Virtual particle

Feynman diagram of the Coulomb scattering of two electrons. The four straight lines symbolize the incoming or outgoing electrons in real states, the wavy line the virtual photon that mediates the electromagnetic interaction. (Time runs from bottom to top.)

A virtual particle , intermediate particle or particle in a virtual state is a concept from quantum field theory , where it is needed for the theoretical description of the fundamental interactions of elementary particles . The virtual state of a particle can be imagined as a short-lived intermediate state that occurs during an interaction between two particles that are in “normal”, i.e. real states . As an exchange particle, the virtual particle actually creates this interaction, but is never visible to the outside in the virtual state. So z. B. in theQuantum electrodynamics mediates the electromagnetic interaction of two electrons through the exchange of a virtual photon . The proof is indirect: the values calculated using this concept are confirmed in the experiment with an accuracy of up to 1: 10 billion. In principle, every particle can assume real states and virtual states.

Virtual particles occur in each of the three types of interaction that can be described by quantum field theory. Virtual particles are components of Feynman diagrams , each of which reproduces a certain term in a quantum field theoretical perturbation calculation . A Feynman diagram consists of different lines that meet at nodes, the vertices . A distinction is made between the outer lines (those that have a free end) for incoming and outgoing particles in a real state, and the inner lines (those that connect two vertices) for virtual particles. In the context of vacuum fluctuations , Feynman diagrams without external lines are also considered, in which particles arise from the vacuum and then disintegrate again and thus contribute to the vacuum energy . Only virtual particles appear here.

properties

The essential difference between the (real observable) real particles and the unobservable virtual particles is that energy and momentum in the virtual state do not fulfill the energy-momentum relationship if the well-defined mass of the same particle is in the real state. One can therefore say that virtual particles have no defined mass, in technical jargon: “They are not limited to the mass shell ” (or they are not “on-shell”). For example, with the elastic scattering of two electrons, viewed in the center of gravity system, the virtual photon only transmits momentum, but no energy. ${\ displaystyle E}$ ${\ displaystyle p}$ ${\ displaystyle E ^ {2} -p ^ {2} c ^ {2} = m ^ {2} c ^ {4}}$ ${\ displaystyle m}$

This property can help to visualize the behavior of a virtual particle: Since the law of conservation of energy and momentum are not violated for a virtual particle either, these have values for energy and momentum that are forbidden for a real state according to the energy-momentum relationship are. The reason that is often to be read that, according to the energy-time uncertainty relation, the conservation of energy may be temporarily violated, is rather misleading. The distance that the particle could travel at the speed of light in this time limits the conceivable radius of any effects. In the case of low-energy processes, the range is precisely the Compton wavelength of the particle in question. In this way the finite range of the nuclear forces or the weak interaction can be roughly understood. Accordingly, z. For example, radioactive beta decay is possible because the exchange particle in question (the W boson ) can arise as a virtual particle even without an energy supply. Due to its large mass, however, it can only have an effect in the range of a thousandth of a proton radius, which explains the comparatively low transition probability and thus added the adjective "weak" to the interaction. In the same way, it is also possible that indications of the existence of very heavy particles are already observed before the collision energy achieved in particle accelerators is sufficient to produce them in a real state. ${\ displaystyle E}$ ${\ displaystyle p}$

Formally, virtual states can be recognized by adding up over them in perturbation theory . In contrast, the initial and final states of perturbation theory are referred to as the real states. As an example, consider the second order of quantum mechanical perturbation expansion:

{\ displaystyle E_ {n} ^ {2} = \ sum _ {k (\ neq n)} {\ frac {\ langle n ^ {0} | H_ {1} | k ^ {0} \ rangle \ langle k ^ {0} | H_ {1} | n ^ {0} \ rangle} {E_ {n} ^ {0} -E_ {k} ^ {0}}} = \ sum _ {k (\ neq n)} {\ frac {| \ langle k ^ {0} | H_ {1} | n ^ {0} \ rangle | ^ {2}} {E_ {n} ^ {0} -E_ {k} ^ {0}} } \ ,.}

This would be a real state, but the states are used as virtual states. ${\ displaystyle | n ^ {0} \ rangle}$ ${\ displaystyle | k ^ {0} \ rangle}$

Quotes

“Virtual particles are spontaneous fluctuations in a quantum field. Real particles are excitations of a quantum field with a persistence useful for observation. Virtual particles are transients that appear in our equations, but not in measuring devices. By supplying energy, spontaneous fluctuations can be amplified above a threshold value, which has the effect that (actually otherwise) virtual particles become real particles. "

- Frank Wilczek : The lightness of being: mass, ether, and the unification of forces

Individual evidence

^ Frank Wilczek: The lightness of being: mass, ether, and the unification of forces . Basic books, New York 2008, ISBN 978-0-465-00321-1 , Glossary, pp. 241 .

literature

B. Povh, K. Rith, Chr. Scholz, F. Zetsche: Particles and nuclei: an introduction to the physical concepts . 8th edition. Springer, Berlin 2009, ISBN 978-3-540-68075-8 ( limited preview in Google book search).

H. Frauenfelder, EM Henley: Particles and Cores . 4th edition. Oldenbourg, Munich 1999, ISBN 3-486-24417-5 , pp. 98 ff., 318 .

W. Demtröder: Experimental Physik 4 . 1st edition. Springer, Berlin 1998, ISBN 3-540-57097-7 , pp. 109 ff .

J. Bleck-Neuhaus: Elementary Particles . 2nd Edition. Springer, Berlin 2013, ISBN 978-3-642-32578-6 , pp. 482 ff ., doi : 10.1007 / 978-3-642-32579-3 ( limited preview in the Google book search).

[1] Frank Wilczek: The lightness of being: mass, ether, and the unification of forces . Basic books, New York 2008, ISBN 978-0-465-00321-1 , Glossary, pp. 241 .