Editing Force carrier (section)

== Forces from the particle viewpoint ==
{{main|Static forces and virtual-particle exchange}}
[[Image:Electron-scattering.png|thumb|220px|A [[Feynman diagram]] of scattering between two electrons by emission of a virtual [[photon]].]]
When one particle [[scattering|scatters]] off another, altering its trajectory, there are two ways to think about the process.  In the field picture, we imagine that the field generated by one particle caused a force on the other.  Alternatively, we can imagine one particle emitting a [[virtual particle]] which is absorbed by the other.  The virtual particle transfers [[momentum]] from one particle to the other.  This particle viewpoint is especially helpful when there are a large number of complicated quantum corrections to the calculation since these corrections can be visualized as [[Feynman diagrams]] containing additional virtual particles.

Another example involving virtual particles is [[beta decay]] where a virtual [[W&nbsp;boson]] is emitted by a [[nucleon]] and then decays to e<sup>±</sup> and (anti)neutrino.

The description of forces in terms of virtual particles is limited by the applicability of the [[perturbation theory (quantum mechanics)|perturbation theory]] from which it is derived.  In certain situations, such as low-energy [[quantum chromodynamics|QCD]] and the description of [[bound states]], perturbation theory breaks down.