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Einstein Online is a web portal with comprehensible information on Einstein's theories of relativity and their most exciting applications from the smallest particles to cosmology.

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Definitioner

ESA (European Space Agency)

Europäische Weltraumagentur; beteiligt an Projekten wie dem Weltraumteleskop Hubble oder dem Gravitationswellendetektor LISA.

wavelength

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.

time

It is a fact of life that not all events in our universe happen concurrently - instead, there is a certain order. Defining a time coordinate or defining time, the way physicists do it, is to define a prescription to associate with each event a number so as to reflect that order - if event B happens after event A, then the number associated with B should be larger than that associated with A. The first step of this definition is to construct a clock: Choose a simple process that repeats regularly. (What is "regular"? Luckily, in our universe, all elementary processes such as a swinging pendulum, the oscillations of atoms or of electronic circuits lead to the same concept of regularity.) As a second step, install a counter: A mechanism that, with every repetition of the chosen process, raises the count by one. With this definition, one can at least assign a time (the numerical value of the counter) to events happening at location of the clock. For events at different locations, an additional definition is necessary: One needs to define simultaneity. After all, the statement that some far-away event A happens at 12 o'clock is the same as saying that event A and "our clock counter shows 12:00:00" are simultaneous. The how and why of defining simultaneity - a centre-piece of Einstein's special theory of relativity - are described in the spotlight topic Defining "now". With all these preparations, physicists can, in principle, assign a time coordinate value ("a time") to any possible events, and describes how fast or how slow processes happen, compared to that time coordinate.

temperature

In systems consisting of many particles, be they solid bodies, fluids or gases, the constitutents are in constant, chaotic motion: the atoms in a solid crystal oscillate a bit, the molecules of a gas are in rapid, disordered motion, and so on. The average energy with which each constitutent contributes to every part of the disorderly motion is the same, and it is called the temperature of the system. High average energy corresponds to high temperature - atoms vibrating wildly, gas molecules zipping around very fast -, low average energy to low temperature. In a slightly different context, certain mixtures of electromagnetic radiation can be assigned a temperature ("radiation temperature"), a single parameter that completely defines the basic properties of the radiation (more precisely, its spectrum). It corresponds to the thermal radiation emitted by a hot body with precisely that temperature. In physics, temperature is measured in Kelvin, in everyday life, depending on the country, in Fahrenheit or Celsius.

surface

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 metres per square second, the so-called standard acceleration.

sun

The central (and most massive) body of our solar system; the star closest to us; a ball of gas with a radius of ca. 700000 km (for comparison the radius of the earth: 12756 km) and a mass of 1.989·10³⁰ kilograms [see exponential notation]. In the interior of the sun, nuclear fusion processes run their course; they are responsible for the sun's impressive brightness.

star

A cosmic gas ball that is massive enough for pressure and temperatur in its core to reach values where self-sustained nuclear fusion reactions set in. The energy set free in these reactions makes stars into very bright sources of light and other forms of electromagnetic radiation. Once the nuclear fuel is exhausted, the star becomes a white dwarf, a neutron star or a black hole.

stars (star)

A cosmic gas ball that is massive enough for pressure and temperatur in its core to reach values where self-sustained nuclear fusion reactions set in. The energy set free in these reactions makes stars into very bright sources of light and other forms of electromagnetic radiation. Once the nuclear fuel is exhausted, the star becomes a white dwarf, a neutron star or a black hole.

speed

An object's average speed is the distance it moves during a given period of time, divided by the length of the time interval. If you make the time interval infinitely small, the result is the object's speed at one particular moment in time. The notion of speed can be applied to waves in different ways; for instance, for a simple wave, the phase speed is the speed at which any given wave crest or wave propagates through space. See also the more general entry velocity.

spectrum

The electromagnetic radiation reaching us from an astronomical object or other source is a mix of electromagnetic waves with a great variety of frequencies. The spectrum lists the composition of this mix: For every frequency, it states the amount of radiation energy contributed by waves of that particular frequency.

spectra (spectrum)

