The decay of the vacuum in supercritical fields of giant nuclear systems

The structure of the vacuum has become of general importance in modern physics. In this context the vacuum of quantum electrodynamics plays a special role because it changes drastically in strong external electric fields and these changes can be followed in a controllable way in the laboratory. The author reviews how in supercritical electric fields the electron-positron-vacuum, i.e., the ground state of QED is changing from a neutral to a charged vacuum. The theoretical concepts, developed during the last 15 years, can be tested in collisions of very heavy ions, like U+U, U+Cm, etc. The important theoretical and experimental stages are described; in particular it is shown that the vacuum decay might have recently been experimentally verified: Sharp line structures in the positron spectra with pronounced threshold effects are the signals of this fundamental phase transition. Furthermore, the observed γ-ray and δ-electron spectra, the non-appearance of a massive line structure in undercritical systems like U+Ta and the quantitative understanding of the inner-shell ionization phenomena in giant quasimolecules (their agreement with theoretical predictions) give confidence into this interpretation