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More than 350 keywords from relativity and related topics, from "absolute zero" to "X-rays" - please use the menu on the left to choose a letter.

 states (of matter)

Depending on parameters like temperature or pressure, one and the same portion of matter can occur in different states with greatly varying physical properties. The most important states of matter are the solid, liquid, gaseous and plasma states (the latter three forms of matter are also called fluid). At very low temperatures, atoms bind together to form a solid body with a definite shape. As temperature increases, many solid bodies melt, giving a liquid or, at even higher temperatures, a gas, an ensemble of atoms and/or molecules scurrying wildly to and fro. With a further increase in temperature, a plasma state can be reached, atoms desintegrating into atomic nuclei and electrons.


Roughly speaking, in relativity, a situation or a spacetime is stationary if there is no change over time.

To be more precise, one has to take into account that, in general relativity, time can be defined in many different ways, all of them equally valid for formulating the laws of physics. This leads to a modified definition: A situation or a spacetime is stationary if it is possible to define time in a way so that there is no change over time - if you follow the properties of a given region of space over time, they will not change.

 Stefan-Boltzmann law

One of the laws governing the properties of the simplest form of thermal radiation - that emitted by a blackbody: The total energy emitted by such a body is proportional to the fourth power of its temperature (measured in Kelvin).

 stellar black holes

Stellar black holes are black holes with between a few and a dozen solar masses that are formed when the core of a massive star collapses.

Basic information about black holescan be found in the chapter Black holes & Co. of Elementary Einstein.

 straight line

In the plane, in three-dimensional everyday space or in more general flat spaces: Any line that forms the shortest connection between two given points.

In the space-time of special relativity: The world-line of an object moving with constant speed on a path that is straight in space.

 string(s), string theory

Candidate for a theory of quantum gravity; a quantum theory, where the fundamental building blocks are tiny, one-dimensional, oscillating entities, called strings.

A brief description can be found on the page String theory in the chapter  Relativity and the quantum of Elementary Einstein.

 strong force

One of the four fundamental forces in our universe (the others are electromagnetism, weak nuclear force and gravity). The strong force binds the quarks to form compound particles such as protons and neutrons. Indirectly, it is also responsible for holding together protons and neutrons in atomic nuclei.


The central (and most massive) body of our solar system; the star closest to us; a ball of gas with a radius of 700000 km and a mass of 1.989·1030 kilograms.

[Problems reading expressions such as 1030? See exponential notation.]

In the interior of the sun, nuclear fusion processes run their course; they are responsible for the sun's impressive brightness.


Class of modells that generalize Einstein's general theory of relativity in a way that satisfies the requirements of supersymmetry.

Today, supergravity is of interest in the context of string theory: In the limiting case of low energies (where low energies includes everything accessible with modern particle accelerators), the physics of string theories can be described with the help of certain supergravity models.

 supermassive black holes

are black holes with masses of more than a million solar masses. As far as we know, such holes can be found in the central regions of almost all galaxies. Central black holes are the energy source for radio galaxies and other active galactic nuclei.

Basic informations about black holes in general can be found in the chapter Black holes & Co. of Elementary Einstein.


Highly energetic explosion that ends the life of stars with more than about ten solar masses. In this explosion, the outer layers of the stars are ejected into space, while the core regions collapse to form a neutron star or even a black hole.

 superstring theory

Synonym: supersymmetric string theory. String theory that satisfies the requirements of an abstract symmetry called superymmetry. All models of string theory that are realistic candidates for a theory of quantum gravity, are superstring theories.


Abstract symmetrie that some of the models of particle physics satisfy: in such models, for every species of particles, there is a partner species with the same mass. If the particles are matter particles (fermions), then the partner particles are force particles (bosons), and vice versa.

Geometric space with two dimensions. Examples include the plane or the surface of a sphere.
 surface gravity

The acceleration due to gravity which is experienced by an object resting on the surface of some solid body is called the body's surface gravity (as most of the solid bodies in question are shaped by gravity, the value for the surface gravity tends to be the same everywhere on the body's surface). For the Earth, the surface gravity is 9.81 meters per square second, the so-called standard acceleration.


A situation has a symmetry if certain changes make no difference. For instance: a mirror-symmetric image that you view in a mirror looks the same. A perfect sphere looks the same, even if it is rotated around an arbitrary axis through its centre point ("spherical symmetry").

In particle physics, there are more abstract symmetries, less readily pictured, such as supersymmetry.


A particle accelerator, in which particles are accelerated with the help of electric fields, while strong magnetic fields keep them on track (the fact that ever-stronger magnetic fields are needed as acceleration proceeds is a consequence of the fact that relativistic mass increases with speed.)

 synchrotron radiation

Electromagnetic radiation produced when electrically charged particles (for instance, electrons) are made to follow a curved trajectory in a particle accelerator, or when these particles undergo comparable accelerations in nature.

Due to effects that can be derived from special relativity, synchrotron radiation is densely concentrated and very intense. These properties, together with the fact that it is very easy to produce synchrotron radiation with a clearly defined frequency, make this type of radiation a valuable tool for research not only in basic physics, but also in biology and medicine.

When it was first discovered, synchrotron radiation was an (annoying!) side effect, observable at particle accelerators which were used for research into the basic properties of elementary particles. Nowadays, there many accelerators whose main purposes is the production of this radiation! One of them can be seen on the page E=mc2 in the chapter Special relativity of Elementary Einstein: the VUV ring at Brookhaven National Laboratory. Here are some links to the webpages of some synchrotron radiation laboratories:

National Synchrotron Light Source at the Brookhaven National Laboratory
Hamburg Synchrotron Radiation Laboratory (HASYLAB)

 Syracuse University

Research university (enrolment ca. 20,000) in New York State. Research topics of the physics department include classical and quantum gravity, cosmology and gravitational waves.

Particle and Gravitational Physics (Fundamental Theory Group) at Syracuse University
Gravitational Wave Group at Syracuse University

 Système International d'Unités
See SI, International System of Units