The electromagnetic radiation reaching us from an astronomical object or other source is a mix of electromagnetic waves with a great variety of frequencies. The spectrum lists the composition of this mix: For every frequency, it states the amount of radiation energy contributed by waves of that particular frequency.

space

In a strict sense: Space as we know it from everyday life: the totality of all locations in which objects can sit, with three dimensions. In a more general sense used by mathematicians, all kinds of sets of points are spaces - a line for instance, which has but a single dimension, or a two-dimensional surface, but also higher-dimensional spaces. Also, in such more general spaces, geometry can be different from the standard Euclidean geometry taught in high schools - such spaces can be curved.

solar mass

The sun has a mass of 1.989·10³⁰ kilograms [see exponential notation] what is equal to 332 946 earth masses. In astronomy, the solar mass is frequently used as a unit of mass ("Neutron stars typically have a mass of 1.4 solar masses"), sometimes written as M_⊙.

second

In the International System of units: the basic unit of time. Defined as a certain multiple of the oscillation period of electromagnetic radiation set free in a certain transition within the electron shell of atoms of the type Cesium-133.

relativistic

Models, effects or phenomena in which special relativity or general relativity play a crucial role are called relativistic. Examples are relativistic quantum field theories as theories based on special relativity, or the relativistic perihelion shift as a consequence of general relativity. In addition, conditions under which the difference between relativistic physics and ordinary, classical physics are especially pronounced, are also called relativistic. For instance, when material objects reach speeds close to speed of light, one talks of relativistic speeds, while speeds that are so small compared to light as to make relativistic effects undetectably small are non-relativistic.

redshift

The frequency of a simple light wave is directly related to its colour (cf. spectrum). For the lowest frequencies of visible light, that colour is red, light of the highest frequencies appears blue. If the frequency of a light wave is shifted towards lower frequencies (for instance by the doppler shift), that corresponds to a colour shift towards the red end of the spectrum, and is hence called a redshift. Consequently, a shift towars higher frequences is called blueshift. From this, “redshift” has come to acquire a more general meaning. It is used to denote any shift towards lower frequencies, even for types of electromagnetic radiation where the frequencies do not correspond to any visible colour, and more generally still, for other types of waves as well (for instance for gravitational waves). In the context of general relativity, the gravitational and the cosmological redshift are of particular interest.

radiation

In a general sense: Collective name for all phenomena in which energy is transported through space in the form of waves or particles. In a more restricted sense, the word is often used synonymously with electromagnetic radiation.

point

Elementary "building block" of geometrical entities such as surfaces or more general spaces. For instance, a surface is the set of all its points, of all possible locations on the surface, and all geometrical objects in that surface are defined by the points that belong to them - for instance, a line on the surface is the set of (infinitely many) points.

plane

A surface within which the axioms of Euclidean geometry (synonym: plane geometry) hold - the rules of geometry as they are taught in high school, with well-known formulae such as Pythagoras's theorem and "the perimeter of a circle is 2 times pi times its radius" hold.

relativistic (perihelion advance, relativistic)

For planetary orbits, there is a small difference between the predictions of Newtonian gravity and general relativity. For instance, in Newton's theory, the orbital curve of a lonely planet orbiting a star is an ellipse. In general relativity, it is a kind of rose or rhodonea curve. Such a curve is similar to an ellipse curve, which shifts a bit with each additional orbit. The shift can be defined by looking at the point which is closest to the sun (perihelion) on each orbit. The additional relativistic shift is, hence, called relativistic perihelion shift or relativistic perihelion advance. A picture can be seen on the page A planet goes astray in the chapter General relativity of Elementary Einstein.

oxygen

Chemical element whose atoms have eight protons each in their nuclei. In the context of relativity, more concretely: cosmology, oxygen is interesting as an indicator of chemical evolution: Oxygen nuclei are not produced during Big Bang Nucleosynthesis, but they are produced by nuclear fusion reactions in the interior of stars. The presence of oxygen in an astronomical object is an indicator that stellar fusion has taken place, and that the abundances of the different elements thus do not reflect the element abundances in the early universe.

observer

In the context of relativity, "observer" can mean two different things. Often, observer is synonymous with reference frame or (spacetime-)coordinate system: An observer in this sense is someone who assigns coordinates to everything that happens around him. In particular, all events are assigned space coordinate values and a time coordinate value. In the context of special relativity, it is often the case that when one talks about an observer, what is really meant is an inertial observer, corresponding to a special type of reference frame. On other occasions, the term is used in a more narrow sense - in those cases, an observer is someone sitting at a certain point in space and using the light signals reaching that location to construct an image of his surroundings. In the context of optical effects in relativity, for instance gravitational lensing, observer is usually meant in this way.

