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gravitational force niru-ye gerâneši (#) Fr.: force gravitationnelle The weakest of the four fundamental forces of nature. Described by → Newton's law of gravitation and subsequently by Einstein's → general relativity. → gravitational; → force. |
gravitational instability nâpâydâri-ye gerâneši (#) Fr.: instabilité gravitationnelle The process by which fluctuations in an infinite medium of size greater than a certain length scale (the Jeans length) grow by self-gravitation. → gravitational; → instability. |
gravitational interaction andaržireš-e gerâneši Fr.: interaction gravitationnelle Mutual attraction between any two bodies that have mass. → gravitational; → interaction. |
gravitational lens adasi-ye gerâneši (#) Fr.: lentille gravitationnelle A concentration of matter, such as a galaxy or a cluster of galaxies, that bends light rays from a background object, resulting in production of multiple images. If the two objects and the Earth are perfectly aligned, the light from the distant object appears as a ring from Earth. This is called an Einstein Ring, since its existence was predicted by Einstein in his theory of general relativity. → gravitational; → lens. |
gravitational lens equation hamugeš-e adasi-ye gerâneši Fr.: équation de lentille gravitationnelle The main equation of gravitational lens theory that sets a relation between the angular position of the point source and the observable position of its image. → gravitational; → lens; → equation. |
gravitational lensing lenzeš-e gerâneši Fr.: effet de lentille gravitationelle The act of producing or the state of a → gravitational lens. → gravitational; → lensing. |
gravitational lensing time delay derang-e zâyide-ye lenzeš-e gerâneši Fr.: retard dû à l'effet de lentille gravitationnelle The difference in light travel times along the various light paths from the source to the observer when the source image is divided into several images because of → gravitational lensing. According to the theory of → general relativity, light rays are deflected in the vicinity of massive objects. If the light source and the deflector are sufficiently well aligned with the observer, and obey some conditions on their distances (→ Einstein radius), we can observe several (generally distorted and magnified) images of the source. A property of → strong lensing is that the light travel time from the source to the observer is generally not identical for the different images. In other words, we not only see several images of one same object, but we also see this object, in each image, at different times. This means, in one image the lensed object will be observed before the other image. Given a physical model of the gravitational lens, the light travel time for each image can be computed. The expression giving the time delay has two components: a term is called → geometric delay, and the second term, known as the → Shapiro time delay. The latter is due to time dilation by the gravitational field of the lens, a direct consequence of general relativity. See also → time delay distance. → gravitational; → lensing; → time; → delay. |
gravitational mass jerm-e gerâneši (#) Fr.: masse gravitationnelle The mass of an object measured using the effect of a gravitational field on the object. → gravitational; → mass. |
gravitational potential energy kâruž-e tavand-e gerâneši Fr.: énergie potentielle gravitationnelle 1) The energy that an object possesses because of its position in a
→ gravitational field, especially an object near the
surface of the Earth where the → gravitational acceleration
can be assumed to be constant, at about 9.8 m s^{-2}. → gravitational; → potential; → energy. |
gravitational radiation tâbeš-e gerâneši (#) Fr.: rayonnement gravitationnel The → energy transported by → gravitational waves. Gravitational radiation is to → gravity what light is to → electromagnetism. → gravitational; → radiation. |
gravitational redshift sorxkib-e gerâneši Fr.: décalage vers le rouge gravitationnel The change in the wavelength or frequency of electromagnetic radiation in a gravitational field predicted by general relativity. → gravitational; → redshift. |
gravitational settling niyâšeš-e gerâneši Fr.: décantation par gravité A physical process occurring in stellar atmospheres whereby in a very stable atmosphere → heavy elements are gravitationally pulled down preferentially. If such an atmosphere is stable for long periods of time, the absorption lines of heavy elements may therefore become very weak. Observationally, the star seems to contain only hydrogen and helium. Gravitational settling takes place in the Sun at the bottom of the outer → convective zone where helium is dragged down, leading to a surface He abundant smaller than the cosmic value. It occurs also in the atmospheres of → brown dwarfs and → planets. → gravitational; → settling. |
gravitational slingshot falâxan-e gerâneši Fr.: fronde gravitationnelle Same as → gravity assist. → gravitational; slingshot, from sling, from M.E. slyngen, from O.N. slyngva "to sling, fling" + shot, from M.E., from O.E. sc(e)ot, (ge)sceot; cf. Ger. Schoss, Geschoss. Falâxan "sling;" from Av. fradaxšana- "sling," fradaxšanya- "sling, sling-stone;" → gravitational. |
gravitational wave mowj-e gerâneši (#) Fr.: ondes gravitationnelles A → space-time oscillation created by the motion of matter,
as predicted by Einstein's → general relativity.
