An Etymological Dictionary of Astronomy and Astrophysics
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فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory

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Number of Results: 50 Search : theory
grand unified theory (GUT)
  نگره‌ی ِ یگانش ِ بزرگ   
negare-ye yegâneš-e bozorg (#)

Fr.: théorie de la grande unification   

Any physical theory that unites the strong, electromagnetic, and weak interactions at high energy. It is hoped that GUTs can ultimately be extended to incorporate gravity. → theory of everything.

M.E. graunt, from O.Fr. grant, grand, from L. grandis "big, great," also "full-grown;" unified, p.p. of → unify; → theory.

Negâré, → theory; yegâneš, verbal noun of yegânestan, → unify; bozorggreat.

graph theory
  نگره‌ی ِ نگاره   
negare-ye negâré

Fr.: théorie des graphes   

The branch of → mathematics dealing with → graphs. In particular, it involves the ways in which sets of points (→ vertex) can be connected by lines or arcs (→ edge).

graph; → theory.

gravitational-field theory
  نگره‌ی ِ میدان ِ گرانشی   
negare-ye meydân-e gerâneši (#)

Fr.: théorie de champ gravitationnel   

A theory that treats gravity as a field rather than a force acting at a distance.

gravitational; → field.

group theory
  نگره‌ی ِ گروه   
negare-ye goruh (#)

Fr.: théorie des groupes   

A branch of mathematics concerned with structures called → groups and the description of their properties. Group theory provides a powerful formal method of analyzing abstract and physical systems in which → symmetry is present. It has a very considerable use in physics, especially → quantum mechanics, notably in analyzing the → eigenstates of energy of a physical system.

group; → theory.

information theory
  نگره‌ی ِ ازدایش   
negare-ye azdâyeš (#)

Fr.: théorie de l'information   

The mathematical theory that defines, quantifies, and analyzes the concept of → information. It involves → probability theory in → transmission of → messages when the → bits of information are subject to various distortions. Its goal is to enable as much information as possible to be reliably stored on a medium, retrieved, or communicated.
See also:
information content, → information entropy, → information flow, → information paradox, → information science, → information theory, → quantum information, → entropy.

information; → theory.

Jordan-Brans-Dicke theory
  نگره‌ی ِ یوردان-برانز-دایک   
negare-ye Jordan-Brans-Dicke

Fr.: théorie de Jordan-Brans-Dicke   

A relativistic theory of gravitation which involves a → scalar field in addition to the → metric (→ tensor field) used in rarr; general relativity. It obeys the → equivalence principle, but tries at the same time to comply with → Mach's principle owing to possible spatial and temporal variations of the → gravitational constant, which is inversely proportional to the scalar field. The theory uses a new dimensionless parameter to determine the discrepancy between its predictions and those of general relativity. So far there is no firm indication of its validity. Same as → scalar-tensor theory.

Named after the creators, Carl Brans (1935-) and Robert Dicke (1916-1997), who presented the theory in 1961, based on the initial work of Pascual Jordan (1902-1980); → theory.

kinetic theory of gases
  نگره‌ی ِ جنبشی ِ گاز‌ها   
negare-ye jonbeši-ye gâzhâ (#)

Fr.: théorie cinétique des gaz   

A theory that explains macroscopic properties of gases, such as pressure, temperature, or volume, by considering their molecular composition and motion.

kinetic; → theory; → gas.

linear perturbation theory
  نگره‌ی ِ پرتورش ِ خطی   
negare-ye partureš-e xatti

Fr.: théorie de perturbation linéaire   

Assumption that the variations in the plasma parameters, due to the presence of waves, are small (to the first order) as compared to the undisturbed parameters. This makes it possible to linearize equations by dropping out second order (and higher) nonlinear terms.

linear; → perturbation; → theory.

Mestel theory
  نگره‌ی ِ مستل   
negare-ye Mestel

Fr.: théorie de Mestel   

The first quantitative model showing that the energy of → white dwarfs is the leftover heat from the star's past nuclear fusion that leaks slowly into space. In this analytic model constructed by Mestel (1952), a white dwarf consists of two layers. The inner layer, which contains most of the mass, is assumed to be → isothermal because of efficient thermal conductivity by the → degenerate electrons. Moreover, it is supposed that the electrons do not contribute significantly to the → heat capacity. The heat capacity comes entirely from the ions, which are assumed to behave as a classical → ideal gas. The thin non-degenerate outer layer forms an insulating blanket and controls the rate at which the energy from the ion reservoir is leaked out into space. The specific rate is controlled by the radiative opacity at the boundary between these two layers, and is assumed to obey → Kramers' opacity law. The Mestel theory shows that the cooling rate of a white dwarf is proportional to its temperature (hotter white dwarfs cool faster), and gives a relationship between the luminosity (L) of the white dwarf and the cooling time: t ∝ L-5/7. More recent models take into account some or all of the following processes neglected in the Mestel theory: neutrino cooling (important for L > 10-1.5 Lsun), latent heat of crystallization release (important for L < 10-4 Lsun), nuclear energy generation via proton-proton burning (important when MH ≥ 10-4 M*), and gravitational energy release from surface layers. The Mestel theory is a very good approximation of more recent calculations. For a review of the Mestel theory see Van Horn (1971, IAU Symp. 42, 97; W. J. Luyten, Editor), Wood (1990, J. Roy. Astro. Soc. Canada 84, 150), and Kepler and Brdaley (1995, Baltic Astron. 4, 166).

