Fr.: théorie de bifurcation
1) A theory which studies how, in certain nonlinear systems, there may be paths
and shifts in behavior dependent on small changes in circumstances or the current position
of the system.
→ bifurcation; → theory.
Big Bang theory
negare-ye Meh Bâng, ~ Big Bang
Fr.: théorie du big bang
Fr.: théorie de capture
One of the first scientific hypotheses about the formation of the Moon, according to which the Moon formed elsewhere in the solar system and was pulled into a stable orbit by Earth's gravity. Observational facts do not confirm this hypothesis. For example, analysis of rocks from the Apollo landings confirm the Moon is made of similar material and rock as the Earth from about the same time and have almost identical oxygen isotopes in them. Moreover, a captured moon, like Mars' → Phobos and → Deimos do not have a spherical shape. See also → giant impact hypothesis, → fission theory, → co-formation theory.
Cartesian vortex theory
negare-ye gerdšâr-e Descartes
Fr.: théorie des vortex de Descartes
A mechanical model put forward before Newton's theory of gravity to explain the revolution of the planets around the Sun. Descartes in his 1644 Principia Philosophiae postulated that the space between the Sun and the planets is filled with matter in the form of a fluid. The fluid rotates in countless whirlpools, one for each planet, thus carrying the planets along in their flow. The vortices vary in size and are contiguous as well as nested. Descartes believed that two objects can exert force on each other only when they are in physical contact. This is why he postulated that space is filled with matter. Newton refuted the vortex theory, using the principle of → action at a distance on which relies his → law of universal gravitation.
Fr.: théorie des catégories
A theory that deals with the concept of → category and generalizes the → set theory.
Fr.: théorie du chaos
The theory of unpredictable behavior that can arise in systems obeying deterministic scientific laws.
classical field theory
negare-ye klâsik-e meydân
Fr.: théorie classique des champs
The theory that studies distributions of → energy, → matter, and other physical quantities under circumstances where their discrete nature is unimportant. Classical field theory traditionally includes → Newtonian mechanics, Maxwell's → electromagnetic theory, and Einstein's theory of → general relativity. The main scope of classical field theory is to construct the mathematical description of → dynamical systems with an infinite number of degrees of freedom. The word "classical" is used in contrast to those field theories that incorporate → quantum mechanics (→ quantum field theory). Classical field theories are usually categorized as → non-relativistic and → relativistic.
Fr.: théorie de co-formation
A theory according to which the Earth and Moon formed from the → protoplanetary disk at the same time. See also → giant impact hypothesis, → capture theory, → fission theory.
corpuscular theory of light
negare-ye karpuli-ye nur
Fr.: théorie corpusculaire de la lumière
Newton's theory according to which light is made up of point-like particles without any mass. It failed to explains several phenomena: simultaneous reflection and refraction at a semi-transparent boundary, interference, diffraction and polarization. Moreover, it requested that the speed of light be greater in a denser medium than in a rarer medium; this prediction is contrary to experimental results. In 1924 Louis de Broglie postulated that matter has not only a corpuscular nature but also a wave nature, and subsequent experiments confirmed de Broglie's model.
density wave theory
negare-ye mowj-e cagâli
Fr.: théorie des ondes de densité
One possible explanation for → spiral arms,
first put forward by B. Lindblad in about 1925 and developed later by
C.C. Lin and F. H. Shu. According to this theory, spiral arms are not material
structures, but regions of somewhat enhanced density, created by
→ density waves. Density waves are perturbations amplified by
the self-gravity of
the → galactic disk. The perturbation results from natural
non-asymmetry in the disk and enhanced by environmental processes, such as galaxy encounters.
Density waves rotate around the → galactic center and periodically
compress the disk material upon their passage. If the spiral arms were
rigid structures rotating like a pinwheel,
the → differential rotation
of the galaxy would wind up the arms completely in a relatively
short time (with respect to the age of the galaxy), → winding problem.
Inside the region defined by the → corotation radius,
density waves rotate more slowly than the galaxy's stars and gas; outside that
region they rotate faster.
