"The inside of the Petten test reactor where the thorium salt is being tested is shining due to charged particles traveling faster than the speed of light in water."
As I understand it, nothing travels faster than the speed of light. The author is mistaken, right?
But the index of refraction of water is 1.33 . So the speed of light through water is much slower than through vacuum. And electrons can travel through water faster than light through water.
The effect is similar to a massive object traveling faster than the speed of sound through air.
https://en.wikipedia.org/wiki/Cherenkov_radiation
Edit: I love that HN is so smart that a dogpile can form from all the people rushing in to explain Cherenkov radiation.
"Speed of light" seems simple, but is a bit more complicated than that.