Difference between revisions of "Quantum electrodynamics"

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{{About|the physical theory|the Latin phrase|quod erat demonstrandum|other uses|QED (disambiguation)}}
 
{{Quantum field theory}}
 
{{Quantum field theory}}
In particle physics, '''quantum electrodynamics''' ('''QED''') is the [[relativity theory|relativistic]] [[quantum field theory]] of [[electrodynamics]]. In essence, it describes how [[light]] and [[matter]] interact and is the first theory where full agreement between [[quantum mechanics]] and [[special relativity]] is achieved. QED mathematically describes all [[phenomenon|phenomena]] involving [[electric charge|electrically charged]] particles interacting by means of exchange of [[photon]]s and represents the [[quantum mechanics|quantum]] counterpart of [[classical electromagnetism]] giving a complete account of matter and light interaction.  
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In particle physics, '''quantum electrodynamics''' ('''QED''') is the [[relativity theory|relativistic]] [[quantum field theory]] of [[electrodynamics]]. In essence, it describes how [[light]] and [[matter]] interact and is the first theory where full agreement between [[quantum mechanics]] and [[special relativity]] is achieved. QED mathematically describes all [[phenomenon|phenomena]] involving [[electric charge|electrically charged]] particles interacting by means of exchange of [[photon]]s and represents the [[quantum mechanics|quantum]] counterpart of [[classical electromagnetism]] giving a complete account of matter and light interaction.
  
In technical terms, QED can be described as a [[perturbation theory (quantum mechanics)|perturbation theory]] of the electromagnetic [[Vacuum state|quantum vacuum]].  [[Richard Feynman]] called it "the jewel of physics" for its [[precision tests of QED|extremely accurate predictions]] of quantities like the [[anomalous magnetic moment]] of the electron and the [[Lamb shift]] of the [[energy level]]s of [[hydrogen]].<ref name=feynbook1>{{cite book |last=Feynman |first=Richard |authorlink=Richard Feynman |year=1985 |isbn=978-0-691-12575-6 |title=QED: The Strange Theory of Light and Matter |chapter=Chapter 1 |page=6 |publisher=Princeton University Press}}</ref>
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In technical terms, QED can be described as a [[perturbation theory (quantum mechanics)|perturbation theory]] of the electromagnetic [[Vacuum state|quantum vacuum]].  [[Richard Feynman]] called it "the jewel of physics" for its [[precision tests of QED|extremely accurate predictions]] of quantities like the [[anomalous magnetic moment]] of the electron and the [[Lamb shift]] of the [[energy level]]s of [[hydrogen]].<ref name=feynbook>{{cite book |last=Feynman |first=Richard |authorlink=Richard Feynman |year=1985 |isbn=978-0-691-12575-6 |title=QED: The Strange Theory of Light and Matter |publisher=Princeton University Press}}</ref>{{rp|Ch1}}
  
 
==History==
 
==History==
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  | doi=10.1103/PhysRev.75.1736
 
  | doi=10.1103/PhysRev.75.1736
 
|bibcode = 1949PhRv...75.1736D
 
|bibcode = 1949PhRv...75.1736D
  | issue=11 }}</ref> it was finally possible to get fully [[Lorentz covariance|covariant]] formulations that were finite at any order in a perturbation series of quantum electrodynamics. [[Sin-Itiro Tomonaga]], [[Julian Schwinger]] and [[Richard Feynman]] were jointly awarded with a [[Nobel prize in physics]] in 1965 for their work in this area.<ref name=nobel65>{{cite web | title = The Nobel Prize in Physics 1965 | publisher = Nobel Foundation | url = http://nobelprize.org/nobel_prizes/physics/laureates/1965/index.html|accessdate=2008-10-09}}</ref> Their contributions, and those of [[Freeman Dyson]], were about [[Lorentz covariance|covariant]] and [[gauge invariant]] formulations of quantum electrodynamics that allow computations of observables at any order of [[Perturbation theory (quantum mechanics)|perturbation theory]]. Feynman's mathematical technique, based on his [[Feynman diagram|diagrams]], initially seemed very different from the field-theoretic, [[Operator (physics)|operator]]-based approach of Schwinger and Tomonaga, but [[Freeman Dyson]] later showed that the two approaches were equivalent.<ref name="dyson1"/> [[Renormalization]], the need to attach a physical meaning at certain divergences appearing in the theory through [[integral]]s, has subsequently become one of the fundamental aspects of [[quantum field theory]] and has come to be seen as a criterion for a theory's general acceptability. Even though renormalization works very well in practice, Feynman was never entirely comfortable with its mathematical validity, even referring to renormalization as a "shell game" and "hocus pocus".<ref name=feynbook2>{{cite book |last=Feynman |first=Richard |authorlink=Richard Feynman |year=1985 |isbn=978-0-691-12575-6 |title=QED: The Strange Theory of Light and Matter |page=128 |publisher=Princeton University Press}}</ref>
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  | issue=11 }}</ref> it was finally possible to get fully [[Lorentz covariance|covariant]] formulations that were finite at any order in a perturbation series of quantum electrodynamics. [[Sin-Itiro Tomonaga]], [[Julian Schwinger]] and [[Richard Feynman]] were jointly awarded with a [[Nobel prize in physics]] in 1965 for their work in this area.<ref name=nobel65>{{cite web | title = The Nobel Prize in Physics 1965 | publisher = Nobel Foundation | url = http://nobelprize.org/nobel_prizes/physics/laureates/1965/index.html|accessdate=2008-10-09}}</ref> Their contributions, and those of [[Freeman Dyson]], were about [[Lorentz covariance|covariant]] and [[gauge invariant]] formulations of quantum electrodynamics that allow computations of observables at any order of [[Perturbation theory (quantum mechanics)|perturbation theory]]. Feynman's mathematical technique, based on his [[Feynman diagram|diagrams]], initially seemed very different from the field-theoretic, [[Operator (physics)|operator]]-based approach of Schwinger and Tomonaga, but [[Freeman Dyson]] later showed that the two approaches were equivalent.<ref name="dyson1"/> [[Renormalization]], the need to attach a physical meaning at certain divergences appearing in the theory through [[integral]]s, has subsequently become one of the fundamental aspects of [[quantum field theory]] and has come to be seen as a criterion for a theory's general acceptability. Even though renormalization works very well in practice, Feynman was never entirely comfortable with its mathematical validity, even referring to renormalization as a "shell game" and "hocus pocus".<ref name=feynbook/>{{rp|128}}
  
QED has served as the model and template for all subsequent quantum field theories. One such subsequent theory is [[quantum chromodynamics]], which began in the early 1960s and attained its present form in the 1975 work by [[H. David Politzer]], [[Sidney Coleman]], [[David Gross]] and [[Frank Wilczek]]. Building on the pioneering work of [[Schwinger]], [[Gerald Guralnik]], [[C. R. Hagen|Dick Hagen]], and [[Tom W. B. Kibble|Tom Kibble]],<ref>
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QED has served as the model and template for all subsequent quantum field theories. One such subsequent theory is [[quantum chromodynamics]], which began in the early 1960s and attained its present form in the 1975 work by [[H. David Politzer]], [[Sidney Coleman]], [[David Gross]] and [[Frank Wilczek]]. Building on the pioneering work of [[Julian Schwinger|Schwinger]], [[Gerald Guralnik]], [[C. R. Hagen|Dick Hagen]], and [[Tom W. B. Kibble|Tom Kibble]],<ref>
 
{{cite journal
 
{{cite journal
 
  | last1=Guralnik | first1=G. S.
 
  | last1=Guralnik | first1=G. S.
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==Feynman's view of quantum electrodynamics==
 
==Feynman's view of quantum electrodynamics==
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===Introduction===
 
===Introduction===
  
Near the end of his life, [[Richard P. Feynman]] gave a series of lectures on QED intended for the lay public. These lectures were transcribed and published as Feynman (1985), [[QED (book)|''QED: The strange theory of light and matter'']],<ref name="feynbook1"/><ref name="feynbook2"/> a classic non-mathematical exposition of QED from the point of view articulated below.
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Near the end of his life, [[Richard P. Feynman]] gave a series of lectures on QED intended for the lay public. These lectures were transcribed and published as Feynman (1985), [[QED (book)|''QED: The strange theory of light and matter'']],<ref name=feynbook/> a classic non-mathematical exposition of QED from the point of view articulated below.
  
