Exchange particle

In particle physics, the quantum field theory called the Standard Model describes the strong, weak and electromagnetic fundamental forces. In such theories, each type of interaction has a characteristic set of force carrier particles associated with quantum excitation of the force field related to that interaction. These kinds of particles are always exchanged between matter.
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A particle of the field of force that transmits that force over a distance. For instance a charged particle (such as a proton) is assumed to be continuously emitting and absorbing virtual photons; when in the proximity of another charged particle the photons are exchanged. With the input of energy, for example from a high speed charged particle, a photon can be materialised, as for example in bremmstrahlung .