�ber die Streuung von Licht an Licht nach der Diracschen Theorie

First results from the new PVLAS apparatus: A new limit on vacuum magnetic birefringence

“…Non linear electrodynamic effects in vacuum have been predicted since the earliest days of Quantum Electrodynamics (QED), a few years after the discovery of positrons [1][2][3]. One such effect is vacuum magnetic birefringence [4], closely connected to elastic light-by-light interaction.…”

Construction of the Pauli–Villars-Regulated Dirac Vacuum in Electromagnetic Fields

“…The situation is much more complicated in Dirac's vacuum. It has been known for a long time that, in reality, the vacuum is not an empty space, but rather a quantum fluctuating medium which behaves as a nonlinear polarizable material [14,13,27,36,21]. In this medium, virtual electron-positron pairs induce a polarization in response to external fields.…”

“…The nonlinear effects are in practice rather small since e has a small physical value, but they become important in the presence of strong external sources, which can produce electron-positron pairs in the vacuum. Already in 1933, Dirac computed in [9] the first order term obtained by expanding ρ vac (eA * ) in powers of e. The nonlinear equations (1.3) was then studied by Euler, Heisenberg, Kockel and Weisskopf among others [14,13,27,46]. In a celebrated paper, Schwinger [36] used (1.3) (and a time-dependent version of it) to derive the probability of pair creation by tunneling in a strong electrostatic field.…”

From effective actions to actual effects

“…Let us begin with a brief sketch of the classic example: the Heisenberg-Euler action [2]. Here, the physical system is defined simply by soft electromagnetic fields being placed in the vacuum.…”