mowj-e desimetri (#)
Fr.: onde décimétrique
An electromagnetic radio wave having wavelengths between 10 cm and 1m, corresponding to a frequency between 300 and 3,000 Mega Hertz. It is naturally emitted by various astronomical objects.
Mowj, → wave; desimetri, from décimétrique, as above.
mowj-e cagâli (#)
Fr.: onde de densité
A wave phenomenon in which the density fluctuations of a physical quantity propagates in a compressible medium. For example, the → spiral arms of a → galaxy are believed to be due to a density wave which results from the natural instability of the → galactic disk caused by its own gravitational force. A common example of a density wave concerns traffic flow. A slow-moving vehicle on a narrow two-lane road causes a high density of cars to pile up behind it. As it moves down the highway the "traffic density wave" moves slowly too. But the density wave of cars does not keep the same cars in it. Instead, the first cars leave the density wave when they pass the slow vehicle and continue on at a more normal speed and new ones are added as they approach the density wave from behind. Moreover, the speed with which the density wave moves is lower than the average speed of the traffic and that the density wave can persist well after its original cause is gone. See → density wave theory.
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.
mowj-e kešâyand (#)
Fr.: onde élatique
A wave that propagates by → elastic deformation of the medium. The → propagation takes place by a change in shape that disappears when the forces are removed. In other words, the displaced particles transfer momentum to adjoining particles, and are themselves restored to their original position. A → seismic wave is a type of elastic wave.
Fr.: onde électromagnétique
A wave produced by oscillation or acceleration of an electric charge. → electromagnetic radiation.
Fr.: onde électrostatique
In a → plasma, a disturbance that is devoid of magnetic field, and hence can be expressed by an electrostatic potential. The electric field is always parallel to the propagation vector, so that the electrostatic wave is → longitudinal.
Fr.: onde évanescente
A wave whose → amplitude → decreases → exponentially with distance from the → interface at which it is formed. Evanescent waves are formed when → sinusoidal waves are internally reflected off an interface at an angle greater than the → critical angle so that → total internal reflection occurs.
frequency to wavelength conversion
hâgard-e basâmad bé mowj-tul
Fr.: conversion fréquence / longueur d'onde
Deriving the → wavelength of an undulatory phenomenon from
its → frequency, and vice versa.
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).
Fr.: onde de gravité
1) A wave that forms and propagates at the free → surface
of a body of → fluid
after that surface has been disturbed and the fluid particles
have been displaced from their original positions.
The motion of such waves is controlled by the restoring force of gravity rather
than by the surface tension of the fluid.
tiqe-ye nin-mowj (#)
Fr.: lame demi-onde
A plate of optical material whose thickness is such that the phase difference between the extraordinary and ordinary rays after passing through the place is exactly one-half cycle. It can serve to rotate the plane of polarization of a light beam.
celle-ye tâbestân (#)
Meteorology: A period of several successive days of abnormally hot and usually humid weather occurring in summer.
mowjhâ-ye nâhamdus (#)
Fr.: ondes incohérentes
The lack of a fixed phase relationship between two or more waves. → coherent.
internal gravity wave (IGW)
mowj-e gerâni-ye daruni
Fr.: onde de gravité interne
Fr.: onde de Langmuir
A disturbance of a → plasma in the form of a longitudinal, → electrostatic wave that propagates in the plasma due to variations in the plasma's electron density. More specifically, Langmuir waves are collective oscillations of inhomogeneous bunches of electrons displaced from their natural equilibrium, in which the inertia of the relatively massive ions serves to establish an electrostatic restoring force that tries to bring the electrons back to their equilibrium positions. → Landau damping causes dissipation of Langmuir waves as the electrons are either accelerated or decelerated so as to be in resonance with the phase velocity of the waves themselves.
Irving Langmuir (1881-1957), American chemist and physicist, Nobel Prize in Chemistry 1932; → wave.
Laser Interferometer Gravitational-Wave Observatory (LIGO)
nepâhešgâh-e mowjhâ-ye gerâneši bâ andarzaneš-sanji-ye leyzeri
Fr.: Observatoire d'ondes gravitationnelles par interférométrie laser
A facility dedicated to the detection and measurement of cosmic → gravitational waves. It consists of two widely separated installations, or detectors, within the United States, operated in unison as a single observatory. One installation is located in Hanford (Washington) and the other in Livingston (Louisiana), 3,000 km apart. Funded by the National Science Foundation (NSF), LIGO was designed and constructed by a team of scientists from the California Institute of Technology, the Massachusetts Institute of Technology, and by industrial contractors. Construction of the facilities was completed in 1999. Initial operation of the detectors began in 2001. Each LIGO detector beams laser light down arms 4 km long, which are arranged in the shape of an "L." If a gravitational wave passes through the detector system, the distance traveled by the laser beam changes by a minuscule amount -- less than one-thousandth of the size of an atomic nucleus (10-18 m). Still, LIGO should be able to pick this difference up. LIGO directly detected gravitational waves for the first time from a binary → black hole merger (GW150914) on September 14, 2015 (Abbott et al., 2016, Phys. Rev. Lett. 116, 061102). The Nobel Prize in physics 2017 was awarded to three physicists (Rainer Weiss, Barry C. Barish, and Kip S. Thorne) for decisive contributions to the LIGO detector and the observation of gravitational waves. LIGO had a prominent role in the detection of → GW170817, the first event with an → electromagnetic counterpart.
Fr.: onde longitudinale
Fr.: onde de Mach
The envelope of wave fronts created by a → supersonic source.
Electromagnetic radiation having wavelengths in the 1 to 300 mm range.
microwave background radiation
tâbeš-e paszamine-ye rizmowj
Fr.: rayonnement micro-onde du fond cosmique
Thermal radiation with a temperature of 2.73 K that is apparently uniformly distributed in the Universe. It is believed to be a redshifted remnant of the hot radiation that was in thermal equilibrium with matter during the first hundred thousand years after the Big Bang. Same as → cosmic microwave background (CMB) radiation.