Pair production

Pair production is the creation of a subatomic particle and its antiparticle from a neutral boson. Examples include creating an electron and a positron, a muon and an antimuon, or a proton and an antiproton. Pair production often refers specifically to a photon creating an electron–positron pair near a nucleus. For pair production to occur, the incoming energy of the interaction must be above a threshold of at least the total rest mass energy of the two particles, and the situation must conserve both energy and momentum.[1] However, all other conserved quantum numbers (angular momentum, electric charge, lepton number) of the produced particles must sum to zero – thus the created particles shall have opposite values of each other. For instance, if one particle has electric charge of +1 the other must have electric charge of −1, or if one particle has strangeness of +1 then another one must have strangeness of −1.

The probability of pair production in photon–matter interactions increases with photon energy and also increases approximately as the square of atomic number of the nearby atom.[2]

Photon to electron and positron

Pair production Cartoon
Diagram showing the process of electron–positron pair production. In reality the produced pair are nearly collinear.

For photons with high photon energy (MeV scale and higher), pair production is the dominant mode of photon interaction with matter. These interactions were first observed in Patrick Blackett's counter-controlled cloud chamber, leading to the 1948 Nobel Prize in Physics.[3] If the photon is near an atomic nucleus, the energy of a photon can be converted into an electron–positron pair:


γ
→ 
e
 + 
e+

The photon's energy is converted to particle mass in accordance with Einstein’s equation, E=mc2; where E is energy, m is mass and c is the speed of light. The photon must have higher energy than the sum of the rest mass energies of an electron and positron (2 × 0.511 MeV = 1.022 MeV) for the production to occur. The photon must be near a nucleus in order to satisfy conservation of momentum, as an electron–positron pair produced in free space cannot both satisfy conservation of energy and momentum.[4] Because of this, when pair production occurs, the atomic nucleus receives some recoil. The reverse of this process is electron positron annihilation.

Basic kinematics

These properties can be derived through the kinematics of the interaction. Using four vector notation, the conservation of energy-momentum before and after the interaction gives:[5]

where is the recoil of the nuclei. Note the modulus of the four vector is:

which implies that for all cases and . We can square the conservation equation:

However, in most cases the recoil of the nuclei is much smaller compared to the energy of the photon and can be neglected. Taking this approximation of to simplify and expanding the remaining relation:

Therefore, this approximation can only be satisfied if the electron and positron are emitted in very nearly the same direction, that is, .

This derivation is a semi-classical approximation. An exact derivation of the kinematics can be done taking into account the full quantum mechanical scattering of photon and nucleus.

Energy transfer

The energy transfer to electron and positron in pair production interactions is given by:

where is Planck's constant, is the frequency of the photon and the is the combined rest mass of the electron–positron. In general, ignoring the nuclei recoil, the electron and positron can be emitted with different kinetic energies, but the average transferred to each is:

Cross section

Electron-Positron nuclear Pair production Feynman Diagram
Feynman diagram of electron–positron pair production. One can calculate multiple diagrams to get the cross section

The exact analytic form for the cross section of pair production must be calculated through quantum electrodynamics in the form of Feynman diagrams and results in a complicated function. To simplify, the cross section can be written as:

where is the fine structure constant, is the classical electron radius, is the atomic number of the material and is some complex function that depends on the energy and atomic number. Cross sections are tabulated for different materials and energies.

In 2008 the Titan laser aimed at a 1-millimeter-thick gold target was used to generate positron–electron pairs in large numbers.[6]

Aestheticity

The symmetry of the phenomena conforms to the definition of a fractal. Therefore if produces quantities of pairs of electrons by a factor of 2, the production itself would become symmetric such that , where n is the times the phenomena occurs.

Astronomy

Pair production is invoked to predict the existence of hypothetical Hawking radiation. According to quantum mechanics, particle pairs are constantly appearing and disappearing as a quantum foam. In a region of strong gravitational tidal forces, the two particles in a pair may sometimes be wrenched apart before they have a chance to mutually annihilate. When this happens in the region around a black hole, one particle may escape while its antiparticle partner is captured by the black hole.

Pair production is also the mechanism behind the hypothesized pair-instability supernova type of stellar explosion, where pair production suddenly lowers the pressure inside a supergiant star, leading to a partial implosion, and then explosive thermonuclear burning. Supernova SN 2006gy is hypothesized to have been a pair production type supernova.

