Overview

Electromagnetic fields are among the most fundamental systems in physics. Without electromagnetism, no observation could be made and no scientific instrument would function. Although electromagnetism is the most extensively studied and technologically exploited interaction in nature, ranging from classical electrodynamics to quantum optics and emerging quantum technologies, important theoretical and experimental questions remain open. Persistent issues such as the radiation reaction of charged particles, divergences in self-energy, and the consistent description of electromagnetic phenomena continue to motivate new investigations into the foundations of electrodynamics.

At the same time, increasingly precise experiments now allow us to probe subtle quantum properties of the vacuum itself. Phenomena such as vacuum birefringence, photon-photon scattering, and small deviations from classical Maxwell dynamics in ultra strong electromagnetic fields provide promising observational windows into the physics beyond the standard linear description of electromagnetism. These developments suggest that several long-standing problems may be addressed through non-linear, non-local, or massive extensions of Maxwell equations, including approaches inspired by the Standard Model Extension (SME).

The importance of these questions extends far beyond laboratory physics and reaches into cosmology and astrophysics. Fritz Zwicky's work, 93 years ago, on the stability of galaxy clusters led to the hypothesis of Dark Matter, while the discovery, 28 years ago, of the non-linear relation between redshift and distance motivated the concept of Dark Energy. Although both interpretations are compatible with General Relativity (GR), the dark sector still lacks direct experimental confirmation and remains unsupported by the Standard Model (SM).

At the same time, photons remain the principal messengers of the Universe and the only freely propagating massless particles in the SM. Nevertheless, their behaviour is still commonly interpreted within the framework of nineteenth century Maxwell theory, despite the fact that Quantum Electrodynamics (QED) predicts corrections such as photon photon interactions and other non-linear effects.

Generalised Maxwellian theories therefore open the possibility of reinterpreting astrophysical and cosmological observations. Within these approaches, electromagnetic signals may propagate differently because of non-linearities, effective photon masses, violations of Lorentz symmetry, or other effects described by the SME framework. Such theories may provide alternative insights into observations traditionally attributed exclusively to dark components of the Universe, while at the same time deepening our understanding of the electromagnetic interaction itself.

This meeting is the successor to the Heraeus seminar held in 2024.