Virtual particle: Difference between revisions

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Virtual particles do not necessarily carry the same [[mass]] as the corresponding ordinary particle, although they always conserve [[energy]] and [[momentum]]. The closer its characteristics come to those of ordinary particles, the longer the virtual particle exists. They are important in the physics of many processes, including particle scattering and [[Casimir force]]s. In quantum field theory, forces—such as the [[electromagnetic repulsion]] or attraction between two charges—can be thought of as resulting from the exchange of [[virtual photon]]s between the charges. Virtual photons are the [[exchange particle]]s for the [[Electromagnetism|electromagnetic interaction]].

The term is somewhat loose and vaguely defined,<ref>{{Cite journal |last=Martinez |first=Jean-Philippe |date=2024-06-01 |title=Virtuality in Modern Physics in the 1920s and 1930s: Meaning(s) of an Emerging Notion |url=https://direct.mit.edu/posc/article-abstract/32/3/350/116521/Virtuality-in-Modern-Physics-in-the-1920s-and?redirectedFrom=fulltext |journal=Perspectives on Science |volume=32 |issue=3 |pages=350–371 |doi=10.1162/posc_a_00610 |issn=1063-6145}}</ref> in that it refers to the view that the world is made up of "real particles". "Real particles" are better understood to be excitations of the underlying quantum fields. 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]]. 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.<ref>{{cite journal|last1=Jaeger|first1=Gregg|title=Are virtual particles less real?|journal=Entropy |volume=21 |issue=2|page=141|date=2019|doi=10.3390/e21020141|pmid=33266857 |pmc=7514619|bibcode=2019Entrp..21..141J|url=http://philsci-archive.pitt.edu/15858/1/Jaeger%20Are%20Virtual%20Particles%20Less%20Real_%20entropy-21-00141-v3.pdf|doi-access=free}}</ref> 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.{{cn|date=May 2025}}

== Properties ==

The concept of virtual particles arises in the [[Perturbation theory (quantum mechanics)|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. Such calculations are often performed using schematic representations known as [[Feynman diagram]]s, in which virtual particles appear as internal lines. By expressing the interaction in terms of the exchange of a virtual particle with [[four-momentum]] {{mvar|q}}, where {{mvar|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.<ref name=Thomson>{{cite book|last1=Thomson|first1=Mark|title=Modern particle physics|date=2013|publisher=Cambridge University Press|___location=Cambridge|isbn=978-1107034266}}</ref>{{rp|119}}

A virtual particle ''does not precisely obey the [[energy–momentum relation]]'' {{math|''m''²''c''⁴ {{=}} ''E''² − ''p''²''c''²}}. Its kinetic energy may not have the usual relationship to [[velocity]]. It can be negative.<ref>{{cite book|last1=Hawking|first1=Stephen|title=A brief history of time|date=1998|publisher=Bantam Books|___location=New York|isbn=9780553896923|edition=Updated and expanded tenth anniversary}}</ref>{{rp|110}} This is expressed by the phrase ''[[On shell and off shell|off mass shell]]''.<ref name=Thomson/>{{rp|119}} The probability amplitude for a virtual particle to exist tends to be canceled out by [[destructive interference]] over longer distances and times. 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.

[[Quantum tunnelling]] may be considered a manifestation of virtual particle exchanges.<ref>{{cite book|last1=Walters|first1=Tony Hey; Patrick|title=The new quantum universe|date=2004|publisher=Cambridge Univ. Press|___location=Cambridge [u.a.]|isbn=9780521564571|edition=Reprint.|bibcode=2003nqu..book.....H}}</ref>{{rp|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.<ref name=Calle>{{cite book|last1=Calle|first1=Carlos I.|title=Superstrings and other things : a guide to physics|date=2010|publisher=CRC Press/Taylor & Francis|___location=Boca Raton|isbn=9781439810743|edition=2nd |pages=443&ndash;444}}</ref>

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* [[Vacuum polarization]], which involves [[pair production]] or the [[decay of the vacuum]], which is the spontaneous production of particle-antiparticle pairs (such as electron-positron).

* [[Lamb shift]] of positions of atomic levels.

* The [[impedance of free space]], which defines the ratio between the [[electric field strength]] {{math|{{abs|'''E'''}}}} and the [[magnetic field strength]] {{math|{{abs|'''H'''}}}}: {{math||1=''Z''{{sub|0}} = {{abs|'''E'''}} / {{abs|'''H'''}}}}.<ref>{{cite news |url=https://phys.org/news/2013-03-ephemeral-vacuum-particles-speed-of-light-fluctuations.html |title=Ephemeral vacuum particles induce speed-of-light fluctuations |website=Phys.org |access-date=2017-07-24}}</ref>

* Much of the so-called [[Near and far field|near-field]] of radio antennas, where the magnetic and electric effects of the changing current in the antenna wire and the charge effects of the wire's capacitive charge may be (and usually are) important contributors to the total EM field close to the source, but both of which effects are [[dipole]] effects that decay with increasing distance from the antenna much more quickly than do the influence of "conventional" [[electromagnetic waves]] that are "far" from the source.{{efn|"Far" in terms of ratio of antenna length or diameter, to wavelength.}} These far-field waves, for which {{mvar|E}} is (in the limit of long distance) equal to {{mvar|cB}}, are composed of actual photons. Actual and virtual photons are mixed near an antenna, with the virtual photons responsible only for the "extra" magnetic-inductive and transient electric-dipole effects, which cause any imbalance between {{mvar|E}} and {{mvar|cB}}. As distance from the antenna grows, the near-field effects (as dipole fields) die out more quickly, and only the "radiative" effects that are due to actual photons remain as important effects. Although virtual effects extend to infinity, they drop off in field strength as {{math|1/''r''{{sup|2}}}} rather than the field of EM waves composed of actual photons, which drop as {{math|1/''r''}}.{{efn|The electrical power in the fields, respectively, decrease as {{math|1/''r''{{sup|4}}}} and {{math|1/''r''{{sup|2}}}}.}}{{efn|See [[near and far field]] for a more detailed discussion. See [[near-field communication]] for practical communications applications of near fields.}}

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{{div col}}

* [[Anomalous photovoltaic effect]]

* [[False vacuum]]

* [[Force carrier]]

* [[Quasiparticle]]

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* [[Quantum foam]]

* [[Virtual black hole]]

* [[Added mass]]

{{div col end}}