The key components of Feynman's presentation of QED are three basic actions.
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The key components of Feynman's presentation of QED are three basic actions.<ref name=feynbook/>{{rp|85}}
  
 
* A [[photon]] goes from one place and time to another place and time.
 
* A [[photon]] goes from one place and time to another place and time.
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* An electron emits or absorbs a photon at a certain place and time.
 
* An electron emits or absorbs a photon at a certain place and time.
  
[[File:Qed elementary rules.jpg|thumb|right|300px|[[Feynman diagrams|Feynman diagram]] elements]]These actions are represented in a form of visual shorthand by the three basic elements of [[Feynman diagrams]]: a wavy line for the photon, a straight line for the electron and a junction of two straight lines and a wavy one for a vertex representing emission or absorption of a photon by an electron. These can all be seen in the adjacent diagram.
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[[File:Feynman Diagram Components.svg|thumb|right|300px|[[Feynman diagrams|Feynman diagram]] elements]]These actions are represented in a form of visual shorthand by the three basic elements of [[Feynman diagrams]]: a wavy line for the photon, a straight line for the electron and a junction of two straight lines and a wavy one for a vertex representing emission or absorption of a photon by an electron. These can all be seen in the adjacent diagram.
  
 
It is important not to over-interpret these diagrams. Nothing is implied about ''how'' a particle gets from one point to another. The diagrams do ''not'' imply that the particles are moving in straight or curved lines. They do ''not'' imply that the particles are moving with fixed speeds. The fact that the photon is often represented, by convention, by a wavy line and not a straight one does ''not'' imply that it is thought that it is more wavelike than is an electron. The images are just symbols to represent the actions above: photons and electrons do, somehow, move from point to point and electrons, somehow, emit and absorb photons. We do not know how these things happen, but the theory tells us about the probabilities of these things happening.
 
It is important not to over-interpret these diagrams. Nothing is implied about ''how'' a particle gets from one point to another. The diagrams do ''not'' imply that the particles are moving in straight or curved lines. They do ''not'' imply that the particles are moving with fixed speeds. The fact that the photon is often represented, by convention, by a wavy line and not a straight one does ''not'' imply that it is thought that it is more wavelike than is an electron. The images are just symbols to represent the actions above: photons and electrons do, somehow, move from point to point and electrons, somehow, emit and absorb photons. We do not know how these things happen, but the theory tells us about the probabilities of these things happening.
  
As well as the visual shorthand for the actions Feynman introduces another kind of shorthand for the numerical quantities which tell us about the probabilities. If a photon moves from one place and time—in shorthand, A—to another place and time—in shorthand, B—the associated quantity is written in Feynman's shorthand as P(A to B). The similar quantity for an electron moving from C to D is written E(C to D).  The quantity which tells us about the probability for the emission or absorption of a photon he calls 'j'. This is related to, but not the same as, the measured [[Elementary charge|electron charge]] 'e'.
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As well as the visual shorthand for the actions Feynman introduces another kind of shorthand for the numerical quantities called [[Quantum electrodynamics#Probability amplitudes|probability amplitudes]]. The probability is the square of the total probability amplitude. If a photon moves from one place and time—in shorthand, A—to another place and time—in shorthand, B—the associated quantity is written in Feynman's shorthand as P(A to B). The similar quantity for an electron moving from C to D is written E(C to D).  The quantity which tells us about the probability amplitude for the emission or absorption of a photon he calls 'j'. This is related to, but not the same as, the measured [[Elementary charge|electron charge]] 'e'.<ref name=feynbook/>{{rp|91}}
  
QED is based on the assumption that complex interactions of many electrons and photons can be represented by fitting together a suitable collection of the above three building blocks, and then using the probability quantities to calculate the probability of any such complex interaction.  It turns out that the basic idea of QED can be communicated while making the assumption that the quantities mentioned above are just our everyday [[Probability|probabilities]]. (A simplification of Feynman's book.) Later on this will be corrected to include specifically quantum mathematics, following Feynman.
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QED is based on the assumption that complex interactions of many electrons and photons can be represented by fitting together a suitable collection of the above three building blocks, and then using the probability amplitudes to calculate the probability of any such complex interaction.  It turns out that the basic idea of QED can be communicated while making the assumption that the square of the total of the probability amplitudes mentioned above (P(A to B), E(A to B) and 'j') is just our everyday [[probability]]. (A simplification of Feynman's book.) Later on this will be corrected to include specifically quantum mathematics, following Feynman.
  
The basic rules of probabilities that will be used are that a) if an event can happen in a variety of different ways then its probability is the '''sum''' of the probabilities of the possible ways and b) if a process involves a number of independent subprocesses then its probability is the '''product''' of the component probabilities.
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The basic rules of probability amplitudes that will be used are that a) if an event can happen in a variety of different ways then its probability amplitude is the '''sum''' of the probability amplitudes of the possible ways and b) if a process involves a number of independent sub-processes then its probability amplitude is the '''product''' of the component probability amplitudes.<ref name=feynbook/>{{rp|93}}
  
 
===Basic constructions===
 
===Basic constructions===
  
Suppose we start with one electron at a certain place and time (this place and time being given the arbitrary label A) and a photon at another place and time (given the label B). A typical question from a physical standpoint is: 'What is the probability of finding an electron at C (another place and a later time) and a photon at D (yet another place and time)?'. The simplest process to achieve this end is for the electron to move from A to C (an elementary action) and that the photon moves from B to D (another elementary action). From a knowledge of the probabilities  of each of these subprocesses – E(A to C) and P(B to D) – then we would expect to calculate the probability of both happening by multiplying them, using rule b) above. This gives a simple estimated answer to our question. [[File:Compton Scattering.svg|thumb|left|200px|[[Compton scattering]]]] But there are other ways in which the end result could come about. The electron might move to a place and time E where it absorbs the photon; then move on before emitting another photon at F; then move on to C where it is detected,  while the new photon moves on to D. The probability of this complex process can again be calculated by knowing the probabilities of each of the individual actions: three electron actions, two photon actions and two vertexes – one emission and one absorption. We would expect to find the total probability by multiplying the probabilities of each of the actions, for any chosen positions of E and F. We then, using rule a) above, have to add up all these probabilities for all the alternatives for E and F. (This is not elementary in practice, and involves [[Integral|integration]].) But there is another possibility: that is that the electron first moves to G where it emits a photon which goes on to D, while the electron moves on to H, where it absorbs the first photon, before moving on to C. Again we can calculate the probability of these possibilities (for all points G and H). We then have a better estimation for the total probability by adding the probabilities of these two possibilities to our original simple estimate. Incidentally the name given to this process of a photon interacting with an electron in this way is [[Compton Scattering]].
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Suppose we start with one electron at a certain place and time (this place and time being given the arbitrary label A) and a photon at another place and time (given the label B). A typical question from a physical standpoint is: 'What is the probability of finding an electron at C (another place and a later time) and a photon at D (yet another place and time)?'. The simplest process to achieve this end is for the electron to move from A to C (an elementary action) and for the photon to move from B to D (another elementary action). From a knowledge of the probability amplitudes of each of these sub-processes – E(A to C) and P(B to D) – then we would expect to calculate the probability amplitude of both happening together by multiplying them, using rule b) above. This gives a simple estimated overall probability amplitude, which is squared to give an estimated probability. [[File:Compton Scattering.svg|thumb|left|200px|[[Compton scattering]]]] But there are other ways in which the end result could come about. The electron might move to a place and time E where it absorbs the photon; then move on before emitting another photon at F; then move on to C where it is detected,  while the new photon moves on to D. The probability of this complex process can again be calculated by knowing the probability amplitudes of each of the individual actions: three electron actions, two photon actions and two vertexes – one emission and one absorption. We would expect to find the total probability amplitude by multiplying the probability amplitudes of each of the actions, for any chosen positions of E and F. We then, using rule a) above, have to add up all these probability amplitudes for all the alternatives for E and F. (This is not elementary in practice, and involves [[Integral|integration]].) But there is another possibility, which is that the electron first moves to G where it emits a photon which goes on to D, while the electron moves on to H, where it absorbs the first photon, before moving on to C. Again we can calculate the probability amplitude of these possibilities (for all points G and H). We then have a better estimation for the total probability amplitude by adding the probability amplitudes of these two possibilities to our original simple estimate. Incidentally the name given to this process of a photon interacting with an electron in this way is [[Compton Scattering]].
  