Pair production does not occur in X-ray imaging because the machines are usually rated ~ 150kV.

See also

References

  1. ^ Das, A.; Ferbel, T. (2003-12-23). Introduction to Nuclear and Particle Physics. World Scientific. ISBN 9789814483339.
  2. ^ Stefano, Meroli. "How photons interact with matter". Meroli Stefano Webpage. Retrieved 2016-08-28.
  3. ^ Bywater, Jenn (29 October 2015). "Exploring dark matter in the inaugural Blackett Colloquium". Imperial College London. Retrieved 29 August 2016.
  4. ^ Hubbell, J. H. (June 2006). "Electron positron pair production by photons: A historical overview". Radiation Physics and Chemistry. 75 (6): 614–623. Bibcode:2006RaPC...75..614H. doi:10.1016/j.radphyschem.2005.10.008.
  5. ^ Kuncic, Zdenka, Dr. (12 March 2013). "PRadiation Physics and Dosimetry" (PDF). Index of Dr. Kuncic's Lectures. PHYS 5012. The University of Sydney. Retrieved 2015-04-14.
  6. ^ "Laser technique produces bevy of antimatter". MSNBC. 2008. Retrieved 2019-05-27. The LLNL scientists created the positrons by shooting the lab's high-powered Titan laser onto a one-millimeter-thick piece of gold.

External links

Absolute hot

Absolute hot is a theoretical upper limit to the thermodynamic temperature scale, conceived as an opposite to absolute zero.

Contemporary models of physical cosmology postulate that the highest possible temperature is the Planck temperature, which has the value 1.416785(71)×1032 kelvins. Above about 1032 K, particle energies become so large that gravitational forces between them would become as strong as other fundamental forces according to current theories. There is no existing scientific theory for the behavior of matter at these energies. A quantum theory of gravity would be required. The models of the origin of the universe based on the Big Bang theory assume that the universe passed through this temperature about 10−42 s (one Planck time) after the Big Bang as a result of enormous entropy expansion.Another theory of absolute hot is based on the Hagedorn temperature, where the thermal energies of the particles exceed the mass-energy of a hadron particle-antiparticle pair. Instead of temperature rising, at the Hagedorn temperature more and heavier particles are produced by pair production, thus preventing effective further heating, given that only hadrons are produced. However, further heating is possible (with pressure) if the matter undergoes a phase change into a quark–gluon plasma. Therefore, this temperature is more akin to a boiling point rather than an insurmountable barrier. For hadrons, the Hagedorn temperature is 2×1012 K, which has been reached and exceeded in LHC and RHIC experiments. However, in string theory, a separate Hagedorn temperature can be defined, where strings similarly provide the extra degrees of freedom. However, it is so high (1030 K) that no current or foreseeable experiment can reach it.Considering only certain degrees of freedom in matter, such as nuclear spins, systems with a negative temperature can be produced. They occur because equipartitioning is too slow to allow communication between the degrees of freedom where thermal energy is stored, such as the vibrational, rotational, and nuclear spin states of molecules. These systems are familiar from lasers. For these, theory predicts a mathematical singularity in temperature. When a spin system is excited with electromagnetic radiation and undergoes population inversion into an excited state, quantum physics formally assumes that its temperature function goes through a singularity. The spin temperature tends to positive infinity, before discontinuously switching to negative infinity. However, this applies only to specific degrees of freedom (the spin temperature in this case) in the system, while others would have normal temperature dependency. Thus, this singularity cannot be observed as ordinary sensible heat. If equipartitioning is possible, the system undergoes relaxation into a thermally uniform state with release of a finite quantity of heat. Before a physical infinite temperature could be reached, realistic ordinary matter would undergo phase transitions, such as evaporation, and never actually reach the infinite temperature.

Arkady Migdal

Arkady Beynusovich (Benediktovich) Migdal (Russian: Арка́дий Бе́йнусович (Бенеди́ктович) Мигда́л; Lida, Russian Empire, 11 March 1911 – Princeton, United States, 9 February 1991) was a Soviet physicist and member of the USSR Academy of Sciences. He developed the formula that accounts for the Landau–Pomeranchuk–Migdal effect, a reduction of the bremsstrahlung and pair production cross sections at high energies or high matter densities.His son, Alexander Arkadyevich Migdal, is also a renowned physicist.

