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| Max Plank (1858-1947) |
Quantum theory is the theoretical basis of modern physics that explains the nature and behavior of matter and energy on the atomic and subatomic level. In 1900, physicist Max Planck presented his quantum theory to the German Physical Society. Planck had sought to discover the reason that radiation from a glowing body changes in color from red, to orange, and, finally, to blue as its temperature rises. He found that by making the assumption that energy existed in individual units in the same way that matter does, rather than just as a constant electromagnetic wave - as had been formerly assumed - and was therefore quantifiable, he could find the answer to his question. The existence of these units became the first assumption of quantum theory.
Planck wrote a mathematical equation involving a figure to represent these individual units of energy, which he called quanta. The equation explained the phenomenon very well; Planck found that at certain discrete temperature levels (exact multiples of a basic minimum value), energy from a glowing body will occupy different areas of the color spectrum. Planck assumed there was a theory yet to emerge from the discovery of quanta, but, in fact, their very existence implied a completely new and fundamental understanding of the laws of nature. Planck won the Nobel Prize in Physics for his theory in 1918, but developments by various scientists over a thirty-year period all contributed to the modern understanding of quantum theory.
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| photograph of 5th Solvay Conefernce oct 1927 chaired by Hendrik Lorentz |
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Introduction
Quantum mechanics "QM; also known as quantum physics, or quantum theory" is a branch of physics which deals with physical phenomena at nanoscopic scales where the action is on the order of the Planck constant. It departs from classical mechanics primarily at the quantum realm of atomic and subatomic length scales. Quantum mechanics provides a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. Quantum mechanics provides a substantially useful framework for many features of the modern periodic table of elements including the behavior of atoms during chemical bonding and has played a significant role in the development of many modern technologies.
In advanced topics of quantum mechanics, some of these behaviors are macroscopic (see macroscopic quantum phenomena) and emerge at only extreme (i.e., very low or very high) energies or temperatures (such as in the use of superconducting magnets). For example, the angular momentum of an electron bound to an atom or molecule is quantized. In contrast, the angular momentum of an unbound electron is not 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 earliest versions of quantum mechanics were formulated in the first decade of the 20th century. About this time, the atomic theory and the corpuscular theory of light (as updated by Einstein)[1] first came to be widely accepted as scientific fact; these latter theories can be viewed as quantum theories of matter and electromagnetic radiation, respectively. 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 (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[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 permeated throughout many aspects of 20th-century physics and other disciplines including 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.
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.
Quantum mechanics: is the science of the very small: the body of scientific principles that explains the behaviour of matter and its interactions with energy on the scale of atoms and subatomic particles.
Classical physics explains matter and energy on a scale familiar to human experience, including the behaviour of astronomical bodies. It remains the key to measurement for much of modern science and technology. However, toward the end of the 19th century, scientists discovered phenomena in both the large (macro) and the small (micro) worlds that classical physics could not explain.
[1] Coming to terms with these limitations led to two major revolutions in physics – one being the theory of relativity, the other being the development of quantum mechanics. This article describes how physicists discovered the limitations of classical physics and developed the main concepts of the quantum theory that replaced it in the early decades of the 20th century. These concepts are described in roughly the order in which they were first discovered; for a more complete history of the subject, see History of quantum mechanics.
The word "quantum" in this sense means the minimum amount of any physical entity involved in an interaction. Certain characteristics of matter can take only discrete values.
Light behaves in some respects like particles and in other respects like waves. Matter—particles such as electrons and atoms—exhibits wavelike behaviour too. Some light sources, including neon lights, give off only certain discrete frequencies of light. Quantum mechanics shows that light and all other forms of electromagnetic radiation comes in discrete units, called photons, and predicts its energies, colours, and spectral intensities.
Some aspects of quantum mechanics can seem counterintuitive or even paradoxical, because they describe behaviour quite different from that seen at larger length scales. In the words of Richard Feynman, quantum mechanics deals with "nature as She is – absurd." For example, the uncertainty principle of quantum mechanics means that the more closely one pins down one measurement (such as the position of a particle), the less precise another measurement pertaining to the same particle (such as its momentum) must become.
