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 a general sense: any travelling pattern, whether or not it involves matter being transported as well. Simple examples are water-waves – wave crests and troughs travelling over a water surface, and a Mexican wave in a football stadium, with fans alternately standing up and sitting down – the pattern moves throught the stadium, not the fans themselves.

An especially simple form for a wave is a sinus wave, a regular pattern of wave crests and troughs.

Synonyms: waves


For simple waves, where maxima and minima – wave crests and troughs – follow each other with perfect regularity, one can define a characteristic wavelength: the never-changing distance between two subsequent wave crests.

weak force

One of the basic interactions in the standard model of particle physics. It is responsible for certain radioactive decays, such as the one where a neutron is transformed into a proton, sending out an electron and a neutrino.

Synonyms: weak nuclear force weak interaction


From everyday life, we’re used to the Earth’s gravity pulling every body down to the ground, and the strength of that force is called its weight. If no such force is present, then bodies placed at a certain location in space simply stay where they are, even without any support. Whenever that is the case, we are in a situation of weightlessness.

There are two types of situation in which weightlessness occurs. First of all, one could move far, far away into space, distancing oneself from all massive bodies so far that their gravitational influence becomes negligible. This would be a truly gravity-free situation. The second type of situation is more common. In free fall – be it an elevator plunging to earth, be it a space-station like the ISS in orbit around the Earth – bodies are weightless. The fact that, at least locally, there is no way to distinguish between these two types of weightlessness is embodied in the equivalence principle, one of the fundamental building blocks of the general theory of relativity.

Weyl singularity

A type of spacetime singularity (i.e. a boundary of spacetime) that is associated with infinitely strong tidal forces.

More information about the different types of singularities can be found in the spotlight text Spacetime singularities.

White dwarf

When stars with up to ten solar masses have exhausted the fuel of light atomic nuclei they need to sustain nuclear fusion reactions, they collaps to form a White Dwarf: a comparatively small ball of gas, prevented from further collapse by a quantum mechanical phenomenon, the so-called degeneracy pressure of its electrons.

One way of determining the masses of White Dwarfs uses an effect of general relativity, namely the gravitational redshift – more information about this can be found in the spotlight text Gravitational redshift and White Dwarf stars.

Synonyms: White dwarf star

Why now? puzzle

From astronomical observations, it follows that the density associated with dark energy in our universe has the same order of magnitude as the density of the matter content of the universe. That is remarkable – in the past, the matter density will have been much larger than that of the dark energy, and in the far future, the roles will be reversed. Is it coincidence that we make our observations precisely at the time when the two densities are of comparable size, or is there a physical explanation for it?

Synonyms: "Why now?" puzzle

Wien’s law

One of the laws governing the properties of the simplest form of thermal radiation – that emitted by a blackbody: the product of such a body’s temperature (measured in Kelvin) and the wavelength at which it emits maximal amounts of energy is a universal constant.

Wilkinson Microwave Anisotropy Probe

NASA satellite telescope dedicated to observations of the cosmic background radiation.

WMAP website

Synonyms: WMAP


The path of a pointlike object in four-dimensional spacetime is a line called the object’s world-line. To every moment in time corresponds one point of that world-line giving the position of the object in space at that particular moment.