The Vacuum in Quantum Electrodynamics

It is generally accepted that the physical vacuum has a nontrivial structure. This conclusion was first made by Dirac on the basis of his famous equation for a fermion field which describes simultaneously particles and antiparticles. The Dirac equation in the vacuum has a simple form 

One of the most fascinating aspects is the structure of the vacuum in QED and its change into charged vacuum states under the influence of strong (supercritical) electric fields [58]. We shortly remind of this phenomenon. 

FIGURE 8.33 Time dependence of the quasi-molecular energy levels in a supercritical heavy-ion collision. The arrows denote various excitation processes which lead to the production of holes and positrons. 

FIGURE 8.34 Positron energy spectra measured in collisions of Th + Ta, Th + Th, and U + Cm at energies of about 6 MeV per nucleon. The QED predictions (dashed lines) and the experimentally determined background from nuclear pair conversion (dotted lines) add up to the full lines which are in close agreement with experiment. 

About twenty years ago transfer reactions of heavy ions with target have 

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