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−  '''Quantum field theory''' is a mathematical theory in physics which extends [[quantum mechanics]]. Field theory describes the interactions between subatomic particles, such as electrons, protons, quarks and photons.  +  '''Quantum field theory''' extends [[quantum mechanics]], which deals with particles, to a generalized domain having infinite degrees of freedom. 
   
−  The differences between basic QM and Field Theory are these: in QM, the interactions between more than two particles are increasingly difficult to model, and the creation and destruction of particles cannot be modeled at all. In contrast, Field Theory can describe states containing arbitrary numbers of particles of different energies, masses, charges and types. Field Theory also provides an elegant framework for describing the interactions between particles, and the creation of new particles and destruction of old ones for example, the emission and absorption of photons by electrons, and vice versa.
 +  '''Quantum field theory''' is compatible with the [[theory of relativity]], while [[quantum mechanics]] is not. '''Quantum field theory''' attempts to explain and describe the interactions between an electromagnetic field and charged particles in what is sometimes called quantum electrodynamics. 
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−  For a given interaction of particles, field theoretical calculations generally cannot be solved in a closed, analytical form that is, the predicted probabilities cannot be described by one simple equation; however, physicists have developed numerous approximation methods which produce estimates of everincreasing precision (compared to experiment), with the precision depending upon how much mathematical work is put into the analysis.
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−  Field theoretical calculations, while extremely laborintensive, have yielded predictions of unparalleled precision compared to experimental measurements. The most famous of these is the anomalous magnetic moment of the electron, a very difficult calculation that has (so far) yielded a prediction accurate to one part in a trillion compared to experiment.
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−  Field theoretical calculations can be extremely laborintensive, if math is employed by hand; or computer timeintensive, if software approximations are used. Computational attempts to model the field theoretical equations of the strong nuclear force within protons, neutrons and mesons are among the most intensive computational research projects ever attempted, but have produced good agreement with experiment.
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−  Field theoretical calculations become increasingly difficult as the number of (incoming or outgoing) particles increases; for very large numbers of particles (e.g. macroscopic liquids and solids), the methods of [[Solid State Physics]] (also based on QM) are employed instead.
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−  Quantum Field Theory has [[Special Relativity]] built in as an intrinsic part of the theory. However, the most naive application of Field Theory to [[General Relativity]] is known to be unworkable. This has led to attempts at alternative formulations to reconcile Field Theory and GR. In contrast to the other three forces of nature (electromagnetic, weak nuclear, strong nuclear), which have extensive and impressive experimental confirmation, no alternative formulations of quantum gravity have experimental confirmation.
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−  == The Forces of Nature ==
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−  In the Standard Model of physics, there are four forces of nature: electromagnetic, weak nuclear, strong nuclear, and gravity.
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−  The electromagnetic force models interactions between electrically charged particles, and historically resulted from a unification of the electrical and magnetic fields, which were once thought to be separate fields.
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−  The weak nuclear force is most wellknown for mediating radioactive atomic decays, in which (for example) a proton in a nucleus will turn into a neutron (which remains in the nucleus), and a positron and neutrino, which are emitted. Nonnuclear particles such as electrons also participate in the weak force. Neutrinos only participate in the weak force, and have
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−  extremely low mass, making their observation very difficult. The weak nuclear force only exerts force when particles are extremely close together.
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−  The strong nuclear force holds together the protons and neutrons in an atomic nucleus, and the quarks within protons, neutrons, and mesons. Because protons all have the same charge, they repel each other strongly, and the strong nuclear force is necessary to overcome this and hold them together in a nucleus; likewise for the quarks inside protons, neutrons and mesons. Unlike electromagnetism, which can extend over long distances, the strong nuclear force only exerts force when particles are extremely close together; but at close range, it is enormously stronger than electromagnetism.
