More than 400 keywords from relativity and related topics, from “absolute zero” to “XMM Newton” – please use the menu on the left to choose a letter.

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Abbreviation for “radio detection and ranging” – how to detect objects, and measure their distance, by means of sending out radio waves and detecting their reflection. Widely used in air and sea traffic; for general relativity, radar is of interest as radar signals reflected by planets can be used to measure the Shapiro delay – the fact that it takes those signals passing close to the sun a little longer to reach us than expected by classical physics.


In a general sense: Collective name for all phenomena in which energy is transported through space in the form of waves or particles. In a more restricted sense, the word is often used synonymously with electromagnetic radiation.

radio astronomy

Branch of astronomy dedicated to the detection and evaluation of radio waves from outer space. Such observations have led to the discoveries of radio galaxies, quasars and pulsars.

radio galaxy

One variety of the active galactic nuclei of young galaxies, whose central region radiates extremely great amounts of energy. Radio galaxies are distinguished by radiating off extremely high energies in the form of radio waves (more than 1035 Watts), from sources that are often located outside the visible part of those galaxies. Usually, the sources are radio bubbles, huge regions of gas whose radiation is stimulated by jets.

To the best of current knowledge, what’s behind all that energy output is a supermassive black hole in the galactic core.

radio signals

See radio waves

radio telescope

Any antenna used for radio astronomy, for instance to observe pulsars or radio galaxies.

radio waves

Variety of electromagnetic radiation with frequencies of a few thousand to a few billion oscillations per second, corresponding to wave-lengths of a few kilometres to a few centimetres. True to their name, these are the electromagnetic waves that bring radio and TV programs from the broadcast towers to our personal antennas and receivers. Cosmic radiowaves also make for interesting observations – see radio astronomy.

Synonyms: radio signals


In the early universe as described by the big bang models: transition that occurs at cosmic time around 380 000 years. At this point, the universe has cooled down sufficiently for atomic nuclei and electrons to form atoms – without immediately being ripped apart by electromagnetic radiation. The etymology of recombination does not quite correspond to the physics – this is not a re-combination, it is the first such combination in the universe as we know it. In the process, the cosmic background radiation decouples from the material universe.

Synonyms: recombination phase


The frequency of a simple light wave is directly related to its colour (cf. spectrum). For the lowest frequencies of visible light, that colour is red, light of the highest frequencies appears blue. If the frequency of a light wave is shifted towards lower frequencies (for instance by the doppler shift), that corresponds to a colour shift towards the red end of the spectrum, and is hence called a redshift. Consequently, a shift towars higher frequences is called blueshift.

From this, “redshift” has come to acquire a more general meaning. It is used to denote any shift towards lower frequencies, even for types of electromagnetic radiation where the frequencies do not correspond to any visible colour, and more generally still, for other types of waves as well (for instance for gravitational waves).

In the context of general relativity, the gravitational and the cosmological redshift are of particular interest.

redshift, gravitational

In general relativity, light moving away from a mass or other source of gravity experiences a shift towards lower frequencies. This is called the gravitational redshift; on the other hand, light falling towards a mass is blueshifted. Both effects together are called the gravitational frequency shift. The gravitational redshift is closely related to gravitational time dilation.

Information about an astrophysical application of this effect can be found in the spotlight text Gravitational redshift and White Dwarf stars.

reference frame

Already in special relativity, motion is relative, and whenever there is talk about a moving clock, one must give the additional information: Moving relative to whom or what? Such a “whom or what”, in other words: An object together with a recipe to determine locations relative to that object and to measure time, is called a reference frame.

In special relativity, there exists a special and very important class of reference frame, so-called inertial reference frames, in short: inertial frames.


Models, effects or phenomena in which special relativity or general relativity play a crucial role are called relativistic.

Examples are relativistic quantum field theories as theories based on special relativity, or the relativistic perihelion shift as a consequence of general relativity.

In addition, conditions under which the difference between relativistic physics and ordinary, classical physics are especially pronounced, are also called relativistic. For instance, when material objects reach speeds close to speed of light, one talks of relativistic speeds, while speeds that are so small compared to light as to make relativistic effects undetectably small are non-relativistic.

Relativistic Heavy Ion Collider

A particle accelerator operated by Brookhaven National Laboratory on Long Island, New York. It brings heavy ions – the nuclei of atoms that have been stripped of all their electrons – into collision at high energies; the resulting states of matter give valuable information about the very early high temperature universe, and about the physics that should be included in the big bang models of relativistic cosmology.

RHIC webpages

Synonyms: RHIC

relativistic mass

One prediction of special relativity is that, the faster an object already is, the more difficult it is to accelerate it even further. One consequence of this is that it is impossible to accelerate a material object to the speed of light: The faster the object already is, the more force has to be used to increase its speed, and close to the speed of light, this effect becomes so strong that, finally, one would have to use infinite force to effect the final, decisive acceleration.

Traditionally, in classical physics, the resistance of an object to changes of its state of motion is its (inertial) mass. The relativistic mass of an object is defined in the same way, and the value an observer measures for this relativistic mass increases as an object moves faster and faster relative to that observer.

relativity principle

Basic principle of special relativity: for two observers moving relative to each other with constant relative velocity (more specifically: for two inertial observers) the laws of physics are the same. There is no key experiment by which one could argue that one of the observers is “at rest” in an absolute sense – as far as physics is concerned, all such observers are equal, no one has more right than another to regard himself as being at rest, and motion (at least: motion with constant velocity) is defined only as relative motion of observers with respect to each other.

For an introduction to some of the consequences Einstein derived from the relativity principle, check out the chapter special relativity of Elementary Einstein.

relativity theory

The modern theories of space and time that go back to Albert Einstein: His special theory of relativity, which ignores the effects of gravitation, and his general theory of relativity, in which gravitation is included as a distortion of space and time.

For an introduction to the basics of both theories of relativity, check out the chapters Special relativity and General relativity in Elementary Einstein.

Synonyms: theory of relativity, theories of relativity

resonant detector

Detector for gravitational waves in which it is attempted to measure the influence of these waves on an oscillating test mass.

More information about how such detectors work can be found in the spotlight topic Small vibrations.

rest mass

In special relativity, the (inertial) mass of an object depends on how fast that object moves relative to the observer. The rest mass is the inertial mass of an object, measured by an observer relative to whom the object is at rest. With this definition, the rest mass is a kind of measure for how much matter is contained in a body.


See Relativistic Heavy Ion Collider (RHIC).

Ricci singularity

A type of spacetime singularity (i.e. a boundary of spacetime) that is associated with infinitely high energy density.

More information about the different types of singularities can be found in the spotlight text Spacetime singularities.


Just like a bell ‚rings‘ for some time after it’s been struck, the collision of two black holes resonates in the form of gravitational waves. They are fainter than those released during the merger and occur due to a brief oscillation of the new black hole while it settles to a stationary configuration. This phase, during which gravitational waves are still being released, is called the ‚ringdown‘.