Light-matter interactions at extreme intensities and densities: reaching the Schwinger limit

Due to the smallness of the fine-structure constant alpha the perturbative sector of quantum electrodynamics (QED) is well understood. In the presence of strong electromagnetic fields, however, the photon density can become so large that it compensates for the suppression induced by alpha and nonperturbative effects are predicted to occur. In this sector ab-initio calculations become intractable, commonly applied approximations are experimentally untested, and, eventually, a complete breakdown of perturbation theory is conjectured. Such conditions exist close to enigmatic astrophysical objects like magnetars or during some of the most violent cosmic events like neutron-star mergers. The Experiment 320 at SLAC's FACET-II aims at probing the strong-field sector of QED in electron-laser collisions. Due to the large Lorentz boost of the 10-13 GeV electron beam E-320 will be able to probe the QED critical (Schwinger) field in the beam rest frame even with a moderate 10 TW-class near infrared laser. The talk will give an introduction into strong-field QED and its relation to other scientific fields such as strong-field atomic physics, laser-plasma interactions in the QED regime, next-generation linear lepton collider, and astrophysics; it will present the current status of E-320 (initial-stage commissioning completed and ready for first beamtime), its relation to other planned experiments, and outline possible future directions of the field.