Breit–Wheeler process

The Breit–Wheeler process or Breit–Wheeler pair production is a physical process in which a positron–electron pair is created from the collision of two photons. It is the simplest mechanism by which pure light can be potentially transformed into matter. The process can take the form γ γ′ → e+ e− where γ and γ′ are two light quanta.The multiphoton Breit–Wheeler process, also referred to as nonlinear Breit–Wheeler or strong field Breit–Wheeler in the literature, is the extension of the pure photon–photon Breit–Wheeler process when a high-energy probe photon decays into pairs propagating through an electromagnetic field (for example, a laser pulse). In contrast with the previous process, this one can take the form of γ + n ω → e+ e−, where ω represents the coherent photons of the laser field.

The inverse process, e+ e− → γ γ′, in which an electron and a positron collide and annihilate to generate a pair of gamma photons, is known as electron–positron annihilation or the Dirac process for the name of the physicist who first described it theoretically and anticipated the Breit–Wheeler process.

Although the pure photon–photon Breit–Wheeler process was one of the first sources of pairs to be described, its experimental validation has yet to be accomplished. This mechanism is theoretically characterized by a very weak probability, so producing a significant number of pairs requires two extremely bright, collimated sources of photons having photon energy close or above the electron and positron rest mass energy. Manufacturing such a source, a gamma-ray laser, is still a technological challenge. In many experimental configurations, pure Breit–Wheeler is dominated by other more efficient pair creation processes that screen pairs produced via this mechanism. The Dirac process (pair annihilation) has nonetheless been by far verified experimentally. It is also the case of the multiphoton Breit–Wheeler at the Stanford Linear Accelerator Center in 1997 by colliding a high-energy electrons with a counter-propagating terawatt laser pulse.Although this mechanism is still one of the most difficult to be observed experimentally on Earth, it is of considerable importance for the absorption of high-energy photons traveling cosmic distances.The photon–photon and the multiphoton Breit–Wheeler processes are described theoretically by the theory of quantum electrodynamics.

Chung-Yao Chao

Chung-Yao Chao (simplified Chinese: 赵忠尧; traditional Chinese: 趙忠堯; pinyin: Zhào Zhōngyáo; Wade–Giles: Chao Chung-yao; 27 June 1902 – 28 May 1998) was a Chinese physicist. He studied the scattering of gamma rays in lead by pair production in 1930, without knowing that positrons were involved in the anomalously high scattering cross-section. When the positron was discovered by Carl David Anderson in 1932, confirming the existence of Paul Dirac's "antimatter", it became clear that positrons could explain Chung-Yao Chao's earlier experiments, with the gamma rays being emitted from electron-positron annihilation.

He entered Nanjing Higher Normal School (later renamed National Southeastern University, National Central University and Nanjing University), in 1920 and earned a B.S. in physics in 1925. Then he earned a Ph.D. degree in physics under supervision of Nobel Prize laureate Robert Andrews Millikan at California Institute of Technology in 1930. Later he went back to China and joined the physics faculty of Tsinghua University in Beijing.

Compton scattering

Compton scattering, discovered by Arthur Holly Compton, is the scattering of a photon by a charged particle, usually an electron. It results in a decrease in energy (increase in wavelength) of the photon (which may be an X-ray or gamma ray photon), called the Compton effect. Part of the energy of the photon is transferred to the recoiling electron. Inverse Compton scattering occurs when a charged particle transfers part of its energy to a photon.

Electron Pair Production

Electron Pair Production may refer to:

Cooper pairing of electrons in superconductor

Electron-positron pair production

Electron-hole pair generation in semiconductor

Pairing of electrons on the same atomic or molecular orbital

Energetic Gamma Ray Experiment Telescope

The Energetic Gamma Ray Experiment Telescope (EGRET) was one of four instruments outfitted on NASA's Compton Gamma Ray Observatory satellite. Since lower energy gamma rays cannot be accurately detected on Earth's surface, EGRET was built to detect gamma rays while in space. EGRET was created for the purpose of detecting and collecting data on gamma rays ranging in energy level from 30 MeV to 30 GeV.

