Spontaneous particle creation in time-dependent overcritical fields

Abstract

It is believed that in the presence of some strong electromagnetic fields, called overcritical, the (Dirac) vacuum becomes unstable and decays, leading to a spontaneous production of an electron–positron pair. However, most of the arguments are based on the analysis of static fields and are insufficient to explain this phenomenon completely. Therefore, we consider time-dependent overcritical fields and show, within the external field formulation, how spontaneous particle creation can be defined and measured in physical processes. We prove that the effect exists always when a overcritical field is switched on, but it becomes unstable and hence generically only approximate and non-unique when the field is switched on and off. In the latter case, it becomes unique and stable only in the adiabatic limit.