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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Hawking radiation

Thermal radiation emitted by black holes due to quantum effects. First calculated by the British physicist Stephen Hawking in the 1970s. The characteristic temperature of the radiation, which depends on the mass and spin of the black hole, is called Hawking temperature.

Hawking temperature

Characteristic temperature of the Hawking radiation of a black hole. For simple, spherically symmetric black holes, it is

TH = 6· 10-8 (solar mass/mass of the black hole) Kelvin. [Problems reading expressions such as 10-8? See exponential notation.]

Heisenberg’s uncertainty principle

Is a fundamental law of quantum theory, which defines the limit of precision with which two complementary physical quantities can be determined. If one of the quantities is measured with high precision, the corresponding other quantity can necessarily only be determined vaguely. In other words, it is impossible to measure simultaneously both complementary quantities with greater precision than the limit defined by the Heisenberg’s uncertainty principle.

An example for such complementary quantities are the location and the momentum of a quantum particle: Very precise determination of the location make precise statements about its momentum impossible and vice versa.


After hydrogen, the second lightest chemical element. Its atomic nucleus consists of two protons and, ordinarily, two neutrons (“helium-4”); such helium nuclei are also called alpha particles. Another variety of helium, helium-3, has only one neutron in its nucleus.

In the context of general relativity, both helium-3 and helium-4 are is of interest as two species of light atomic nuclei that formed in the early universe during Big Bang Nucleosynthesis.


Unit of frequency, abbreviation: Hz. One Hertz corresponds to one oscillation per second.


In general relativity: A closed surface that is the boundary of a black hole. Whatever enters through this boundary from the outside can never again leave the inside.

Synonym: event horizon.

Synonyms: event horizon

Hubble constant

In an expanding universe such as that of the big bang models, every observer will find: The apparent velocity with which the galaxies around him recede is proportional to their distance; the more distant a galaxy, the more its distance increases in a given time. This relation was first found by the astronomer Edwin Hubble in the 1920s from observations of far-away galaxies; it is hence called Hubble relation or Hubble’s law, and the constant of proportionality between speed and distance is the Hubble constant.

A visualisation of the Hubble relation can be found on the page The expanding universe in the chapter on Cosmology of Elementary Einstein.

The Hubble relation only holds for all galaxies in an idealized universe whose expansion neither accelerates nor slows down. In more realistic universes, it is true in good approximation only for galaxies that are not too far away.

Synonyms: Hubble effect Hubble relation Hubble's law

Hubble relation

See Hubble constant

Hubble space telescope

Cooperative project of NASA and ESA: Space telescope that was put into orbit in 1990. Orbiting 600 kilometres above the earth, it leaves behind the densest parts of the earth’s atmosphere, allowing an unrivalled, undisturbed view into space.

Outreach website for the Hubble Space Telescope

Website of the Space Telescope Science Institute


The lightest (and, in our universe, the most abundant) chemical element. The atomic nucleus of an ordinary hydrogen atom is a single proton. If the atomic nucleus contains an additional neutron, the atom is called heavy hydrogen or deuterium.