An Etymological Dictionary of Astronomy and Astrophysics
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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.

The act of producing or the state of a → gravitational lens.

→ gravitational; → lensing.

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.

The mass of an object measured using the effect of a gravitational field on the object.

→ gravitational; → mass.

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.
2) In a two body system. It is the amount of work done in bringing the mass m to the distance R from M: E_P = -GMm/R, where G is the → gravitational constant.
3) For a uniform sphere. It is E_P = -(3/5)GM²/R, where G is the gravitational constant and M is the mass contained in the sphere of radius R.

→ gravitational; → potential; → energy.

The → energy transported by → gravitational waves. Gravitational radiation is to → gravity what light is to → electromagnetism.

→ gravitational; → radiation.

The change in the wavelength or frequency of electromagnetic radiation in a gravitational field predicted by general relativity.

→ gravitational; → redshift.

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. See also → radiative levitation, → element diffusion, → thermal diffusion.

→ gravitational; → settling.

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.

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).
2) Not to be confounded with → gravity wave.

→ gravitational; → wave.

A theory that treats gravity as a field rather than a force acting at a distance.

→ gravitational; → field.

Objects held in orbit about each other by their → gravitational attraction. Such objects are part of a → bound system.

→ gravitational; → bound.

A parameter representing the product of the → gravitational constant by the → solar mass. It is 13.27 x 10¹⁹ m³ s^-2.

→ heliocentric; → gravitational; → constant.

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.

→ laser; → interferometer; → gravitational; → wave; → observatory.

The universal law which states that the force of attraction between any two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them: F = G (m₁.m₂)/r², where G is the → gravitational constant.

→ Newton; → law; → gravitation.

Same as the → gravitational constant.

→ Newtonian; → constant; → gravitation.

A parameter representing the product of the → gravitational constant by the → lunar mass. It is 49.03 x 10¹¹ m³ s^-2.

→ selenocentric; → gravitational; → constant.

A → gravitational lensing phenomenon in which the image distortion is strong enough to be readily recognized, such as in the case of the → Einstein cross or when giant luminous arcs show up in → galaxy clusters (e.g. Abell 2218). Opposite to → weak gravitational lensing.

→ strong; → gravitational; → lensing.

A parameter representing the product of the → gravitational constant by the Earth's mass. It is 3.987 x 10¹⁴ m³s^-2 or 3.987 x 10⁵ km³s^-2.

→ terrestrial; → gravitational; → constant.

A gravitational bending of light by structures in the Universe that distorts the images of distant galaxies. The distortion allows the distribution of → dark matter and its evolution with time to be measured, thereby probing the influence of → dark energy on the growth of structures. Weak gravitational lensing is generally difficult to identify in individual images, in contrast to → strong gravitational lensing (see, e.g., Bartelmann & Peter Schneider, 2001, Phys. Rept. 340, 291).

→ weak; → gravitational; → lensing.