"Real particles" are better understood to be excitations of the underlying quantum fields. The term is somewhat loose and vaguely defined, in that it refers to the view that the world is made up of "real particles". Virtual photons are the exchange particle for the electromagnetic interaction. In quantum field theory, even classical forces-such as the electromagnetic repulsion or attraction between two charges-can be thought of as due to the exchange of many virtual photons between the charges. They are important in the physics of many processes, including particle scattering and Casimir forces.
The longer the virtual particle exists, the closer its characteristics come to those of ordinary particles. Virtual particles do not necessarily carry the same mass as the corresponding real particle, although they always conserve energy and momentum. A process involving virtual particles can be described by a schematic representation known as a Feynman diagram, in which virtual particles are represented by internal lines. The concept of virtual particles arises in perturbation theory of quantum field theory where interactions between ordinary particles are described in terms of exchanges of virtual particles. In physics, a virtual particle is a transient quantum fluctuation that exhibits some of the characteristics of an ordinary particle, while having its existence limited by the uncertainty principle.
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