Editing Quantum electrodynamics (section)

===Nonperturbative phenomena===
The predictive success of quantum electrodynamics largely rests on the use of perturbation theory, expressed in Feynman diagrams. However, quantum electrodynamics also leads to predictions beyond perturbation theory. In the presence of very strong electric fields, it predicts that electrons and positrons will be spontaneously produced, so causing the decay of the field. This process, called the [[Schwinger effect]],<ref name="Schwinger">{{cite journal | last=Schwinger | first=Julian | title=On Gauge Invariance and Vacuum Polarization | journal=Physical Review | publisher=American Physical Society (APS) | volume=82 | issue=5 | date=1951-06-01 | issn=0031-899X | doi=10.1103/physrev.82.664 | pages=664–679| bibcode=1951PhRv...82..664S }}</ref> cannot be understood in terms of any finite number of Feynman diagrams and hence is described as [[Non-perturbative|nonperturbative]]. Mathematically, it can be derived by a semiclassical approximation to the [[Path integral formulation|path integral]] of quantum electrodynamics.