Fr.: formule de Bekenstein
The mathematical expression giving the → entropy, S, of a → black hole as a function of the area of its → event horizon, A: S = (kc3A)/(4Għ), where k is → Boltzmann's constant, ħ is the → reduced Planck's constant, and G the → gravitational constant. It can also be expressed by S = (kA)/(4lP2), where lP is the → Planck length. The existence of this entropy led to the prediction of the → Hawking radiation, because an entropy is associated with a temperature and a temperature to a → thermal radiation. The entropy of a black hole increases continuously because the fall of material into it increases its area.
For Jacob D. Bekenstein (1947-), an Israeli theoretical physicist, who contributed to the foundation of black hole thermodynamics; → formula.
A dimensionless unit to measure sound. One bel designates a ratio 10:1 between two quantities, P1 and P0, which have the dimension of a power: n [B] = log (P1/P0), in Bel units, with → natural logarithm. If one sound is 2 bels louder than another, this means the louder sound is 100 times more intense than the fainter one. It is also common to use this definition for quantities that are proportional to a power, such as energy, work, intensity, or voltage. The bel was too large for everyday use, so the → decibel (dB), equal to 0.1 bel, is more commonly used.
This unit was put forward by engineers of the Bell telephone network in 1923 and named in honor of the inventor of the telephone, Alexander Graham Bell (1847-1922), who also invented techniques for teaching speech to the deaf.
Belinda (Uranus XIV)
One of the small satellites of → Uranus discovered from the Voyager 2 photographs taken during its encounter with the planet in 1986.
Named after the heroine in Alexander Pope's The Rape of the Lock.
Fr.: inégalité de Bell
Any of a large number of inequality relations developed to study the → hidden variable hypothesis suggested in the → EPR paradox. Using Bell's inequalities, the → Aspect experiment showed that no local hidden variable theory can make predictions in agreement with those of quantum mechanics. If, in a measurement, the inequality is violated, the measurement is in agreement with the predictions of the quantum theory. If the equality is satisfied, it suggests that a classical, causal, and local model is adequate to explain the outcome of the measurements. See also → quantum entanglement.
John Stewart Bell (1928-1990); → inequality.
Bellatrix (γ Orionis)
A bright, blue → giant star ( → spectral type B2 III), one of the main stars of the constellation → Orion. With a visual magnitude of 1.64, it is about 1000 times more luminous than the Sun, and lies at a distance of 243 → light-years.
From L. bellatrix "a female warrior," fem. of bellator, from bellum "war."
Merzam, Ar. name of the star; its other name is Nâjed.
pargetidan, parget dâštan
1) (with preposition to) To be the property of.
M.E. belongen, from be- intensive prefix, + longen "to go," from O.E. langian "pertain to, to go along with;" akin to Du. belangen, Ger. belangen; of unknown origin.
Pargetidan, literally "to surround, to relate with" (on the model of L. pertinere "pertain," Skt. parigraha- "surrounding; relation to"), from parget "to hold, seize, take around," from par- "around," → peri-, + get "to take, sieze," as in Tâleši gate "to take," Târi gata, Sorxeyi, Lâsgardi, Semnâni, Šâhmerzâdi -git- "take, seize," variants of gereftan "take, hold," → concept.
mipargetad, parget dârad
Third person present verb of → belong.
O.E. belt, from P.Gmc. *baltjaz, from L. balteus "girdle;" → Orion.
Kamarband "belt," from kamar "waist" (Mid.Pers. kamar "waist; belt, girdle," Av. kamarâ- "belt") + band "a band, tie, belt."
Belt of Orion
kamarband-e Šekârgar, ~ Orion
Fr.: Baudrier d'Orion
Belt of Venus
Fr.: Ceinture de Vénus
A pink to brownish border above the horizon separating the Earth's dark shadow on the sky from the sky above it. The Belt of Venus appears during a cloudless twilight just before sunrise or after sunset. It is due to scattered red sunlight in the atmosphere. Also called anti-twilight arc.
A → European Space Agency (ESA) mission aimed at studying the composition, geophysics, atmosphere, magnetosphere, and history of → Mercury, the least explored planet in the inner → Solar System. It will be launched in 2018. Among several investigations, BepiColombo will make a complete map of Mercury at different wavelengths. It will chart the planet's mineralogy and elemental composition, determine whether the interior of the planet is molten or not, and investigate the extent and origin of Mercury's magnetic field. BepiColombo is a joint mission between ESA and the Japanese Aerospace Exploration Agency (JAXA), executed under ESA leadership.
Named after Giuseppe (Bepi) Colombo (1920-1984), a scientist who studied Mercury's orbital motion in detail as well as orbits and interplanetary travel in general.
Fr.: Chevelure de Bérénice
Fr.: équation de Bernoulli
The equation expressing → Bernoulli's theorem: P + (1/2)ρV2 + ρgz = constant, where P is the fluid → pressure, V is → velocity, ρ is → density, g is the acceleration due to → gravity, and z is the vertical reference → level. The theree terms are called → static pressure, → dynamic pressure, and → hydrostatic pressure, respectively. The Bernoulli equation states that the total pressure along a → streamline is → constant.
Fr.: théorème de Bernoulli
A statement of the → conservation of energy in the → steady flow of an → incompressible, → inviscid fluid. Accordingly, the quantity (P/ρ) + gz + (V2/2) is → constant along any → streamline, where P is the fluid → pressure, V is the fluid → velocity, ρ is the mass → density of the fluid, g is the acceleration due to → gravity, and z is the vertical → height. This equation affirms that if the internal velocity of the flow goes up, the internal pressure must drop. Therefore, the flow becomes more constricted if the velocity field within it increases. Same as the → Bernoulli equation.
After Daniel Bernoulli (1700-1782), the Swiss physicist and mathematician who put forward the theorem in his book Hydrodynamica in 1738; → theorem.
A grey, very hard metallic chemical element; symbol Be. → Atomic number 4; → atomic weight 9.01218; → melting point about 1,278°C; → boiling point 2,970°C (estimated); → specific gravity 1.85 at 20°C; → valence +2. Beryllium occurs as beryl, from which it is obtained by electrolysis. Used for light alloys which are corrosion resistant. Beryllium was discovered by Louis Nicolas Vauquelin (1763-1829) in 1798. First isolated by Friedrich Wöhler (1800-1882) in 1828.
From L. beryll(us), from beryl, a mineral, beryllium aluminum silicate, Be3Al2Si6O18, M.E. beril, from O.Fr., from L. berillus, from Gk. beryllos, + → -ium.
From Friedrich Wilhelm Bessel (1784-1846), German astronomer and mathematicians, who made fundamental contributions to positional and spherical astronomy.
Fr.: équation de Bessel
A linear second-order differential equation, the solutions to which are called Bessel functions.
Hamugeš, → equation.
Fr.: étoile de Bessel
Same as → 61 Cygni, the first star whose distance was measured, by Friedrich Bessel in 1838.
Fr.: besselien, de Bessel
Besselian day number
šomâre-ye ruz-e Besseli