(This is the final part of a series on Cherenkov radiation — the "light boom." Read Part 1 , Part 2 , and Part 3 first.) So we know what Cherenkov radiation is. We know how it works. We know that Pavel Cherenkov spent three years poking a glowing bottle of water before anyone believed him. Now: what is it good for? The answer, it turns out, is quite a lot. Cherenkov radiation shows up in some of the most dramatic, extreme, and important contexts in modern physics. And also, wonderfully, in hospitals. Let's start with the most visceral image in all of nuclear physics. You've probably seen photographs of nuclear reactors — the ones where the fuel rods are submerged in a deep pool of water, and the water glows. That electric, otherworldly blue. It looks almost supernatural. Like something from a science fiction film. Like the reactor has a soul, and it's blue. That glow is Cherenkov radiation.…