An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

The decrease in the length of a body moving in the direction of its length as measured by an observer situated in that direction. The shortening factor is [1 - (v/c)²]^1/2, where v is the relative velocity and c light speed.

→ Lorentz; → contraction.

In → special relativity, an important parameter which appears in several equations, including → time dilation, → length contraction, and → relativistic mass. It is defined as γ = 1 / [1 - (v/c)²]^1/2 = dt/dτ, where v is the velocity as observed in the reference frame where time t is measured, τ is the proper time, and c the → velocity of light. Same as Lorentz γ factor.

→ Lorentz; → factor.

The force acting upon a → charged particle as it moves in a → magnetic field. It is expressed by F = q.v x B, where q is the → electric charge, v is its → velocity, and B the → magnetic induction of the field. This force is perpendicular both to the velocity of the charge and to the magnetic field. The magnitude of the force is F = qvB sinθ, where θ is the angle between the velocity and the magnetic field. This implies that the magnetic force on a stationary charge or a charge moving parallel to the magnetic field is zero. The direction of the force is given by the → right-hand rule.

→ Lorentz; → force.

Of a physical law, invariance with respect to a → Lorentz transformation.

→ Lorentz; → invariance.

A repeated electromagnetic force on an electrically charged ring particle, nudging the particle in the same direction and at the same point in its orbit. Lorentz resonances are especially important for tiny ring particles whose charge-to-mass ratio is high and whose orbit periods are a simple integer fraction of the rotational period of the planet's magnetic field (Ellis et al., 2007, Planetary Ring Systems, Springer).

→ Lorentz; → resonance.

A set of linear equations that expresses the time and space coordinates of one → reference frame in terms of those of another one when one frame moves at a constant velocity with respect to the other. In general, the Lorentz transformation allows a change of the origin of a coordinate system, a rotation around the origin, a reversal of spatial or temporal direction, and a uniform movement along a spatial axis. If the system S'(x',y',z',t') moves at the velocity v with respect to S(x,y,z,t) in the positive direction of the x-axis, the Lorentz transformations will be: x' = γ(x - vt), y' = y, z' = z, t' = γ [t - (vx/c²)], where c is the → velocity of light and γ = [1 - (v/c)²]^-1/2. For the special case of velocities much less than c, the Lorentz transformation reduces to → Galilean transformation.

→ Lorentz; → transformation.

A spectral profile in which the intensity distribution follows a specific mathematical function (Lorentz or Cauchy probability). Compared to the normal or Gaussian profile, Lorentzian has a pointed peak and more important wings.

→ Lorentz; → profile.

The first ever discovered → fast radio burst. It was done during a search of archival data from a 1.4-GHz survey of the → Magellanic Clouds using the multi-beam receiver on the 64-m Parkes Radio Telescope in Australia.

D. R. Lorimer et al., 2007, Science, 318, 777; → burst.

The number of molecules in 1 cm³ of an ideal gas (2.687 x 10¹⁹ per cm³).

Joseph Loschmidt (1821-1895), Austrian physicist.

In physics, a measure of the energy, mass, or other physical quantities lost in a system, by conversion or external effects.

From O.E. los "loss, destruction," from P.Gmc. *lausa, from PIE base *leu- "to loosen, divide, cut apart, untie, separate" (cf. Gk. lyein "to loosen, untie, slacken," lysus "a loosening;" L. luere "to loose, release, atone for;" Skt. lunati "cuts, cuts off," lavitram "sickle;" Pers. las "loose," lâ "slit, cut;" → analysis).

Dastraft, literally "gone from hand," from dast "hand" (Mid.Pers. dast; O.Pers. dasta-; Av. zasta-; cf. Skt. hásta-; Gk. kheir; L. praesto "at hand;" Arm. jern "hand;" Lith. pa-žastis "arm-pit;" PIE *ghes-to-) + raft p.p. of raftan "to go, elapse, glide by, depart" (Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- "to go; to attack").

High in volume of sound.

M.E., O.E. hlud "making noise, sonorous" (cf. M.Du. luut, Du. luid, O.H.G. hlut, Ger. laut "loud"), from PIE *klutos- (cf. Skt. sruta-, Gk. klytos "heard of, celebrated," Arm. lu "known."

Boland, → high.

The magnitude of the sensation produced by a sound wave when it reaches the ear. The loudness of a sound depends upon the effective → acoustic pressure and → frequency. The basis of loudness scales is the → Weber-Fechner law.

→ loud + → -ness.

Bolandi, → height; sedâ, → sound.

A device in which electric signals are converted into audible sound.

→ loud; → speak; → -er.

1) Below the average or expected degree, amount, or intensity. Having or containing a relatively small amount.
2) Situated, placed, or occurring not far above the ground, floor, or base. Relatively little in height.

From M.E. lah, from O.N. lagr "low," from P.Gmc. *lægaz (cf. O.Fris. lech, Du. laag, Ger. läge "low"), literally "that which is lying flat;" related to E. lie (v.).

Kam "little, few; deficient, wanting; scarce," from Mid.Pers. kam "little, small, few," O.Pers./Av. kamna- "small, few."
Keh "small, little, slender" (related to kâstan, kâhidan "to decrease, lessen, diminish," from Mid.Pers. kâhitan, kâstan, kâhênitan "to decrease, diminish, lessen;" Av. kasu- "small, little;" Proto-Iranian *kas- "to be small, diminish, lessen"); kutâh, → small; pâyin, → bottom-up structure.

A redshift characterizing a near-by receding object.

→ low; → redshift.

The quality of an instrument that lacks sufficient resolution for a specific observation. This is a relative quality, but presently a resolution below about 1 arcsecond.

→ low; → resolution.

A member of a particularly faint population of galaxies with a central → surface brightness below the brightness of the background sky. The central regions of many of them resemble a → dwarf galaxy, but most of the mass is contained in a large gaseous disk of low density that is observable only with long-exposure optical images or at radio wavelengths. Some are as massive as a large → spiral galaxy, for example Malin 1. The proportion of LSBGs relative to normal galaxies is unknown. They may however represent a significant fraction of mass in the Universe. LSBGs are thought to be primitive systems because they have total masses similar to normal galaxies, but have typically converted less than 10% of their gas into stars. Spiral LSBGs do not obey → Freeman's law.

→ low; → surface; → brightness; → galaxy.

The state of the → tide when at its lowest level.

→ low; → tide.

Owpas, from Persian Gulf dialects, literally "backward water," from ow, variant of âb, → water, + pas "→ back, behind."
Jazr, loan from Ar.

Also known as → low tide.

→ low; → water.

A neutrino which is mainly produced in → nuclear processes, such as the ones in the → Sun (→ solar neutrino), or in the center of an exploding → supernova. Such neutrinos are, however, more energetic than those making up the → cosmic neutrino background.

→ low; → energy; → neutrino.