To accomplish its task, EGRET was equipped with a spark chamber, calorimeter, and plastic scintillator anti-coincidence dome. The spark chamber was used to induce a process called electron-positron pair production as a gamma ray entered the telescope. The calorimeter on the telescope was then used to record the data from the electron or positron. To reject other energy rays that would skew the data, scientists covered the telescope with a plastic scintillator anti-coincidence dome. The dome acted as a shield for the telescope and blocked out any unwanted energy rays.

The telescope was calibrated to only record gamma rays entering the telescope at certain angles. As these gamma rays entered the telescope, the rays went through the telescopes spark chamber and started the production of an electron and positron. The calorimeter then detected the electron or positron and recorded its data, such as energy level.

From EGRET's finds, scientists have affirmed many long-standing theories about energy waves in space. Scientists have also been able to categorize and characterize four pulsars. Scientists were able to map an entire sky of gamma rays with EGRET's results as well as find out interesting facts about Earth's Moon and the Sun.

EGRET is a predecessor of the Fermi Gamma-ray Space Telescope LAT.

Gamma ray

A gamma ray, or gamma radiation (symbol γ or ), is a penetrating electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves and so imparts the highest photon energy. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900 while studying radiation emitted by radium. In 1903, Ernest Rutherford named this radiation gamma rays based on their relatively strong penetration of matter; he had previously discovered two less penetrating types of decay radiation, which he named alpha rays and beta rays in ascending order of penetrating power.

Gamma rays from radioactive decay are in the energy range from a few kiloelectron volts (keV) to approximately 8 Megaelectronvolts (~8 MeV), corresponding to the typical energy levels in nuclei with reasonably long lifetimes. The energy spectrum of gamma rays can be used to identify the decaying radionuclides using gamma spectroscopy. Very-high-energy gamma rays in the 100–1000 teraelectron volt (TeV) range have been observed from sources such as the Cygnus X-3 microquasar.

Natural sources of gamma rays originating on Earth are mostly as a result of radioactive decay and secondary radiation from atmospheric interactions with cosmic ray particles. However, there are other rare natural sources, such as terrestrial gamma-ray flashes, which produce gamma rays from electron action upon the nucleus. Notable artificial sources of gamma rays include fission, such as that which occurs in nuclear reactors, and high energy physics experiments, such as neutral pion decay and nuclear fusion.

Gamma rays and X-rays are both electromagnetic radiation, and since they overlap in the electromagnetic spectrum, the terminology varies between scientific disciplines. In some fields of physics, they are distinguished by their origin: Gamma rays are created by nuclear decay, while in the case of X-rays, the origin is outside the nucleus. In astrophysics, gamma rays are conventionally defined as having photon energies above 100 keV and are the subject of gamma ray astronomy, while radiation below 100 keV is classified as X-rays and is the subject of X-ray astronomy. This convention stems from the early man-made X-rays, which had energies only up to 100 keV, whereas many gamma rays could go to higher energies. A large fraction of astronomical gamma rays are screened by Earth's atmosphere.

Gamma rays are ionizing radiation and are thus biologically hazardous. Due to their high penetration power, they can damage bone marrow and internal organs. Unlike alpha and beta rays, they pass easily through the body and thus pose a formidable radiation protection challenge, requiring shielding made from dense materials such as lead or concrete.

Higgsino

In particle physics, for models with N=1 supersymmetry a higgsino, symbol H͂, is the superpartner of the Higgs field. A higgsino is a Dirac fermionic field with spin ​1⁄2 and it refers to a weak isodoublet with hypercharge half under the Standard Model gauge symmetries. After electroweak symmetry breaking higgsino fields linearly mix with U(1) and SU(2) gauginos leading to four neutralinos and two charginos that refer to physical particles. While the two charginos are charged Dirac fermions (plus and minus each), the neutralinos are electrically neutral Majorana fermions. In an R-parity-conserving version of the Minimal Supersymmetric Standard Model, the lightest neutralino typically becomes the lightest supersymmetric particle (LSP). The LSP is a particle physics candidate for the dark matter of the universe since it cannot decay to particles with lighter mass. A neutralino LSP, depending on its composition can be bino, wino or higgsino dominated in nature and can have different zones of mass values in order to satisfy the estimated dark matter relic density. Commonly, a higgsino dominated LSP is often referred as a higgsino, in spite of the fact that a higgsino is not a physical state in the true sense.