There are an ''infinite number'' of other intermediate processes in which more and more photons are absorbed and/or emitted. For each of these possibilities there is a Feynman diagram describing it. This implies a complex computation for the resulting probabilities, but provided it is the case that the more complicated the diagram the less it contributes to the result, it is only a matter of time and effort to find as accurate an answer as one wants to the original question. This is the basic approach of QED. To calculate the probability of ''any'' interactive process between electrons and photons it is a matter of first noting, with Feynman diagrams, all the possible ways in which the process can be constructed from the three basic elements. Each diagram involves some calculation involving definite rules to find the associated probability.
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There are an ''infinite number'' of other intermediate processes in which more and more photons are absorbed and/or emitted. For each of these possibilities there is a Feynman diagram describing it. This implies a complex computation for the resulting probability amplitudes, but provided it is the case that the more complicated the diagram the less it contributes to the result, it is only a matter of time and effort to find as accurate an answer as one wants to the original question. This is the basic approach of QED. To calculate the probability of ''any'' interactive process between electrons and photons it is a matter of first noting, with Feynman diagrams, all the possible ways in which the process can be constructed from the three basic elements. Each diagram involves some calculation involving definite rules to find the associated probability amplitude.
  
That basic scaffolding remains when one moves to a quantum description but some conceptual changes are needed. One is that whereas we might expect in our everyday life that there would be some constraints on the points to which a particle can move, that is ''not'' true in full quantum electrodynamics. There is a possibility of an electron at A, or a photon at B, moving as a basic action to ''any other place and time in the universe''. That includes places that could only be reached at speeds greater than that of light and also ''earlier times''. (An electron moving backwards in time can be viewed as a [[positron]] moving forward in time.)
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That basic scaffolding remains when one moves to a quantum description but some conceptual changes are needed. One is that whereas we might expect in our everyday life that there would be some constraints on the points to which a particle can move, that is ''not'' true in full quantum electrodynamics. There is a possibility of an electron at A, or a photon at B, moving as a basic action to ''any other place and time in the universe''. That includes places that could only be reached at speeds greater than that of light and also ''earlier times''. (An electron moving backwards in time can be viewed as a [[positron]] moving forward in time.)<ref name=feynbook/>{{rp|89, 98–99}}
  
 
===Probability amplitudes===
 
===Probability amplitudes===
  
[[File:Feynmans QED probability amplitudes.gif|frame|right|Feynman replaces complex numbers with spinning arrows, which start at emission and ends at detection of a particle. The sum of all resulting arrows represents the total probability of the event. In this diagram, light emitted by the source '''''S''''' bounces off a few segments of the mirror (in blue) before reaching the detector at '''''P'''''. The sum of all paths must be taken into account. The graph below depicts the total time spent to traverse each of the paths above.]]
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[[File:Feynmans QED probability amplitudes.gif|frame|right|Feynman replaces complex numbers with spinning arrows, which start at emission and end at detection of a particle. The sum of all resulting arrows represents the total probability of the event. In this diagram, light emitted by the source '''''S''''' bounces off a few segments of the mirror (in blue) before reaching the detector at '''''P'''''. The sum of all paths must be taken into account. The graph below depicts the total time spent to traverse each of the paths above.]]
  
[[Quantum mechanics]] introduces an important change on the way probabilities are computed. It has been found that the quantities which we have to use to represent the probabilities are not the usual real numbers we use for probabilities in our everyday world, but [[complex number]]s which are called [[probability amplitude]]s.
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[[Quantum mechanics]] introduces an important change in the way probabilities are computed. Probabilities are still represented by the usual real numbers we use for probabilities in our everyday world, but probabilities are computed as the square of probability amplitudes. [[Probability amplitude]]s are [[complex number]]s.
  
 
Feynman avoids exposing the reader to the mathematics of complex numbers by using a simple but accurate representation of them as arrows on a piece of paper or screen. (These must not be confused with the arrows of Feynman diagrams which are actually simplified representations in two dimensions of a relationship between points in three dimensions of space and one of time.) The amplitude arrows are fundamental to the description of the world given by quantum theory. No satisfactory reason has been given for ''why'' they are needed. But pragmatically we have to accept that they are an essential part of our description of all quantum phenomena. They are related to our everyday ideas of probability by the simple rule that the probability of an event is the '''square''' of the length of the corresponding amplitude arrow. So, for a given process, if two probability amplitudes, '''v''' and '''w''', are involved, the probability of the process will be given either by
 
Feynman avoids exposing the reader to the mathematics of complex numbers by using a simple but accurate representation of them as arrows on a piece of paper or screen. (These must not be confused with the arrows of Feynman diagrams which are actually simplified representations in two dimensions of a relationship between points in three dimensions of space and one of time.) The amplitude arrows are fundamental to the description of the world given by quantum theory. No satisfactory reason has been given for ''why'' they are needed. But pragmatically we have to accept that they are an essential part of our description of all quantum phenomena. They are related to our everyday ideas of probability by the simple rule that the probability of an event is the '''square''' of the length of the corresponding amplitude arrow. So, for a given process, if two probability amplitudes, '''v''' and '''w''', are involved, the probability of the process will be given either by
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Addition and multiplication are familiar operations in the theory of complex numbers and are given in the figures. The sum is found as follows. Let the start of the second arrow be at the end of the first. The sum is then a third arrow that goes directly from the start of the first to the end of the second.  The product of two arrows is an arrow whose length is the product of the two lengths. The direction of the product is found by adding the angles that each of the two have been turned through relative to a reference direction: that gives the angle that the product is turned relative to the reference direction.
 
Addition and multiplication are familiar operations in the theory of complex numbers and are given in the figures. The sum is found as follows. Let the start of the second arrow be at the end of the first. The sum is then a third arrow that goes directly from the start of the first to the end of the second.  The product of two arrows is an arrow whose length is the product of the two lengths. The direction of the product is found by adding the angles that each of the two have been turned through relative to a reference direction: that gives the angle that the product is turned relative to the reference direction.
  
That change, from probabilities to probability amplitudes, complicates the mathematics without changing the basic approach. But that change is still not quite enough because it fails to take into account the fact that both photons and electrons can be polarized, which is to say that their orientations in space and time have to be taken into account. Therefore P(A to B) actually consists of 16 complex numbers, or probability amplitude arrows. There are also some minor changes to do with the quantity "j", which may have to be rotated by a multiple of 90° for some polarizations, which is only of interest for the detailed bookkeeping.
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That change, from probabilities to probability amplitudes, complicates the mathematics without changing the basic approach. But that change is still not quite enough because it fails to take into account the fact that both photons and electrons can be polarized, which is to say that their orientations in space and time have to be taken into account. Therefore P(A to B) actually consists of 16 complex numbers, or probability amplitude arrows.<ref name=feynbook/>{{rp|120–121}} There are also some minor changes to do with the quantity "j", which may have to be rotated by a multiple of 90° for some polarizations, which is only of interest for the detailed bookkeeping.
  