Fr.: théorie déterministe
A theory in which specification of the initial value of all relevant variables of the system is sufficient to calculate the past values and to predict the future values of such variables for any arbitrary time. Moreover, it is possible, for any arbitrary time, to assign a value to all the variables characterizing the system. In quantum mechanics, the time evolution of the → wave function, governed by the → Schrodinger equation, is deterministic. Quantum mechanics, however, is a non deterministic theory because of the probabilistic nature of the predictions for the values of the → observables of a quantum system.
→ deterministic; → theory.
Dvali-Gabadadze-Porrati theory (DGP)
Fr.: théorie de Dvali-Gabadadze-Porrati
A → braneworld theory in which the → space-time is locally embedded in a five dimensional space, the → bulk, and has as a key aspect leakage of gravitational energy into the bulk. More specifically, the → graviton is pinned to a four-dimensional braneworld by intrinsic curvature terms induced by quantum matter fluctuations. But as it propagates over large distances, the graviton eventually evaporates off the brane into an infinite volume, five-dimensional Minkowski bulk. Therefore, the DGP braneworld theory is a model in a class of theories in which gravity deviates from conventional → General Relativity not at short distances, but rather at long distances. This means that at those distances General Relativity cannot correctly describe gravitational interactions. This model has various cosmologically interesting features. Particularly in the model with five dimensional bulk, the → accelerating expansion of the Universe at late epoch is realized without introducing the → cosmological constant (see, e.g., A. Lue, 2002, arxiv.0208169, T. Tanaka, 2003, arXiv.0305031).
G. Dvali, G. Gabadadze and M. Porrati, 2000, Phys. Lett. 485B, 208.
Fr.: théorie de la dynamo
Branch of magnetohydrodynamics concerned with self-excitation of magnetic fields in any large rotating mass of conducting fluid in motion (usually turbulent). Self-exciting dynamo action is believed to account for magnetic fields at the planetary, stellar, and galactic scales.
Einstein's theory of specific heat
negare-ye garmâ-ye âbize-ye Einstein
Fr.: théorie de la chaleur spécifique d'Einstein
Same as → Einstein model.
→ Einstein; → theory; → specific heat.
Fr.: théorie électromagnétique
The description of combined electric and magnetic fields mainly by → Maxwell's equations. Same as → electromagnetism.
→ electromagnetic; → theory.
electromagnetic theory of light
negare-ye barqâmeqnâti-ye nur
Fr.: théorie électromagnétique de la lumière
The theory describing light as a wave phenomenon resulting from the combination of two electric and magnetic fields vibrating transversely and mutually at right angles. → electromagnetic radiation; → electromagnetic wave; → Maxwell's equations.
→ electromagnetic; → theory; → light.
Fr.: théorie épicyclique
The theory that describes the Galactic dynamics, that is the orbits of stars and gas clouds in the → Galactic disk, as well as the spiral → density wave. Formulated by Bertil Lindblad (1895-1965), the epicyclic theory assumes that orbits are circular with small deviations. Star orbits are described by the superposition of two motions: i) a rotation of the star (epicenter) around the Galactic center at the circular angular velocity, Ω, and ii) a retrograde elliptical motion at → epicyclic frequency, κ. The epicyclic motion in the Galactic plane occurs in a retrograde sense to conserve → angular momentum. In general Ω and κ are different and, therefore, orbits do not close. However, seen by an observer who rotates with the epicenter, orbits are closed ellipses.
Fr.: théorie des champs
1) A theory which uses the concept of → field
to describe physical phenomena. It consists of two types:
→ classical field theory
and → quantum field theory.
Fr.: théorie de fission
A theory that suggests the Moon was formed at the same time as Earth. A spinning Earth ejected a large piece of its material into space which then developed into the shape and orbit of the Moon. This event was also thought to be at the origin of the Pacific Ocean. This first modern idea about the formation of the Moon is due to George Darwin, the son of the great naturalist Charles Darwin. The fission theory explained the lack of volatile substances on the Earth. The volatile materials on the Earth would have been thrown out into space The fission theory is almost completely abandoned today. The analysis of lunar rocks brought to Earth by NASA astronauts showed that the Moon rocks are older than the rocks at the bottom of the Pacific Ocean. Moreover, modern → plate tectonics gives a better explanation of the origin of the Pacific Ocean. See also → giant impact hypothesis, → capture theory, → co-formation theory.
negare-ye gaz (#)
Fr.: théorie de jauge
A field theory in which it is possible to perform a transformation without altering any measurable physical quantity.