Quantum electrodynamics of strong fields

Quantum electrodynamics of strong external fields is investigated in the context of atomic physics. If the electromagnetic coupling constant Zα becomes sufficiently large (Z > 1/α approximately 137 for point sources or Z > 172 for extended nuclei) bound electron states can join the negative-energy continuum of the Dirac equation. The resulting possibility of spontaneous positron production and the new concept of a charge electron-positron vacuum is discussed under several aspects. The autoionization model and the exact overcritical solutions of the single-particle Dirac equation are contrasted with a quantum-field-theoretical approach. Non-linear field effects are shown to have no influence. Vacuum polarization and the self-screening of extremely strong electric charges are treated explicitly. Adiabatic collisions of heavy ions leading to the transient formation of quasimolecular electron orbitals are suggested as a means for an experimental test of the special features of strong-field QED. After an introduction to the relativistic two-centre Dirac equation, the x-ray spectroscopy of quasimolecules and the spontaneous and induced positron emission in, for example, U-U collisions are discussed. The importance of several background processes is stressed. Finally, strong-field aspects for nuclear matter and in gravitation are mentioned. (author)