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More than 350 keywords from relativity and related topics, from "absolute zero" to "X-rays" - please use the menu on the left to choose a letter.

 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 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 = 6ยท 10-8 (solar mass/mass of the black hole) Kelvin. [Problems reading expressions such as 10-8? See exponential notation.]
 Heisenberg's uncertainty principle

Fundamental law of quantum theory: All physical quantities that can be measured come in pairs. If one of the quantities in a pair is measured with high precision, the corresponding other quantity is necessarily determined only very vaguely. It is impossible to measure precisely and simultaneously both quantities in one and the same pair.

An example for such a pair are the location and the velocity of a quantum particle: Very precise measurements of the location disturb the velocity; if the velocity is measured precisely, it is automatically unclear where exactly the particle is located.


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.

 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 effect, and the constant of proportionality between speed and distance is the Hubble constant.

A visualisation of the Hubble effect 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.

 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.

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