In physics, a virtual particle is a particle which exists for such a short time and space that its energy and momentum do not have to obey the usual relationship. Virtual particles exhibit some of the phenomena that real particles do, such as obedience to the conservation laws. If a single particle is detected, then the consequences of its existence are prolonged to such a degree that it cannot be virtual. Virtual particles are viewed as the quanta which describe fields of the basic force interactions, which cannot be described in terms of real particles. Examples of these are static force fields, such as a simple electric or magnetic fields, or any field which exists without excitations which result in it carrying information from place to place. Physics (Greek: (phúsis), nature and (phusiké), knowledge of nature) is the science concerned with the discovery and characterization of universal laws which govern matter, energy, space, and time. ... In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not made up of smaller particles. ...

The concept of virtual particles necessarily arises in the perturbation theory of quantum field theory, where interactions (essentially, forces) between real particles are described in terms of exchanges of virtual particles. Any process involving virtual particles admits a schematic representation known as a Feynman diagram which facilitates understanding of calculations. Perturbation theory comprises mathematical methods that are used to find an approximate solution to a problem which cannot be solved exactly, by starting from the exact solution of a related problem. ... In this Feynman diagram, an electron and positron annihilate and become a quark-antiquark pair. ...

A virtual particle is one that does not obey the m²c⁴ = E² − p²c² relationship. In other words, their kinetic energy may not have the usual relationship to velocity, indeed, it can be negative. The probability amplitude for them to exist tends to be canceled out by destructive interference over longer distances and times. They can be considered a manifestation of quantum tunneling. 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 limited range due to a limited travel time at the speed of light. Interference of two circular waves - Wavelength (decreasing bottom to top) and Wave centers distance (increasing to the right). ... Quantum tunneling is the quantum-mechanical effect of transitioning through a classically-forbidden energy state. ...

There is no bright-line separating virtual particles from real particles — the equations of physics just describe particles (which includes both equally). The amplitude that a virtual particle exists interferes with the amplitude for its non-existence; whereas for a real 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, "real particles" are viewed as being detectable excitations of underlying quantum fields. As such, virtual particles are also excitations of the underlying fields, but are detectable only as forces but not particles. They are "temporary" in the sense that they appear in calculations, but are not detected as single particles. Thus, in mathematical terms, they never appear as indexes to the scattering matrix, which is to say, they never appear as the observable inputs and outputs of the physical process being modeled. In this sense, virtual particles are an artifact of perturbation theory, and do not appear in a nonperturbative treatment. As such, their objective existence as "particles" is questionable; however, the term is useful in informal, casual conversation, or in rendering concepts into layman's terms. To meet Wikipedias quality standards, this article or section may require cleanup. ... In quantum mechanics, perturbation theory is a set of approximation schemes directly related to mathematical perturbation for describing a complicated quantum system in terms of a simpler one. ... In quantum mechanics, perturbation theory is a set of approximation schemes for describing a complicated quantum system in terms of a simpler one. ...

There are two principal ways in which the notion of virtual particles appear in modern physics. They appear as intermediate terms in Feynman diagrams; that is, as terms in a perturbative calculation. They also appear as an infinite set of states to be summed or integrated over in the calculation of a semi-non-perturbative effect. In the latter case, it is sometimes said that virtual particles cause the effect, or that the effect occurs because of the existence of virtual particles. In this Feynman diagram, an electron and positron annihilate and become a quark-antiquark pair. ...

Manifestations

There are many observable physical phenomena resulting from interactions involving virtual particles. All tend to be characterized by the relatively short range of the force interaction produced them.

