Editing Virtual particle (section)

== Manifestations ==
There are many observable physical phenomena that arise in interactions involving virtual particles. For bosonic particles that exhibit [[rest mass]] when they are free and actual, virtual interactions are characterized by the relatively short range of the force interaction produced by particle exchange. [[Color confinement|Confinement]] can lead to a short range, too. Examples of such short-range interactions are the strong and weak forces, and their associated field bosons.

For the gravitational and electromagnetic forces, the zero rest-mass of the associated boson particle permits long-range forces to be mediated by virtual particles. However, in the case of photons, power and information transfer by virtual particles is a relatively short-range phenomenon (existing only within a few wavelengths of the field-disturbance, which carries information or transferred power), as for example seen in the characteristically short range of inductive and capacitative effects in the [[Near and far field|near field]] zone of coils and antennas.

Some field interactions which may be seen in terms of virtual particles are:
* The [[Coulomb force]] (static electric force) between electric charges. It is caused by the exchange of virtual [[photon]]s. In symmetric 3-dimensional space this exchange results in the [[inverse square law]] for electric force.  Since the photon has no mass, the coulomb potential has an infinite range.
* The [[magnetic field]] between magnetic [[dipole]]s. It is caused by the exchange of virtual [[photon]]s. In symmetric 3-dimensional space, this exchange results in the inverse cube law for magnetic force.  Since the photon has no mass, the magnetic potential has an infinite range. Even though the range is infinite, the time lapse allowed for a virtual photon existence is not infinite.
* [[Electromagnetic induction]]. This phenomenon transfers energy to and from a magnetic coil via a changing (electro)magnetic field.
* The [[strong nuclear force]] between [[quark]]s is the result of interaction of virtual [[gluon]]s. The residual of this force outside of quark triplets (neutron and proton) holds neutrons and protons together in nuclei, and is due to virtual mesons such as the [[pi meson]] and [[rho meson]].
* The [[weak nuclear force]] is the result of exchange by virtual [[W and Z bosons]].
* The [[spontaneous emission]] of a [[photon]] during the decay of an excited atom or excited nucleus; such a decay is prohibited by ordinary quantum mechanics and requires the quantization of the electromagnetic field for its explanation.
* The [[Casimir effect]], where the [[ground state]] of the quantized electromagnetic field causes attraction between a pair of electrically neutral metal plates.
* The [[van der Waals force]], which is partly due to the Casimir effect between two atoms.
* [[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.}}

Most of these have analogous effects in [[solid-state physics]]; indeed, one can often gain a better intuitive understanding by examining these cases.  In [[semiconductor]]s, the roles of electrons, positrons and photons in field theory are replaced by electrons in the [[conduction band]], holes in the [[valence band]], and [[phonon]]s or vibrations of the crystal lattice. A virtual particle is in a [[Two-photon absorption|virtual state]] where the [[probability amplitude]] is not conserved. Examples of macroscopic virtual phonons, photons, and electrons in the case of the tunneling process were presented by [[Günter Nimtz]]<ref name=Nimtz1>{{cite journal |first=G. |last=Nimtz |year=2009 |title=On virtual phonons, photons, and electrons |journal=Found. Phys. |volume=39 |issue=12 |pages=1346–1355|doi=10.1007/s10701-009-9356-z |arxiv=0907.1611 |bibcode=2009FoPh...39.1346N |s2cid=118594121 }}</ref> and Alfons A. Stahlhofen.<ref name=Nimtz2>{{cite journal |first1=A. |last1=Stahlhofen |first2=G. |last2=Nimtz |year=2006 |title=Evanescent modes are virtual photons |journal=Europhys. Lett. |volume=76 |issue=2 |page=198|doi=10.1209/epl/i2006-10271-9 |bibcode=2006EL.....76..189S |s2cid=250758644 }}</ref>