Virtual particle: Difference between revisions

Revision as of 19:51, 26 October 2023

Revision as of 22:50, 18 November 2023

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{{Use American English|date = February 2019}}

{{Short description|Transient quantum fluctuation (physics)}}

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{{For|related articles|Quantum vacuum (disambiguation)}}

{{Use American English|date = February 2019}}

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A '''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]].{{vague|reason=What does it mean that the particle has {{''}}its existence limited by the [[uncertainty principle]]{{''}}?|date=November 2021}} The concept of virtual particles arises in the [[perturbation theory (quantum mechanics)|perturbation theory]] of [[quantum field theory]] where interactions between ordinary particles are described in terms of exchanges of virtual particles. 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.<ref>Peskin, M.E., Schroeder, D.V. (1995). ''An Introduction to Quantum Field Theory'', Westview Press, {{ISBN|0-201-50397-2}}, p. 80.</ref><ref>Mandl, F., Shaw, G. (1984/2002). ''Quantum Field Theory'', John Wiley & Sons, Chichester UK, revised edition, {{ISBN|0-471-94186-7}}, pp. 56, 176.</ref>

Virtual particles do not necessarily carry the same [[mass]] as the corresponding real 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 due to the exchange of virtual [[Photon|photonsphoton]]s between the charges. Virtual photons are the [[exchange particle]] for the [[Electromagnetism|electromagnetic interaction]].

The term is somewhat loose and vaguely defined, 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.

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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>

Written in the usual mathematical notations, in the equations of physics, there is no mark of the distinction between virtual and actual particles. 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. In the quantum field theory view, actual particles are viewed as being detectable excitations of underlying quantum fields. Virtual particles are also viewed as excitations of the underlying fields, but appear only as forces, not as detectable particles. They are "temporary" in the sense that they appear in some calculations, but are not detected as single particles. 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.

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==Compared to actual particles==

As a consequence of quantum mechanical [[uncertainty principle|uncertainty]], any object or process that exists for a limited time or in a limited volume cannot have a precisely defined energy or momentum. For this reason, virtual particles &ndash; which exist only temporarily as they are exchanged between ordinary particles &ndash; do not typically obey the [[On shell and off shell|mass-shell relation]]; the longer a virtual particle exists, the more the energy and momentum approach the mass-shell relation.

The lifetime of real particles is typically vastly longer than the lifetime of the virtual particles. Electromagnetic radiation consists of real photons which may travel light years between the emitter and absorber, but (Coulombic) electrostatic attraction and repulsion is a relatively short-range{{Dubious|reason=It is not a short-range force, the range is infinite. E.g., in a system of only two electrons separated by light years, the electrons *will* repel each other|date=June 2023}} force that is a consequence of the exchange of virtual photons {{Citation needed|reason=Need an explicit source for why real particles cannot mediate short-range forces such as Coulombic interactions|date=February 2020}}.