Dictionary
More than 350 keywords from relativity and related topics, from "absolute zero" to "Xrays"  please use the menu on the left to choose a letter.
 California Institute of Technology (Caltech)

Major research university located in Pasadena, California. Areas of research include general relativity, particle physics, quantum gravity and cosmology; in addition, Caltech is one of the main sites for the researchers (although not the detectors!) of the LIGO project, which operates the most sensitive gravitational wave detectors to date. Also, the Einstein Papers Project, which is working on an edition of Einstein's collected papers, is based at Caltech.
Caltech webpages
Webpages of the Theoretical Astrophysics and Relativity group at Caltech  carrier particle

In the framework of relativistic quantum field theories (which form the theoretical basis of the physics of elementary particles, the forces by which matter particles interact are transmitted by socalled carrier particles travelling back and forth between them. For instance, the electric force between two electrons would come about through the exchange of photons, the carrier particles of the electromagnetic interaction. Carrier particles always have integer spin, such as spin 1 or 2 (which means they belong to the class of particles called bosons). Synonym: force particles.
 causal

In the context of relativity: causality concerns the questions which events can, in principle, cause which other events and which events are two far apart for one to influence the other. In special relativity, nothing, no moving object, no information, no influence can move faster than light. Thus, no event can influence another if the two events happen too far apart for light to travel between from the first event to reach the location of the second event in time. In other words, light propagation determins the causal structure of spacetime (cf. lightcone). Models or theories respecting this causal structure are themselves called causal  an example are the relativistic quantum field theories.
In general relativity, the cosmic speed limit light speed is only defined locally: In a sidebyside race, no object, no influence can overtake a light signal. This, too, leads to a causal structure, to strict rules which event can influence which other event. As gravity deflects and delays light signals, matters are more complicated than in special relativity, but it's still true that the causal structure is completely determined by how light propagates in the spacetime in question.
 causal sets

Approach to the problem of finding a theory of quantum gravity: In causet models, spacetime is composed of elementary building blocks that represent elementary events.
More information about causal sets can be found in the spotlight topic Geometry from order: causal sets.
 celestial mechanics

That discipline in physics (and astronomy) dealing with the laws that govern the motions of heavenly bodies. Originally seen as distinct from the motions of bodies on earth (see Kepler's laws of motion), it has been a subdiscipline of mechanics ever since Newton derived the cosmic laws of motions from more general mechanical laws.
For most astronomical applications, Newtonian, classical mechanics works perfectly well, however, as soon as highprecision measurements or strong gravitational fields come into play, celestial mechanics is governed by laws of relativistic mechanics derived from Einstein's theory of general relativity.
 Celsius

Most European countries use the Celsius temperature scale in everyday life. Temperatures are given in "degrees Celsius" (abbreviated as °C). By definition, the zero point of this scale (0°C) is the melting point of water, while the temperature 100°C corresponds to its boiling point (both parts of the definition assume the same standard value for air pressure).
Relation to the Fahrenheit scale: X degrees Celsius correspond to (9/5 times X) +32 Fahrenheit, Y Fahrenheit are (Y32)*5/9 degrees Celsius.
Relation to the Kelvin temperature scale used in physics: X degree Celsius are X plus 273.15 Kelvin, Y Kelvin are Y minus 273.15 degrees Celsius. In particular, differences in temperature are the same in Kelvin and in degrees Celsius; the only difference between the two scales is their choice of zero point.
 censorship

See cosmic censorship, below.
 centrifugal force

An inertial force which an observer in a rotating reference frame needs to introduce in order to explain why nearly all objects in the vicinity appear to undergo acceleration away from the axis of rotation.
 CERN

European research centre for nuclear and particle physics (Centre Européen pour la Récherche Nucleaire  pardon my French), located near Geneva on both sides of the francoswiss border, founded 1954.
CERN isn't famous just because of particle accelerators like its proton synchrotron, the Large Electron Positron Collider (LEP) and the Large Hadron Collider (LHC) currently under construction, but also as the birthplace of the World Wide Web.
 Chandrasekhar mass

Upper bound for the masses of white dwarfs, in other words: for what lowmass stars become when they have used up their nuclear fuel. The first to calculate this upper bound was the Indian astrophysicist Subramanian Chandrasekhar.
The Chandrasekhar mass is 1.4 times as large as the solar_mass. The reason that no white dwarf can have more mass follows from its need to maintain equilibrium between the gravitational force working towards further collapse and the interior pressure of the star acting to prevent collapse. For larger masses, the degeneracy pressure on which a white dwarf's stability depends is overcome, and further collapse ensues.
 charge

On the one hand: a measure of the strength of a force (actionatadistance) originating from a body, and of how susceptible it is to being influenced by other bodies via the same force. The most famous example is electric charge: Electrically charged bodies act on other electrically charged bodies via an electric force whose strength is proportional to the electric charges of the bodies involved.
It is a characteristic property of charges that they are conserved; they can neither be created from nothing nor simply disappear. For instance, when a positron with electric charge +1 (in suitable units) and an electron with electric charge 1 annihilate to give electromagnetic radiation, overall charge conservation is satisfied: Before the annihilation, the sum of the electric charges was 1+(1)=0, and afterwards, when there is only uncharged electromagnetic radiation left, it is also zero.
In the context of particle physics, there are more abstract charges not directly connected with forces and interaction, but subject to similar conservation laws.
 chemical element

