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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In physics, velocity is a combination of two aspects: First of all, how fast an object is, in other words: how long a distance it moves in a given time (its “speed”). Secondly, the direction in which an object moves. Physicists combine these two informations into a single mathematical object, called a “vector”, and this is what is called the velocity. For instance, when a car goes around a curve with 100 miles per hour, its speed is constant, but as it changes its direction of movement, its velocity changes correspondingly.


Virgo is a gravitational wave detector that was collaboratively built by French and Italian labs and was in operation until 2011. Then, the instrument was decommissioned and adapted to achieve 10 times higher sensitivity. Advanced Virgo started operation in 2017. Together with LIGO, GEO600 and KAGRA, it has since served the purpose of investigating space using gravitational wave signals.

Virgo Cluster

The galaxy cluster closest to our own, roughly 50 million light years away. It consists of about 2000 galaxies. In the night sky, as viewed from Earth, it is located in the constellation virgo.

Synonyms: virgo galaxy cluster

visible light

In astronomy, the word light is often used to denote any kind of electromagnetic radiation, from infrared radiation to X-rays and beyond. If, in contrast, an astronomer is talking about the ordinary light to which our eyes are susceptible – electromagnetic radiation with wavelengths between about 400 and 700 nanometers (400 to 700 billionths of a metre) – he or she will use the expression “visible light”.