We construct a minimal one-parameter deformation of the Hayward regular black hole by introducing a self-saturating mass profile that preserves asymptotic flatness and a finite ADM mass. The deformation interpolates smoothly between the ordinary Hayward geometry and the Schwarzschild solution, without introducing additional conserved charges or modifying the asymptotic vacuum structure. The resulting spacetime is everywhere regular and exhibits a de Sitter–type core supported by an effective nonlinear electrodynamics source, which we reconstruct explicitly. We analyze the horizon structure and Hawking temperature, showing that the evaporation process generically terminates at a finite-mass, zero-temperature remnant whose properties depend on the deformation parameter. The nonlinear electrodynamic interpretation naturally induces an effective optical metric, leading to controlled corrections to the photon sphere and critical impact parameter. Our results provide a unified and minimal framework for exploring strong-field and observational signatures of regular black holes beyond the Hayward model.

We present the Green function method in the context of higher- and infinite-derivative field theory, and apply it to (i) static, (ii) ultrarelativistic, and (iii) uniformly accelerated charge distributions across a wide range of scenarios. We close by emphasizing universal imprints in topological quantities that could serve as a smoking gun indicator for such deviations from standard Maxwell theory.

Massive electrodynamics with derivative self-interactions provide a rich playground for the study of stability, even in the first-order arena. I will point out subtleties on the constraint algorithm of such theories, particularly when it comes to ensuring functional independence among constraints. Rarely taken into consideration, such independence is nonetheless essential to correctly assess the stability of field theories. The wise lesson applies to gravity as well.

The standard minimally coupled Einstein-Maxwell action can be modified in two different ways: by introducing nonminimal couplings between the gravitational and electromagnetic sectors, or by replacing linear Maxwell theory with nonlinear electrodynamics (NLE). A natural question is whether such modifications can remove black hole singularities that are generically present in the Einstein-Maxwell case. NLE fields, originally introduced as effective QED corrections or to regularise the divergent self-energy of point charges, have also been regarded as possible mechanisms for black hole regularisation. However, this possibility faces fundamental obstructions. We first show that broad families of NLE Lagrangians cannot give rise to regular black hole solutions [1]. Moreover, even when the presented no-go results are circumvented, regular black holes sourced by NLE fields must satisfy restrictive mass-charge relations [2]. We then turn to nonminimally coupled Einstein-Maxwell models and consider several classes of interaction terms. We prove that magnetically charged black holes remain singular [3] and show that, under mild additional assumptions, the same conclusion holds for the electrically charged case. Overall, our results suggest that these classical modifications of the Einstein-Maxwell theory do not provide generic or physically viable models of regular black holes. [1] A. Bokulić, T. Jurić, I. Smolić: Constraints on singularity resolution by nonlinear electrodynamics, Phys. Rev. D 106, 064020 (2022) [2] A. Bokulić, T. Jurić, I. Smolić: Conundrum of regular black holes with nonlinear electromagnetic fields, Phys. Rev. D 113, 024044 (2026) [3] A. Bokulić, T. Jurić, L. K. Pejić, F. Požar, I. Smolić: Constraints on regular black holes with nonminimally coupled electromagnetic fields, arXiv:2606.13773 [gr-qc] (2026)

In this presentation I explain the classical theory of relativistic particles in a covariant hamiltonian frame work. This applies both to space-time covariance and gauge covariance of abelian and non-abelian type. A scheme for deriving conservation laws for charged and spinning particles in non-trivial backgrounds such as monopoles and black holes is developed. The scheme applies in particular also to the conservation of expressions of second or higher degree in momenta. Extending the procedure to quantum theory is straightforward.

Does the observed lepton mass family (e, mu, tau,...) arise from a spectrum of excited quantum states of a Dirac-Bopp-Lande-Podolsky-Thomas (Dirac-BLPT) membrane with intrinsic spin? In 1958, Paul Dirac outlined a mathematical relativistic membrane model for an extended electron. He suggested, but was unable to verify, that the muon might arise as an excited spherically symmetric stationary quantum state of the membrane without intrinsic spin, in Minkowski spacetime. Since then, there have several attempts to improve his model within the confines of Maxwellian electrodynamics using various approximation schemes and various generalisations. In this talk we suggest a new approach to this problem that accommodates relativistic membranes with intrinsic spin and BLPT Electrodynamics, within the context of a Rayleigh-Ritz estimate. Particular emphasis is placed on the relevance of fundamental parameters needed to non-dimensionalise the quantum Rayleigh-Ritz functional equation.

