Scientists have once again taken aim at one of physics’ most ironclad rules, Einstein’s assertion that nothing can outpace the speed of light, using ultra-high-energy gamma rays from distant cosmic explosions to hunt for cracks in special relativity, only to tighten the noose on potential violations by an order of magnitude.
The effort, detailed in a new study published in Physical Review D, stems from theories of quantum gravity that hint at subtle tweaks to light’s behavior at extreme energies, potentially allowing photons to travel at slightly varying speeds based on their energy levels. Led by researchers at Spain’s Universitat Autònoma de Barcelona (UAB) and Portugal’s University of Algarve, the team analyzed existing observations of very-high-energy gamma rays blasting from far-off astrophysical sources, like gamma-ray bursts or active galactic nuclei. If higher-energy photons lagged or led their lower-energy counterparts over cosmic distances, spanning billions of light-years, even minuscule speed differences would accumulate into detectable timing delays upon reaching Earth.
Employing a fresh statistical method, the scientists scoured data for signs of Lorentz invariance violation (LIV), a key prediction in some quantum gravity models that could reconcile quantum mechanics with general relativity by allowing tiny deviations from Einstein’s constant speed of light. Lorentz invariance, the principle that physics laws hold steady regardless of an observer’s motion, underpins special relativity and has withstood over a century of tests, including the famous 1887 Michelson-Morley experiment that found no variation in light speed tied to Earth’s movement.
No such violations turned up. Instead, the analysis sharpened existing bounds on LIV parameters within the Standard Model Extension framework by a factor of ten, pushing the effective energy scale for quantum gravity effects to unprecedented heights and leaving even less room for new physics to emerge. The paper, first posted on arXiv in August 2025 and published in November, highlights how these null results echo the Michelson-Morley outcome, which inadvertently paved the way for Einstein’s breakthroughs.
The research team, including former UAB student Mercè Guerrero, IEEC PhD candidate Anna Campoy-Ordaz, University of Algarve’s Robertus Potting, and UAB lecturer Markus Gaug (also affiliated with the Institute of Space Sciences), drew on measurements from current telescopes but eyes future upgrades like the Cherenkov Telescope Array Observatory for even sharper probes.
This isn’t the first cosmic litmus test for Einstein. Just months earlier, in November 2025, separate studies using gamma-ray bursts reconfirmed light’s unwavering speed across energies and colors, aligning with relativity’s predictions and quashing energy-dependent variations. Other experiments, like those simulating near-light-speed objects at the Vienna University of Technology, have visualized relativity’s weird visual distortions without breaching the limit.
While clickbait headlines occasionally tout “faster-than-light” feats, like quantum entanglement or light pulses in special media, these don’t actually violate relativity, as no information or mass exceeds c (about 299,792 kilometers per second in vacuum). Theoretical loopholes, such as warp drives or wormholes, remain speculative and energy-prohibitive.
As quantum gravity quests intensify, these ever-stricter tests could either crown Einstein eternally or finally unveil the next paradigm shift, but for now, the speed of light stands unchallenged, guiding everything from GPS to black hole imaging.
