Probing the quantum vacuum

This book investigates the vacuum of quantum electrodynamics (QED) using the perturbative effective action approach. By probing the vacuum with external perturbations, the system's response can be analyzed after averaging over high energy degrees of freedom, leading to an effective description of vacuum properties akin to those of a classical medium. The focus is on slowly varying fields or soft photons, achieved by integrating out high energy degrees of freedom, specifically electrons, through Schwinger's proper time method. A new representation of the one-loop photon polarization tensor is derived, which couples to all orders with an arbitrary constant electromagnetic field while maintaining dependence on a complete set of invariants. Effective Lagrangians are employed to derive the light cone condition for low-frequency photons in strong fields. The formalism can be extended to various external perturbations, including temperature and Casimir effects. A proof of the "unified formula" for low energy phenomena is presented, describing the refractive indices of various perturbed quantum vacua. In the high energy domain, similarities are noted between a vacuum with a superstrong magnetic field and a magnetized plasma. The book also addresses the measurability of various effects, confirming that no violation of causality is found.