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

It is reviewed 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 delta-electron spectra, the non-appearance of a massive line structure in undercritical systems like U+Ta and the quantitative understanding of the innershell ionization phenomena in giant quasimolecules (their agreement with theoretical predictions) give confidence into this interpretation. At the same time the positron line structures possibly indicate the existence of rather long living giant nuclear systems with Z proportional 180-190. It is shown that most likely giant nuclear molecules have been formed as doorway states, but that other new not yet completely understood nuclear structure of giant complexes is indicated. Finally the overcritical phenomenon in other areas of physics (e.g., overcritical gluo-electric fields, overcritical gravitational fields) is shortly discussed. (orig./HSI)