When an object accelerates, it creates ripples in space-time, just
like a boat causes ripples in a lake.
Gravitational waves are extremely weak even for the most massive objects like
→ supermassive black holes.
They had been inferred from observing a → binary pulsar
in which the components slow down, due to losing energy from
emitting gravitational waves. Gravitational waves were directly detected for the
first time on September 14, 2015 by the
→ Laser Interferometer Gravitational-Wave Observatory (LIGO)
(Abbott et al., 2016, Phys. Rev. Lett. 116, 061102).
Since then several other events have been detected by LIGO and
→ Laser Interferometer Space Antenna (LISA).
The Nobel Prize in physics 2017 was awarded to three physicists who had leading
roles in the first detection of gravitational waves using LIGO. They were
Rainer Weiss (MIT), Barry C. Barish, and Kip S. Thorne (both Caltech). → gravitational; → wave. |
gravitational-field theory 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. |
gravitationally bound gerânešâné bandidé Fr.: gravitationnellement lié Objects held in orbit about each other by their → gravitational attraction. Such objects are part of a → bound system. → gravitational; → bound. |
gravitino gerâvitino (#) Fr.: gravitino A hypothetical force-carrying particle predicted by supersymmetry theories. The gravitino's spin would be 1/2; its mass is unknown. From gravit(on) + (neutr)ino. |
graviton gerâviton (#) Fr.: graviton A hypothetical elementary particle associated with the gravitational interactions. This quantum of gravitational radiation is a stable particle, which travels with the speed of light, and has zero rest mass, zero charge, and a spin of ± 2. From gravit(y), → gravity + → -on a suffix used in the names of subatomic particles. |
gravity gerâni (#) Fr.: gravité 1) The apparent force of → gravitation on an object at or
near the surface of a star, planet, satellite, etc. From L. gravitatem (nom. gravitas) "weight, heaviness," from gravis "heavy," from PIE base *g^{w}rə- "heavy" (cf. Mod.Pers. gerân "heavy;" Av. gouru- "heavy;" Skt. guru- "heavy, weighty, venerable;" Gk. baros "weight," barys "heavy;" Goth. kaurus "heavy"). Gerâni, noun of gerân "heavy, ponderous, valuable," from Mid.Pers. garân "heavy, hard, difficult;" Av. gouru- "heavy" (in compounds), from Proto-Iranian *garu-; cognate with gravity, as above. |
gravity assist yâri-ye gerâneši Fr.: gravidéviation An important astronautical technique whereby a → spacecraft takes up a tiny fraction of the → orbital energy of a planet it is flying by, allowing it to change → trajectory and → speed. Since the planet is not at rest but gravitating around the Sun, the spacecraft uses both the orbital energy and the gravitational pull of the planet. Also known as the slingshot effect or → gravitational slingshot. More specifically, as the spacecraft approaches the planet, it is accelerated by the planet's gravity. If the spacecraft's velocity is too low, or if it is heading too close to the planet, then the planet's → gravitational force will pull it down to the planet. But if its speed is large enough, and its orbit does not bring it too close to the planet, then the gravitational attraction will just bend the spacecraft's trajectory around, and the accelerated spacecraft will pass rapidly by the planet and start to move away. In the absence of other gravitational forces, the planet's gravity would start to slow down the spacecraft as it moves away. If the planet were stationary, the slow-down effect would be equal to the initial acceleration, so there would be no net gain in speed. But the planets are themselves moving through space at high speeds, and this is what gives the "slingshot" effect. Provided the spacecraft is traveling through space in the same direction as the planet, the spacecraft will emerge from the gravity assist maneuver moving faster than before. → gravity; assist, from M.Fr. assister "to stand by, help, assist," from L. assistere "assist, stand by," from → ad- "to" + sistere "to cause to stand," from PIE *siste-, from *sta- "to stand" (cognate with Pers. istâdan "to stand"). Yâri "assistance, help; friendship," from yâr "assistant, helper, friend," from Mid.Pers. hayyâr "helper," hayyârêh "help, aid, assistance," Proto-Iranian *adyāva-bara-, cf. Av. aidū- "helpful, useful." |
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