Newtonian gravity

In pre-Einstein mechanics, which goes back to the English physicist and mathematician Isaac Newton (1643-1727), gravity is a force with which masses act on each other. As other forces do, they cause bodies to accelerate. In its simplest form, Newton's law of gravity describes the force acting between two spherical, symmetric masses: The force with which the first sphere acts on the second is equal to the mass of the first sphere times the mass of the second sphere times Newton's gravitational constant, divided by the square of the distance between the centre-points of the two spheres. How to remove from this law more complicated gravitational effects, see the article The gravitation of gravitation. The differences between Newton's gravitation and Einstein's theory of gravitation, the theory of General Relativity, can be described systematically in the frame of the so called post-Newtonian approximation.

NASA (National Aeronautics and Space Administration (NASA))

Part of the US government in charge not only of manned space missions, but also responsible for numerous highly successful satellite and probe missions. NASA is a partner in projects such as the Hubble space telescope or the gravitational wave detector LISA. NASA website

matter

In general relativity: All contents of spacetime that contribute to its curvature: particles, dust, gases, fluids, electromagnetic and other fields. In particle physics: All elementary particles with half-integer spin, such as electrons and quarks, as well as their composites such as protons and neutrons, in contrast with force particles.

mass

In classical physics, mass plays a triple role. First of all, it is a measure for how easy it is to influence the motion of a body. Imagine that you're drifting in emtpy space. Drifting by are an elephant and a mouse, and you give each of them a push of equal strength. The fact that the mouse abruptly changes its path, while the elephant's course is as good as unaltered, is a sure sign that the mass (or, in the language of physics, the inertia or inertial mass) of the elephant is much greater than that of the mouse. Secondly, mass is a measure of how many atoms there are in a body, and of what type they are. All atoms of one and the same type have the same mass, and adding up all those tiny component masses, the total mass of the body results. Thirdly, in Newton's theory of gravity, mass determines how strongly a body attracts other bodies via the gravitational force, and how strongly these bodies attract it (in this sense, mass is the charge associated with the gravitational force). In special relativity, one can also define a mass that is a measure for a bodies resistance to changing its motion. However, the value of this relativistic mass depends on the relative motion of the body and the observer. The relativistic mass is the "m" in Einstein's famous E=mc² (cf. equivalence of mass and energy). The relativistic mass has a minimum for an observer that is at rest relative to the body in question. This value is the so-called rest mass of the body, and when particle physicists talk of mass, this is usually what they mean. Just as in classical physics, the rest mass is a kind of measure for how much matter the body is made up of - with one caveat: For composite bodies, the energies associated with the forces holding the body together contribute to the total mass, as well (another consequence of the equivalence of mass and energy). In general relativity, mass still plays a role as a source of gravity; however, it has been joined by physical quantities such as energy, momentum and pressure.

line

Geometric object with a single dimension. A line can either be an independent one-dimensional space (in the abstract mathematical space where a space does not need to have three dimensions), or it can be embedded into a more general space, like a line drawn onto a piece of paper (i.e. a surface).

light

Light in the strict sense of the word is electromagnetic radiation the human eye can detect, with wave-lengths between 400 and 700 nano metres. In relativity theory and in astronomy, the word is often used in a more general sense, encompassing all kinds of electromagnetic radiation. For instance, astronomers might talk about "infrared light" or "gamma light"; in this context, light in the stricter sense is referred to as "visible light". Within classical physics, the properties of light are governed by Maxwell's equations; in quantum physics, it turns out that light is a stream of energy packets called light quanta or photons. In the context of relativistic physics, light is of great interest, and for a number of reasons. First of all, the speed of light plays a central role in both special and general relativity. Also, there are a number of interesting effects in general relativity which are associated with the propagation of light, namely deflection, the Shapiro effect and the gravitational redshift.

interaction

Interactions are all the different ways in which elementary or compound particles can influence each other. In elementary particle physics, "interaction" and "force" are used synonymously.