Time Line
1897 : Pieter Zeeman shows that light is radiated by the motion of charged particles in an atom, and Joseph John (J.J.) Thomson discovers the electron.
1900 : Max Planck explains blackbody radiation in the context of quantized energy emission: Quantum theory is born.
1905 : Albert Einstein proposes that light, which has wavelike properties, also consists of discrete, quantized bundles of energy, which are later called photons.
1911 : Ernest Rutherford proposes the nuclear model of the atom.
1913 : Niels Bohr proposes his planetary model of the atom, along with the concept of stationary energy states, and accounts for the spectrum of hydrogen.
1914 : James Franck and Gustav Hertz confirm the existence of stationary states through an electron-scattering experiment.
1923 : Arthur Compton observes that x-rays behave like miniature billiard balls in their interactions with electrons, thereby providing further evidence for the particle nature of light.
1923 : Louis de Broglie generalizes wave-particle duality by suggesting that particles of matter are also wavelike.
1924 : Satyendra Nath Bose and Albert Einstein find a new way to count quantum particles, later called Bose-Einstein statistics, and they predict that extremely cold atoms should condense into a single quantum state, later known as a Bose-Einstein condensate.
1925
• Wolfgang Pauli enunciates the exclusion principle.
• Werner Heisenberg, Max Born, and Pascual Jordan develop matrix mechanics, the first version of quantum mechanics, and make an initial step toward quantum field theory.
1926
• Erwin Schrodinger develops a second description of quantum physics, called wave mechanics. It includes what becomes one of the most famous formulas of science, which is later known as the Schrodinger equation.
• Enrico Fermi and Paul A.M. Dirac find that quantum mechanics requires a second way to count particles, Fermi-Dirac statistics, opening the way to solid-state physics.
• Dirac publishes a seminal paper on the quantum theory of light.
1927 : Heisenberg states his Uncertainty Principle, that it is impossible to exactly measure the position and momentum of a particle at the same time.
1928 : Dirac presents a relativistic theory of the electron that includes the prediction of antimatter.
1932 : Carl David Anderson discovers antimatter, an antielectron called the positron.
1934 : Hideki Yukawa proposes that nuclear forces are mediated by massive particles called mesons, which are analogous to the photon in mediating electromagnetic forces.
1946-48 :Experiments by Isidor I. Rabi, Willis Lamb, and Polykarp Kusch reveal discrepancies in the Dirac theory.
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| Richard Feynman "(Richard Phillips Feynman) " May 11, 1918 – February 15, 1988 was an American theoretical physicist known for his work in the path integral formulation of quantum mechanics |
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1948 : Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga develop the first complete theory of the interaction of photons and electrons, quantum electrodynamics, which accounts for the discrepancies in the Dirac theory.
1957 : John Bardeen, Leon Cooper, and J. Robert Schrieffer show that electrons can form pairs whose quantum properties allow them to travel without resistance, providing an explanation for the zero electrical resistance of superconductors. This theory was later termed the BCS theory (after the surname initials of the three physicists).
1959 : Yakir Aharonov and David Bohm predict that a magnetic field affects the quantum properties of an electron in a way that is forbidden by classical physics. The Aharonov-Bohm effect is observed in 1960 and hints at a wealth of unexpected macroscopic effects.
1960 : Building on work by Charles Townes, Arthur Schawlow, and others, Theodore Maiman builds the first practical laser.
1964 : John S. Bell proposes an experimental test, "Bell's inequalities," of whether quantum mechanics provides the most complete possible description of a system.
1970 : Foundations are laid for the standard model of particle physics, in which matter is said to be built of quarks and leptons that interact via the four physical forces.
1982 : Alain Aspect carries out an experimental test of Bell's inequalities and confirms the completeness of quantum mechanics.
1995: Eric Cornell, Carl Wieman, and Wolfgang Ketterle trap clouds of metallic atoms cooled to less than a millionth of a degree above absolute zero, producing Bose-Einstein condensates, which were first predicted 70 years earlier. This accomplishment leads to the creation of the atom laser and superfluid gase