• The Coulomb force between electric charges. It is caused by exchange of virtual photons. In symmetric 3-dimensional space this exchange results in inverse square law for force.
• The pure magnetic field, which also exists as virtual photons, but due to its dipole nature, results in an inverse cube law for force. The force between two magnetic poles falls off as the inverse square of the distance. But magnetic monopoles do not exist (as far as we know). The force field generated by a magnetic dipole falls off as the inverse cube of the distance; and the net force between two magnetic dipoles falls off as the inverse fourth power of the distance.
• The so-called near field of radio antennas, where the magnetic effects of the current in the antenna wire and the charge effects of the wire's capacitative charge are detectable, but both of which effects disappear with increasing distance from the antenna much more quickly than do the influence of conventional electromagnetic waves, for which E is always equal to cB, and which are composed of real photons.
• Various magnetic induction fields (such as exist in an electrical transformer) and alternating electric fields (such as might be found in the vicinity of a Tesla coil). These are rather similar to the near field in antennas, but typically are produced with frequencies which result in comparatively little radiation of conventional radio waves (real photons). The effects of such devices (example, a metal detector coil) are therefore of relatively short range. It is a characteristic of near-field effects that energy drawn from them produces a counter-field which results in increased power consumption at the antenna (or transformer). However, with conventional electromagnetic waves (real photons) this is not the case, and the power drawn from the antenna to produce them is the same whether they are absorbed or not.
• The strong nuclear force between quarks - it is the result of interaction of virtual gluons. 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 - it is the result of exchange by virtual W 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.
• Hawking radiation, where the gravitational field is so strong that it causes the spontaneous production of photon pairs (with black body energy distribution) and even of particle pairs.

Most of these have analogous effects in solid-state physics; indeed, one can often gain a better intuitive understanding by examining these cases. In semiconductors, the roles of electrons, positrons and photons in field theory are replaced by electrons in the conduction band, holes in the valence band, and phonons or vibrations of the crystal lattice. The word light is defined here as electromagnetic radiation of any wavelength; thus, X-rays, gamma rays, ultraviolet light, infrared radiation, microwaves, radio waves, and visible light are all forms of light. ... In physics, an inverse-square law is any physical law stating that some quantity is inversely proportional to the square of the distance from a point. ... In the study of diffraction and antenna design, the near field is that part of the radiated field that is within a small number of wavelengths of the diffracting edge or antenna. ... Electromagnetic radiation or EM radiation is a combination (cross product) of oscillating electric and magnetic fields perpendicular to each other, moving through space as a wave, effectively transporting energy and momentum. ... Look up induction in Wiktionary, the free dictionary. ... Tesla Coil at Questacon, the Australian National Science Centre museum A Tesla coil is a type of resonant transformer, named after its inventor, Nikola Tesla. ... The strong nuclear force or strong interaction (also called color force or colour force) is a fundamental force of nature which affects only quarks and antiquarks, and is mediated by gluons in a similar fashion to how the electromagnetic force is mediated by photons. ... These are the six flavors of quarks and their most likely decay modes. ... In particle physics, gluons are subatomic particles that cause quarks to interact, and are indirectly responsible for the binding of protons and neutrons together in atomic nuclei. ... In particle physics, pion (short for the Greek pi meson = P middle) is the collective name for three subatomic particles discovered in 1947: π0, π+ and π−. Pions are the lightest mesons. ... In particle physics, a rho meson is a short-lived hadronic particle that is an isospin triplet whose three states are denoted as . After the pions and kaons, the rho mesons are the lightest strongly interacting particle with a mass of roughly 770 MeV for all three states. ... The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ... In physics, the W and Z bosons are the elementary particles that mediate the weak nuclear force. ... Spontaneous emission is the process by which a molecule in an excited state drops to the ground state, resulting in the creation of a photon. ... The word light is defined here as electromagnetic radiation of any wavelength; thus, X-rays, gamma rays, ultraviolet light, infrared radiation, microwaves, radio waves, and visible light are all forms of light. ... In physics, the Casimir effect is a physical force exerted between separate objects, which is due to neither charge, gravity, nor the exchange of particles, but instead is due to resonance of all-pervasive energy fields in the intervening space between the objects. ... In physics, the ground state of a quantum mechanical system is its lowest-energy state. ... In chemistry, the term van der Waals force originally referred to all forms of intermolecular forces; however, in modern usage it tends to refer to intermolecular forces that deal with forces due to the polarization of molecules. ... In quantum physics, if we expand about the Fock vacuum, the true vacuum contains short-lived virtual particle-antiparticle pairs which are created in pairs out of the Fock vacuum and then annihilate each other. ... Pair production refers to the creation of an elementary particle and its antiparticle, usually from a photon (or another neutral boson). ... Pair production refers to the creation of an elementary particle and its antiparticle, usually from a photon (or neutral bosonic fields). ... In physics, the Lamb shift, named after Willis Lamb, is a small difference in energy between two energy levels and of the hydrogen atom in quantum mechanics. ... In physics, Hawking radiation (also known as Bekenstein-Hawking radiation) is a thermal radiation thought to be emitted by black holes due to quantum effects. ... Solid-state physics, the largest branch of condensed matter physics, is the study of rigid matter, or solids. ... A semiconductor is a solid whose electrical conductivity can be controlled over a wide range, either permanently or dynamically. ... In semiconductors and insulators, the conduction band is the range of electron energy, higher than that of the valence band, sufficient to make the electrons free to accelerate under the influence of an applied electric field and thus constitute an electric current. ... In solids, the valence band is the highest range of electron energies where electrons are normally present at zero temperature. ... Normals modes of vibration progression through a crystal. ...