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−  Field theory can model the first three forces with a high degree of precision and success, but fails for gravity, for reasons described below. Gravity is by far the weakest of all the forces of nature, when measured in absolute units. This may seem to us to be paradoxical. Electromagnetism (for example) appears to us to be weaker than gravity, because most matter we encounter is nearly equal in positive and negative charges, so that the opposing charges nearly cancel at long distances. However, with gravity, there is no observed "negative mass" to cancel out the effect of positive mass, so the gravitational forces of particles, while individually very tiny, are cumulatively enormous over long distances. (Antimatter has positive mass, but opposite properties to matter.)
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−  == Field Bosons Mediate Action At a Distance ==
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−  How is it that particles affect each other for example, the repulsion of like charges, or attraction of unlike charges?
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−  Field theory can incorporate "action at a distance" models by the direct incorporation of an external potential field, e.g. V(x) where x is the distance from a fixed, unmoving charge, and V is the potential energy of the interaction. However, such models are of limited use because x becomes dependent on the motion of the other particle, if the other particle moves; and in addition this method ignores fluctuations of virtual particles expected from [[Heisenberg's Uncertainty Principle]]. Thus, most commonly field theory employs other methods.
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−  The forces of nature are mediated by particles called field [[bosons]]. Particles exert forces on each other, which appears to be action at a distance, by passing back and forth between them virtual field bosons, which exchange their momenta. As an analogy, consider two people on ice skates on an ice rink, playing football. The first throws a football, and the reaction force of the throw pushes him backward. The second catches the football, and the catch pushes her backward. The two ice skaters are now "repelled" and moving away from each other, with the football as mediator of the force. Thus, virtual bosons mediate attractive and repulsive forces.
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−  Bosons in general are particles of integral spin (0, 1, 2, etc.) "Spin" here refers to quantum mechanical angular momentum of a particle spinning about its axis. In quantum mechanics, all elementary particles can only have angular momenta which are multiples or halfmultiples of [[Planck's constant]]. In particular, the first three forces (but not gravity) are considered to be mediated by bosons of spin 1.
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−  The electromagnetic force is mediated by the photon, a particle of light, which is massless and uncharged. The theory which describes the interactions of charged particles and photons is called Quantum Electrodynamics (QED), attributed primarily to Richard Feynman and Julian Schwinger.
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−  The weak nuclear force is mediated by particles called W and Z bosons. These are very massive particles, as massive as a heavy atomic nucleus. The W particles are charged (W+ and W) and Z is uncharged.
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−  The electromagnetic and weak forces have been mathematically unified into a single formalism, called [[Electroweak Theory]]. The unification means that photons, W and Z bosons are all considered to be different aspects of a more fundamental doublet of field bosons. The electroweak theory can describe electromagnetic and weak phenomena with fewer tuneable free parameters as the single most important goal of physics is to describe all forces with as few free parameters as possible. Electroweak Theory has been extremely successful, and predicted the existence and approximate mass of the Z boson before its observation in experiments.
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−  The strong nuclear force is mediated by particles called gluons. Particles such as quarks are said to have "color charge", which gives them an ability to exchange gluons, in the same way that particles with electrical charge can exchange photons. The theory that describes quarks and gluons is called Quantum Chromodynamics, or QCD. A major complication in QCD is that the gluons themselves have "color charge", unlike, say, photons which have no electrical charge. This makes QCD calculations extremely difficult. On the other hand, it has the advantage of eliminating the "screening problem" identified by Landau. Also, the coupling between quarks and gluons (color charge) is, measured in absolute units, much larger than the electrical charges of charged particles like electrons.
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−  These differences make QCD much harder than QED. However, all the forces of nature involve their own theoretical challenges.
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−  == Perturbative and NonPerturbative Methods ==
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−  A difference between QED/Electroweak and QCD is that the coupling of electron and photons (electrical charge) is much less than 1 when measured in absolute units, while the color charges of quarks in QCD is greater than 1. This means that predictions in QED can be approximated by a set of mathematical methods called "perturbative", while QCD calculations must be usually approached by methods called "nonperturbative."
