In a reactor pool, particles outrun light
Light in water travels at about three quarters of its speed in vacuum. A charged particle from a reactor can go faster than that. The result is a blue glow.
A particle cannot exceed the speed of light in empty space. It can exceed the speed of light in water, which is slower by the refractive index, about 1.33, so about three quarters of the vacuum speed. When a charged particle does this it polarises the molecules as it passes, and they radiate. The radiation adds along a cone, the optical equivalent of a sonic boom. Pavel Cherenkov saw the blue glow in the 1930s. Ilya Frank and Igor Tamm explained the cone. The three shared the Nobel prize in 1958.
The glow in a reactor pool is spent fuel and fission products, their fast electrons outrunning light in the water that shields them. The same effect, in ice and in air, is how some telescopes detect cosmic rays. The colour is blue because the shorter wavelengths are favoured. Nothing about the glow breaks the vacuum limit. The limit is what makes the glow informative: the particle is between two speeds, faster than light in that water and slower than light in the dark.
Caveats
- ‘Outrun light’ here means light in water. The particle is still slower than light in vacuum. An entry in this library is named for that second fact, and both are true.
- The blue is not fluorescence of the water, though water can fluoresce as well. Cherenkov light has a direction and a spectrum. That is how the two are told apart.
Sources
- 1Cherenkov P. A.. Nobel Lecture: Radiation of particles moving at a velocity exceeding that of light. Nobel Foundation, 1958.