Why nuclear reactors glow an eerie blue

Peer into the water pool of a nuclear reactor and you'll see an otherworldly blue glow. It looks like something out of science fiction, but it's a real optical effect called Cherenkov radiation — and it hides a beautifully strange twist on one of physics' most famous rules.

You've probably heard that nothing can travel faster than light. That's true for light moving through a vacuum. But light slows down when it passes through a medium like water, dropping to about 75% of its vacuum speed. Charged particles — such as the high-energy electrons flung out during radioactive decay — face no such slowdown and can actually zip through the water faster than light travels in that water.

When a charged particle outruns light in a medium, it creates the optical equivalent of a sonic boom. Just as a jet breaking the sound barrier drags a cone of built-up sound behind it, the speeding electron leaves a shockwave of light in its wake. This light is emitted more strongly at shorter wavelengths, which is why the glow appears blue rather than red or green.

Far from being just a pretty sight, Cherenkov radiation is a useful tool. Scientists use it to detect elusive particles like neutrinos in giant underground water tanks, and the glow's brightness helps engineers gauge the intensity of radiation in a reactor's core.