# Glossary

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 in the research centre CERN. From the point of view of relativity theory, it has several points of interest: First of all, it accelerates protons to 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.

Synonyms: LHC

laser

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

See LIGO

Laser Interferometer Space Antenna

See LISA

lead

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).

light

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.

Synonyms: origin of light elements

light speed

See speed of light.

light-cone

In special as well 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 to travel faster than light – cf. causal structure). Graphically, the boundary between the two sets of events where influence is possible or 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, would be absorbed there.

Light-second

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

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.

Synonyms: light-minute light-hour light-day light-year

light-wave

Light is a mixture of elementary waves, each a regular succession of maxima and minima of the electromagnetic field. The same is true for all other sorts of electromagnetic radiation.

LIGO

Laser Interferometer Gravitational-Wave Observatory : A detector project for the measurement of gravitational waves in the United States, which was upgraded to the Advanced LIGO between 2010 and 2015 .The detector includes two interferometric gravitational wave detectors, with an arm-length of four kilometers each. One of them is located in Hanford, Washington, the other one is located in Livingston, Louisiana. The first ever direct measurement of gravitational waves was made at LIGO.

Synonyms: Laser Interferometer Gravitational-Wave Observatory

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).

liquid

State of matter in which the constituent atoms and molecules are bound to each other, 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.

LISA

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. The planned interferometric gravitational wave detector should be composed of three satellites, positioned to form a triangle, with each side 2.5 million kilometers long. While called eLISA for some time, the project named LISA is expected to launch in 2034.

Synonyms: Laser Interferometer Space Antenna

LISA Pathfinder

LISA Pathfinder was a test mission of the European Space Agency ESA for the LISA mission. LISA Pathfinder demonstrated the functionality of crucial LISA technologies with which the first gravitational wave observatory in space will observe low-frequency gravitational waves.

lithium

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.

logarithmic

In graphic representations of physical data, a way of plotting values according to their common logarithm. In mathematics, the logarithm is the inverse function to exponentiation. That means the logarithm of a given number x is the exponent to which another fixed number, the base b, must be raised, to produce that number x. If the base is 10 then 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.

Synonyms: logarithmic scale

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 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.