A photon with an energy of about 300 teraelectronvolts (TeV)—the highest ever recorded from a gamma-ray burst—has sparked new questions about Einstein’s theory of relativity. This photon was detected by the Carpet ultra-high-energy cosmic-ray detector in the Russian Caucasus. It traveled over 2 billion light-years from the gamma-ray burst GRB 221009A, known as BOAT (Brightest Of All Time), which was observed on October 9, 2022. According to existing physics, such a high-energy photon should have interacted with photons from the cosmic microwave background during its journey, transforming into other particles and disappearing before reaching Earth. However, the photon survived its journey, challenging current models of how high-energy particles behave in space. The study, led by Giorgio Galanti (INAF) and Marco Roncadelli (INFN), proposes a possible explanation involving axion-like particles (ALPs), hypothetical particles that could allow photons to temporarily convert into ALPs during their journey and then convert back into photons near Earth. This mechanism, however, is not sufficient to fully explain the survival of the 300 TeV photon. The researchers combined the ALP idea with a possible violation of Lorentz invariance, a fundamental property of Einstein’s special theory of relativity. Some quantum-gravity models suggest that at the highest energies, this property could be slightly modified, allowing the photon to avoid interactions that would have destroyed it during its journey. This would make the universe more "transparent" at such high energies, allowing the photon to reach Earth. The model also predicts that the 300 TeV photon should arrive about one hour later than lower-energy photons detected by the Chinese Large High Altitude Air Shower Observatory (LHAASO). This delay was indeed observed, supporting the researchers’ hypothesis. If future observations confirm this scenario, the universe could become a natural laboratory for studying quantum gravity at energies much higher than those achievable by any Earth-based accelerator. The findings, published in Physical Review Letters, combine two previously separate ideas and offer a single explanation for the survival of the highest-energy photon and its delayed arrival compared to lower-energy photons. Further observations will be needed to test the proposed theoretical scenario. This discovery highlights the importance of studying extreme cosmic events to probe the fundamental laws of physics, potentially revealing new insights into the nature of the universe at its most energetic scales.