An Etymological Dictionary of Astronomy and Astrophysics
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Same as → gravitational potential energy.

→ gravitational; → energy.

The condition in a celestial body when gravitational forces acting on each point are balanced by some outward pressure, such as radiation pressure or electron degeneracy pressure, so that no vertical motion results.

→ gravitational; → equilibrium.

The region of space in which → gravitational attraction exists.

→ gravitational; → field.

The weakest of the four fundamental forces of nature. Described by → Newton's law of gravitation and subsequently by Einstein's → general relativity.

→ gravitational; → force.

The process by which fluctuations in an infinite medium of size greater than a certain length scale (the Jeans length) grow by self-gravitation.

→ gravitational; → instability.

Mutual attraction between any two bodies that have mass.

→ gravitational; → interaction.

A concentration of matter, such as a galaxy or a cluster of galaxies, that bends light rays from a background object, resulting in production of multiple images. If the two objects and the Earth are perfectly aligned, the light from the distant object appears as a ring from Earth. This is called an Einstein Ring, since its existence was predicted by Einstein in his theory of general relativity.

→ gravitational; → lens.

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 hypothetical elementary particle associated with the gravitational interactions. This quantum of gravitational radiation is a stable particle, which travels with the speed of light, and has zero rest mass, zero charge, and a spin of ± 2.

From gravit(y), → gravity + → -on a suffix used in the names of subatomic particles.