In the standard model of elementary particles, there are three fundamental interactions: electromagnetism, the strong nuclear force and the weak nuclear force. For another interaction, gravity, there is no quantum description yet.

hydrogen

The lightest (and, in our universe, the most abundant) chemical element. The atomic nucleus of an ordinary hydrogen atom is a single proton. If the atomic nucleus contains an additional neutron, the atom is called heavy hydrogen or deuterium.

helium

After hydrogen, the second lightest chemical element. Its atomic nucleus consists of two protons and, ordinarily, two neutrons ("helium-4"); such helium nuclei are also called alpha particles. Another variety of helium, helium-3, has only one neutron in its nucleus. In the context of general relativity, both helium-3 and helium-4 are is of interest as two species of light atomic nuclei that formed in the early universe during Big Bang Nucleosynthesis.

gravity

See gravitation

gravitational redshift

According to general relativity, light flying away from a massive body (or other source of gravity) experience a redshift - its frequency decreases and the light becomes less energetic. On the other hand, light flying towards a massive body gets blueshifted - its frequency and energy increase.

gravity (gravitation)

In classical physics: An action-at-a-distance force by which all bodies that possess mass attract each other (see Newtonian theory of gravity), synonym: gravitational force. In Einstein's general theory of relativity: The fact that matter that possesses mass, energy, pressure or similar properties distorts spacetime, and that this distortion in turn influences whatever matter might be present. An introduction to the basic ideas of general relativity is provided by the section General relativity of Elementary Einstein. More information about the nature of gravity in general relativity can be found in the spotlight text Gravity: From weightlessness to curvature.

general relativity (general theory of relativity)

Albert Einstein's theory of gravity; a generalization of his special theory of relativity. For information about the concepts and applications of this theory, we recommend the chapter general relativity of our introductory section Elementary Einstein. Further information about many different aspects of general relativity and its applications can be found in our section Spotlights on relativity.

gas

In a strict sense: A state of matter in which the atoms and/or molecules wildly careen and collide, without being bound to each other. This movement leads to an inner pressure, while the average kinetic energy of the moving particles is a measure for the temperature of the gas. Compare the other states of matter: solid state, liquid, plasma. In a broader sense, gas is also used to denote other mixtures of freely careening particles, for instance in the case of the electron gas whose pressure stabilizes a white dwarf against further collapse.

gamma rays

The most highly energetic variety of electromagnetic radiation , with over a quintillion oscillations per second, corresponding to wave-lengths of less than a hundredth billionth of a metre.

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.

field

A field describes how a physical quantity is distributed in space and time. For instance, the area where electric forces act on a test particle is subject to an electric field. Or the gravitational forces which act on the mass of a test body define a gravitational field. In general a field contains energy, occupies space and can change over time.

ESA (European Space Agency)

The European Space Agency coordinates the space-faring activities of the European countries. It is a partner in projects such as the Hubble space telescope and the gravitational wave detector LISA. ESA website

equivalence principle

One of the postulates at the basis of general relativity: A freely-falling observer in a gravitational field does not feel gravity. More precisely: In a small region of space around an observer in free fall in a gravitational field, the laws of physics are approximately the same as without gravitation (i.e. in special relativity) - at least for a time-limited observation period. This is sometimes called Einstein's equivalence principle, which includes a more restricted version called the weak equivalence principle, namely that, in a gravitational field, objects which are at the same location are subject to the same gravitationalacceleration - they fall at the same rate ("universality of free fall"). More information about the equivalence principle can be found in the spotlight topic The elevator, the rocket, and gravity: the equivalence principle, while the path from there to Einstein's geometric gravity is traced in Gravity: From weightlessness to curvature.

electromagnetic radiation

Electromagnetic influences (in the language of physics: electric and magnetic field) which, even with no electric charges present, are locked in a state of mutual excitation so that they form a wave that propagates through space. As this wave transports energy, it is, by the usual physics definition, a form of radiation, called electromagnetic radiation. Depending on frequency, there are special names for different types of electromagnetic radiation; going from lower to higher frequencies: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays. In the context of quantum theory, it turns out that electromagnetic radiation consists of tiny energy packets, called light particles or photons.

