= Virtual particle
{wiki}
A virtual particle is an internal contribution to a perturbative <scattering amplitude>, commonly represented by an internal line of a <Feynman diagram>. The line contributes a <propagator>, and its internal <four-momentum> is summed or integrated as required by the diagram. It need not satisfy the on-shell <relativistic energy-momentum relation> of an external detected particle.
For example, a <virtual photon> connects charged currents in electron scattering. This is a calculational contribution to the full interaction amplitude, rather than a directly observed photon flying between the electrons. <Energy conservation> and <momentum conservation> still hold at the vertices; off-shell propagation does not mean energy has been borrowed in violation of conservation. Individual virtual contributions can depend on the gauge or perturbative organization, whereas an observable prediction requires the appropriate complete amplitude.
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