Antiparticles have been proven to exist and should not be confused with virtual particles or virtual antiparticles. Corresponding to most kinds of particle, there is an associated antiparticle with the same mass and opposite charges. ...

History

Paul Dirac was the first to propose that empty space (the vacuum) can be visualized as consisting of a sea of virtual electron-positron pairs, known as the Dirac sea. The Dirac sea has a direct analog to the structure of electronic bands in crystalline solids as described in solid state physics. Here, particles correspond to conduction electrons, and antiparticles to holes. A variety of interesting phenomena can be attributed to this structure. Paul Adrien Maurice Dirac, OM, FRS (IPA: [dɪræk]) (August 8, 1902 – October 20, 1984) was a British theoretical physicist and a founder of the field of quantum physics. ... The Dirac sea is a theoretical model of the vacuum as an infinite sea of particles possessing negative energy. ... In semiconductors and insulators, the conduction band is the range of electron energy, higher than that of the valence band, sufficient to make the electrons free to accelerate under the influence of an applied electric field and thus constitute an electric current. ... Crystal (disambiguation) Insulin crystals A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. ... Solid-state physics, the largest branch of condensed matter physics, is the study of rigid matter, or solids. ... Electrical conduction is the current (movement of charged particles) through a material in response to an electric field. ... For the following two reasons the electron hole was introduced into calculations: If an electron is excited into higher state it leaves a hole in its old state. ...

Virtual particles in Feynman diagrams

One particle exchange scattering diagram

The calculation of scattering amplitudes in theoretical particle physics requires the use of some rather large and complicated integrals over a large number of variables. These integrals do, however, have a regular structure, and may be represented as Feynman diagrams. The appeal of the Feynman diagrams is strong, as it allows for a simple visual presentation of what would otherwise be a rather arcane and abstract formula. In particular, part of the appeal is that the outgoing legs of a Feynman diagram can be associated with real, on-shell particles. Thus, it is natural to associate the other lines in the diagram with particles as well, called the "virtual particles". Mathematically, they correspond to the propagators appearing in the diagram. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... The S-matrix is the matrix in quantum mechanics or quantum field theory that relates the final state in the infinite future and the initial state in the infinite past. ... Thousands of particles explode from the collision point of two relativistic (100 GeV per ion) gold ions in the STAR detector of the Relativistic Heavy Ion Collider. ... In this Feynman diagram, an electron and positron annihilate and become a quark-antiquark pair. ... In physics, particularly in classical field theory, configurations of a physical system that satisfy classical equations of motion are called on shell, and those that do not are called off shell. ... In quantum mechanics and quantum field theory, the propagator gives the probability amplitude for a particle to travel from one place to another in a given time, or to travel with a certain energy and momentum. ...

