conference" title="Solvay Conference">Solvay Conference</a> in 1927.
The other <a href="http://en.wikipedia.org/wiki/Exemplar" title="Exemplar">exemplar</a> that led to quantum mechanics was the study of <a href="http://en.wikipedia.org/wiki/Electromagnetic_wave" title="Electromagnetic wave" class="mw-redirect">electromagnetic waves</a>, such as visible <a href="http://en.wikipedia.org/wiki/Light" title="Light">light</a>. When it was found in 1900 by Max Planck that the energy of waves could be described as consisting of small packets or "quanta", Albert Einstein further developed this idea to show that an electromagnetic wave such as <a href="http://en.wikipedia.org/wiki/Light" title="Light">light</a> could be described as a particle (later called the <a href="http://en.wikipedia.org/wiki/Photon" title="Photon">photon</a>) with a discrete quantum of energy that was dependent on its frequency.<sup id="cite_ref-3" class="reference"><a href="http://en.wikipedia.org/wiki/Quantum_physics#cite_note-3">[4]</a></sup> This led to a <a href="http://en.wikipedia.org/wiki/Photon_polarization" title="Photon polarization">theory of unity</a> between <a href="http://en.wikipedia.org/wiki/Subatomic_particles" title="Subatomic particles" class="mw-redirect">subatomic particles</a> and electromagnetic waves, called <a href="http://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality" title="Wave–particle duality">wave–particle duality</a>, in which particles and waves were neither one nor the other, but had certain properties of both.
While quantum mechanics traditionally described the world of the very small, it is also needed to explain certain recently investigated <a href="http://en.wikipedia.org/wiki/Macroscopic" title="Macroscopic" class="mw-redirect">macroscopic</a> systems such as <a href="http://en.wikipedia.org/wiki/Superconductivity" title="Superconductivity">superconductors</a> and <a href="http://en.wikipedia.org/wiki/Superfluid" title="Superfluid" class="mw-redirect">superfluids</a>.
The word <i>quantum</i> derives from the <a href="http://en.wikipedia.org/wiki/Latin_language" title="Latin language" class="mw-redirect">Latin</a>, meaning "how great" or "how much".<sup id="cite_ref-4" class="reference"><a href="http://en.wikipedia.org/wiki/Quantum_physics#cite_note-4">[5]</a></sup> In quantum mechanics, it refers to a discrete unit that quantum theory assigns to certain <a href="http://en.wikipedia.org/wiki/Physical_quantity" title="Physical quantity">physical quantities</a>, such as the <a href="http://en.wikipedia.org/wiki/Energy" title="Energy">energy</a> of an <a href="http://en.wikipedia.org/wiki/Atom" title="Atom">atom</a> at rest (see Figure 1). The discovery that particles are discrete packets of energy with wave-like properties led to the branch of physics dealing with atomic and sub-atomic systems which is today called quantum mechanics. It is the underlying <a href="http://en.wikipedia.org/wiki/Mathematical" title="Mathematical" class="mw-redirect">mathematical</a> framework of many fields of <a href="http://en.wikipedia.org/wiki/Physics" title="Physics">physics</a> and <a href="http://en.wikipedia.org/wiki/Chemistry" title="Chemistry">chemistry</a>, including <a href="http://en.wikipedia.org/wiki/Condensed_matter_physics" title="Condensed matter physics">condensed matter physics</a>, <a href="http://en.wikipedia.org/wiki/Solid-state_physics" title="Solid-state physics">solid-state physics</a>, <a href="http://en.wikipedia.org/wiki/Atomic_physics" title="Atomic physics">atomic physics</a>, <a href="http://en.wikipedia.org/wiki/Molecular_physics" title="Molecular physics">molecular physics</a>, <a href="http://en.wikipedia.org/wiki/Computational_physics" title="Computational physics">computational physics</a>, <a href="http://en.wikipedia.org/wiki/Computational_chemistry" title="Computational chemistry">computational chemistry</a>, <a href="http://en.wikipedia.org/wiki/Quantum_chemistry" title="Quantum chemistry">quantum chemistry</a>, <a href="http://en.wikipedia.org/wiki/Particle_physics" title="Particle physics">particle physics</a>, <a href="http://en.wikipedia.org/wiki/Nuclear_chemistry" title="Nuclear chemistry">nuclear chemistry</a>, and <a href="http://en.wikipedia.org/wiki/Nuclear_physics" title="Nuclear physics">nuclear physics</a>.<sup id="cite_ref-5" class="reference"><a href="http://en.wikipedia.org/wiki/Quantum_physics#cite_note-5">[6]</a></sup> Some fundamental aspects of the theory are still actively studied<sup id="cite_ref-6" class="reference"><a href="http://en.wikipedia.org/wiki/Quantum_physics#cite_note-6">[7]</a></sup>
Quantum mechanics is essential to understanding the behavior of systems at <a href="http://en.wikipedia.org/wiki/Atom" title="Atom">atomic</a> length scales and smaller. For example, if <a href="http://en.wikipedia.org/wiki/Classical_mechanics" title="Classical mechanics">classical mechanics</a> truly governed the workings of an atom, <a href="http://en.wikipedia.org/wiki/Electron" title="Electron">electrons</a> would rapidly travel toward, and collide with, the <a href="http://en.wikipedia.org/wiki/Atomic_nucleus" title="Atomic nucleus">nucleus</a>, making stable atoms impossible. However, in the natural world electrons normally remain in an uncertain, non-deterministic, "smeared", <a href="http://en.wikipedia.org/wiki/Probability" title="Probability">probabilistic</a> wave–particle wavefunction orbital path around (or through) the nucleus, defying <a href="http://en.wikipedia.org/wiki/Classical_electromagnetism" title="Classical electromagnetism">classical electromagnetism</a>.<sup id="cite_ref-7" class="reference"><a href="http://en.wikipedia.org/wiki/Quantum_physics#cite_note-7">[8]</a></sup>
Quantum mechanics was initially developed to provide a better explanation of the atom, especially the differences in the <a href="http://en.wikipedia.org/wiki/Spectrum" title="Spectrum">spectra</a> of <a href="http://en.wikipedia.org/wiki/Light" title="Light">light</a> emitted by different <a href="http://en.wikipedia.org/wiki/Isotopes" title="Isotopes" class="mw-redirect">isotopes</a> of the same <a href="http://en.wikipedia.org/wiki/Chemical_element" title="Chemical element">element</a>. The quantum theory of the atom was developed as an explanation for the electron remaining in its <a href="http://en.wikipedia.org/wiki/Atomic_orbital" title="Atomic orbital">orbit</a>, which could not be explained by <a href="http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion" title="Newton's laws of motion">Newton's laws of motion</a> and <a href="http://en.wikipedia.org/wiki/Maxwell%27s_equations" title="Maxwell's equations">Maxwell's laws</a> of (classical) electromagnetism.
Broadly speaking, quantum mechanics incorporates four classes of phenomena for which classical physics cannot account:</p>