# Glossary

First of all, quantization is the process describing the transition from a classical theory to the corresponding quantum theory. For instance, if you quantize classical electrodynamics, you will end up with its quantum version, quantum electrodynamics.

On the other hand, to be quantized means for a physical quantity to be divided into little building blocks or packets. For instance, in quantum theory, the energy of light is quantized: a given quantity of light consists of a finite number of energy packets called photons.

Synonyms: quantization

Quantum chromodynamics

Quantum theory of the strong nuclear interactions between quarks (or compound particles made of quarks). These interactions are described via the exchange of carrier particles called gluons, for instance: when two quarks attract each other via the strong nuclear force, that influence is transmitted by gluons flying back and forth between them.

Synonyms: QCD

quantum cosmology

According to the big bang models, the energy density of the very early universe was extremely high, with the contents of the observable universe compressed into a volume much smaller than that of an atomic nucleus. Under such circumstances, the effects of quantum physics and of general relativity should become equally important, in other words: This era of our cosmic evolution should be described using a theory of quantum gravity. Quantum cosmology encompasses all attempts to apply various candidate theories of quantum gravity to the physics of the early universe, and to describe the universe as a whole as a quantum system.

More information can be found in the spotlight topic Searching for the quantum beginning of the universe. Some quantum cosmological applications of loop quantum gravity can be found in the spotlight texts Avoiding the big bang and Taming infinities with loops.

quantum effects

All effects and phenomena that follow from the fact that, deep down, our world obeys not the laws of classical physics, but those of quantum theory. Examples are tunneling and consequences of the Pauli exclusion principle for the structure of atoms.

Quantum electrodynamics

Quantum theory of electromagnetic interactions. These interactions are described via the exchange of carrier particles called photons, for instance: when two electrons repel each other via the electromagnetic force, that influence is transmitted by photons flying back and forth between them.

Quantum electrodynamics is one facet of the standard model of particle physics. It is also the simplest example for a relativistic quantum field theory – a theory that is both a quantum theory and based on the principles of special relativity.

Synonyms: QED

quantum field theory, relativistic

Collective name for theories that are quantum theories based on the principles of special relativity. Typically, in relativistic quantum theory there exists for every species of particle a corresponding species of anti-particle; forces are transmitted by the exchange of carrier particles.

The simplest example of a relativistic quantum field theory is quantum electrodynamics.

quantum gravity

Theory based both on the effects, concepts and laws of quantum theory and on those of general relativity. To date, no complete such theory exists; the best-known candidate theories are string theory and loop quantum gravity.

Some information on the question of quantum gravity can be found in the chapter Relativity and the quantum of Elementary Einstein, starting with the page Border regions of gravity.

Selected aspects of quantum gravity are described in the category Relativity and the quantum of our Spotlights on relativity.

quantum mechanics

In a more general sense: synonymous with quantum theory. In a more restricted sense: The quantum theory of particles moving under the influence of forces – wherein the particles are described as quantum objects, while the forces are not. An important application of quantum mechanics is the physics of the electron shells of atoms (“atomic physics”, for short). Attempts to extend the quantum laws to govern the forces themselves lead to relativistic quantum field theories.

quantum particles

In classical physics, one can picture particles as little clumps of matter. At every time, such a clump has a definite location in space. In quantum theory, on the other hand, (quantum) particles are much more elusive. Their most complete description involves an abstract “state” that allows one to calculate probabilities, in particular: how likely it is to detect the particle, at a given time, at a given location.

quantum physics

Quantum physics comprises all theories, models, experiments and applications that are based on the laws of quantum theory.

quantum theory

The framework for formulating the physical laws that govern the world at microscopic length-scales – the physics of the micro-world, for instance of atoms, atomic nuclei or elementary particles, but also the physics of ultra-precise measurements such as those made by gravitational wave detectors.

The laws of quantum theory are fundamentally different from our everyday experience and from those of classical physics.

The first unusual feature is that, in many cases, quantum theory merely allows statements about *probabilities*. For instance, in classical physics, one can assign to every particle, at every point in time, a location and a velocity. Whosoever can measure those quantities precisely can, in principle, predict where the particle in question can be found at every point in the future. In quantum theory, all one can assign to a system of particles is an abstract quantum state from which can be derived no precise predictions, but merely the probabilities for detecting a specific particle at a given time in a given place. Whether or not one will really find the particle at that location is governed by chance.

The second unusual feature is a fundamental restriction placed on the exactness of certain measurements (Heisenberg uncertainty relation). For instance, the more precise one measures a particle’s location, the less definite any statements one can make about its velocity.

The third feature is how quantum theory came by its name: A number of physical quantities in nature come in little packets, in quanta. For instance, according to quantum theory, electromagnetic radiation is made of tiny quanta of energy called photons.

Examples for quantum theories are quantum mechanics and relativistic quantum field theories such as Quantum electrodynamics or other parts of the standard model of particle physics.

quark

Elementary particle that is subject to the strong interaction. Quarks come in six species: up, down, strange, charme, bottom and top (with the last two sometimes called beauty and truth).

Quarks are the constituents of protons and neutrons, and thus the material atomic nuclei are made of.

Synonyms: quarks

quark-gluon-plasma

An exotic form of matter which was in all probability present in the early universe shortly after the big bang. Under ordinary circumstances, quarks only occur inside larger particles, such as protons or neutrons, tightly bound together as they are by the carrier particles of the strong nuclear force, which are called gluons. However, at extremely high densities and temperatures it is thought that these larger particles break up, creating a dense and strongly interacting soup of quarks and gluons – a quark-gluon-plasma.

It should be possible to create a quark-gluon-plasma artificially at particle accelerators; in fact, there are strong indications that particle physicists have managed to do just that using the Relativistic Heavy Ion Collider.

quasar

Class of active galactic nuclei. First noticed by radio astronomers as exceedingly bright radio sources that, in the night sky, did not appear more extended than ordinary stars – thus their name, a contractino of quasi-stellar radio source.

Synonyms: quasars

quasi-normal modes

Quasi-normal modes are a mathematical description of the energy dissipation of a perturbed black hole while it settles towards stationary configuration, which corresponds to its ringdown frequencies.