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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absolute zero

The lowest possible temperature. It is the point where particles reach their minimum of vibrational motion. For instance, the temperature of a gas is proportional to the average kinetic energy of the moving molecules or atoms. In classical terms, absolute zero would be reached when the gas particles do not move anymore. According to quantum mechanics the particles still retain a minimum motion, induced by lowest possible energy state that a quantum mechanical system may have and which fluctuates according to the Heisenberg uncertainty principle.

On the usual temperature scales, absolute zero is at -459.67 degrees Fahrenheit or -273.15 degrees Celsius. By definition of the Kelvin scale, absolute zero is at zero Kelvin.

abundances of elements

What portion of atomic matter in the universe is made up of hydrogen atoms? What fraction takes the form of helium, and how abundant are the other chemical elements? Questions like these are interesting in the context of relativity theory because the relativistic big bang models predict how many nuclei of light elements (mainly deuterium, helium, lithium) should have formed in the early universe during the phase known as Big Bang Nucleosynthesis. A brief account of this phase can be found in the spotlight text Big Bang Nucleosynthesis, while Equilibrium and change provides more information about the physical processes involved.

Measuring the abundances for these elements and subtracting the estimate for how many such nuclei formed inside stars (stellar nucleosynthesis) makes for an important test of this prediction and thus of the big bang models themselves. More information is provided by the spotlight text Elements of the past.


Every change of velocity with time is an acceleration.

This definition is slightly different from our everyday usage of the word. Ordinarily, we talk of an object accelerating when it becomes faster and faster. The physics definition covers two more situations. An object that decelerates, becomes slower, thus changes its velocity and, in the physics sense, undergoes a (negative) acceleration. Also, in physics, velocity is not the same as speed. A constant velocity implies not only constant speed, but also a constant direction of movement. Once the direction changes, so does the velocity – the change in velocity is associated with the change in the direction of movement. Thus, in the physics sense, even a car going around a curve of the road at constant speed undergoes acceleration.


When gas, dust or other kinds of matter fall towards a compact object (such as a black hole or a neutron star), a disk of infalling matter forms around the central object called the accretion disk.

The energy that matter gains in its fall is transformed into heat energy of the disk matter. Consequently, accretion disks are, as a rule, extremely hot. Their thermal radiation they emit is an important tool for indirect observation of neutron stars and black hole.

Within the disk, matter spirals around and around, coming closer and closer to the central object until at last it falls onto its surface (or, in the case of a black hole, through its event horizon).

Synonyms: accretion disk


Forces acting from one location to another without the need for any material connection, and without any delay – for instance, the Newtonian gravitational force with which even distant bodies in empty space can exert influence on each other.

active galactic nuclei (AGN)

The innermost regions of young galaxies can be very active and radiate considerable amounts of energy. Examples for such active galactic nuclei are radio galaxies and quasars.

In current models, the energy source powering activities of such nuclei, is a supermassive black hole in the galactic centre.

age of the universe

Another word for cosmic time, the time coordinate of the big bang models: 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.

aggregate state

See under states of matter.

Albert Einstein Institute

One of the research institutes of the Max Planck Society; an international centre for research on Einstein’s theory of gravity – from the mathematical fundamental, astrophysics and gravitational waves to quantum gravity. Founded in 1995, the institute is situated in Golm near Potsdam in Germany. In 2002, the experimental branch of the institute was opened in Hannover. It is dedicated to research with the gravitational wave detector GEO 600 .

AEI webpages
Website of AEI-Hannover

Synonyms: Max Planck Institute for gravitational physics AEI

alpha particle

Alternative expression for the naked (i.e. stripped of electrons) atomic nucleus of the element helium consisting of two protons and two neutrons.


For a physical quantity that changes periodically, the amplitude is a measure of how much the quantity changes from maximum to minimum. The simplest example is a sine oscillation. Over time, the sine curve oscillates between its minimum and its maximum values and the amplitude measures how big this oscillation is. There are different ways of defining amplitude. Some definitions use the Peak-to-peak difference for the amplitude (Maximum of the signal minus Minimum of the signal). Other definitions for signals with values centered symmetrically around zero specify the amplitude as the maximum value of the signal (Half of the Peak-to-peak amplitude).