In natural scenarios of SUSY, top squarks, bottom squarks, gluinos, and higgsino-enriched neutralinos and charginos are expected to be relatively light, enhancing their production cross sections. Higgsino searches have been performed by both the ATLAS and CMS experiments at the Large Hadron Collider at CERN, where physicists have searched for the direct electroweak pair production of Higgsinos. As of 2017, no experimental evidence for Higgsinos has been reported.

Landau–Pomeranchuk–Migdal effect

In high-energy physics, the Landau–Pomeranchuk–Migdal effect, also known as the Landau–Pomeranchuk effect and the Pomeranchuk effect, or simply LPM effect, is a reduction of the bremsstrahlung and pair production cross sections at high energies or high matter densities. It is named in honor to Lev Landau, Isaak Pomeranchuk and Arkady Migdal.

Matter creation

Even restricting the discussion to physics, scientists do not have a unique definition of what matter is. In the currently known particle physics, summarised by the standard model of elementary particles and interactions, it is possible to distinguish in an absolute sense particles of matter and particles of antimatter. This is particularly easy for those particles that carry electric charge, such as electrons or protons or quarks, while the distinction is more subtle in the case of neutrinos, fundamental elementary particles that do not carry electric charge. In the standard model, it is not possible to create a net amount of matter particles—or more precisely, it is not possible to change the net number of leptons or of quarks in any perturbative reaction among particles. This remark is consistent with all existing observations.

However, similar processes are not considered to be impossible and are expected in other models of the elementary particles, that extend the standard model. They are necessary in speculative theories that aim to explain the cosmic excess of matter over antimatter, such as leptogenesis and baryogenesis. They could even manifest themselves in laboratory as proton decay or as creations of electrons in the so-called neutrinoless double beta decay. The latter case occurs if the neutrinos are Majorana particles, being at the same time matter and antimatter, according to the definition given just above.In a wider sense, one can use the word matter simply to refer to fermions. In this sense, matter and antimatter particles (such as an electron and a positron) are a priori identified. The process inverse to particle annihilation can be called matter creation; more precisely, we are considering here the process obtained under time reversal of the annihilation process. This process is also known as pair production, and can be described as the conversion of light particles (i.e., photons) into one or more massive particles. The most common and well-studied case is the one where two photons convert into an electron–positron pair.

Pair-instability supernova

A pair-instability supernova occurs when pair production, the production of free electrons and positrons in the collision between atomic nuclei and energetic gamma rays, temporarily reduces the internal pressure supporting a supermassive star's core against gravitational collapse. This pressure drop leads to a partial collapse, which in turn causes greatly accelerated burning in a runaway thermonuclear explosion, resulting in the star being blown completely apart without leaving a black hole remnant behind. Pair-instability supernovae can only happen in stars with a mass range from around 130 to 250 solar masses and low to moderate metallicity (low abundance of elements other than hydrogen and helium – a situation common in Population III stars). The recently observed objects SN 2006gy, SN 2007bi, SN 2213-1745, and SN 1000+0216 are hypothesized to have been pair-instability supernovae.

Particle shower

In particle physics, a shower is a cascade of secondary particles produced as the result of a high-energy particle interacting with dense matter. The incoming particle interacts, producing multiple new particles with lesser energy; each of these then interacts, in the same way, a process that continues until many thousands, millions, or even billions of low-energy particles are produced. These are then stopped in the matter and absorbed.

Positron

The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1 e, a spin of 1/2 (same as electron), and has the same mass as an electron. When a positron collides with an electron, annihilation occurs. If this collision occurs at low energies, it results in the production of two or more gamma ray photons.

Positrons can be created by positron emission radioactive decay (through weak interactions), or by pair production from a sufficiently energetic photon which is interacting with an atom in a material.

Quantum field theory in curved spacetime

In particle physics, quantum field theory in curved spacetime is an extension of standard, Minkowski space quantum field theory to curved spacetime. A general prediction of this theory is that particles can be created by time-dependent gravitational fields (multigraviton pair production), or by time-independent gravitational fields that contain horizons.