Associated with the fact that the electron can be polarized is another small necessary detail which is connected with the fact that an electron is a [[fermion]] and obeys [[Fermi–Dirac statistics]]. The basic rule is that if we have the probability amplitude for a given complex process involving more than one electron, then when we include (as we always must) the complementary Feynman diagram in which we just exchange two electron events, the resulting amplitude is the reverse – the negative – of the first. The simplest case would be two electrons starting at A and B ending at C and D. The amplitude would be calculated as the "difference", {{nowrap|E(A to D) × E(B to C) − E(A to C) × E(B to D)}}, where we would expect, from our everyday idea of probabilities, that it would be a sum.
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Associated with the fact that the electron can be polarized is another small necessary detail which is connected with the fact that an electron is a [[fermion]] and obeys [[Fermi–Dirac statistics]]. The basic rule is that if we have the probability amplitude for a given complex process involving more than one electron, then when we include (as we always must) the complementary Feynman diagram in which we just exchange two electron events, the resulting amplitude is the reverse – the negative – of the first. The simplest case would be two electrons starting at A and B ending at C and D. The amplitude would be calculated as the "difference", {{nowrap|E(A to D) × E(B to C) − E(A to C) × E(B to D)}}, where we would expect, from our everyday idea of probabilities, that it would be a sum.<ref name=feynbook/>{{rp|112–113}}
  
 
===Propagators===
 
===Propagators===
  
Finally, one has to compute P(A to B) and E (C to D) corresponding to the probability amplitudes for the photon and the electron respectively. These are essentially the solutions of the [[Dirac Equation]] which describes the behavior of the electron's probability amplitude and the [[Klein–Gordon equation]] which describes the behavior of the photon's probability amplitude. These are called [[Propagator|Feynman propagators]]. The translation to a notation commonly used in the standard literature is as follows:
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Finally, one has to compute P (A to B) and E (C to D) corresponding to the probability amplitudes for the photon and the electron respectively. These are essentially the solutions of the [[Dirac Equation]] which describes the behavior of the electron's probability amplitude and the [[Klein–Gordon equation]] which describes the behavior of the photon's probability amplitude. These are called [[Propagator|Feynman propagators]]. The translation to a notation commonly used in the standard literature is as follows:
  
 
:<math>P(\mbox{A to B}) \rightarrow D_F(x_B-x_A),\quad  E(\mbox{C to D}) \rightarrow S_F(x_D-x_C) </math>
 
:<math>P(\mbox{A to B}) \rightarrow D_F(x_B-x_A),\quad  E(\mbox{C to D}) \rightarrow S_F(x_D-x_C) </math>
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===Mass renormalization===
 
===Mass renormalization===
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{{main|Self-energy}}
  
 
[[File:Electron self energy loop.svg|thumb|right|200px|[[Electron self-energy]] loop]]
 
[[File:Electron self energy loop.svg|thumb|right|200px|[[Electron self-energy]] loop]]
A problem arose historically which held up progress for twenty years: although we start with the assumption of three basic "simple" actions, the rules of the game say that if we want to calculate the probability amplitude for an electron to get from A to B we must take into account '''all''' the possible ways: all possible Feynman diagrams with those end points.  Thus there will be a way in which the electron travels to C, emits a photon there and then absorbs it again at D before moving on to B. Or it could do this kind of thing twice, or more. In short we have a [[fractal]]-like situation in which if we look closely at a line it breaks up into a collection of "simple" lines, each of which, if looked at closely, are in turn composed of "simple" lines, and so on ''ad infinitum''. This is a very difficult situation to handle. If adding that detail only altered things slightly then it would not have been too bad, but disaster struck when it was found that the simple correction mentioned above led to ''infinite'' probability amplitudes. In time this problem was "fixed" by the technique of [[renormalization]] (see below and the article on [[Self-energy|mass renormalization]]). However, Feynman himself remained unhappy about it, calling it a "dippy process".<ref name="feynbook2"/>
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A problem arose historically which held up progress for twenty years: although we start with the assumption of three basic "simple" actions, the rules of the game say that if we want to calculate the probability amplitude for an electron to get from A to B we must take into account '''all''' the possible ways: all possible Feynman diagrams with those end points.  Thus there will be a way in which the electron travels to C, emits a photon there and then absorbs it again at D before moving on to B. Or it could do this kind of thing twice, or more. In short we have a [[fractal]]-like situation in which if we look closely at a line it breaks up into a collection of "simple" lines, each of which, if looked at closely, are in turn composed of "simple" lines, and so on ''ad infinitum''. This is a very difficult situation to handle. If adding that detail only altered things slightly then it would not have been too bad, but disaster struck when it was found that the simple correction mentioned above led to ''infinite'' probability amplitudes. In time this problem was "fixed" by the technique of [[renormalization]]. However, Feynman himself remained unhappy about it, calling it a "dippy process".<ref name=feynbook/>{{rp|128}}
  
 
===Conclusions===
 
===Conclusions===
  
Within the above framework physicists were then able to calculate to a high degree of accuracy some of the properties of electrons, such as the [[anomalous magnetic dipole moment]]. However, as Feynman points out, it fails totally to explain why particles such as the electron have the masses they do. "There is no theory that adequately explains these numbers. We use the numbers in all our theories, but we don't understand them – what they are, or where they come from. I believe that from a fundamental point of view, this is a very interesting and serious problem."<ref name=feynbook3>{{cite book |last=Feynman |first=Richard |authorlink=Richard Feynman |year=1985 |isbn=978-0-691-12575-6 |title=QED: The Strange Theory of Light and Matter |page=152 |publisher=Princeton University Press}}</ref>
+
Within the above framework physicists were then able to calculate to a high degree of accuracy some of the properties of electrons, such as the [[anomalous magnetic dipole moment]]. However, as Feynman points out, it fails totally to explain why particles such as the electron have the masses they do. "There is no theory that adequately explains these numbers. We use the numbers in all our theories, but we don't understand them – what they are, or where they come from. I believe that from a fundamental point of view, this is a very interesting and serious problem."<ref name=feynbook/>{{rp|152}}
 
 
==Quantum Field Theory==
 
{{Main|Quantum field theory}}
 
 
 
Theory that brings [[quantum mechanics]] and [[special relativity]] together to account for subatomic theory.
 
  
 
==Mathematics==
 
==Mathematics==
  
 
Mathematically, QED is an [[abelian group|abelian]] [[gauge theory]] with the symmetry group [[U(1)]]. The [[gauge field]], which mediates the interaction between the charged [[spin (physics)|spin-1/2]] [[field (physics)|field]]s, is the [[electromagnetic field]].
 
Mathematically, QED is an [[abelian group|abelian]] [[gauge theory]] with the symmetry group [[U(1)]]. The [[gauge field]], which mediates the interaction between the charged [[spin (physics)|spin-1/2]] [[field (physics)|field]]s, is the [[electromagnetic field]].
The QED [[Lagrangian]] for a spin-1/2 field interacting with the electromagnetic field is given by the real part of
+
The QED [[Lagrangian]] for a spin-1/2 field interacting with the electromagnetic field is given by the real part of<ref name=Peskin>{{cite book | last1 =Peskin | first1 =Michael | last2 =Schroeder | first2 =Daniel | title =An introduction to quantum field theory | publisher =Westview Press | edition =Reprint | date =1995 | isbn =978-0201503975}}</ref>{{rp|78}}
 
 
 
{{Equation box 1
 
{{Equation box 1
 
|indent =:
 
|indent =:
Line 306: Line 303:
 
:<math> \gamma^\mu </math> are [[Dirac matrices]];
 
:<math> \gamma^\mu </math> are [[Dirac matrices]];
 
:<math>\psi</math> a [[bispinor]] [[field (physics)|field]] of [[spin-1/2]] particles (e.g. [[electron]]–[[positron]] field);
 
:<math>\psi</math> a [[bispinor]] [[field (physics)|field]] of [[spin-1/2]] particles (e.g. [[electron]]–[[positron]] field);
:<math>\bar\psi\equiv\psi^\dagger\gamma^0</math>, called "psi-bar", is sometimes referred to as [[Dirac adjoint]];
+
:<math>\bar\psi\equiv\psi^\dagger\gamma^0</math>, called "psi-bar", is sometimes referred to as the [[Dirac adjoint]];
 
:<math>D_\mu \equiv \partial_\mu+ieA_\mu+ieB_\mu \,\!</math> is the [[gauge covariant derivative]];
 
:<math>D_\mu \equiv \partial_\mu+ieA_\mu+ieB_\mu \,\!</math> is the [[gauge covariant derivative]];
 
:''e'' is the [[Fine-structure constant|coupling constant]], equal to the [[electric charge]] of the bispinor field;
 
:''e'' is the [[Fine-structure constant|coupling constant]], equal to the [[electric charge]] of the bispinor field;
Line 366: Line 363:
 
|background colour=#F5FFFA}}
 
|background colour=#F5FFFA}}
  
Now, if we impose the [[Lorenz gauge condition]], that the divergence of the four potential vanishes  
+
Now, if we impose the [[Lorenz gauge condition]], that the divergence of the four potential vanishes
  
 
:<math>\partial_{\mu} A^\mu = 0 </math>
 
:<math>\partial_{\mu} A^\mu = 0 </math>
Line 374: Line 371:
 
:<math>\Box A^{\mu}=e\bar{\psi} \gamma^{\mu} \psi\,,</math>
 
:<math>\Box A^{\mu}=e\bar{\psi} \gamma^{\mu} \psi\,,</math>
  
which is a [[wave equation]] for the four potential, the QED version of the classical Maxwell equations in the Lorenz gauge.
+
which is a [[wave equation]] for the four potential, the QED version of the classical Maxwell equations in the Lorenz gauge. (In the above equation, the square represents the [[D'Alembert operator]].)
  