The exchange of new light(ish) particles, scalars, pseudoscalars or vectors, coupled to electrons and nucleons leads to extra forces and corresponding potentials between particles and even macroscopic bodies. We motivate some interesting examples. Based on the type of particle and the fundamental interaction vertex the potential may be spin or velocity dependent. The high accuracy as well as the sheer number of precision measurements in atomic and ionic systems provides a powerful laboratory to test these forces at atomic and even smaller distances. We discuss new tests of feebly interacting bosons in highly charged ions, but also atomic tests of Lorentz and even CPT violating forces. We also compare to astrophysical probes.

Generalised (or Generic) Non-Linear Electrodynamics is a framework that encompasses a broad array of models, which all exhibit non-linearities through background photon interactions. In previous work, we showed that these interactions lead to frequency drifts over astrophysical scales, and that with a suitable field redefinition it is possible to recast these dissipative effects as a mass term. In this presentation, we will show that despite the presence of a mass term, and the apparent loss of the manifest gauge invariance of the theory, that we are still dealing with a gauge theory. We will show through the dispersion relations that there still exist null vectors, which all depend on background corrections, modifying the definition of the transversal projector before deriving the propagator for the theory. We will conclude through a canonical analysis, to definitely rule that the model possesses two degrees of freedom, a massive pole, as well as derive the new gauge transformations and Noether's second identity.

In this work, we explore the geodesic structure and observational appearance of a regular black hole with gravitational self-energy. The underlying geometry describes a neutral regular black-hole solution in which the nonlocal gravitational self-energy, induced by a T-duality-inspired zero-point length, modifies the effective mass function. The spacetime is of Ay\'{o}n-Beato–Garc\'{i}a type, although it carries no electric charge; instead, the regularization is controlled by the zero-point length $l_{0}$. In particular, we examine the orbital structure of null and timelike geodesics, with emphasis on the existence and stability of circular orbits, including the innermost stable circular orbit and the light rings. We then assess how the zero-point length modifies the observable signatures of accretion disks and black hole shadows relative to the corresponding Schwarzschild geometry. Depending on its value, the model parameter $l_{0}$ can enhance or suppress the effective gravitational influence on photon propagation, thereby altering the photon trajectories and the resulting observational features.

The standard (advanced) text-book equations of motion for a point charge that reacts to its own radiation, such as the Lorentz-Abraham-Dirac, the Landau-Lifschitz, and the Eliezer–Ford–O'Connell equations, all yield a vanishing radiation-reaction force in the simplest non-trivial initial value problem that is treated in all elementary mechanics textbooks: a point charge released from rest in a constant external electric field. The Larmor formula on the other hand predicts radiation losses in this situation. Something is amiss. The culprit has long been suspected (by Born and others) to be Maxwell's ``law of the pure ether.'' Everything comes out mathematically consistently, indeed, when Maxwell's law is replaced by the Bopp-Landé-Thomas-Podolsky law of the electromagnetic vacuum. Whether this results in a physically viable theory is still being sorted out. In this presentation I survey the state of affairs. Joint work with S. Tahvildar-Zadeh, H. Carley, R. Mcguigan, L. Sargsyan.