In the image above and to the right, the solid lines correspond to real particles (of momentum p₁ and so on), while the dotted line corresponds to a virtual particle carrying momentum k. For example, if the solid lines were to correspond to electrons interacting by means of the electromagnetic interaction, the dotted line would correspond to the exchange of a virtual photon. In the case of interacting nucleons, the dotted line would be a virtual pion. In the case of quarks interacting by means of the strong force, the dotted line would be a virtual gluon, and so on. In classical mechanics, momentum (pl. ... e- redirects here. ... Electromagnetic interaction is a fundamental force of nature and is felt by charged leptons and quarks. ... The word light is defined here as electromagnetic radiation of any wavelength; thus, X-rays, gamma rays, ultraviolet light, infrared radiation, microwaves, radio waves, and visible light are all forms of light. ... In physics a nucleon is a collective name for two baryons: the neutron and the proton. ... In particle physics, pion (short for pi meson) is the collective name for three subatomic particles: Ï€0, Ï€+ and π−. Pions are the lightest mesons and play an important role in explaining low-energy properties of the strong nuclear force. ... These are the six flavors of quarks and their most likely decay modes. ... The strong nuclear force or strong interaction (also called color force or colour force) is a fundamental force of nature which affects only quarks and antiquarks, and is mediated by gluons in a similar fashion to how the electromagnetic force is mediated by photons. ... In particle physics, gluons are subatomic particles that cause quarks to interact, and are indirectly responsible for the binding of protons and neutrons together in atomic nuclei. ...

One-loop diagram with fermion propagator

It is sometimes said that all photons are virtual photons. This is because the world-lines of photons always resemble the dotted line in the above Feynman diagram: the photon was emitted somewhere (say, a distant star), and then is absorbed somewhere else (say a photoreceptor cell in the eyeball). Furthermore, in a vacuum, a photon experiences no passage of (proper) time between emission and absorption. This statement illustrates the difficulty of trying to distinguish between "real" and "virtual" particles as mathematically they are the same objects and it is only our definition of "reality" which is weak here. In practice, a clear distinction can be made: real photons are detected as individual particles in particle detectors, whereas virtual photons are not directly detected; only their average or side-effects may be noticed. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... The word light is defined here as electromagnetic radiation of any wavelength; thus, X-rays, gamma rays, ultraviolet light, infrared radiation, microwaves, radio waves, and visible light are all forms of light. ... STAR is an acronym for: Organizations Society of Ticket Agents and Retailers], the self-regulatory body for the entertainment ticket industry in the UK. Society for Telescopy, Astronomy, and Radio, a non-profit New Jersey astronomy club. ... Rods and Cones redirects here. ... Wikipedia does not yet have an article with this exact name. ... The Compact Muon Solenoid (CMS) is an example of a large particle detector. ...

Virtual particles need not be mesons or bosons, as in the example above; they may also be fermions. However, in order to preserve quantum numbers, most simple diagrams involving fermion exchange are prohibited. The image to the right shows an allowed diagram, a one-loop diagram. The solid lines correspond to a fermion propagator, the wavy lines to bosons. Mesons of spin 1 form a nonet In particle physics, a meson is a strongly interacting boson, that is, it is a hadron with integral spin. ... In particle physics, bosons, named after Satyendra Nath Bose, are particles having integer spin. ... In particle physics, fermions are particles with half-integer spin, such as protons and electrons. ... In physics, a one-loop Feynman diagram is a connected Feynman diagram with only one cycle (unicyclic). ...

