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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.

 Lamb shift

Typically, electromagnetic radiation is emitted by atoms only at very specific frequencies that depend on the type of atom. For a number of these characteristic frequencies, the relativistic quantum theory of electromagnetism (called quantum electrodynamics) predicts a slight shift, as compared with earlier theories. This is the Lamb shift. Experiments have confirmed the prediction.

 Large Hadron Collider

The Large Hadron Collider is a particle accelerator currently under construction in the research centre CERN. From the point of view of relativity theory, it has several points of interest: First of all, the protons it accelerates will reach higher energies than ever, allowing new tests of the relativistic quantum field theories that are at the core of modern particle physics. Secondly, at such high energies, there should be first traces of an as-yet unproven symmetry of nature called supersymmetry, which plays an important role in string theory, one of the candidates for a theory of quantum gravity (the quantum theory version of Einstein's general relativity). Finally, the high energies are interesting because they give information about the very early high temperature universe, and about the physics that should be included in the big bang models of relativistic cosmology.

CERN website


Abbreviation for "Light Amplification by Stimulated Emission of Radiation". Technique for the production of very concentrated, strong light with a fixed frequency, which propagates as a very simple electromagnetic wave in which wave crests and wave troughs are in perfect step ("coherent light").

 Laser Interferometer Gravitational Wave Observatory
 Laser Interferometer Space Antenna



Chemical element with the symbol Pb; each lead nucleus contains 82 protons.

Lead nuclei, stripped of their electrons, are among the types of heavy ions which are brought into collision in particle accelerators such as the Relativistic Heavy Ion Collider in order to recreate the state of matter in the early universe shortly after the big bang.

 length contraction

Effect of special relativity theory: An observer (more precisely: an inertial observer) measures a shorter length for a moving object than for an identical copy of that object resting beside him (here, length refers to extension in the direction of movement - extension in orthogonal directions remains the same).

 Lense-Thirring effect

See frame-dragging.


Light in the strict sense of the word is electromagnetic radiation the human eye can detect, with wave-lengths between 400 and 700 nanometres. 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.

 light elements

According to the big bang models, the early universe underwent a brief period of primordial nucleosynthesis between a few seconds and a few minutes cosmic time, during which nuclei of light elements such as heavy hydrogen, helium and lithium formed.

A brief account of this Big Bang Nucleosynthesis can be found in the spotlight text Big Bang Nucleosynthesis, while Equilibrium and change provides more information about the physical processes involved and Elements of the past describes how the predictions of Big Bang Nucleosynthesis can be tested against astronomical observation.

 light speed

The speed at which light ore, more generally, electromagnetic radiation propagates through space (especially: through empty space). Central quantity in special relativity: There, the constancy of the speed of light is a basic postulate: ever observer (more precisely: every inertial observer) that measures the speed of light in a vacuum obtains the same constant value, c=299,792,458 metres per second.

Another important relativistic aspect of the speed of light is that it defines an absolute upper speed limit: In special relativity, nothing can move faster than light, and information or influence at most be transmitted at light-speed. In general relativity, the same law is in force locally: No object, no matter, no information can directly overtake or catch up with light (cf. causality).

Basic information about the role of light speed in special relativity can be found in the chapter Special relativity of Elementary Einstein.


In special as in general relativity, the speed of light sets the upper limit for the transmission of influences and signals. Thus, studying the propagation of light, one can find out for an event A which other events can influence A, which can be influenced by A, and where any influence is impossible (because such influence would have had to traval faster than light - cf. causal structure). Graphically, the boundary between the two sets of events where influence is possible/impossible has the form of a double cone (for a sketch, see the page Spacetime in the chapter Special relativity of Elementary Einstein). It is formed by all world-lines of hypothetical light signals that would be emitted at the event A or, coming from an arbitrary direction, absorbed there.


Units of distance: A light-year is the distance a light signal traverses during one year of flight-time, a light-second the distance it traverses in a second, and so on.

The following table translates these units into the more familiar kilometres or seconds:
1 light-second = 300000 km = 186000 mi.
1 light-minute = 18 millionen km = 11 million mi.
1 light-hour = 1.1 billion km = 670 million mi.
1 light-day = 25 billion km = 16 billion mi.
1 light-year = 9.5 trillion km = 6 trillion mi.

Light is a mixture of simple waves, each a regular succession of maxima and minima of the electromagnetic fields. The same is true for all other varieties of electromagnetic radiatio

See light-second etc., above

The largest current project in search of gravitational waves. LIGO includes three interferometric gravitational wave detectors, one with an arm-length of four and one with an arm-length of two kilometres in Hanford, Washington, another one with four-kilometre arms in Livingston, Louisiana.

LIGO website

Geometric object with a single dimension. A line can either be an independent one-dimensional space (in the abstract mathematical space where a space need not 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).

State of matter in which the constituent atoms and molecules are bound to each other (in contrast with a gas, where there is hardly any binding), but so loosely that the matter cannot maintain any shape without external support: If you place a liquid into a container, its shape will adapt to that of the container (in contrast with a solid body, which will keep its shape, and in common with other fluids such as gas or plasma).

Compare the other states of matter: solid state, gas, plasma.


The concept of LISA (Laser Interferometer Space Antenna), a space-based gravitational wave detector, has been studied jointly by the European and US space agencies ESA and NASA for 20 years. Due to shortage of money on the NASA side, currently ESA is studying a variant of the original LISA concept, called eLISA (or sometimes NGO), that ESA could afford on its own, should NASA not be able to come up with a financial contribution.

LISA website (ESA)

LISA website (NASA)


Chemical element whose atomic nucleus contains three protons.

In the context of general relativity, it is of interest as one of the light elements that formed in the early universe during Big Bang Nucleosynthesis.

 local group

The galaxy cluster of which our very own galaxy, the Milky Way, is a member. Cosmically speaking, the local group is rather puny - its only members apart from the Milky Way are the Andromeda galaxy, the galaxy M33 and a number of dwarf galaxies (such as the Magellanic clouds).

 Lockman hole

A region in the sky, located in the constellation Ursa Major (better known as the Big Dipper), and covering an area about 75 times that of the full moon. Pointing their telescopes at this region, astronomers will encounter only negligible amounts of the hydrogen gas that fills significant portions of our galaxy. These are ideal conditions for obtaining a clear and unobstructed view of objects in deep space, far beyond our own galaxy. Not surprisingly, the Lockman hole is one of the most intensively studied regions in the night sky.


In graphic representations of physical data, a way of plotting values according to their common logarithm. The common logarithm y=log(x) of a number x is the number y for which x=10y, so the logarithm of 10 is 1, the logarithm of 100 is 2, and so on.

In ordinary ("linear") plots, the distance between the values 1 and 2 is the same as between the values 2 and 3. In a logarithmic plot, the distance between the values 1 and 10 is the same as the distance between 10 and 100 and the distance between 100 and 1000.

 lookback time

The time that light from a distance body needs to reach us here on earth. Lookback time, defined using the cosmic time coordinate, is one way of defining distances in an expanding universe.

 loop quantum gravity

Candidate theory for a theory of quantum gravity. Loop quantum gravity is an attempt to apply the concepts and laws of quantum theory directly to the geometry that is at the heart of general relativity.

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

 Lorentz transformation

Central set of formulae of special relativity: Formulae that define how to go back and forth between two inertial reference frames that are in relative motion; more precisely: if an event is defined in terms of the space- and time coordinates of one of the observers, one can calculate which coordinates the other observer would assign to that same event.

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