Although compatible with General Relativity (GR), the dark Universe lacks experimental confirmation and support by the Standard Model (SM). Opposing darkness, extensions of GR face the successes of the latter. But photons, main messengers, are still read with the XIX Maxwell theory, although Quantum Electro-Dynamics (QED) requires non-linear corrections, e.g., photon-photon. Extended Theories of Electro-Magnetism (ETEM) induce a reinterpretation of the observations. In presence of a background, the SM Extension (SME) or Non-Linear Electro-Magnetism dress the photon, the only SM free massless particle, with an effective mass [1-3], compatible with the upper limits from Fast Radio Bursts [4-6] and solar wind [7,8]. Birefringence, dispersion, second-order QED are tested or searched ETEM effects at, e.g., BMV Toulouse, ATLAS CERN, DeLLight Paris. All photons either massive - ab initio as in the de Broglie-Proca theory or through the SME or the Born-Infeld, Heisenberg-Euler non-linear type of theories - undergo a frequency shift in presence of an electromagnetic and/or a Lorentz Symmetry Violation background [9,10]. This small additional shift, towards the red or the blue, added to the expansion redshift, determines new cosmological scenarios, e.g., without dark energy and matter [11-14]. Interferometry guarantees testing perspectives [15,16] below 3x10-18 in Δν/ν per metre for an Earth-Moon-like optical length. We aim to determine a dissipative effect free from blurring due to the interaction of the photon energy with the curvature produced by an electromagnetic or gravitational field [17-20]. In parallel, we aim to study photon-photon interactions in the frame of ETEM. Finally, we implement the Heisenberg principle at cosmological distances, where the Hubble tension appears as quantum measurement limit [21,22]. References [1] Bonetti L., dos Santos Filho L.R., Helayël-Neto J.A., Spallicci A.D.A.M., Phys. Lett. B, 764, 203 (2017) [2] Bonetti L., dos Santos Filho L.R., Helayël-Neto J.A., Spallicci A.D.A.M., Eur. Phys. J. C, 78, 811 (2018) [3] Dib A., Helayël-Neto J.A., Spallicci A.D.A.M., Eur. Phys. J. C, 86, 3 (2026) [4] Bonetti L., Ellis J., Mavromatos N.E., Sakharov A.S., Sarkisyan-Grinbaum E.K.G., Spallicci A.D.A.M., Phys. Lett. B, 757, 548 (2016) [5] Bonetti L., Ellis J., Mavromatos N.E., Sakharov A.S., Sarkisyan-Grinbaum E.K.G., Spallicci A.D.A.M., Phys. Lett. B, 768, 326 (2017) [6] Bentum M.J., Bonetti L., Spallicci A.D.A.M., Adv. Space Res., 59, 736 (2017) [7] Retinò A., Spallicci A.D.A.M., Vaivads A., Astropart. Phys., 82, 49 (2016) [8] Spallicci A.D.A.M., Sarracino G., Randriamboarison O., Helayël-Neto J.A., Dib A., Eur. Phys. J. Plus, 139, 551 (2024) [9] Helayël-Neto J.A., Spallicci A.D.A.M., Eur. Phys. J. C, 79, 590 (2019) [10] Spallicci A.D.A.M., Dib A., Helayël-Neto J.A., Phys. Lett. B, 885, 138873 (2024). [11] Spallicci A.D.A.M., Helayël-Neto J.A., López-Corredoira M., Capozziello S., Eur. Phys. J. C, 81, 4 (2021) [12] Spallicci A.D.A.M., Sarracino G., Capozziello S., Eur. Phys. J. Plus, 137, 253 (2022) [13] Sarracino G., Spallicci A.D.A.M., Capozziello S., Eur. Phys. J. Plus, 137, 1386 (2022) [14] Dib A., Djeghloul N., Spallicci A.D.A.M., submitted [15] Abend S. et al., AVS Quantum Sci., 6, 024701 (2024) [16] Abdalla A. et al., Eur. Phys. J. Quantum Techn., 12, 42 (2025) [17] Blanchet L., Spallicci A., Whiting B., Mass and motion in general relativity, Fundamental Theory of Physics 162 (Springer, 2011) [18] Spallicci A.D.A.M., Ritter P., Aoudia S., Int. J. Geom. Meth. Mod. Phys., 11, 1450072 (2014) [19] Ritter P., Aoudia S., Spallicci A.D.A.M., Cordier S., Int. J. Geom. Meth. Mod. Phys., 13, 1650019 (2016) [20] Dib A., Garnier A., Spallicci A.D.A.M., Int. J. Geom. Meth. Mod. Phys., 23, 2550278 (2026) [21] Capozziello S., Benetti M., Spallicci A.D.A.M., Found. Phys. 50, 893 (2020) [22] Spallicci A.D.A.M., Benetti M., Capozziello S., Found. Phys. 52, 23 (2022)

In this talk I will present the current understanding of the canonical structure of several popular modified teleparallel gravities based on torsion and nonmetricity of a teleparallel connection, and their intimate link to the structure of Maxwell electrodynamics and Yang-Mills theories. I will show how the Hamiltonian formulation of these modified gravity theories reveal internal consistency problems, and how some of these lessons could apply to generalized electrodynamics as well.

