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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Fahrenheit scale

Usual temperature scale in the US. Temperatures are given in degrees Fahrenheit (°F); the scale is defined historically, by using as its zero point the lowest temperature measured in winter 1708/1709 in Danzig (today Gdansk, Poland, Daniel Gabriel Fahrenheit’s hometown), while 100 Fahrenheit is human body temperature.

Relation with the Celsius scale widely used in Europe: X degrees Fahrenheit are (X-32)*5/9 degrees Celsius, Y degrees Celsius are (Y*9/5) +32 degrees Fahrenheit.

Relation with the Kelvin scale widely used in science: X degrees Fahrenheit sind (X+459.67)*5/9 Kelvin, Y Kelvin are (Y*9/5)-459.67 degrees Fahrenheit.

false colour

Only a small part of astronomical observations is concerned with visible light, in other words: with electromagnetic radiation visible to the human eye. In order to visualize obervations made at invisible wave-length, such as infra-red light, radio waves or X-rays, the different wave-lengths are mapped to visible colours following some arbitrarily chosen scheme.

Similarly, physical quantities that are not connected with electromagnetic radiation can be mapped to colours; for instance, one can produce images of a star’s interior where different colours stand for different densities.


“Fermion” is the collective word for all quantum particles with half-integer spin, such as spin 1/2, 3/2 or 5/2.

Among elementary particles, fermions are the matter particles, for example electrons or quarks, while the carrier particles responsible for transmitting the elementary forces between particles belong to a different class – they are bosons.

Quite generally, fermions are subject to the Pauli exclusion principle. Roughly: no two electrons can rest at the same location. A bit more precisely: no two electrons can ever be in the same state. This property is decisive for what we call matter: Only the fact that it is impossible for all electrons of an atom to occupy the lowest energy state close to the atomic nucleus, but that instead the electrons have to spread out and occupy different states leads to the difference between atoms with different numbers of electrons and, in particular, to the fact that different elements have different chemical properties.


The totality of possible force influences acting on test particles in a given region. For instance, the electric forces with which an electrically charged body would act on test particles brought into its vicinity define its electric field, the gravitational forces with which a mass acts on test bodies define its gravitational field.


See nuclear fission


A space is called flat if its geometry is the direct generalization of Euclidean geometry, the standard geometry taught in schools. By this definition, the simplest two-dimensional flat space is the plane, and ordinary, everyday three-dimensional space is also flat, to very good approximation.

A space that isn’t flat is curved.

FLRW universe

See Friedman-Lemaître-Robertson-Walker universe.


State of matter in which the constituent atoms and molecules are connected so loosely that the matter cannot maintain any shape without external support: If you place a fluid into a container, its shape will adapt to that of the container (in contrast with a solid body, which will keep its shape). Examples of fluids are gases, liquids and plasma.


In optics: point where light rays meet after travelling through a lense.

In geometry: The foci of an ellipse; two points in the interior of such a curve for which the following holds: For any curve on the ellipse, adding its distance to one focus and its distance to the other gives the same sum.


In mechanics: Influence acting on a body, trying to accelerate it.

More generally: All influences by which elementary or other particles can interact; in this sense, force and interaction are synonymous. In the standard model of particle physics, there are three elementary forces: electromagnetism, the weak (nuclear) force and the strong (nuclear) force, while there is no quantum description of the fourth fundamental interaction, gravity.

force particle

In the framework of relativistic quantum field theories (which form the theoretical basis of the physics of elementary particles, the forces by which matter particles interact are transmitted by so-called carrier particles travelling back and forth between them. For instance, the electric force between two electrons would come about through the exchange of photons, the carrier particles of the electromagnetic interaction. Carrier particles always have integer spin, such as spin 1 or 2 (which means they belong to the class of particles called bosons). Synonym: carrier particles.

fourth test of general relativity

Another name for measurements of the Shapiro time delay as an addition to the three classical tests of general relativity.

Synonyms: fourth test of general relativity

frame of reference

See reference frame


In Newtonian gravity , the gravitational field of a mass is independent of whether or not that mass rotates. In general relativity , a mass’s rotation influences the motion of objects in its neighbourhood. Put simply, the rotating mass “drags along” spacetime in the vicinity.

This is known as Lense-Thirring effect or frame-dragging. Sometimes, frame-dragging is also used in a more general sense that includes additional general-relativistic effects associated with the movement of sources of gravity. There is an analogy between gravity and  electromagnetism in which ordinary gravity corresponds to the electrostatic force , and the field components responsible for frame-dragging to *magnetism*. For this reasons, these effects also go by the name of gravitomagnetism.

Synonyms: Lense-Thirring effect, Gravitomagnetism


In the context of relativity theory, a particle (object, observer…) that is not acted upon by any force except gravity is said to be free or, a bit more specific, to be in free fall. Free test particles play an important role in understanding the structure of general relativity.

Synonyms: free fall free particle


Measure for the rapidity of an oscillation, defined as the inverse of the period of oscillation: A process that, in oscillating, repeats itself after 0.1 seconds has the frequency 1/(0.1 seconds)= 10 Hz. (The unit Hertz, abbreviated as Hz, is defined as 1 Hz = 1/second.)

For a simple wave, the frequency is given by the number of maxima going by a stationary observer in a second. Ten maxima going by per second correspond to a frequency of 10 Hz.

Friedmann-Lemaître-Robertson-Walker universe

The simplest assumptions one can make about a universe are that it is homogeneous and isotropic. Homogeneity means that the properties of matter and of the geometry of spacetime are the same at every point in space. Isotropy means that all spatial directions are on the same footing, and that to a hypothetical observer, such a universe looks exactly the same, in whatever direction he or she might be looking. These assumptions are quite restrictive; in fact, it is possible to write down an expression characterizing the spacetime geometry of all homogeneous and isotropic solutions of Einstein’s equations. The result is a family of spacetimes known as Friedmann-Lemaître-Robertson-Walker universes. Typically, these universes are either in a state of expansion or a state of collapse. The best-known example is the expanding universe described by big bang cosmology.

Sometimes, these model universes are also referred to as Friedmann-Lemaître universes, Robertson-Walker universes or Friedmann-Robertson-Walker universes.


See nuclear fusion