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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naked singularity

In general relativity, singularities – ragged edges of spacetime – that form in the collapse of massive bodies or in similar processes are typically hidden inside black holes, in other words: spacetime in their vicinity is distorted so much that no information about the singularity can ever reach the outside world. Hypothetical singularities which are not cloaked in this way, and thus are visible to the rest of the cosmos, are called “naked”. By the cosmic censorship hypothesis, no realistic kind of collapse can lead to the formation of a naked singularity.


“Nano” as a prefix denotes “one billionth”, making a nanometre one billionth of a metre.

National Aeronautics and Space Administration (NASA)

Part of the US government in charge not only of manned space missions, but also responsible for numerous highly successful satellite and probe missions. NASA is a partner in projects such as the Hubble space telescope or the gravitational wave detector LISA.

NASA website

Synonyms: NASA

National Radio Astronomy Observatory

US national institute for radio astronomy, located in Charlottesville, Virginia. Responsible for operating the Very Large Array of radio telescopes in New Mexico and the Very Large Baseline Array, an array of ten far-apart radio telescopes.

NRAO website

Synonyms: NRAO


Type of elementary particle that is related to the electron, but carries no electric charge and has an extremely small mass. There are three types of neutrinos, called electron-neutrino, muon-neutrino and tau-neutrino.

Synonyms: neutrinos


Particle that is electrically neutral and comparatively massive; the atomic nuclei consist of neutrons and protons.

Neutrons are not elementary particles, they are compound particles consisting of quarks that are bound together through the strong nuclear interaction . Collectively, neutrons, protons and a number of similar particles are called baryons.

Neutron stars are mainly made of neutrons.

Synonyms: neutrons

neutron star

Final stage of massive stars that explode as a supernova. In the explosion process, the core of the star collapses to form a compact object with roughly 1.4 solar masses that mostly consists of nuclear matter, predominantly of neutrons.

For astronomers, neutron stars are of interest as there exists a variety called pulsars from which they receive highly regular pulses of electromagnetic radiation. For relativists, they are interesting as the typical effects of general relativity are very pronounced in objects that compact (compare PSR 1913+16).


English physicist of the 16th/17th century, see Newtonian gravity, below.

In the international system of units (SI) the unit of force, abbreviation N. One Newton is the force needed to impart on a body of mass one kilogram an acceleration of one metre per second-squared.

Newton’s constant

See gravitational constant

Newtonian gravity

In pre-Einstein mechanics, which goes back to the English physicist and mathematician Isaac Newton (1643-1727), gravity is a force with which masses act on each other. As other forces do, they cause bodies to accelerate.

In its simplest form, Newton’s law of gravity describes the force acting between two spherical, symmetric masses: The force with which the first sphere acts on the second is equal to the mass of the first sphere times the mass of the second sphere times Newton’s gravitational constant, divided by the square of the distance between the centre-points of the two spheres.

Synonyms: Newton's law of gravity


Chemical element whose atoms have seven protons each in their nuclei.

In the context of relativity, more concretely: cosmology, nitrogen is interesting as an indicator of chemical evolution: Its nuclei are not produced during Big Bang Nucleosynthesis, but they are produced by nuclear fusion reactions in the interior of stars. The presence of nitrogen (or, for that matter, of other elements such as oxygen or iron) in an astronomical object is an indicator that stellar fusion has taken place, and hence that the abundances of the different elements do not reflect the element abundances in the early universe.

no-hair theorem

See black hole uniqueness theorems.


In some physical theories, influences can simply be added up – take the electric force associated with one particular charged body, the electric force associated with a different body, and their sum will be the electric force felt by a test particle when both bodies are present. Such theories are called linear; theories where separate contributions do not simply add up are called nonlinear, an important example being Einstein’s general relativity.

Synonyms: nonlinearity

nuclear fission

Processes in which a heavier atomic nucleus splits up into several lighter nuclei. If the initial nucleus is heavy enough, energy is set free in the split. Nuclear fission reactions are used in nuclear reactors to produce electrical energy, and in nuclear weapons to power an energetic explosion.

Some more information about nuclear fission can be found in the spotlight topic Is the whole the sum of its parts?. The role played in nuclear fission and fusion by Einstein’s famous formula E=mc² is the subject of the spotlight topic From E=mc² to the atomic bomb.

nuclear force

Force that binds protons and neutrons together to form atomic nuclei; a side-effect of the strong interaction.

See also strong interaction and weak interaction.

nuclear fusion

Processes in which two lighter atomic nuclei merge to form a more massive nucleus. For nuclei lighter than those of iron, energy is released in fusion. This is the main source of energy of ordinary stars like our sun.

Some more information about nuclear fusion can be found in the spotlight topic Is the whole the sum of its parts?. The role played in nuclear fusion and fission by Einstein’s famous formula E=mc² is the subject of the spotlight topic From E=mc² to the atomic bomb.

nuclear physics

That branch of physics dealing with the properties of atomic nucleus. One connection to relativistic physics is the fact that nuclear physics is needed to describe the properties of matter in the early universe of the big bang models and in the interior of neutron stars.


See Big Bang Nucleosynthesis.


Extremely dense central region of an atom, consisting of protons and neutrons held together by nuclear forces. The number of protons determines what chemical element a nucleus represents.

Typical diameter for atomic nuclei are in the region of a quadrillionth of a metre= 10-15 metres. This makes nuclei about a hundredth of a thousandth as large as atoms.

Synonyms: nuclei

numerical relativity

Subdiscipline of physics devoted to the use of computer simulations for exploring the structure and consequences of Einstein’s theories, special and general relativity.

Notably, the centerpiece of general relativity are Einstein’s equations, which relate certain properties of the matter contained in a spacetime to that spacetime’s geometry. A model universe in which matter distorts the geometry – and is in turn influenced by those distortions – in exactly the way prescribed by Einstein’s equations is called a solution of these equations. Some simple solutions can simply be written down on a piece of paper (“exact solutions”). More complicated situations can only be described by simulating space, time and matter in a computer (“numerical solution”), and this is one of the main tasks of numerical relativity.

Numerical relativity has led to interesting results about black holes and gravitational waves, for instance about the gravitational wave produced when two black holes collide and merge. They have also shed light on what general relativity predicts for the properties of spacetime close to a black hole’s central singularity (further information about this can be found in the spotlight text Of singularities and breadmaking). The branch of numerical relativity that is of interest for the study of phenomena such as supernovae, jets, and merging or collapsing neutron stars is relativistic (magneto-)hydrodynamics, more about which can be found in the spotlight text The realm of relativistic hydrodynamics.