The blue glow in the water surrounding a nuclear reactor core does not mean that a particle has broken Einstein’s speed limit. Relativity forbids a massive particle from reaching or exceeding the speed of light in vacuum.

In a material such as water, however, light’s phase velocity is lower. A sufficiently energetic charged particle, usually an electron, can therefore move faster than light’s phase velocity in the water while still travelling slower than light in vacuum. The electromagnetic disturbances it creates then add coherently along a cone: the optical analogue of a sonic boom.

In 1934, Pavel Cherenkov systematically investigated this unusual blue light while studying liquids exposed to radioactive radiation under Sergei Vavilov. Cherenkov showed that the emission was directional and polarized, evidence that it was not ordinary fluorescence. Ilya Frank and Igor Tamm supplied the theoretical explanation in 1937. Cherenkov, Frank and Tamm shared the 1958 Nobel Prize “for the discovery and the interpretation of the Cherenkov effect.”

Cherenkov radiation is now used to identify and measure fast particles in nuclear and particle physics, cosmic-ray experiments and neutrino observatories. The glow is not an exception to relativity; it appears because relativity’s speed limit has been stated precisely.

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