Virtual particles in the vacuum

Formally, a particle is considered to be an eigenstate of the particle number operator where is the particle annihilation operator and the particle creation operator (sometimes collectively called ladder operators). In many cases, the particle number operator does not commute with the 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 observables is represented by a probability distribution. Since these particles do not have a permanent existence, 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 time-energy uncertainty principle in the vacuum. In linear algebra, the eigenvectors (from the German eigen meaning inherent, characteristic) of a linear operator are non-zero vectors which, when operated on by the operator, result in a scalar multiple of themselves. ... The number operator is a mathematical operator used in quantum mechanics. ... In physics, an annihilation operator is the operator in quantum field theory that lowers the number of particles in a given state by one. ... In physics, an annihilation operator is the operator in quantum field theory that lowers the number of particles in a given state by one. ... The quantum harmonic oscillator is the quantum mechanical analogue of the classical harmonic oscillator. ... In mathematics, the commutator gives an indication of the extent to which a certain binary operation fails to be commutative. ... The quantum Hamiltonian is the physical state of a system, which may be characterized as a ray in an abstract Hilbert space (or, in the case of ensembles, as a trace class operator with trace 1). ... In physics, particularly in quantum physics, a system observable is a property of the system state that can be determined by some sequence of physical operations. ... In mathematics and statistics, a probability distribution is a function of the probabilities of a mutually exclusive and exhaustive set of events. ... Vacuum energy is an underlying background energy that exists in space even when devoid of matter (known as free space). ... In quantum physics, the Heisenberg uncertainty principle is a mathematical property of a pair of canonical conjugate quantities - usually stated in a form of reciprocity of spans of their spectra. ...

An important example of the "presence" of virtual particles in the vacuum is the Casimir effect. Here, the explanation of the effect requires that the total energy of all of the virtual particles in the vacuum 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 dielectrics. In physics, the Casimir effect is a physical force exerted between separate objects, which is due to neither charge, gravity, nor the exchange of particles, but instead is due to resonance of all-pervasive energy fields in the intervening space between the objects. ... In physics, the zero-point energy is the lowest possible energy that a quantum mechanical physical system may possess; it is the energy of the ground state of the system. ...

Pair production

In order to conserve the total fermion number of the universe, a fermion cannot be created without also creating its antiparticle; thus many physical processes lead to pair creation. The need for the normal ordering of particle fields in the vacuum can be interpreted by the idea that a pair of virtual particles may briefly "pop into existence", and then annihilate each other a short while later. Pair production is a nuclear physics process which occurs where a high-energy photon, generally interacting with an atomic nucleus, produces a particle and an antiparticle. ... Since quantum mechanical Hamiltonians consist of operators, they depend on the order of these. ...

Thus, virtual particles are often popularly described as coming in pairs, a particle and antiparticle, which can be of any kind. These pairs exist for an extremely short time, and mutually annihilate in short order. In some cases, however, it is possible to boost the pair apart using external energy so that they avoid annihilation and become real particles. In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not made up of smaller particles. ... Corresponding to most kinds of particle, there is an associated antiparticle with the same mass and opposite charges. ...

This may occur in one of two ways. In an accelerating frame of reference, the virtual particles may appear to be real to the accelerating observer; this is known as the Unruh effect. In short, the vacuum of a stationary frame appears, to the accelerated observer, to be a warm gas of real particles in thermodynamic equilibrium. The Unruh effect is a toy model for understanding Hawking radiation, the process by which black holes evaporate. This article or section is in need of attention from an expert on the subject. ... The Unruh effect, discovered in 1976 by Bill Unruh of the University of British Columbia, is the prediction that an accelerating observer will observe black-body radiation where an inertial observer would observe none, that is, the accelerating observer will find themselves in a warm background. ... This article or section does not cite its references or sources. ... Insert non-formatted text hereInsert non-formatted text hereInsert non-formatted text hereInsert non-formatted text hereInsert non-formatted text hereInsert non-formatted text hereInsert non-formatted text hereInsert non-formatted text hereInsert non-formatted text hereInsert non-formatted text hereInsert non-formatted text hereInsert non-formatted text hereInsert non... In physics, Hawking radiation (also known as Bekenstein-Hawking radiation) is a thermal radiation thought to be emitted by black holes due to quantum effects. ... Simulated view of a black hole in front of the Milky Way A black hole is an object with a gravitational field so powerful that a region of space becomes cut off from the rest of the universe – no matter or radiation (including light) that has entered the region can... In physics, Hawking radiation is thermal radiation emitted by black holes due to quantum effects. ...

