In quantum description, vacuum is the state of lowest-possible energy density. In contrast to classical physics, that is at finite, non-zero energy density – due to quantum nature. Secondly, Heisenberg’s uncertainty principle allows for energy fluctuations on very short time scales, and observations are the outcome of many possibilities. In essence, vacuum contains fleeting fields and particles, know as virtual particles or vacuum fluctuations.
A classic thought experiment considering those pairs is the “Schwinger effect”: Imagine an external electric field strong enough to pull a transient virtual particle-antiparticle pair apart in such way that the work done is equivalent to the pair’s rest mass. Due to mass-energy equivalence, the pair becomes real and matter is effectively created out of nothing (well, converted from the field’s energy) – as inversion of particle-antiparticle annihilation.
Vacuum Birefringence — a Helmholtz lighthouse experiment
Before reaching field strengths for such direct observation of those virtual pairs they have indirect and rather weak effect. In particular, the virtual pairs can be partially aligned, rendering vacuum as optical medium with a refractive index different from unity. This is in general coined light-by-light scattering, in contrast to classic electrodynamics with its superposition principle. Light-by-light scattering contributes to many quantum effects as minor contribution. For isolated detection, many pathways are conceivable like frequency conversion (change of color), refraction (change of direction) or birefringence (change of polarization), upon which we focus together with HI Jena, DESY and XFEL.