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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magnetic field

The magnetic force is a force by which electric currents (i.e. moving electric charges) act on each other; the magnetic field is the associated field. All phenomena related to the magnetic force or magnetic field are subsumed under the heading of magnetism. Magnetic fields cannot be understood separate from electric fields – their complete description is possible only within the more general context of electromagnetism.

Synonyms: magnetic force magnetism


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.

mass defect

Whenever two or more objects are bound together by strong forces, there is a binding energy – the energy needed to be able to get these objects apart. Since Einstein, we know that energy and mass are equivalent. To this binding energy there corresponds a mass. It is called the mass defect because, by this amount, the mass of the component object is less than the sum of the masses of its parts.

Some more information about binding energies and the mass defect can be found in the spotlight topic Is the whole the sum of its parts?

masses in astronomy

While mass is surely one of the most basic properties of an astronomical object, it is not that easy to actually determine that mass. Most methods utilize the laws of celestial mechanics to deduce from the way that two (or more) objects orbit each other their respective masses. In other cases, relativistic effects such as the deflection of light or the Shapiro delay can be used to determine an object’s mass.

Synonyms: determination of masses in astronomy


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.

Max Planck Institute for Astrophysics

Research institute of the Max Planck Society, dedicated to astrophysical subjects such as the evolution of stars, the physics of supernovae, the evolution of galaxies and cosmology. Founded in 1958 in Germany, located in Garching, near Munich.

MPA website

Synonyms: MPA

Max Planck Institute for Extraterrestrial Physics

Research institute of the Max Planck Society, dedicated to astronomy and astrophysics with observations in the infrared, X-ray and gamma ray part of the electromagnetic spectrum. Founded in 1963 in Germany, located in Garching, near Munich.

MPE website

Synonyms: MPE

Max Planck Institute for Gravitational Physics/Albert Einstein Institute

See Albert Einstein Institute.

Synonyms: AEI

Max Planck Institute for Radio Astronomy

Research institute of the Max Planck Society, dedicated to infrared and radio astronomy. Founded in 1966, the institute is located in Bonn and Bad Münstereifel-Effelsberg, Germany.

MPIfR website

Max Planck Society

German organisation dedicated to basic research; operates 84 Max Planck Institutes (as of January 2018) dedicated to research in specific fields of science – see the entries directly above. Founded in 1948 as the successor of the Kaiser Wilhelm Society; administrative headquarters are located in Munich, Germany.

MPG website

Synonyms: MPG

Maxwell’s equations

The four fundamental equations of electromagnetism that describe how magnetic and electric influences (in physics language: electric and magnetic fields) are produced: Electric fields are produced whenever there are electric charges or, alternatively, when magnetic fields change over time. Magnetic fields are produced whenever there are electric currents (moving electric charges), but also whenever electric fields change over time. The fact that electric and magnetic fields can exist without the presence of charges or currents, simply by mutual excitation where a change in the magnetic field produces an electric field and vice versa, is the basic phenomenon behind electromagnetic waves.


Branch of physics dealing with the motions of objects and how they react to forces acting on them. Depending on the framework used, there is classical mechanics, relativistic mechanics and quantum mechanics.

mechanics, classical

Synonym: Newtonian mechanics. According to classical mechanics, the movements of bodies are regulated by Newton’s three laws of mechanics. The first law states that bodies on which no external force acts stay at rest or move with constant speed along a straight path (“law of inertia”). The second law relates the force acting on a body, the body’s mass and the acceleration caused by the force: Force is equal to mass times acceleration. The third law is the law of “action equals reaction”: If a body A acts on a body B with a certain force, then A itself experiences B acting on it with a force that is equal in strength but has the opposite direction.

An alternative version of the second law uses the concept of momentum: The force acting on a body is equal to the change of that body’s momentum over time.

Synonyms: classical mechanics

mechanics, relativistic

The generalization of classical mechanics that takes into account the effects of special relativity. The basic laws are almost unchanged: First of all, bodies on which no external forces act stay at rest or move with constant speed along straight paths – in the language of special relativity: such bodies move on straight lines in spacetime. Secondly: The total force acting on a body is equal to the change of its momentum over time (but notice: this momentum is defined using the body’s relativistic mass, which depends on the bodies speed relative to the observer). Thirdly, mass and momentum are conserved quantities – their total sum is the same whenever particles interact (this is equivalent to a slightly modified version of the “action equals reaction” principle of classical mechanics).

There exists an elegant reformulation of these laws of mechanics using four-dimensional concepts adapted to the geometry of spacetime, such as the “four-momentum”.

Synonyms: relativistic mechanics


The planet closest to the sun. In the context of general relativity it is of interest because, for this planet, the deviation from the orbits of Newtonian gravity the theory predicts, the relativistic perihelion shift, is especially great. The concord between prediction and observation for this shift constitutes the first successful test of general relativity.


A merger occurs as a consequence of an inspiral, when two black holes collide to form one single bigger black hole. The emission of gravitational waves peaks during a merger.


In the international system of units (SI), the basic unit for length. Since 1983, the official definition uses the constancy of the speed of light as postulated in special relativity: the metre is defined with the help of the basic unit for time, the second: a metre is the distance that light travels in a vacuum in one 299792458th of a second.

Synonyms: metre


See electron volt

Synonyms: Mega-electronvolt


“Micro” as a prefix denotes “one millionths”, making a micrometre a millionth of a metre.


An astronomical object comparable in size to a star, which emits enormous amounts of energy; the processes responsible for the emission are similar to those which happen in quasars or other active galactic nuclei. The key component of a microquasar is a central stellar black hole; more information about how black holes can lead to such enormous energy output can be found in the spotlight text Luminous disks: How black holes light up their surroundings.


Electromagnetic radiation with wave-lengths between a millimetre and 30 centimetres, corresponding to frequencies between some and some hundred billion oscillations per second.

Looking up to the night sky, almost all energy that reaches us of the cosmic background radiation is in the form of microwaves.

Milky Way

1. Our home galaxy – a spiral galaxy, a disk of stars with a diameter of roughly hundred thousand light-years and a thickness between three- and six thousand light-years, containing about 100 billions of stars. It also contains a supermassive black hole in its centre – more about that in our spotlight topic The black heart of the Milky Way.

2. As our sun is located within the disk of the Milky Way galaxy, there are directions in which we can observe comparatively few of the other stars in our galaxy (namely as we look perpendicularly to the disk, or nearly so) and directions in which we can see a large amount of stars (as we look approximately in parallel with the disk plane). The result is that there is a dim band in the night sky marking the disk plane (the directions where we see a lot of the other stars). This band is also known as the Milky Way.


“Milli” as a prefix denotes “one thousandth”, making a millimetre a thousandth of a metre.

minute of arc

See arcminute, arcseond.


Composite object consisting of two or more atoms, bound together by electromagnetic forces.

Synonyms: molecules


Physical quantity that is equal to a body’s mass times its velocity (in special relativity: its relativistic mass times its velocity).

What makes momentum a useful quantity is that it is conserved – if several bodies interact, the sum of their momenta before and after the interaction is the same. Momentum is neither created nor destroyed, merely passed on from one body to another.

In general relativity, momentum is one of a number of quantities (like mass and energy) that acts as a source of gravity.


An elementary particle which is a somewhat heavier version of the electron. Its electric charge is the same as that of the electron, and like that particle, it does not interact via the strong force. The muon’s mass is about 207 times that of the electron.