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More than 350 keywords from relativity and related topics, from "absolute zero" to "X-rays" - please use the menu on the left to choose a letter.


Stars are rarely found alone - usually, they congregate in conglomerates of millions, billions or even more stars called galaxies. A case in point is our sun, part of a galaxy we call the milky way.

Young galaxies can be quite wild - examples for young, active galactic nuclei can be found under radio galaxy and quasar.

 galaxy cluster

Galaxies are not solitary objects - usually, they cluster together. Our own galaxy for instance, the Milky Way, is part of a small cluster called the local group of galaxies. The next-closest large galaxy cluster is the Virgo cluster.

 gamma rays

The most highly energetic variety of electromagnetic radiation , mit above a quintillion oscillations per second, corresponding to wave-lengths of less than a hundredth billionth of a meter.

 gamma-ray burst

Astronomical events visible as extremely strong flashes of gamma rays. Their origin is still unclear; in the context of general relativity they are interesting because they could signal the mergers of neutron stars and/or black holes - and because gravitational waves should help decide whether this is indeed the case.


In a strict sense: A state of matter in which the atoms and/or molecules wildly careen and collide, without being bound to each other. This movement leads to an inner pressure, while the average kinetic energy of the moving particles is a measure for the temperature of the gas.

Compare the other states of matter: solid state, liquid, plasma.

In a broader sense, gas is also used to denote other mixtures of freely careening particles, for instance in the case of the electron gas whose pressure stabilizes a white dwarf against further collapse.

 general theory of relativity

Albert Einstein's theory of gravity; a generalization of his special theory of relativity.

For information about the concepts and applications of this theory, we recommend the chapter general relativity of our introductory section Elementary Einstein. Further information about many different aspects of general relativity and its applications can be found in our section Spotlights on relativity.


British-German gravitational wave detector located in Ruthe (close to Hannover, Germany). GEO600 is an interferometric gravitational wave detector with an arm-length of 600 metres.

GEO600 website


A straightest-possible line in a surface or a more general space. In the plane, the geodesics are straight lines, on the surface of a sphere they are great circles.

 geodetic precession

In classical mechanics, the rotation axis of a gyroscope on which no external forces are acting will remain constant - a useful property that has found applications in navigation. However, in the presence of spacetime curvature, this is no longer true - the axis direction of a gyroscope in free fall will change over time; an effect predicted by Einstein's general theory of relativity.


That part of mathematics concerning itself with surfaces or more general spaces as well as objects defined on such spaces, such as points or lines as well as the objects constructable from points and lines, such as triangles.

See electron volt
 global positioning system

A system of satellites and mobile receivers that makes it possible to determine each receiver's position with high accuracy. Used by pilots, truckers, car drivers and hikers world-wide, it is an industrial application of Einstein's theories of special and general relativity: Without taking into account the effects predicted by these theories for moving clocks in a gravitational field, there would be errors of roughly 10 kilometres per day of operation in the determination of positions on earth.


The carrier particles of the strong nuclear force. They are responsible for binding (glueing) quarks together into compound particles like protons or neutrons.


Chemical element with the symbol Au; each gold nucleus contains 79 protons.

Gold nuclei, stripped of their electrons, are among the types of heavy ions which are brought into collision in particle accelerators such as the Relativistic Heavy Ion Collider in order to recreate the state of matter in the early universe shortly after the big bang.

 Gowdy spacetimes

Gowdy spacetimes (or universes) are simple expanding model universes. In contrast with the better known Friedmann-Lemaître-Robertson-Walker universes (the basis for the big bang models), Gowdy universes are not homogeneous. Instead, they are filled with a regular pattern of gravitational waves. A Gowdy T3 universe is the simplest kind of Gowdy universe, in which space has the shape of a three-dimensional torus.

More information about Gowdy universes can be found in the spotlight text Of gravitational waves and spherical chickens.


In classical physics: An action-at-a-distance force by which all bodies that possess mass attract each other (see Newtonian theory of gravity), synonym: gravitational force.

In Einstein's general theory of relativity: The fact that matter that possesses mass, energy, pressure or similar properties distorts space-time, and that this distortion in turn influences whatever matter might be present.

An introduction to the basic ideas of general relativity is provided by the section General relativity of Elementary Einstein. More information about the nature of gravity in general relativity can be found in the spotlight text Gravity: From weightlessness to curvature.

 gravitational constant

Constant of nature; the fundamental Newton's law of gravity and thus a measure for the natural strength of gravity. Analogously, in Einstein's equations in the general theory of relativity, it occurs as the proportionality factor determining how strongly mass, energy and similar properties of matter distort space and time. In formulae, it is usually written as G. The best current value for G is

G = (6.6742± 0.001)·10-11m3kg-1s-2.

Compared with other fundamental constants, G is known only to a comparatively low accuracy.

 gravitational field

The totality of all gravitational influences that one or more massive objects can exert on bodies in their vicinity.

More precisely: At every location in space, the gravitational field is defined as the acceleration that a small test particle present at that location would feel due to the gravitational forces of the masses around it.

 gravitational lens

In Einstein's general relativity, gravity necessarily acts not only on material bodies, but also on light - light passing a massive body is deflected. This deflection can be so strong that light of one and the same cosmic object reaches an observer along multiple paths - corresponding to the observer seeing multiple images of that object in the sky. Masses that, in this sense, act like very special optical lenses are called gravitational lenses.

More information can be found in the spotlight text A brief history of gravitational lenses.

 gravitational redshift

According to general relativity, light flying away from a massive body (or other source of gravity) experience a redshift - its frequency decreases and the light becomes less energetic. On the other hand, light flying towards a massive body gets blueshifted - its frequency and energy increase.

 gravitational wave astronomy

Nascent discipline of astronomy which aims at using gravitational waves to gain information about cosmic objects or the cosmos as a whole - for instance about what's happening in the core region of a supernova, about neutron star or about the heated past of our universe.

So far, though, scientists are still working on the first direct detection of gravitational waves using highly sophisticated gravitational wave detectors, after which gravitational wave astronomy is hoped to begin in earnest.

 gravitational wave detector

Currently, scientists world wide are attempting the direct measurement of gravitational waves reaching us from the depths of space. They are mainly using two types of detectors: interferometric detectors like GEO600 and the LIGO detectors, and resonant detectors.

For more informations about gravitational waves, please consult the chapter Gravitational waves of Elementary Einstein.

 gravitational waves

Distortions of space geometry that propagate through space with the speed of light, analogous to ripples on the surface of a pond propagating as water waves.

For more informations about gravitational waves, please consult the chapter Gravitational waves of Elementary Einstein.

Selected aspects of gravitational wave physics are described in the category Gravitational waves of our Spotlights on relativity.


See frame-dragging .


Hypothetical carrier particle in a quantum theory of gravity. However, as of yet physicists have but a rough idea of how a complete theory of quantum gravity will look like.

See gravitation
 great circle

Circle on the surface of a sphere whose center coincides with that of the sphere itself. On the globe, the equator is a great circle, while every meridian corresponds to half of a great circle.

If you want to move on a spherical surface in the straightest possible way, choose a path along a great circle - in the language of mathematics this is equivalent to saying: great circles are geodesics of a spherical surface.

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