Editing Coupling (physics) (section)

== Particle physics and quantum field theory ==
[[File:Gluon coupling.svg|thumb|Examples of gluon coupling]]
[[Particle]]s which interact with each other are said to be coupled. This interaction is caused by one of the fundamental forces, whose strengths are usually given by a dimensionless [[coupling constant]]. In [[quantum electrodynamics]], this value is known as the [[fine-structure constant]] α, approximately equal to 1/137. For [[quantum chromodynamics]], the constant changes with respect to the distance between the particles. This phenomenon is known as ''[[asymptotic freedom]].'' Forces which have a coupling constant greater than 1 are said to be "strongly coupled" while those with constants less than 1 are said to be "weakly coupled."<ref>{{Cite book|title=Elementary Particle-Second, Revised Edition|last=Griffiths|first=David|publisher=Wiley-VCH|year=2010|isbn=978-3-527-40601-2}}</ref>