Named after Leon Mestel (1927-), British astrophysicist, who put forward this theory in 1952 (MNRAS, 112, 583); → theory.

Mie theory
  نگره‌ی ِ می   
negare-ye Mie

Fr.: théorie de Mie   

The explanation of the → scattering of → electromagnetic waves by → homogeneous spheres of arbitrary → size and → composition using analytical solutions of → Maxwell's equations. See also: → Mie scattering, → Rayleigh scattering.

Mie scattering; → theory.

mixing length theory (MLT)
  نگره‌ی ِ درازای ِ آمیزش   
negare-ye derâzâ-ye âmizeš

Fr.: théorie de la longueur de mélange   

A theory dealing with heat transport by → turbulence which includes an elementary treatment of → convection. The central idea is that an unbalanced → buoyancy force drives a → convective cell to move through a distance, called the → mixing length, before the cell dissolves and joins the ambient medium. In this theory an adjustable → mixing length parameter  α is used. The theory, originally due to L. Prandtl (1925), was first applied to the Sun by L. Biermann (1932, Z. Astrophys. 5, 117).

mixing; → length; → theory.

number theory
  نگره‌ی ِ عددها   
negare-ye adadhâ

Fr.: théories des nombres   

The branch of mathematics that studies the relationship between integers and their generalization.

number; → theory.

Orion correlation theory
  نگره‌ی ِ هم‌باز‌آنش ِ اوریون   
negare-ye hambâzâneš-e Oryon

Fr.: théorie de la corrélation d'Orion   

A controversial proposition according to which a coincidence would exist between the mutual positions of the three stars of → Orion's Belt and those of the main Giza pyramids. More specifically, Khufu, Khafre, and Menkaure would be the monumental representation of → Alnitak, → Alnilam, and → Mintaka, respectively.

Orion; → correlation; → theory.

phlogiston theory
  نگره‌ی ِ فلوژیستون   
negare-ye fložiston

Fr.: phlogistique   

An obsolete theory of combustion in which all flammable objects were supposed to contain a substance called → phlogiston, which was released when the object burned. The existence of this hypothetical substance was proposed in 1669 by Johann Becher, who called it terra pinguis "fat earth." For example, as wood burns it releases phlogiston into the air, leaving ash behind. Ash was therefore wood minus phlogiston. In the early 18th century Georg Stahl renamed the substance phlogiston. The theory was disproved by Antoine Lavoisier in 1783, who proved the principle of conservation of mass, refuted the phlogiston theory and proposed the oxygen theory of burning.

phlogiston; → theory.

probability theory
  نگره‌ی ِ شوانایی   
negare-ye šavânâyi

Fr.: théorie des probabilités   

A branch of → mathematics with its own axioms and methods, which is based on the concept of → randomness and is concerned with the possible outcome of given → events and their relative → likelihoods and → distributions.

probability; → theory.

quantum field theory
  نگره‌ی ِ کو‌آنتومی ِ میدان   
negare-ye kuântomi-ye meydân

Fr.: théorie quantique des champs   

The quantum mechanical theory based on the assumption that the interactions between particles and fields are mediated by messenger particles. Accordingly, particles are → quanta of a field, just s photons are quanta of light. All fields display a granular structure in interaction. QFT is the framework in which quantum mechanics and → special relativity are successfully reconciled (→ Dirac equation). It forms the basis of today's particle physics.

quantum; → field; → theory.

quantum theory
  نگره‌ی ِ کو‌آنتومی   
negare-ye kuântomi (#)

Fr.: théorie quantique   

The theoretical basis of modern physics which describes the behavior and interactions of elementary particles or energy states based on the assumptions that energy is subdivided into discrete amounts and that matter possesses wave properties. → quantum mechanics; → quantum field theory.

quantum; → theory.

quasi-linear theory
  نگره‌ی ِ چونان-خطی   
negare-ye cunân-xatti

Fr.: théorie quasi-linéaire   

In plasma physics, the theory that considers the interactions between waves and particles are of first order only. It ignores all terms of second order in the fluctuating quantities.

quasi-; → linear; → theory.

relativity theory
  نگره‌ی ِ بازانیگی   
negare-ye bâzânigi

Fr.: théorie de la relativité   

The → theory of relativity.

theory; → relativity.

scalar-tensor theory
  نگره‌ی ِ مرپلی-تانسوری   
negare-ye marpel-tânsori

Fr.: théorie scalaire-tensorielle   

An alternative to the standard → general relativity of gravity that contains not only the → tensor field (or → metric), but also a → scalar field. In this formalism, the → gravitational constant is considered to vary over time. As a consequence, the measured strength of the gravitational interaction is a function of time. Same as → Jordan-Brans-Dicke theory.

scalar; → tensor; → theory.

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