Radioactive material glowing blue in water meant particles outrunning light itself
In 1934, Pavel Cherenkov noticed a faint blue glow from radioactive sources in water and correctly linked it to charged particles moving faster than light travels in that medium — light's universal speed limit still holds, just lower in water than in a vacuum. The effect is why nuclear reactor cores glow blue. Cherenkov shared the 1958 Nobel Prize in Physics for the discovery.