 
===Interaction picture===
 
===Interaction picture===
  
This theory can be straightforwardly quantized by treating bosonic and fermionic sectors as free. This permits us to build a set of asymptotic states which can be used to start a computation of the probability amplitudes for different processes. In order to do so, we have to compute an [[Hamiltonian (quantum mechanics)|evolution operator]] that, for a given initial state <math>|i\rangle</math>, will give a final state <math>\langle f|</math> in such a way to have
+
This theory can be straightforwardly quantized by treating bosonic and fermionic sectors as free. This permits us to build a set of asymptotic states which can be used to start a computation of the probability amplitudes for different processes. In order to do so, we have to compute an [[Hamiltonian (quantum mechanics)|evolution operator]] that, for a given initial state <math>|i\rangle</math>, will give a final state <math>\langle f|</math> in such a way to have<ref name=Peskin/>{{rp|5}}
  
 
:<math>M_{fi}=\langle f|U|i\rangle.</math>
 
:<math>M_{fi}=\langle f|U|i\rangle.</math>
  
This technique is also known as the [[S-Matrix]]. The evolution operator is obtained in the [[interaction picture]] where time evolution is given by the interaction Hamiltonian, which is the integral over space of the second term in the Lagrangian density given above:
+
This technique is also known as the [[S-Matrix]]. The evolution operator is obtained in the [[interaction picture]] where time evolution is given by the interaction Hamiltonian, which is the integral over space of the second term in the Lagrangian density given above:<ref name=Peskin/>{{rp|123}}
  
 
:<math>V=e\int d^3x\bar\psi\gamma^\mu\psi A_\mu</math>
 
:<math>V=e\int d^3x\bar\psi\gamma^\mu\psi A_\mu</math>
  
and so, one has
+
and so, one has<ref name=Peskin/>{{rp|86}}
  
 
:<math>U=T\exp\left[-\frac{i}{\hbar}\int_{t_0}^tdt'V(t')\right]</math>
 
:<math>U=T\exp\left[-\frac{i}{\hbar}\int_{t_0}^tdt'V(t')\right]</math>
Line 395: Line 392:
 
Despite the conceptual clarity of this Feynman approach to QED, almost no early textbooks follow him in their presentation. When performing calculations it is much easier to work with the [[Fourier transform]]s of the [[propagator]]s. Quantum physics considers particle's [[Momentum|momenta]] rather than their positions, and it is convenient to think of particles as being created or annihilated when they interact. Feynman diagrams then ''look'' the same, but the lines have different interpretations. The electron line represents an electron with a given energy and momentum, with a similar interpretation of the photon line. A vertex diagram represents the annihilation of one electron and the creation of another together with the absorption or creation of a photon, each having specified energies and momenta.
 
Despite the conceptual clarity of this Feynman approach to QED, almost no early textbooks follow him in their presentation. When performing calculations it is much easier to work with the [[Fourier transform]]s of the [[propagator]]s. Quantum physics considers particle's [[Momentum|momenta]] rather than their positions, and it is convenient to think of particles as being created or annihilated when they interact. Feynman diagrams then ''look'' the same, but the lines have different interpretations. The electron line represents an electron with a given energy and momentum, with a similar interpretation of the photon line. A vertex diagram represents the annihilation of one electron and the creation of another together with the absorption or creation of a photon, each having specified energies and momenta.
  
Using [[Wick theorem]] on the terms of the Dyson series, all the terms of the [[S-matrix]] for quantum electrodynamics can be computed through the technique of [[Feynman diagrams]]. In this case rules for drawing are the following
+
Using [[Wick theorem]] on the terms of the Dyson series, all the terms of the [[S-matrix]] for quantum electrodynamics can be computed through the technique of [[Feynman diagrams]]. In this case rules for drawing are the following<ref name=Peskin/>{{rp|801–802}}
  
 
<center>[[Image:qed rules.jpg|488px]]</center>
 
<center>[[Image:qed rules.jpg|488px]]</center>
Line 401: Line 398:
 
<center>[[Image:qed2e.jpg|488px]]</center>
 
<center>[[Image:qed2e.jpg|488px]]</center>
  
To these rules we must add a further one for closed loops that implies an integration on momenta <math>\int d^4p/(2\pi)^4</math>, since these internal ("virtual") particles are not constrained to any specific energy–momentum – even that usually required by special relativity (see [[Propagator#Propagators_in_Feynman_diagrams | this article]] for details).
+
To these rules we must add a further one for closed loops that implies an integration on momenta <math>\int d^4p/(2\pi)^4</math>, since these internal ("virtual") particles are not constrained to any specific energy–momentum – even that usually required by special relativity (see [[Propagator#Propagators in Feynman diagrams|this article]] for details).
From them, computations of [[probability amplitude]]s are straightforwardly given. An example is [[Compton scattering]], with an [[electron]] and a [[photon]] undergoing [[elastic scattering]]. Feynman diagrams are in this case
+
From them, computations of [[probability amplitude]]s are straightforwardly given. An example is [[Compton scattering]], with an [[electron]] and a [[photon]] undergoing [[elastic scattering]]. Feynman diagrams are in this case<ref name=Peskin/>{{rp|158–159}}
  
 
<center>[[Image:compton qed.jpg|300px]]</center>
 
<center>[[Image:compton qed.jpg|300px]]</center>
Line 415: Line 412:
 
==Renormalizability==
 
==Renormalizability==
  
Higher order terms can be straightforwardly computed for the evolution operator but these terms display diagrams containing the following simpler ones
+
Higher order terms can be straightforwardly computed for the evolution operator but these terms display diagrams containing the following simpler ones<ref name=Peskin/>{{rp|ch 10}}
  
<center><gallery>
+
<gallery class="center">
 
Image:vacuum_polarization.svg | One-loop contribution to the [[vacuum polarization]] function <math>\Pi\,</math>
 
Image:vacuum_polarization.svg | One-loop contribution to the [[vacuum polarization]] function <math>\Pi\,</math>
 
Image:electron_self_energy.svg | One-loop contribution to the electron [[self-energy]] function <math>\Sigma \,</math>
 
Image:electron_self_energy.svg | One-loop contribution to the electron [[self-energy]] function <math>\Sigma \,</math>
 
Image:vertex_correction.svg | One-loop contribution to the [[vertex function]] <math>\Gamma\,</math>
 
Image:vertex_correction.svg | One-loop contribution to the [[vertex function]] <math>\Gamma\,</math>
</gallery></center>
+
</gallery>
  
 
that, being closed loops, imply the presence of diverging [[integral]]s having no mathematical meaning. To overcome this difficulty, a technique called [[renormalization]] has been devised, producing finite results in very close agreement with experiments. It is important to note that a criterion for theory being meaningful after renormalization is that the number of diverging diagrams is finite. In this case the theory is said to be '''renormalizable'''. The reason for this is that to get observables renormalized one needs a finite number of constants to maintain the predictive value of the theory untouched. This is exactly the case of quantum electrodynamics displaying just three diverging diagrams. This procedure gives observables in very close agreement with experiment as seen e.g. for electron [[gyromagnetic ratio]].
 
that, being closed loops, imply the presence of diverging [[integral]]s having no mathematical meaning. To overcome this difficulty, a technique called [[renormalization]] has been devised, producing finite results in very close agreement with experiments. It is important to note that a criterion for theory being meaningful after renormalization is that the number of diverging diagrams is finite. In this case the theory is said to be '''renormalizable'''. The reason for this is that to get observables renormalized one needs a finite number of constants to maintain the predictive value of the theory untouched. This is exactly the case of quantum electrodynamics displaying just three diverging diagrams. This procedure gives observables in very close agreement with experiment as seen e.g. for electron [[gyromagnetic ratio]].
Line 439: Line 436:
 
}}</ref> The basic argument goes as follows: if the [[fine structure constant|coupling constant]] were negative, this would be equivalent to the [[Coulomb force constant]] being negative. This would "reverse" the electromagnetic interaction so that ''like'' charges would ''attract'' and ''unlike'' charges would ''repel''. This would render the vacuum unstable against decay into a cluster of electrons on one side of the universe and a cluster of positrons on the other side of the universe. Because the theory is 'sick' for any negative value of the coupling constant, the series do not converge, but are an [[asymptotic series]].
 
