Physics Glossary – Q

Quantum Chromodynamics (QCD)

Is the physical theory describing one of the fundamental forces, the strong interaction. It was first proposed in the early 1970s by Frank Wilczek and Dieter Gross.

Quantum Electrodynamics (QED)

Is a quantum field theory of electromagnetism. QED describes all phenomena exhibited by charged point particles, such as electrons and positrons, and the particles of light (photons), interacting by electromagnetism


The smallest discrete amount of any quantity (plural: quanta). The word comes from the Latin for quantity. Electric charges comes in fixed multiples of a basic unit, the quantum of charge.

Quantum Mechanics or Quantum Physics

Is a physical theory formulated in the first half of the twentieth century which successfully describes the behavior of matter at small distance scales. It is in contrast to classical mechanics which deals with larger scales.


Quarks are one of the two families of particles thought to be elemental and indivisible (the other being the leptons). Quarks never exist alone but only in groups. Quarks are differentiated from leptons, the other family of elemental particles, by electric charge.


A quasar (from quasi-stellar radio source) is an astronomical object that looks like a star in optical telescopes (i.e. it is a point source), but has a very high redshift. The general consensus is that this high redshift is cosmological, the result of Hubble’s law and that their redshift indicates that they are typically very distant from Earth; we observe them as they were several billions of years ago.

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