Dictionary
More than 350 keywords from relativity and related topics, from "absolute zero" to "Xrays"  please use the menu on the left to choose a letter.
 Fahrenheit scale

Usual temperature scale in the US and other Englishspeaking countries. 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 (X32)*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 wavelength, such as infrared light, radio waves or Xrays, the different wavelengths 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

"Fermion" is the collective word for all quantum particles with halfinteger 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.
 field

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.
 fission
 See nuclear fission
 flat

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 twodimensional flat space is the plane, and ordinary, everyday threedimensional space is also flat, to very good approximation.
A space that isn't flat is curved.
 FLRW universe
 See FriedmanLemaîtreRobertsonWalker universe.
 fluid

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.
 focus

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.
 force

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 socalled 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

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

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 LenseThirring effect or framedragging. Sometimes, framedragging is also used in a more general sense that includes additional generalrelativistic 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 framedragging to *magnetism*. For this reasons, these effects also go by the name of gravitomagnetism.  free

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.
 frequency

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.
 FriedmannLemaîtreRobertsonWalker 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 FriedmannLemaîtreRobertsonWalker universes. Typically, these universes are either in a state of expansion or a state of collapse. The bestknown example is the expanding universe described by big bang cosmology.
Sometimes, these model universes are also referred to as FriedmannLemaître universes, RobertsonWalker universes or FriedmannRobertsonWalker universes.
 fusion
 See nuclear fusion