}}</ref> The basic argument goes as follows: if the [[fine structure constant|coupling constant]] were negative, this would be equivalent to the [[Coulomb force constant]] being negative. This would "reverse" the electromagnetic interaction so that ''like'' charges would ''attract'' and ''unlike'' charges would ''repel''. This would render the vacuum unstable against decay into a cluster of electrons on one side of the universe and a cluster of positrons on the other side of the universe. Because the theory is 'sick' for any negative value of the coupling constant, the series do not converge, but are an [[asymptotic series]].
  
From a modern perspective, we say that QED is not well defined as a QFT to arbitrarily high energy.<ref>{{Cite web
+
From a modern perspective, we say that QED is not well defined as a quantum field theory to arbitrarily high energy.<ref>{{cite arXiv
 
   | last = Espriu and Tarrach
 
   | last = Espriu and Tarrach
 
   | first =  
 
   | first =  
Line 446: Line 443:
 
   | publisher =  
 
   | publisher =  
 
   | date =  
 
   | date =  
   | url = http://arxiv.org/pdf/hep-ph/9604431.pdf
+
   | eprint = hep-ph/9604431
  | accessdate = 11-11-13
 
 
}}</ref> The coupling constant runs to infinity at finite energy, signalling a [[Landau pole]]. The problem is essentially that QED is not [[asymptotically free]]. This is one of the motivations for embedding QED within a [[Grand Unified Theory]].
 
}}</ref> The coupling constant runs to infinity at finite energy, signalling a [[Landau pole]]. The problem is essentially that QED is not [[asymptotically free]]. This is one of the motivations for embedding QED within a [[Grand Unified Theory]].
  
Line 495: Line 491:
  
 
==Further reading==
 
==Further reading==
 +
 
===Books===
 
===Books===
* {{cite book |last=De Broglie |first=Louis |authorlink=Louis de Broglie |title=Recherches sur la theorie des quanta [Research on quantum theory] |year=1925 |publisher=Wiley-Interscience |location=France}}
+
* {{cite book |last=De Broglie |first=Louis |authorlink=Louis de Broglie |title=Recherches sur la theorie des quanta [Research on quantum theory] |year=1925 |publisher=Wiley-Interscience |location=France}}
 
* {{cite book |last=Feynman |first=Richard Phillips |authorlink=Richard Feynman |title=Quantum Electrodynamics |year=1998 |publisher=Westview Press |edition=New |isbn=978-0-201-36075-2}}
 
* {{cite book |last=Feynman |first=Richard Phillips |authorlink=Richard Feynman |title=Quantum Electrodynamics |year=1998 |publisher=Westview Press |edition=New |isbn=978-0-201-36075-2}}
 
* {{cite book |last1=Jauch |first1=J.M. |last2=Rohrlich |first2=F. |title=The Theory of Photons and Electrons |year=1980 |publisher=Springer-Verlag |isbn=978-0-387-07295-1}}
 
* {{cite book |last1=Jauch |first1=J.M. |last2=Rohrlich |first2=F. |title=The Theory of Photons and Electrons |year=1980 |publisher=Springer-Verlag |isbn=978-0-387-07295-1}}

Latest revision as of 10:32, 11 January 2015

{{#invoke:Hatnote|hatnote}} {{#invoke: Sidebar | collapsible }}

In particle physics, quantum electrodynamics (QED) is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and special relativity is achieved. QED mathematically describes all phenomena involving electrically charged particles interacting by means of exchange of photons and represents the quantum counterpart of classical electromagnetism giving a complete account of matter and light interaction.

In technical terms, QED can be described as a perturbation theory of the electromagnetic quantum vacuum. Richard Feynman called it "the jewel of physics" for its extremely accurate predictions of quantities like the anomalous magnetic moment of the electron and the Lamb shift of the energy levels of hydrogen.[1]:Ch1

History

{{#invoke:main|main}}

The first formulation of a quantum theory describing radiation and matter interaction is attributed to British scientist Paul Dirac, who (during the 1920s) was first able to compute the coefficient of spontaneous emission of an atom.[2]

Dirac described the quantization of the electromagnetic field as an ensemble of harmonic oscillators with the introduction of the concept of creation and annihilation operators of particles. In the following years, with contributions from Wolfgang Pauli, Eugene Wigner, Pascual Jordan, Werner Heisenberg and an elegant formulation of quantum electrodynamics due to Enrico Fermi,[3] physicists came to believe that, in principle, it would be possible to perform any computation for any physical process involving photons and charged particles. However, further studies by Felix Bloch with Arnold Nordsieck,[4] and Victor Weisskopf,[5] in 1937 and 1939, revealed that such computations were reliable only at a first order of perturbation theory, a problem already pointed out by Robert Oppenheimer.[6] At higher orders in the series infinities emerged, making such computations meaningless and casting serious doubts on the internal consistency of the theory itself. With no solution for this problem known at the time, it appeared that a fundamental incompatibility existed between special relativity and quantum mechanics.

Difficulties with the theory increased through the end of 1940. Improvements in microwave technology made it possible to take more precise measurements of the shift of the levels of a hydrogen atom,[7] now known as the Lamb shift and magnetic moment of the electron.[8] These experiments unequivocally exposed discrepancies which the theory was unable to explain.

A first indication of a possible way out was given by Hans Bethe. In 1947, while he was traveling by train to reach Schenectady from New York,[9] after giving a talk at the conference at Shelter Island on the subject, Bethe completed the first non-relativistic computation of the shift of the lines of the hydrogen atom as measured by Lamb and Retherford.[10] Despite the limitations of the computation, agreement was excellent. The idea was simply to attach infinities to corrections of mass and charge that were actually fixed to a finite value by experiments. In this way, the infinities get absorbed in those constants and yield a finite result in good agreement with experiments. This procedure was named renormalization.

Feynman (center) and Oppenheimer (right) at Los Alamos.

Based on Bethe's intuition and fundamental papers on the subject by Sin-Itiro Tomonaga,[11] Julian Schwinger,[12][13] Richard Feynman[14][15][16] and Freeman Dyson,[17][18] it was finally possible to get fully covariant formulations that were finite at any order in a perturbation series of quantum electrodynamics. Sin-Itiro Tomonaga, Julian Schwinger and Richard Feynman were jointly awarded with a Nobel prize in physics in 1965 for their work in this area.[19] Their contributions, and those of Freeman Dyson, were about covariant and gauge invariant formulations of quantum electrodynamics that allow computations of observables at any order of perturbation theory. Feynman's mathematical technique, based on his diagrams, initially seemed very different from the field-theoretic, operator-based approach of Schwinger and Tomonaga, but Freeman Dyson later showed that the two approaches were equivalent.[17] Renormalization, the need to attach a physical meaning at certain divergences appearing in the theory through integrals, has subsequently become one of the fundamental aspects of quantum field theory and has come to be seen as a criterion for a theory's general acceptability. Even though renormalization works very well in practice, Feynman was never entirely comfortable with its mathematical validity, even referring to renormalization as a "shell game" and "hocus pocus".[1]:128

QED has served as the model and template for all subsequent quantum field theories. One such subsequent theory is quantum chromodynamics, which began in the early 1960s and attained its present form in the 1975 work by H. David Politzer, Sidney Coleman, David Gross and Frank Wilczek. Building on the pioneering work of Schwinger, Gerald Guralnik, Dick Hagen, and Tom Kibble,[20][21] Peter Higgs, Jeffrey Goldstone, and others, Sheldon Glashow, Steven Weinberg and Abdus Salam independently showed how the weak nuclear force and quantum electrodynamics could be merged into a single electroweak force.

