Fast radio bursts are bright cosmic flashes that, like a starting signal, launch a race of radio waves across the Universe. Astronomers used them to check whether the photon – the particle of light – has mass. If mass existed, the speed of light would depend on its color: blue rays, like heavier runners, would lag behind red ones. But observations show that any delay is fully explained by the drag of intergalactic hydrogen – a sparse gas that only slightly slows the waves.

Scientists considered various scenarios of Universe’s expansion, including the influence of dark energy, and set a new mass limit: the photon is lighter than 5×10⁻⁵¹ kg. This is billions of times less than the mass of the lightest known particle. The precision is astonishing: distant blue light would lead red by only a second – and that tiny shift would already have been noticed, but it’s not there.

The result strengthens the foundation of physics laid by Albert Einstein: light remains the fastest and massless racer.