See element
 chemical evolution

The changes in the abundances of the different chemical elements that have taken place throughout the history of the universe, mainly in the very early universe during the phase called Big Bang Nucleosynthesis and, from a couple of hundred million years later until today, in the interior of stars (stellar nucleosynthesis).
 classical

In physics, the word is used with two meanings. First of all, it denotes physical models or theories that take into account neither the effects of Einstein's theories of relativity nor those of quantum physics, for example classical mechanics. However, it is also used to denote models or theories that are not formulated in the framework of quantum physics; in that sense, general relativity is an example for a classical theory.
 classical mechanics
 classical tests of general relativity
 cluster of galaxies
 See galaxy cluster
 Compton Gamma Ray Observatory

A satellite observatory for astronomical observations of gamma rays operated by NASA from 1991 to 2000. Scientific aims included the study of gamma ray bursts, pulsars, supernovae, and accretion processes around black holes.
 conic singularity

A particular type of spacetime singularity (i.e. a boundary where spacetime ends) that is not associated with curvature, but instead is the analogue of the pointed tip of a cone.
More information about the different types of singularity can be found in the spotlight text Spacetime singularities.
 conservation laws

Some of the most important quantities in physics are conserved : What they measure can neither be created nor destroyed, and their total sum is constant over time. Such statements of constancy over time are called conservation laws.
The most important conserved quantity is energy. Energy can neither be created from nothing nor simply vanish. If the energy contained in a system increases, it must be because energy has been transported into the system (and there is now less energy outside the system); if the energy decreases, it must be because energy has been transferred into the system (and there is now less energy outside).
Another important class of conserved quantities are charges, for instance electric charge. A further conserved quantity in mechanics is angular momentum; a quantity associated with a body's rotation.
 constancy of the speed of light
 One of the basic postulates of special relativity: The speed of light in a vacuum is the same for all observers drifting through gravityfree space (more precisely: for all inertial observers. In particular, its value its independent of an observer's motion relative to the source of the light.
 continuous
 Space as we are used to thinking about it is a continuum or, equivalently, continuous space: Between every two points, there always exists an infinity of other points, and every volume can be divided into smaller and smaller parts without ever reaching a limit.
 coordinates

A rule for assigning to each point of a general space (that is to say: of a line segment, a surface, threedimensional space or higherdimensional analogues) or spacetime a set of numbers for purposes of identification.
Many readers will know two examples from school: In the case of the line of real numbers, every point on the line corresponds to a real number which can be seen as its coordinate. What's important is that these coordinates reflect neighbourly relations: The number 1 lies between the number 0 and the number 2, and so does the point corresponding to it lie between the two points corresponding to 0 and 2. The second example is the usual XYcoordinate system (sometimes called Cartesian coordinates), by which every point in a plane can be characterized by two numbers: the first its X coordinate value, the second its Y coordinate value.
The examples reflect an important property of coordinates: To uniquely identify a point in space, one needs as many coordinate values as the space has dimensions.
Of the four coordinates defining an event in spacetime, three serve to fix its location in threedimensional space, while the fourth gives the point in time for the event.
 Coriolis force

An inertial force which an observer in a rotating reference frame needs to introduce in order to explain why certain moving objects appear to undergo acceleration at right angles to their direction of motion.
The Coriolis force plays an important role in meteorology  from the point of view of an observer at rest on the surface of the earth, it explains the deflection of certain wind flows.
 Cosmic background radiation

"Electromagnetic echo" of the early universe; first predicted by the big bang models in the context of general relativity; later, from the 1960s on, observed with radio telescopes.
The cosmic microwave background contains important information about the properties and the earliest history of the universe. For instance, it can be used to deduce whether space is curved or Euclidean; more information about this can be found in the spotlight text Cosmic sound.
 cosmic censorship

Possibly the most disturbing feature of Einstein's general theory of relativity is the existence of singularities  most commonly, regions of spacetime in which density and curvature go to infinity.
Is is quite likely that singularities are artefacts resulting from the fact that Einstein's theory does not take quantum effects into account, and that they will be absent in a more complete theory of quantum gravity. Yet even if you leave aside quantum theory, and stay strictly within the framework of Einstein's theory, it is likely that most singularities are, if not absent, then at least wellconcealed:
The hypothesis of cosmic censorship states that, whenever a body collapses so completely as to result in the formation of a singularity, a black hole will be formed so that the singularity will be hidden behind the horizon, and thus completely unobservable for anyone outside the black hole.
At the present time, this hypothesis is unproven. Indeed, there are some counterexamples, but they describe idealized situations which are not likely to tell us anything about the real world. Finding a proof that, for all realistic collapse situations, there is indeed cosmic censorship, is one of the great open problems of general relativity research.
 Cosmic microwave background radiation
 See cosmic background radiation, above.
 cosmic rays
 Highly energetic particles reaching the earth from the depths of space, mainly consisting of protons and light atomic nuclei.
 cosmic time

Measure for the progress of the evolution of an expanding universe such as that of the big bang models. It corresponds to time as measured by clocks that are at rest relative to the expanding space, and that have been set to zero at the very beginning, the time of the hypothetical big bang singularity. Synonym: Age of the universe.
The basic features of the cosmological models that use cosmic time are reviewed in the chapter Cosmology of Elementary Einstein.
 cosmological constant
 In the big bang models, an inherent tendency of space to accelerate or decelerate its expansion. From observations, it seems that our own cosmos has a cosmological constant that leads to a slight acceleration of its expansion.