Another example is pair production in very strong electric fields, sometimes called vacuum decay. If, for example, a pair of atomic nuclei are merged together to very briefly form a nucleus with a charge greater than about 140, (that is, larger than about the inverse of the fine structure constant), the strength of the electric field will be such that it will be energetically favorable to create positron-electron pairs out of the vacuum or Dirac sea, with the electron attracted to the nucleus to annihilate the positive charge. This pair-creation amplitude was first calculated by Julian Schwinger in 1951. Pair production refers to the creation of an elementary particle and its antiparticle, usually from a photon (or another neutral boson). ... A vacuum decay region is a large region that allows the Kaons and their decay products to undergo minimal interaction with matter. ... The nucleus of an atom is the very small dense region, of positive charge, in its centre consisting of nucleons (protons and neutrons). ... The fine-structure constant or Sommerfeld fine-structure constant, usually denoted , is the fundamental physical constant characterizing the strength of the electromagnetic interaction. ... The Dirac sea is a theoretical model of the vacuum as an infinite sea of particles possessing negative energy. ... Julian Seymour Schwinger (February 12, 1918 -- July 16, 1994) was an American theoretical physicist. ... 1951 (MCMLI) was a common year starting on Monday; see its calendar. ...

The restriction to particle-antiparticle pairs is actually only necessary if the particles in question carry a conserved quantity, such as electric charge, which is not present in the initial or final state. Otherwise, other situations can arise. For instance, the beta decay of a neutron can happen through the emission of a single virtual, negatively charged W particle that almost immediately decays into a real electron and antineutrino; the neutron turns into a proton when it emits the W particle. The evaporation of a black hole is a process dominated by photons, which are their own antiparticles and are uncharged. In physics, a conservation law states that a particular measurable property of an isolated physical system does not change as the system evolves. ... Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ... In nuclear physics, beta decay (sometimes called neutron decay) is a type of radioactive decay in which a beta particle (an electron or a positron) is emitted. ... This article or section does not adequately cite its references or sources. ... In physics, the W and Z bosons are the elementary particles that mediate the weak nuclear force. ... e- redirects here. ... Antineutrinos, the antiparticles of neutrinos, are neutral particles produced in nuclear beta decay. ... In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... The word light is defined here as electromagnetic radiation of any wavelength; thus, X-rays, gamma rays, ultraviolet light, infrared radiation, microwaves, radio waves, and visible light are all forms of light. ...

It is sometimes suggested that pair production can be used to explain the origin of matter in the universe. In models of the Big Bang, it is suggested that vacuum fluctuations, or virtual particles, briefly appear. Then, due to effects such as CP-violation, an imbalance between the number of virtual particles and antiparticles is created, leaving a surfeit of particles, thus accounting for the visible matter in the universe. The Universe is defined as the summation of all particles and energy that exist and the space-time in which all events occur. ... According to the Big Bang model, the universe emerged from an extremely dense and hot state. ... CP-symmetry is a symmetry obtained by a combination of the C-symmetry and the P-symmetry. ...

External links

Are virtual particles really constantly popping in and out of existence? — Gordon Kane, director of the Michigan Center for Theoretical Physics at the University of Michigan at Ann Arbor, provides an answer at the Scientific American website.