Its research object isElectromagnetic interactionThe quantum properties of (i.ephotonEmission and absorption)charged particleGeneration andannihilation, between charged particlesscattering, scattering between charged particles and photons, etc.It summarizes the basic principles of electromagnetic interaction in various fields of atomic physics, molecular physics, solid physics, nuclear physics and particle physics.
Quantum electrodynamics, abbreviated as QED, is the oldest and most mature branch in the development of quantum field theory.It mainly studieselectromagnetic fieldThe basic process of interaction with charged particles.In principle, its principle summarizesAtomic physics、Molecular physics, solid physicsnuclear physicsAnd particle physicsElectromagnetic interactionProcess.It studies the quantum properties of electromagnetic interaction (i.e. the emission and absorption of photons), the generation and annihilation of charged particles (such as positive and negative electrons), the scattering between charged particles, and the scattering between charged particles and photons.It is outstanding in modern physics in terms of the wide range of applications, the simple and clear basic assumptions, and the highly accurate degree of conformity with experiments.
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1925quantum mechanicsShortly after its establishment,P. A.M. DiracIn 1927W. K. HeisenbergandW. PauliIn 1929, he successively proposed the quantum theory of radiation, which laid the theoretical foundation of quantum electrodynamics.
Within the scope of quantum mechanics, the interaction between charged particles and electromagnetic fields can be regarded as a perturbation to deal with the absorption and stimulated emission of light, but it cannot deal with the self emission of light.Because if the electromagnetic field is regarded as a classical field, there is no radiation field before photons are emitted.The excited electron in an atom is a stationary state in quantum mechanics. Without the radiation field as a perturbation, it will not transition.Self emission is a fact that is certain to exist. In order to explain this phenomenon and quantitatively give its probability of occurrence, we can only use an alternative method to deal with it in quantum mechanics.
One way is to use the correspondence principle, regard the excited electrons in the atom as the sum of many harmonic oscillators, and identify the oscillating current generating radiation as corresponding to some transition matrix elements in quantum mechanics, so as to calculate the transition probability of self emission.From this treatment, we can getmax planck This in turn shows that the treatment of the corresponding principle is feasible.
Another way is to useA. EinsteinThe relationship between self emission probability and absorption probability.Although the results obtained by these methods can be consistent with the experimental results, they are actually contradictory to the quantum mechanical system in theory - the steady-state life of quantum mechanics is infinite.
radiation field
Dirac, Heisenberg andPauliQuantize the radiation field.In addition to gettingwave-particle duality of light In addition to the explicit statement of, it also solves the above contradiction.After quantization of electromagnetic field, the electric field strength
And magnetic field strength
All become operators.Their components meet a certain commutation relationship, and their "expected value" (that is, the average value of the measurement in the experiment) should meet the uncertainty relationship of quantum mechanics. They cannot have a certain value at the same time (that is, the mean square deviation is zero at the same time).As a special case, they cannot be determined to be zero at the same time.In the state where there is no photon (it is called the vacuum state of the radiation field),
and
The average value of is zero.but
And
The average value of is not zero (otherwise, the mean square; the difference will be zero at the same time).This is the vacuum fluctuation of the quantized radiation field.It is very similar to the zero point energy of the harmonic oscillator in quantum mechanics.After quantization, the production and annihilation of field become a universal and basic process.Therefore, when the atom is in the excited state, although there is no photon, the electron can still transition to the low-energy state and generate photons.Starting from the expression of the quantum theory of radiation field, the cross sections of the basic processes of interaction between various charged particles and electromagnetic fields can be calculated, such asCompton effect、photoelectric effect、Bremsstrahlung、Electron pair generationandElectron pair annihilationEtc.These results are obtained by using perturbation theory to approximate the lowest order, which is not zero, and are in good agreement with the experiment.But no matter what kind of process, when calculating the results of higher approximationDivergence difficulty, that is, we can get infinite results.This isJ. R. OppenheimerIt was first pointed out in 1930.In the following ten years, although quantum electrodynamics has continued to develop in the research of many basic electromagnetic processes, as well as the penetration of high-energy radiation in matter and the cascade shower of cosmic rays, it is still at a relative standstill in solving the divergence difficulties in the basic theory.
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Under the new theoretical expression, the calculation of high-order correction of various processes is carried out, and these results meet the increasingly high requirements of the theory due to the improvement of experimental conditions and accuracy.Quantum electrodynamics is a kind ofGauge fieldThe theory of.It is an important development stage of quantum field theory to unify electromagnetic interaction and weak interaction.Unified theory of electroweakThe standard model ofQuantum chromodynamicsBoth belong to the category of gauge field theory.Both of them are based on the theories and methods of quantum electrodynamics.The renormalization theory established from the study of quantum electrodynamics is not only used in particle physics, but also a useful tool for statistical physics (seePhase and phase transition、Renormalization group）。[1]