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−  As a simple case, consider two electrons rushing toward each other. They should be repelled by like charges. In a QED perturbative calculation, this repulsion, while very complex, is modelled by a series of ever more complicated [[Feynman graphs]]. With each step in the calculation, the graphs grow more and more complicated, with more and more internal particles being exchanged. The zeroth. approximation is the outgoing particles are the same as the incoming (no change). The first approximation is that they exchange one virtual photon. The second approximation is that one incoming electron emits a photon, which splits into an electron/position pair, which recombine to form a photon again, which is absorbed by the second incoming electron; and so on. Each new graph is considered an additional "perturbation" of the zeroth. approximation (no change). By the addition of many such graphs, predictions of extraordinary precision can be computed; but this is very laborintensive.
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−  In QCD, perturbative methods are of limited use because the color charge is so strong, that each new graph contributes more than the previous one, so the series may not terminate. Consequently, the interactions of quarks are often modelled by other means, e.g. the use of very powerful computer simulations called "Lattice Gauge QCD" simulations. In these simulations, space and time are approximated as a lattice of points separated by fixed distances. The quarks and gluons do not have fixed positions on the lattice, as they are quantum mechanical particles; rather, the field of each quark and gluon must be continually computed all over the lattice, each point continually changing due to the strong interactions of the particles. These methods require enormous amounts of time on powerful supercomputers, but have produced several important recent successes.
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−  == Experimental Successes ==
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−  Calculations involving field theory are universal throughout highenergy particle physics. Here we will briefly summarize a few more notable successes.
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−  As stated above, one the greatest achievements of QED is the very labor intensive, but extremely precise, calculation of the angular magnetic moment of the electron. In classical quantum mechanics, the magnetic moment of the electron, called g, should be exactly 2.0. The very small deviation from 2 is called the anomalous magnetic moment, and can be experimentally measured to extremely high precision. The very laborintensive theoretical predictions from QED match the experimental measurement to one part in a trillion, a precision unparalleled in all of science. Also, the magnetic moment of the muon can be predicted to one part in a billion.
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−  Also, as stated above, the Electroweak Theory, a unification of QED and weak force, predicted the existence and approximate mass of the Z boson before its observation.
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−  QCD has successfully described jet events at particle colliders. In these events, a particle and antiparticle are smashed together in a collider; the resulting energy then turns into a quark and antiquark pair. The quark and antiquark each then split into vast, complex showers of other highenergy particles, called "jets", which are seen as showers of particles in opposing directions in the particle detector. If the quark and antiquark produce showers immediately, it is called a twojet event. In about 10% of all cases, a quark or antiquark will emit a gluon, which then splits into still more particles, thus displaying a threejet event. QCD successfully models the probability and momentum distributions of the jets.
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−  Nonperturbative methods have recently been very successfully used in QCD. Lattice gauge QCD calculations, requiring vast computer power, have computed the masses of the proton, neutron and mesons, by computing the energy of the interaction E of the quarks and gluons on a lattice, and then employing Einstein's equation m = E/c2. These calculations compute the masses of several important particles entirely a priori, with no tuneable free parameters input to the model, except the strong coupling constant.
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−  == Field Theory and Relativity ==
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−  Although Quantum Field Theory is fully compatible with, and in fact requires [[Special Relativity]], the most naive application of field theory to [[General Relativity]] is known to be unworkable. This is generally considered to be the most difficult outstanding problem in Physics.
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−  The incompatibility is due both to the complications of curved spacetime required by GR, as well as from presumed properties of gravitons. Gravitons are hypothetical bosons which, in the most naive application of Field Theory to GR, would mediate the gravitational force between particles. Unlike the bosons mediating the other forces of nature, which are spin 1, gravitons in the simplest case would be [[bosons]] of spin 2, coupled to the Einstein massenergy tensor. This difference in spin greatly complicates the renormalization of quantum gravity.
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−  Consequently, alternative theories of quantum gravity are an active area of research among physicists, including popular theoretical attempts such as string theory and loop quantum gravity. None of these alternative approaches yet has experimental confirmation.
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 == Sources ==   == Sources == 