Einstein's speed of light theory confirmed again

Physicists have once again tested one of the pillars of Albert Einstein's theory of relativity, confirming that the speed of light remains constant regardless of its energy.

Researchers measure the speed of light with unprecedented precision

A team led by Mercè Guerrero at the Universitat Autònoma de Barcelona used powerful gamma-ray bursts from distant galaxies to scrutinize whether light could travel slightly faster or slower depending on its energy. Their findings, published in 2025, showed that photons of varying energies arrived at Earth nearly simultaneously.

The experiment, which reduced the margin of error tenfold compared to previous measurements, has significant implications for theories attempting to unify Einstein's special relativity with quantum mechanics. The result not only validates Einstein's theory but also sets increasingly stringent limits on models predicting deviations in the behavior of light.

Gamma-ray bursts, the most energetic explosions in the universe, offer an ideal scenario for searching for such an effect. These cataclysmic events, resulting from the collapse of massive stars or the collision of neutron stars, generate powerful radiation that expands across space, carrying photons with diverse energies.

By measuring the exact moment when each particle of light was detected, scientists could determine if higher-energy photons traveled at a different pace. However, the analysis revealed no detectable delay between photons of varying energies, indicating that the speed of light remains absolute, even after traversing vast cosmic distances.

As Anna Campoy-Ordaz, a co-author of the study, remarked,