A team of researchers has tested Einstein's century-old principle that the speed of light remains constant, using observations of distant gamma rays. Their analysis found no violations of this rule but improved existing constraints by an order of magnitude. The study highlights ongoing efforts to reconcile quantum theory with gravity.
In 1887, physicists Albert Michelson and Edward Morley conducted an experiment that unexpectedly showed no variation in the speed of light regardless of direction, paving the way for Albert Einstein's special relativity. This theory posits that the speed of light is constant for all observers, underpinned by Lorentz invariance, a principle central to both quantum field theory and the Standard Model of particle physics.
Despite the successes of special relativity and general relativity—which describes gravity as the curvature of spacetime—these frameworks clash when combining quantum mechanics with gravitational effects. Theories of quantum gravity often predict subtle breaches of Lorentz invariance, particularly suggesting that light's speed might vary slightly with a photon's energy at very high levels.
To probe this, a research team including Mercè Guerrero, a former student at Universitat Autònoma de Barcelona (UAB), Anna Campoy-Ordaz, a current IEEC PhD student at UAB, Robertus Potting from the University of Algarve, and Markus Gaug, a UAB lecturer affiliated with IEEC, analyzed very-high-energy gamma rays from cosmic sources. These photons travel immense distances, so even tiny speed differences based on energy could result in detectable time delays upon reaching Earth.
Employing a novel statistical method, the scientists combined prior measurements to scrutinize parameters from the Standard Model Extension that might indicate Lorentz violations. Their findings, published in Physical Review D in 2025, revealed no such effects, upholding Einstein's predictions. However, the work narrows the possible scope for new physics by a factor of ten.
Future instruments like the Cherenkov Telescope Array Observatory promise even sharper tests, continuing the quest to unify quantum theory and gravity.