Schwinger effect

The Schwinger effect is a predicted physical phenomenon whereby matter is created by a strong electric field. It is also referred to as the (Sauter-)Schwinger effect, Schwinger mechanism, or Schwinger pair production. It is a prediction of quantum electrodynamics (QED) in which electron-positron pairs are spontaneously created in the presence of an electric field, thereby causing the decay of the electric field. The effect was originally proposed by Fritz Sauter in 1931 and further important work was carried out by Werner Heisenberg and Hans Heinrich Euler in 1936, though it was not until 1951 when Julian Schwinger gave a complete theoretical description .

Schwinger limit

In quantum electrodynamics (QED), the Schwinger limit is a scale above which the electromagnetic field is expected to become nonlinear. The limit was first derived in one of QED's earliest theoretical successes by Fritz Sauter in 1931 and discussed further by Werner Heisenberg and his student Hans Heinrich Euler. The limit, however, is commonly named in the literature for Julian Schwinger, who derived the leading nonlinear corrections to the fields and calculated the rate of electron–positron pair production in a strong electric field. The limit is typically reported as a maximum electric field before nonlinearity for the vacuum of

where me is the mass of the electron, c is the speed of light in vacuum, qe is the elementary charge, and ħ is the reduced Planck constant.

In a vacuum, the classical Maxwell's equations are perfectly linear differential equations. This implies – by the superposition principle – that the sum of any two solutions to Maxwell's equations is yet another solution to Maxwell's equations. For example, two intersecting beams of light should simply add together their electric fields and pass right through each other. Thus Maxwell's equations predict the impossibility of any but trivial elastic photon–photon scattering. In QED, however, non-elastic photon–photon scattering becomes possible when the combined energy is large enough to create virtual electron–positron pairs spontaneously, illustrated by the Feynman diagram in the adjacent figure.

A single plane wave is insufficient to cause nonlinear effects, even in QED. The basic reason for this is that a single plane wave of a given energy may always be viewed in a different reference frame, where it has less energy (the same is the case for a single photon). A single wave or photon does not have a center of momentum frame where its energy must be at minimal value. However, two waves or two photons not traveling in the same direction always have a minimum combined energy in their center of momentum frame, and it is this energy and the electric field strengths associated with it, which determine particle-antiparticle creation, and associated scattering phenomena.

Photon–photon scattering and other effects of nonlinear optics in vacuum is an active area of experimental research, with current or planned technology beginning to approach the Schwinger limit. It has already been observed through inelastic channels in SLAC Experiment 144. However, the direct effects in elastic scattering have not been observed. As of 2012, the best constraint on the elastic photon–photon scattering cross section belongs to PVLAS, which reports an upper limit far above the level predicted by the Standard Model. Proposals have been made to measure elastic light-by-light scattering using the strong electromagnetic fields of the hadrons collided at the LHC. Observation of a cross section larger than that predicted by the Standard Model could signify new physics such as axions, the search of which is the primary goal of PVLAS and several similar experiments. Even the planned, funded ELI–Ultra High Field Facility, which will study light at the intensity frontier, is likely to remain well below the Schwinger limit although it may still be possible to observe some nonlinear optical effects. Such an experiment, in which ultra-intense light causes pair production, has been described in the popular media as creating a "hernia" in spacetime.

Tests of relativistic energy and momentum

Tests of relativistic energy and momentum are aimed at measuring the relativistic expressions for energy, momentum, and mass. According to special relativity, the properties of particles moving approximately at the speed of light significantly deviate from the predictions of Newtonian mechanics. For instance, the speed of light cannot be reached by massive particles.

Today, those relativistic expressions for particles close to the speed of light are routinely confirmed in undergraduate laboratories, and necessary in the design and theoretical evaluation of collision experiments in particle accelerators. See also Tests of special relativity for a general overview.

Virtual particle

In physics, a virtual particle is a transient quantum fluctuation that exhibits some of the characteristics of an ordinary particle, while having its existence limited by the uncertainty principle. The concept of virtual particles arises in 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.Virtual particles do not necessarily carry the same mass as the corresponding real particle, although they always conserve energy and momentum. The longer the virtual particle exists, the closer its characteristics come to those of ordinary particles. They are important in the physics of many processes, including particle scattering and Casimir forces. In quantum field theory, even classical forces—such as the electromagnetic repulsion or attraction between two charges—can be thought of as due to the exchange of many virtual photons between the charges. Virtual photons are the exchange particle for the 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": it is not; rather, "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.

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