[nayuki]

> One of those circumstances is being close to a nuclear reactor submerged in water, which allows Cherenkov radiation to be observed. In layman’s terms: gamma radiation exits the reactor at the speed of light in a vaccuum, but the speed of light in water is lower, and photons have to slow down somehow. The direct, observable effect is that water glows blue.

Almost but not quite. The speed of light is always the speed of light. There's nothing wrong with gamma rays.

It's charged particles such as beta and alpha rays that generate the blue Cherenkov radiation. The writer's link to Wikipedia already reflects that.

[NextHendrix]

The speed of light in a vacuum is always the same, but not through dielectric media such as water or glass. In diamond it is less than half the speed.

[nayuki]

You're right, but I knew that. I should've worded my statement in such a way to prevent criticism.

Gamma rays always follow the speed of light in the medium that they travel through, because they are light. They cannot exceed the local speed of light, thus they cannot generate Cherenkov radiation. It must be particles with charge and non-zero mass that are capable of generating Cherenkov radiation.