Thus, in mathematical terms, they never appear as indices to the scattering matrix, which is to say, they never appear as the observable inputs and outputs of the physical process being modelled. They are "temporary" in the sense that they appear in some calculations, but are not detected as single particles. Virtual particles are also viewed as excitations of the underlying fields, but appear only as forces, not as detectable particles. In the quantum field theory view, actual particles are viewed as being detectable excitations of underlying quantum fields. The amplitudes of processes with a virtual particle interfere with the amplitudes of processes without it, whereas for an actual particle the cases of existence and non-existence cease to be coherent with each other and do not interfere any more. Written in the usual mathematical notations, in the equations of physics, there is no mark of the distinction between virtual and actual particles. : 235 The range of forces carried by virtual particles is limited by the uncertainty principle, which regards energy and time as conjugate variables thus, virtual particles of larger mass have more limited range. Quantum tunnelling may be considered a manifestation of virtual particle exchanges. As a consequence, a real photon is massless and thus has only two polarization states, whereas a virtual one, being effectively massive, has three polarization states. : 119 The probability amplitude for a virtual particle to exist tends to be canceled out by destructive interference over longer distances and times. : 110 This is expressed by the phrase off mass shell. Its kinetic energy may not have the usual relationship to velocity. : 119Ī virtual particle does not precisely obey the energy–momentum relation m 2 c 4 = E 2 − p 2 c 2. By expressing the interaction in terms of the exchange of a virtual particle with four-momentum q, where q is given by the difference between the four-momenta of the particles entering and leaving the interaction vertex, both momentum and energy are conserved at the interaction vertices of the Feynman diagram. Such calculations are often performed using schematic representations known as Feynman diagrams, in which virtual particles appear as internal lines. The concept of virtual particles arises in the perturbation theory of quantum field theory, an approximation scheme in which interactions (in essence, forces) between actual particles are calculated in terms of exchanges of virtual particles. Although widely used, they are by no means a necessary feature of QFT, but rather are mathematical conveniences - as demonstrated by lattice field theory, which avoids using the concept altogether. The accuracy and use of virtual particles in calculations is firmly established, but as they cannot be detected in experiments, deciding how to precisely describe them is a topic of debate. Virtual particles are also excitations of the underlying fields, but are "temporary" in the sense that they appear in calculations of interactions, but never as asymptotic states or indices to the scattering matrix. "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, forces-such as the electromagnetic repulsion or attraction between two charges-can be thought of as due to the exchange of virtual photons between the charges. They are important in the physics of many processes, including particle scattering and Casimir forces. The closer its characteristics come to those of ordinary particles, the longer the virtual particle exists. 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 the perturbation theory of quantum field theory where interactions between ordinary particles are described in terms of exchanges of virtual particles. For related articles, see Quantum vacuum (disambiguation).Ī virtual particle is a theoretical transient particle that exhibits some of the characteristics of an ordinary particle, while having its existence limited by the uncertainty principle.
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