Feynman's view of quantum electrodynamics

Introduction

Near the end of his life, Richard P. Feynman gave a series of lectures on QED intended for the lay public. These lectures were transcribed and published as Feynman (1985), QED: The strange theory of light and matter,[1] a classic non-mathematical exposition of QED from the point of view articulated below.

The key components of Feynman's presentation of QED are three basic actions.[1]:85

  • A photon goes from one place and time to another place and time.
  • An electron goes from one place and time to another place and time.
  • An electron emits or absorbs a photon at a certain place and time.

These actions are represented in a form of visual shorthand by the three basic elements of Feynman diagrams: a wavy line for the photon, a straight line for the electron and a junction of two straight lines and a wavy one for a vertex representing emission or absorption of a photon by an electron. These can all be seen in the adjacent diagram.

It is important not to over-interpret these diagrams. Nothing is implied about how a particle gets from one point to another. The diagrams do not imply that the particles are moving in straight or curved lines. They do not imply that the particles are moving with fixed speeds. The fact that the photon is often represented, by convention, by a wavy line and not a straight one does not imply that it is thought that it is more wavelike than is an electron. The images are just symbols to represent the actions above: photons and electrons do, somehow, move from point to point and electrons, somehow, emit and absorb photons. We do not know how these things happen, but the theory tells us about the probabilities of these things happening.

As well as the visual shorthand for the actions Feynman introduces another kind of shorthand for the numerical quantities called probability amplitudes. The probability is the square of the total probability amplitude. If a photon moves from one place and time—in shorthand, A—to another place and time—in shorthand, B—the associated quantity is written in Feynman's shorthand as P(A to B). The similar quantity for an electron moving from C to D is written E(C to D). The quantity which tells us about the probability amplitude for the emission or absorption of a photon he calls 'j'. This is related to, but not the same as, the measured electron charge 'e'.[1]:91

QED is based on the assumption that complex interactions of many electrons and photons can be represented by fitting together a suitable collection of the above three building blocks, and then using the probability amplitudes to calculate the probability of any such complex interaction. It turns out that the basic idea of QED can be communicated while making the assumption that the square of the total of the probability amplitudes mentioned above (P(A to B), E(A to B) and 'j') is just our everyday probability. (A simplification of Feynman's book.) Later on this will be corrected to include specifically quantum mathematics, following Feynman.

The basic rules of probability amplitudes that will be used are that a) if an event can happen in a variety of different ways then its probability amplitude is the sum of the probability amplitudes of the possible ways and b) if a process involves a number of independent sub-processes then its probability amplitude is the product of the component probability amplitudes.[1]:93

Basic constructions

Suppose we start with one electron at a certain place and time (this place and time being given the arbitrary label A) and a photon at another place and time (given the label B). A typical question from a physical standpoint is: 'What is the probability of finding an electron at C (another place and a later time) and a photon at D (yet another place and time)?'. The simplest process to achieve this end is for the electron to move from A to C (an elementary action) and for the photon to move from B to D (another elementary action). From a knowledge of the probability amplitudes of each of these sub-processes – E(A to C) and P(B to D) – then we would expect to calculate the probability amplitude of both happening together by multiplying them, using rule b) above. This gives a simple estimated overall probability amplitude, which is squared to give an estimated probability.

But there are other ways in which the end result could come about. The electron might move to a place and time E where it absorbs the photon; then move on before emitting another photon at F; then move on to C where it is detected, while the new photon moves on to D. The probability of this complex process can again be calculated by knowing the probability amplitudes of each of the individual actions: three electron actions, two photon actions and two vertexes – one emission and one absorption. We would expect to find the total probability amplitude by multiplying the probability amplitudes of each of the actions, for any chosen positions of E and F. We then, using rule a) above, have to add up all these probability amplitudes for all the alternatives for E and F. (This is not elementary in practice, and involves integration.) But there is another possibility, which is that the electron first moves to G where it emits a photon which goes on to D, while the electron moves on to H, where it absorbs the first photon, before moving on to C. Again we can calculate the probability amplitude of these possibilities (for all points G and H). We then have a better estimation for the total probability amplitude by adding the probability amplitudes of these two possibilities to our original simple estimate. Incidentally the name given to this process of a photon interacting with an electron in this way is Compton Scattering.

There are an infinite number of other intermediate processes in which more and more photons are absorbed and/or emitted. For each of these possibilities there is a Feynman diagram describing it. This implies a complex computation for the resulting probability amplitudes, but provided it is the case that the more complicated the diagram the less it contributes to the result, it is only a matter of time and effort to find as accurate an answer as one wants to the original question. This is the basic approach of QED. To calculate the probability of any interactive process between electrons and photons it is a matter of first noting, with Feynman diagrams, all the possible ways in which the process can be constructed from the three basic elements. Each diagram involves some calculation involving definite rules to find the associated probability amplitude.

That basic scaffolding remains when one moves to a quantum description but some conceptual changes are needed. One is that whereas we might expect in our everyday life that there would be some constraints on the points to which a particle can move, that is not true in full quantum electrodynamics. There is a possibility of an electron at A, or a photon at B, moving as a basic action to any other place and time in the universe. That includes places that could only be reached at speeds greater than that of light and also earlier times. (An electron moving backwards in time can be viewed as a positron moving forward in time.)[1]:89, 98–99

Probability amplitudes

Feynman replaces complex numbers with spinning arrows, which start at emission and end at detection of a particle. The sum of all resulting arrows represents the total probability of the event. In this diagram, light emitted by the source S bounces off a few segments of the mirror (in blue) before reaching the detector at P. The sum of all paths must be taken into account. The graph below depicts the total time spent to traverse each of the paths above.

Quantum mechanics introduces an important change in the way probabilities are computed. Probabilities are still represented by the usual real numbers we use for probabilities in our everyday world, but probabilities are computed as the square of probability amplitudes. Probability amplitudes are complex numbers.

Feynman avoids exposing the reader to the mathematics of complex numbers by using a simple but accurate representation of them as arrows on a piece of paper or screen. (These must not be confused with the arrows of Feynman diagrams which are actually simplified representations in two dimensions of a relationship between points in three dimensions of space and one of time.) The amplitude arrows are fundamental to the description of the world given by quantum theory. No satisfactory reason has been given for why they are needed. But pragmatically we have to accept that they are an essential part of our description of all quantum phenomena. They are related to our everyday ideas of probability by the simple rule that the probability of an event is the square of the length of the corresponding amplitude arrow. So, for a given process, if two probability amplitudes, v and w, are involved, the probability of the process will be given either by

or

The rules as regards adding or multiplying, however, are the same as above. But where you would expect to add or multiply probabilities, instead you add or multiply probability amplitudes that now are complex numbers.

Addition of probability amplitudes as complex numbers
Multiplication of probability amplitudes as complex numbers

Addition and multiplication are familiar operations in the theory of complex numbers and are given in the figures. The sum is found as follows. Let the start of the second arrow be at the end of the first. The sum is then a third arrow that goes directly from the start of the first to the end of the second. The product of two arrows is an arrow whose length is the product of the two lengths. The direction of the product is found by adding the angles that each of the two have been turned through relative to a reference direction: that gives the angle that the product is turned relative to the reference direction.

That change, from probabilities to probability amplitudes, complicates the mathematics without changing the basic approach. But that change is still not quite enough because it fails to take into account the fact that both photons and electrons can be polarized, which is to say that their orientations in space and time have to be taken into account. Therefore P(A to B) actually consists of 16 complex numbers, or probability amplitude arrows.[1]:120–121 There are also some minor changes to do with the quantity "j", which may have to be rotated by a multiple of 90° for some polarizations, which is only of interest for the detailed bookkeeping.