I discuss the theoretical foundations for testing nonlinear vacuum electrodynamics with Michelson interferometry. The discussion applies to all nonlinear electrodynamical theories of the Plebanski class, i.e., to all Lagrangians that depend only on the two Lorentz-invariant scalars quadratic in the field strength. The main idea of the experiment proposed here is to use the fact that, according to nonlinear electrodynamics, the phase velocity of light should depend on the strength and on the direction of an electromagnetic background field. There are two possible experimental setups for testing this prediction with Michelson interferometry. The first possibility is to apply a strong electromagnetic field to the beam in one arm of the interferometer and to compare the situation where the field is switched on with the situation where it is switched off. The second possibility is to place the whole interferometer in a strong electromagnetic field and to rotate it. If an electromagnetic field is placed in one arm, the interferometer could have an arm length of several hundred meters. If the whole interferometer is placed in an electromagnetic field, one would have to do the experiment with a tabletop interferometer. A null result of the experiment would place bounds on the parameters of the theory. I specify the general results to some particular theories of the Plebanski class; in particular, I give numerical estimates for Born, Born-Infeld, and Heisenberg-Euler theories. - The talk is largely based on G. Schellstede, VP and C. Laemmerzahl, Phys. Rev. D 92, 025039 (2015).

Ring lasers emit two counter‑propagating beams inside a ring optical cavity; when the cavity rotates, the resonance conditions differ for the two directions and the resulting frequency split is proportional to the rotation rate (Sagnac effect). The GINGER (Gyroscopes IN General Relativity) experiment aims to build an array of large‑frame ring laser gyros for fundamental physics applications. One of the prototypes, GINGERINO, is a square ring laser with 3.6 m sides installed at LNGS. GINGERINO’s noise floor has been evaluated in the femto‑rad/s range, a result that paves the way for using high‑precision optical gyros in fundamental physics tests. We present an overview of the GINGER experiment, covering the working principle and the experimental setup. We also discuss some applications in fundamental physics, including tests of gravitomagnetism.

Precision optical polarimetry provides a powerful experimental approach to search for tiny modifications of light propagation in vacuum. In this talk, I will present the development of high-sensitivity polarimetric experiments at the Max Planck Institute for Gravitational Physics aimed at probing vacuum magnetic birefringence, one of the fundamental predictions of quantum electrodynamics, and at searching for axion-like particles and other light weakly interacting fields. I will discuss the experimental techniques employed to reach extreme polarization sensitivity and outline how the same platform can be used to test a broader class of generalized electrodynamics, including theories predicting vacuum birefringence or modified photon propagation. Finally, I will highlight future opportunities to connect precision laboratory experiments with tests of fundamental physics beyond the Standard Model and classical Maxwell electrodynamics.

In this talk we analyse the self-consistent classical electromagnetic interaction of compact electron bunches undergoing finite hyperbolic accelerated motion. We apply perturbation theory to a model of electron acceleration over a finite length scale in a vacuum to extract the fundamental physics of the interaction, independent of the specific acceleration method. We consider both the far-field and near-field features of this problem and demonstrate that a compact, high charge bunch of charge accelerated by a high-field gradient (>1GV/m) emits a radiation pulse in the THz frequency regime that is significantly enhanced by coherence effects, analogous to Coherent Synchrotron Radiation (CSR), which consequently carries a measurable amount of energy in the nJ-μJ regime, contrary to common expectation. In the near field we derive non-singular integral expressions for the fields of an accelerated, Gaussian charge distribution. As an electron bunch transitions from a low-to-high energy state via a strong applied field the Lienard-Weichert fields are distorted and induce asymmetric forces across the bunch. This leads to a net de-accelerating force on the electron bunch. We extend the analysis via perturbation theory and calculate higher order effects including the correction to the charge density, which describes the evolution of the electron distribution function under the action of the self-forces, as well as calculating additional corrections to the electromagnetic self-field.