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Gravity
Gravity, also called gravitation, is the mutual attraction between masses, the force that keeps the Moon in orbit and gives objects near Earth a downward acceleration.
Gravity anomaly
A gravity anomaly is the difference between measured gravity, reduced to sea level, and the theoretical gravity of a uniform ellipsoidal Earth, revealing rock density variations.
Gravity darkening
Gravity darkening is the variation of brightness and temperature across a rotating star, with hotter, brighter poles and a cooler equator, formulated by von Zeipel in 1924.
Gravity gradiometer
A gravity gradiometer is an instrument that measures the rate at which gravity changes from one point to a nearby point, used in resource exploration and passive navigation.
Gravity gradiometry
Gravity gradiometry is a geophysical surveying method that measures how gravitational acceleration changes from point to point, revealing subsurface density structure from aircraft or satellites.
Gravity inversion
Gravity inversion is the geophysical process of estimating a subsurface density model from measured gravity anomalies. It is non-unique and unstable, so regularization and constraints are needed.
Gravity of Earth
The gravity of Earth, also called little g, is the acceleration objects feel at its surface: a standard 9.80665 m/s² that actually varies by about 0.7 percent with location.
Gravity Probe A
Gravity Probe A was a 1976 NASA suborbital experiment that confirmed general relativity's gravitational redshift by flying a hydrogen maser clock to 10,000 km altitude.
Gravity Probe B
Gravity Probe B, a NASA-Stanford physics satellite launched in 2004, tested Einstein's general relativity by measuring geodetic and frame-dragging effects with four cryogenic gyroscopes, announcing results in 2011.
Gravity wave
A gravity wave is a wave in a fluid, such as the ocean or atmosphere, in which gravity and buoyancy act as the restoring force on displaced fluid.
Gray (unit)
The gray (Gy) is the SI unit of absorbed dose of ionizing radiation, equal to one joule per kilogram, adopted in 1975 and replacing the rad.
Gray cast iron
Gray cast iron, also called flake graphite iron, is a cast iron with 2.5 to 4 percent carbon as graphite flakes, used for engine blocks and machine bases.
Gray code
Gray code, also known as reflected binary code, is an ordering of binary numbers in which successive values differ in exactly one bit, useful in rotary encoders.
Gray molasses
Gray molasses is a sub-Doppler laser cooling technique that combines Sisyphus-type optical pumping with coherent dark states, important for cooling lithium, potassium, sodium, and molecules.
Grazing lunar occultation
A grazing lunar occultation, also called a lunar graze, is an occultation where the Moon's edge passes in front of a star, making it disappear and reappear several times.
Grazing-incidence small-angle X-ray scattering
Grazing-incidence small-angle X-ray scattering (GISAXS) is a surface-sensitive X-ray technique, essentially reflection-mode SAXS, that reveals nanoscale structure in thin films and on surfaces at grazing incidence angles below 2°.
Grazing-incidence wide-angle X-ray scattering
Grazing-incidence wide-angle X-ray scattering (GIWAXS) is an X-ray method in which a beam strikes a flat sample below 1° to determine thin-film crystal structure and orientation.
Grazing-incidence X-ray diffraction
Grazing-incidence X-ray diffraction (GIXD), also called GIWAXS, is an X-ray diffraction technique in which the beam strikes below one degree, revealing crystal structure in nanometer-thin films.
Grazing-incidence X-ray scattering
Grazing-incidence X-ray scattering (GIXS) is a family of X-ray techniques in which the beam strikes a sample below 1°, characterizing thin-film nanostructure through GISAXS and GIWAXS.
GRB 000926
GRB 000926 was a long-duration gamma-ray burst detected by the Interplanetary Network on September 26, 2000, with a bright optical afterglow at redshift z = 2.0369.
GRB 021004
GRB 021004 was a long-duration gamma-ray burst detected on 4 October 2002 by NASA's HETE-2 satellite, its optical afterglow caught nine minutes after the explosion, making it one of the best observed bursts of its era.
GRB 050820A
GRB 050820A was an exceptionally energetic long gamma-ray burst detected by Swift on 20 August 2005 at redshift 2.612, with one of the Swift era's best-sampled broadband data sets.
GRB 050904
GRB 050904 was a long gamma-ray burst detected by Swift on 4 September 2005 at redshift z = 6.295, then the most distant cosmic explosion observed.
GRB 051221
GRB 051221A was a short, hard gamma-ray burst detected by Swift on 21 December 2005, notable for the first clear energy injection into a short-burst afterglow and a jet break.
GRB 060418
GRB 060418 was a long-duration gamma-ray burst detected by the Swift satellite on 18 April 2006, notable for its early afterglow onset and polarization measurement.
GRB 060614
GRB 060614 is a gamma-ray burst detected on 14 June 2006 by Swift, notable for its 102-second duration, long enough to count as long, despite lacking the expected supernova.
GRB 080319B
GRB 080319B, known as the naked-eye burst, was a gamma-ray burst detected by Swift on 19 March 2008 at redshift 0.937, briefly visible to the naked eye.
GRB 080916C
GRB 080916C was a long-duration gamma-ray burst detected by NASA's Fermi telescope in 2008 in Carina, and was for roughly a decade the most energetic burst known.
GRB 090423330
GRB 090423330 is the Fermi trigger designation for GRB 090423, a gamma-ray burst detected in 2009 at redshift 8.2, then the most distant object known.
GRB 090510016
GRB 090510016, also called GRB 090510, was a short, hard gamma-ray burst detected on 10 May 2009 by Fermi and Swift, notable for its measured redshift and GeV emission.