Virtual particle: Difference between revisions

Revision as of 18:08, 25 March 2024

Latest revision as of 20:48, 20 August 2025

(21 intermediate revisions by 13 users not shown)

Line 9:

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>

Line 55:

{{Main article|Quantum fluctuation|QED vacuum|QCD vacuum|Vacuum state}}

In formal terms, a particle is considered to be an [[eigenstate]] of the [[particle number operator]] ''a''^†''a'', where ''a'' is the particle [[annihilation operator]] and ''a''^† the particle [[creation operator]] (sometimes collectively called [[ladder operator]]s). In many cases, the particle number operator does not [[commutator|commute]] with the [[Hamiltonian (quantum mechanics)|Hamiltonian]] for the system. This implies the number of particles in an area of space is not a well-defined quantity but, like other quantum [[observable]]s, is represented by a [[probability distribution]]. Since these particles are not certain to exist, they are called ''virtual particles'' or ''vacuum fluctuations'' of [[vacuum energy]]. In a certain sense, they can be understood to be a manifestation of the [[Uncertainty principle#Robertson.E2.80.93Schr.C3.B6dinger uncertainty relations|time-energy uncertainty principle]] in a vacuum.<ref>{{cite book|last1=Raymond|first1=David J.|title=A radically modern approach to introductory physics: volume 2: four forces|date=2012|publisher=New Mexico Tech Press|___location=Socorro, NM|isbn=978-0-98303-946-4|pages=252&ndash;254|url=http://kestrel.nmt.edu/~raymond/books/radphys/book2/book2.html#x1-2100014.7}}</ref>

An important example of the "presence" of virtual particles in a vacuum is the [[Casimir effect]].<ref>{{cite journal|last1=Choi|first1=Charles Q.|title=A vacuum can yield flashes of light|journal=Nature|date=13 February 2013|doi=10.1038/nature.2013.12430|s2cid=124394711|url=http://www.nature.com/news/a-vacuum-can-yield-flashes-of-light-1.12430|access-date=2 August 2015|doi-access=free}}</ref> Here, the explanation of the effect requires that the total energy of all of the virtual particles in a vacuum can be added together. Thus, although the virtual particles themselves are not directly observable in the laboratory, they do leave an observable effect: Their [[zero-point energy]] results in forces acting on suitably arranged metal plates or [[dielectric]]s.<ref>{{cite journal|last1=Lambrecht|first1=Astrid|title=The Casimir effect: a force from nothing|journal=Physics World|date=September 2002|volume=15|issue=9|pages=29&ndash;32|doi=10.1088/2058-7058/15/9/29}}</ref> On the other hand, the Casimir effect can be interpreted as the [[Casimir effect#Relativistic van der Waals force|relativistic van der Waals force]].<ref>{{cite journal|last1=Jaffe|first1=R. L.|title=Casimir effect and the quantum vacuum|journal=Physical Review D|date=12 July 2005|volume=72|issue=2|pages=021301|doi=10.1103/PhysRevD.72.021301|arxiv = hep-th/0503158 |bibcode = 2005PhRvD..72b1301J |s2cid=13171179}}</ref>

Line 75:

{{div col}}

* [[Anomalous photovoltaic effect]]

* [[False vacuum]]

* [[Force carrier]]

* [[Quasiparticle]]

Line 82 ⟶ 83:

* [[Quantum foam]]

* [[Virtual black hole]]

* [[Added mass]]

{{div col end}}