Earth

Our very own planet in the solar system - the third planet from the sun. The earth has a mass of about 6 trillion trillion (in exponential notation, 6·10²⁴) kilograms.

Doppler effect

Effect named after the Austrian scientist Christian Doppler concerning the emission of waves by moving sources. Consider a wave-source (for instance, a device that sends out sound-waves or light-waves). Also consider two observers A and B, with observer A moving relative to the source, while observer B is at rest relative to it. When a source that moves relative to an observer emits a wave, the frequency measured by this observer is different from what a measuring instrument would record that is at rest relative to the source: If source and observer approach each other, the observer measures a higher frequency, if they move away from each other, a lower frequency. In everyday life, the Doppler effect is readily apparent when we listen to sound waves from moving sources. If a police car or fire truck with blaring horns first races towards us, then passes us and races away, the characteristic horn sounds change dramatically in pitch, the moment the car passes us. This is because, at first, the car is moving towards us, and there is a Doppler shift towards higher pitch compared with a listener in the car. From the moment the car passes us, it becomes a source that moves away from us, with all sounds being shifted to lower pitch. In the context of relativity, the most important Doppler effect is that for light waves. In this context, a shift towards higher frequencies is called blueshift, one to lower frequencies redshift.

Doppler shift (Doppler effect)

Effect named after the Austrian scientist Christian Doppler concerning the emission of waves by moving sources. Consider a wave-source (for instance, a device that sends out sound-waves or light-waves). Also consider two observers A and B, with observer A moving relative to the source, while observer B is at rest relative to it. When a source that moves relative to an observer emits a wave, the frequency measured by this observer is different from what a measuring instrument would record that is at rest relative to the source: If source and observer approach each other, the observer measures a higher frequency, if they move away from each other, a lower frequency. In everyday life, the Doppler effect is readily apparent when we listen to sound waves from moving sources. If a police car or fire truck with blaring horns first races towards us, then passes us and races away, the characteristic horn sounds change dramatically in pitch, the moment the car passes us. This is because, at first, the car is moving towards us, and there is a Doppler shift towards higher pitch compared with a listener in the car. From the moment the car passes us, it becomes a source that moves away from us, with all sounds being shifted to lower pitch. In the context of relativity, the most important Doppler effect is that for light waves. In this context, a shift towards higher frequencies is called blueshift, one to lower frequencies redshift.

classical

In physics, the word is used with two meanings. First of all, it denotes physical models or theories that take into account neither the effects of Einstein's theories of relativity nor those of quantum physics, for example classical mechanics. However, it is also used to denote models or theories that are not formulated in the framework of quantum physics; in that sense, general relativity is an example for a classical theory.

binary (binary star)

A system consisting of two stars in orbit around each other. From a relativistic point of view, there are binaries that are of special interest, namely those in which at least one of the partners is a neutron star or a black hole. Potentially, such systems are effective sources of gravitational waves.

acceleration

Every change of velocity with time is an acceleration.

This definition is slightly different from our everyday usage of the word. Ordinarily, we talk of an object accelerating when it becomes faster and faster. The physics definition covers two more situations. An object that decelerates, becomes slower, thus changes its velocity and, in the physics sense, undergoes a (negative) acceleration. Also, in physics, velocity is not the same as speed. A constant velocity implies not only constant speed, but also a constant direction of movement. Once the direction changes, so does the velocity - the change in velocity is associated with the change in the direction of movement. Thus, in the physics sense, even a car going around a curve of the road at constant speed undergoes acceleration.