Schwinger effect

The Schwinger effect is a predicted physical phenomenon whereby matter is created by a strong electric field. It is also referred to as the Sauter–Schwinger effect, Schwinger mechanism, or Schwinger pair production. It is a prediction of quantum electrodynamics (QED) in which electronpositron pairs are spontaneously created in the presence of an electric field, thereby causing the decay of the electric field. The effect was originally proposed by Fritz Sauter in 1931[1] and further important work was carried out by Werner Heisenberg and Hans Heinrich Euler in 1936,[2] though it was not until 1951 that Julian Schwinger gave a complete theoretical description.[3]

Predicted physical phenomenon in QED
In the presence of a strong, constant electric field, electrons,

e{displaystyle e^{-}}

, and positrons,

e+{displaystyle e^{+}}

, will be spontaneously created.

The Schwinger effect can be thought of as vacuum decay in the presence of an electric field. Although the notion of vacuum decay suggests that something is created out of nothing, physical conservation laws are nevertheless obeyed. To understand this, note that electrons and positrons are each other’s antiparticles, with identical properties except opposite electric charge.

To conserve energy, the electric field loses energy when an electron-positron pair is created, by an amount equal to

2mc2{displaystyle 2mc^{2}}

, where

m{displaystyle m}

is the electron’s rest mass and

c{displaystyle c}

is the speed of light. Electric charge is conserved because an electron-positron pair is charge neutral. Linear and angular momentum are conserved because, in each pair, the electron and positron are created with opposite velocities and spins. In fact, the electron and positron are expected to be created at (close to) rest, and then subsequently accelerated away from each other by the electric field.[4]

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Schwinger pair production in a constant electric field takes place at a constant rate per unit volume, commonly referred to as

Γ{displaystyle Gamma }

. The rate was first calculated by Schwinger[3] and at leading (one-loop) order is equal to

Γ=(eE)24π3c2n=11n2eπm2c3neE{displaystyle Gamma ={frac {(eE)^{2}}{4pi ^{3}chbar ^{2}}}sum _{n=1}^{infty }{frac {1}{n^{2}}}mathrm {e} ^{-{frac {pi m^{2}c^{3}n}{eEhbar }}}}

where

m{displaystyle m}

is the mass of an electron,

e{displaystyle e}

is the charge of an electron, and

E{displaystyle E}

is the electric field strength. This formula cannot be expanded in a Taylor series in

e2{displaystyle e^{2}}

, showing the nonperturbative nature of this effect. In terms of Feynman diagrams, one can derive the rate of Schwinger pair production by summing the infinite set of diagrams shown below, containing one electron loop and any number of external photon legs, each with zero energy.

The infinite set of Feynman diagrams relevant for Schwinger pair production.

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