Light travels across a room, bouncing off mirrors and walls. The emptiness in between seems perfectly neutral: it doesn't hinder the beam or curve its path. But what if, at the deepest foundations of the world, emptiness is more complex? Physicists have long suspected that the symmetry of space might break at tiny scales, and then light would behave differently.
In the new work, scientists calculated how such violations change the laws of electromagnetism. They took the equations describing electrons and photons and added a slight "warp" to the background — a symmetry violation that depends on the point in space. It turned out that the vacuum begins to behave like an inhomogeneous medium: in some places light speeds up, in others it slows down. Moreover, waves can exchange energy with this background — amplifying or damping, like sound in windy weather.
Of particular interest were the so-called "axion-like" terms. The axion is a hypothetical particle that experimentalists are searching for, but here we're talking about mathematical structures that resemble the properties of special materials — Weyl semimetals. In such crystals, electrons behave as if space were curved for them, and similar effects may occur in the vacuum when symmetry is broken.
The calculations showed that for certain combinations of parameters, Furry's theorem is violated — a rule that forbids some processes involving photons. This is consistent with earlier calculations and means that under such conditions, the vacuum can give rise to new types of light interactions. The practical implication: if regions with broken symmetry exist in space, light from distant stars may reach us distorted, and this could be detected by telescopes.
Ultimately, the work shows that emptiness is not an empty box but a complex medium with its own properties. Symmetry breaking makes it akin to glass with variable density, and light passing through it tells us about the hidden structure of the cosmos.
as written by the authors arXiv:2603.20880
This work sets out to compute the corrections to the Euler-Heisenberg effective action that arise from spacetime-dependent background anisotropies that violate Lorentz symmetry. To accomplish our task, we evaluate the functional determinant of the modified Dirac operator using the spectral regularization method. Within the framework of the Standard Model Extension (SME), these Lorentz-violating parameters correspond to the coefficients $$m_5(x)$$, $$a_\mu(x)$$, and $$b_\mu(x)$$. The corrected effective action is attained up to the second-order in the background parameters. Our results indicate a violation of Furry's theorem at second-order for specific CPT-even combinations of these parameters, in agreement with previous analyses based on explicit Feynman diagram calculations in scenarios with Lorentz-symmetry violation. In the kinetic (bilinear) piece of the effective action, non-dynamical axion-like terms emerge, resembling structures commonly encountered in condensed-matter systems, such as Weyl semimetals. We show how this procedure modifies the Maxwell equations, from which we present both the corresponding (non-)conservation of the energy-momentum tensor and the wave equation. We also notice that the vacuum behaves as an inhomogeneous medium, and compute the local dispersion relation by working in the eikonal approximation. As a result of the spacetime-dependence of the background, there appear imaginary contributions in the dispersion relation and, consequently, the wave amplitudes may be amplified or attenuated, which expresses the energy-momentum exchange between the waves and the background.
Weyl semimetals are real crystals in which electrons move as if they had zero mass, like light. They were discovered in 2015 and became a testing ground for ideas born in particle physics.
In Greg Egan's novel "Schild's Ladder," a universe is described where the laws of physics change from point to point, and travelers must account for this to survive.