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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The Kamioka Gravitational Wave Detector (KAGRA) is an underground Japanese gravitational wave detector, the length of its two arms being 3 km. KAGRA uses cyrogenic technology, which means that the interferometer’s mirrors are cooled down to 20 Kelvin in order to reduce thermal noise. On top of that, the underground construction is expected to reduce seismic noise. The detector has started observation in 2020.

Karlstad University

University (enrolment ca. 10,000) in central Sweden. Research areas include a number of relativistic topics, from string theory to the mathematics of general relativity.
Karlstad University website

Kelvin scale

The temperature scale used in physics, synonym: absolute temperature.

The zero point of the Kelvin scale is at absolute zero; a temperature difference of one Kelvin (abbreviated 1 K and, rarely, also called one “degree Kelvin”) is the same as a temperature difference of one degree Celsius, as both scales differ only by their choice of zero point: X degrees Celsius are (X plus 273.15) Kelvin, Y Kelvin are (Y minus 273,15) degrees Celsius.

Relation to the Fahrenheit scale: X degrees Fahrenheit are (X+459,67)*5/9 Kelvin, Y Kelvin are (Y*9/5)-459,67 degrees Fahrenheit.

Kepler’s laws

Basic laws governing the orbital motions of planets around the sun. First law: Each planetary orbit is an ellipse, with the sun in one of its focus points. Second law: If you connect the planet and the sun by an imaginary line then, in equal time intervals, the line will sweep over equally large areas, independent on where the planet is on its orbit. Third law: dividing the square of a planet’s orbital period by the third power of it’s average distance from the sun gives the same value for all planets in the solar system; written as a formula: period²/(average distance to the sun)³ = const.

Kepler’s laws follow directly from the laws of classical mechanics and Newton’s law of gravity. However, they are only valid approximately – the gravitational pull of the planets on each other, as well as the fact that, ultimately, gravity is governed not by Newton’s laws, but by general relativity (see relativistic perihelion shift) lead to small deviations from perfectly elliptic orbits.

Synonyms: Kepler's laws of planetary motion

Kerr black hole

The simplest kind of a rotating black hole: a model universe containing a single rotating black hole and nothing else. This solution to Einstein’s equations was found by Roy Kerr in 1963.

Synonyms: Kerr solution

Kerr-Newman black hole

The simplest kind of a rotating, electrically charged black hole: a model universe containing a single rotating, charged black hole and nothing else. This solution to Einstein’s equations was found independently by Roy Kerr and Ted Newman in 1963.

The Kerr-Newman solution is more general and includes the Kerr solution if the charge of the black hole is zero.

Synonyms: Kerr-Newman solution


See electron volt

Synonyms: kilo-electronvolt


In the international system of units (SI), the unit of mass; until May 2019 defined by a reference mass that is kept in Paris, France. From then on by definition of the Planck constant and of the Avogadro number that has an exact experimental value used for the definition of the Mol.

kinetic energy

A type of energy that has to be ascribed to an object simply because that object moves relative to the reference frame. In classical, pre-Einstein physics, the amount of energy is given by one half, times an object’s mass, times the square of its speed.

Klein-Gordon equation

Equation regulating the behaviour of relativistic quantum particles with spin 0.