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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Variety of electromagnetic radiation with frequencies between a few and a few hundred quadrillion oscillations per second, corresponding to wave-lengths between a few hundred and a few billionths of metres. Known in everyday life as that part of the radiation we receive from the sun that causes our skin to tan

Synonyms: ultraviolet radiation

uncertainty principle

See Heisenberg’s uncertainty principle

unified field theory

Collective designation for Einstein’s unsuccessful attempts to formulate a theory in which gravity and other interactions, notably electromagnetism, are described in a unified manner – a theory in which gravity and electromagnetism would be no more than different facets of one and the same underlying structure, in the same manner in which magnetism and the electrostatic force are facets of a more general description of electromagnetism.

After Einstein, quite a number of scientists have searched for a unified description of all interactions; the best-known modern incarnation of the idea of unification is string theory.


Given a set of physical laws, one interesting class of question is aimed at finding out the variety of situations those laws allow. For example, is there only a single kind of rotating black hole, or do the laws of general relativity admit an infinite variety of such objects? Theorems addressing this kind of question are generally known as uniqueness theorems – in their purest form, they state that, given a certain set of physical laws and a certain set of additional conditions, there is no more than one configuration of spacetime and matter that fits the bill.

In general relativity, the most famous such theorems are the black hole uniqueness theorems. They are explored in the spotlight text How many different kinds of black hole are there? A different aspect of the question of uniqueness is addressed in the spotlight text The many ways of building an empty, unchanging universe.

Synonyms: uniqueness theorems

UV radiation

See ultraviolet radiation.