Depending on the nature of the oscillation or wave, the amplitude will have different meanings. For a pendulum swinging back and forth, the amplitude is the maximum angle between the vertical direction and the pendulum string. For an electromagnetic wave, the amplitude is the maximal value of the electric field or equivalently (since the two maxima are related) the maximum of the magnetic field. For a (weak) gravitational wave, the amplitude is a direct measure of the changes in distance caused by the wave – as a simple gravitational wave of amplitude A passes, there are two directions in which distances are alternately stretched by up to a factor (1+A/2) and compressed by a factor (1-A/2).

The amplitude can change over time. For instance, for an ordinary pendulum, air friction will slow the pendulum bob down, and for each period – for each time the pendulum bob travels back and forth – the amplitude will be less than for the previous period. For a wave, the amplitude will also in general vary with location. Typically, the amplitude of a wave will decrease with the distance from the wave’s source.

angular momentum

A conserved physical quantity associated with the rotation of an object.

In classical physics, the contribution of each part of a body to the body’s total angular momentum is the part’s mass times its distance from the axis of rotation, times the part’s speed component which points in the direction of rotation and is perpendicular to the axis of rotation.

More information can be found in the spotlight text What figure-skaters, planets, and neutron stars have in common.

In the context of general relativity, angular momentum is an interesting quantity in the physics of black holes. More information about this can be found in the spotlight text How many kinds of black holes are there?


As a general rule, theories uniting special relativity and quantum theory predict the existence of a species of anti-particle for every species of particle. For instance, if such a theory contains electrons, then it also contains their anti-particles, called positrons, for protons, there are anti-protons, and so on.

It is a universal feature of anti-particles that they have the same mass as corresponding particles, and equal, but opposite charges; for examples, electrons and positrons have the same mass, but the electrons carry negative electric charge, whereas positrons carry the exact same amount of positive electric charge. For particles that carry no charges of any kind, particles and anti-particles are identical.

Synonyms: anti-matter, anti-particles


For a planet or other heavenly body orbiting the sun on an elliptic orbit, that point of the orbit farthest from the sun. The point closest to the sun is the perihelion. In the context of general relativity, aphelion and perihelion are of great interest as that theory predicts a slight motion of these points around the sun, cf. (relativistic) perihelion shift.


Synonyms: minute of arc, second of arc. Subdivisions of an angle, analogous to subdivisions of time: Sixty arcseconds correspond to one arcminute; sixty arcminutes (or 3600 arcseconds) correspond to one degree. A right angle has 90 degrees, or 5400 arcminutes, or 324000 arcseconds.

To denote fractions of these units, a prefix is added in the usual way – for instance, one thousandth of a second of arc is a millisecond of arc.

Synonyms: arcsecond

astronomical unit

Unit of length used by astronomers for distances in and around the solar system; the average distance from the earth to the sun. Abbreviation: au.

Concrete value:

1 au = 149.597870700 million kilometres
= 92.955807 miles
= 8.3 light minutes.


All matter we encounter in everyday life consists of smallest units called atoms – the air we breath consists of a wildly careening crowd of little groups of atoms, my computer’s keyboard of a tangle of atom chains, the metal surface it rests on is a crystal lattice of atoms. All the variety of matter consists of less than hundred species of atoms (in other words: less than a hundred different chemical elements).

Every atom consists of an nucleus surrounded by a cloud of electrons. Nearly all of the atom’s mass is concentrated in its nucleus, while the structure of the electron cloud determines how the atom can bind to other atoms (in other words: its chemical properties). Every chemical element can be defined via a characteristic number of protons in its nucleus. Atoms that have lost some of their usual number of electrons are called ions. Atoms are extremely small (typical diametres are in the region of tenths of a billionth of a metre = 10-10 metres), and to describe their properties and behaviour, one has to resort to quantum theory.

atomic nucleus

See nucleus.

Auger observatory

See Pierre Auger Observatory.