Associated with the fact that the electron can be polarized is another small necessary detail which is connected with the fact that an electron is a fermion and obeys Fermi–Dirac statistics. The basic rule is that if we have the probability amplitude for a given complex process involving more than one electron, then when we include (as we always must) the complementary Feynman diagram in which we just exchange two electron events, the resulting amplitude is the reverse – the negative – of the first. The simplest case would be two electrons starting at A and B ending at C and D. The amplitude would be calculated as the "difference", E(A to D) × E(B to C) − E(A to C) × E(B to D), where we would expect, from our everyday idea of probabilities, that it would be a sum.[1]:112–113

Propagators

Finally, one has to compute P (A to B) and E (C to D) corresponding to the probability amplitudes for the photon and the electron respectively. These are essentially the solutions of the Dirac Equation which describes the behavior of the electron's probability amplitude and the Klein–Gordon equation which describes the behavior of the photon's probability amplitude. These are called Feynman propagators. The translation to a notation commonly used in the standard literature is as follows:

where a shorthand symbol such as stands for the four real numbers which give the time and position in three dimensions of the point labeled A.

Mass renormalization

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A problem arose historically which held up progress for twenty years: although we start with the assumption of three basic "simple" actions, the rules of the game say that if we want to calculate the probability amplitude for an electron to get from A to B we must take into account all the possible ways: all possible Feynman diagrams with those end points. Thus there will be a way in which the electron travels to C, emits a photon there and then absorbs it again at D before moving on to B. Or it could do this kind of thing twice, or more. In short we have a fractal-like situation in which if we look closely at a line it breaks up into a collection of "simple" lines, each of which, if looked at closely, are in turn composed of "simple" lines, and so on ad infinitum. This is a very difficult situation to handle. If adding that detail only altered things slightly then it would not have been too bad, but disaster struck when it was found that the simple correction mentioned above led to infinite probability amplitudes. In time this problem was "fixed" by the technique of renormalization. However, Feynman himself remained unhappy about it, calling it a "dippy process".[1]:128

Conclusions

Within the above framework physicists were then able to calculate to a high degree of accuracy some of the properties of electrons, such as the anomalous magnetic dipole moment. However, as Feynman points out, it fails totally to explain why particles such as the electron have the masses they do. "There is no theory that adequately explains these numbers. We use the numbers in all our theories, but we don't understand them – what they are, or where they come from. I believe that from a fundamental point of view, this is a very interesting and serious problem."[1]:152

Mathematics

Mathematically, QED is an abelian gauge theory with the symmetry group U(1). The gauge field, which mediates the interaction between the charged spin-1/2 fields, is the electromagnetic field. The QED Lagrangian for a spin-1/2 field interacting with the electromagnetic field is given by the real part of[22]:78

where

are Dirac matrices;
a bispinor field of spin-1/2 particles (e.g. electronpositron field);
, called "psi-bar", is sometimes referred to as the Dirac adjoint;
is the gauge covariant derivative;
e is the coupling constant, equal to the electric charge of the bispinor field;
Aμ is the covariant four-potential of the electromagnetic field generated by the electron itself;
Bμ is the external field imposed by external source;
is the electromagnetic field tensor.

Equations of motion

To begin, substituting the definition of D into the Lagrangian gives us

Next, we can substitute this Lagrangian into the Euler–Lagrange equation of motion for a field:

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to find the field equations for QED.

The two terms from this Lagrangian are then

Substituting these two back into the Euler–Lagrange equation (Template:EquationNote) results in

with complex conjugate

Bringing the middle term to the right-hand side transforms this second equation into

The left-hand side is like the original Dirac equation and the right-hand side is the interaction with the electromagnetic field.

One further important equation can be found by substituting the Lagrangian into another Euler–Lagrange equation, this time for the field, Aμ:

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The two terms this time are

and these two terms, when substituted back into (Template:EquationNote) give us

Now, if we impose the Lorenz gauge condition, that the divergence of the four potential vanishes

then we get

which is a wave equation for the four potential, the QED version of the classical Maxwell equations in the Lorenz gauge. (In the above equation, the square represents the D'Alembert operator.)

Interaction picture

This theory can be straightforwardly quantized by treating bosonic and fermionic sectors as free. This permits us to build a set of asymptotic states which can be used to start a computation of the probability amplitudes for different processes. In order to do so, we have to compute an evolution operator that, for a given initial state , will give a final state in such a way to have[22]:5

This technique is also known as the S-Matrix. The evolution operator is obtained in the interaction picture where time evolution is given by the interaction Hamiltonian, which is the integral over space of the second term in the Lagrangian density given above:[22]:123

and so, one has[22]:86

where T is the time ordering operator. This evolution operator only has meaning as a series, and what we get here is a perturbation series with the fine structure constant as the development parameter. This series is called the Dyson series.

Feynman diagrams

Despite the conceptual clarity of this Feynman approach to QED, almost no early textbooks follow him in their presentation. When performing calculations it is much easier to work with the Fourier transforms of the propagators. Quantum physics considers particle's momenta rather than their positions, and it is convenient to think of particles as being created or annihilated when they interact. Feynman diagrams then look the same, but the lines have different interpretations. The electron line represents an electron with a given energy and momentum, with a similar interpretation of the photon line. A vertex diagram represents the annihilation of one electron and the creation of another together with the absorption or creation of a photon, each having specified energies and momenta.

Using Wick theorem on the terms of the Dyson series, all the terms of the S-matrix for quantum electrodynamics can be computed through the technique of Feynman diagrams. In this case rules for drawing are the following[22]:801–802

Qed rules.jpg
Qed2e.jpg

To these rules we must add a further one for closed loops that implies an integration on momenta , since these internal ("virtual") particles are not constrained to any specific energy–momentum – even that usually required by special relativity (see this article for details). From them, computations of probability amplitudes are straightforwardly given. An example is Compton scattering, with an electron and a photon undergoing elastic scattering. Feynman diagrams are in this case[22]:158–159

Compton qed.jpg

and so we are able to get the corresponding amplitude at the first order of a perturbation series for the S-matrix:

from which we are able to compute the cross section for this scattering.

Renormalizability

Higher order terms can be straightforwardly computed for the evolution operator but these terms display diagrams containing the following simpler ones[22]:ch 10

that, being closed loops, imply the presence of diverging integrals having no mathematical meaning. To overcome this difficulty, a technique called renormalization has been devised, producing finite results in very close agreement with experiments. It is important to note that a criterion for theory being meaningful after renormalization is that the number of diverging diagrams is finite. In this case the theory is said to be renormalizable. The reason for this is that to get observables renormalized one needs a finite number of constants to maintain the predictive value of the theory untouched. This is exactly the case of quantum electrodynamics displaying just three diverging diagrams. This procedure gives observables in very close agreement with experiment as seen e.g. for electron gyromagnetic ratio.

Renormalizability has become an essential criterion for a quantum field theory to be considered as a viable one. All the theories describing fundamental interactions, except gravitation whose quantum counterpart is presently under very active research, are renormalizable theories.

Nonconvergence of series

An argument by Freeman Dyson shows that the radius of convergence of the perturbation series in QED is zero.[23] The basic argument goes as follows: if the coupling constant were negative, this would be equivalent to the Coulomb force constant being negative. This would "reverse" the electromagnetic interaction so that like charges would attract and unlike charges would repel. This would render the vacuum unstable against decay into a cluster of electrons on one side of the universe and a cluster of positrons on the other side of the universe. Because the theory is 'sick' for any negative value of the coupling constant, the series do not converge, but are an asymptotic series.

From a modern perspective, we say that QED is not well defined as a quantum field theory to arbitrarily high energy.[24] The coupling constant runs to infinity at finite energy, signalling a Landau pole. The problem is essentially that QED is not asymptotically free. This is one of the motivations for embedding QED within a Grand Unified Theory.

See also

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References

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Further reading

Books

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  • Milonni, Peter W., (1994) The quantum vacuum - an introduction to quantum electrodynamics. Academic Press. ISBN 0-12-498080-5
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Journals

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External links

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