Even if the name doesn't ring a bell, you've probably seen the distinctive blue glow of Cherenkov radiation before. In many films and documentaries, this glow appears when a high-energy particle passes through a medium where light travels slower than in a vacuum. It bears the name of Nobel Prize-winning physicist Pavel Alekseyevich Cherenkov, who first explained its origin in the early 1950s.
What is Cherenkov radiation?
Everyone knows that nothing can travel faster than light in a vacuum. Those nearly 300,000 kilometers per second are the maximum speed a particle obeying the laws of physics can hope to reach. However, nothing prevents a photon or any other subatomic particle from moving slower than that, and indeed they do all the time. In fact, there are many materials (virtually any you can think of) in which light 'travels' at speeds lower than in a vacuum.
Cherenkov radiation is the electromagnetic radiation produced when a high-energy particle moving through a medium encounters a semitransparent material in which it cannot travel as fast as it did outside. This effect, responsible for the characteristic blue glow in the core of nuclear reactors, was first explained in the early 1950s by the Russian physicist Pavel Alekseyevich Cherenkov, who won the Nobel Prize in Physics for that work in 1958.
A sonic boom of light
Even though explaining the origin of this luminosity without dozens of complicated formulas is nearly impossible, we can try with a much simpler example: the shock wave produced when an aircraft or projectile exceeds the speed of sound. A moving object in the air generates a series of spherical waves that overlap, forming a cone-shaped front. The air molecules have trouble 'getting out of the way in time' and atypical conditions arise around the object, eventually producing the thunderclap you probably already know.
In very particular cases, when the relative humidity and air temperature are right, the air waves create a small condensation cloud due to the Prandtl-Glauert singularity. In the case of light or particles accelerated to that speed, something similar happens, since despite being electromagnetic in nature, they are also a wave. Cherenkov radiation only occurs if the particle crossing the medium (which must be a dielectric) is electrically charged. A proton moving through a medium composed of neutrons would not emit Cherenkov radiation, but it would—and indeed does—if it crosses a medium like water.
The role of cosmic rays
Cosmic rayscosmic rays, which are nothing more than charged particles traveling at the speed of light, constantly bombard our atmosphere. As a result of collisions between them and atmospheric atoms, true 'showers of particles' of many kinds are produced, including several that carry an electric charge. It is very common for some of these particles to asymmetrically polarize the abundant nitrogen and oxygen molecules they encounter. When, after a brief moment, the inevitable spontaneous depolarization of the affected atmospheric molecules occurs, an emission of Cherenkov radiation takes place.
In fact, there are special telescopes, called 'Cherenkov telescopes' that are capable of 'seeing' these flashes. Just as with the air molecules that form the cone-shaped front when a supersonic plane passes, the polarization produced by a charged particle is asymmetric. This means that the molecules in front of its path have not yet polarized when those behind already have. The reason for this behavior is simple: the particle travels—in that medium—faster than its own electric field, so everything in front of it is unaffected until it is overtaken. If the polarization were symmetric, Cherenkov radiation would not occur.
Detecting particles
The Cherenkov effect is used to build particle detectors. This type of sensor can detect the photon produced when a particle crosses the liquid inside it. This effect is also very visible inside nuclear reactors, since during fission neutrons are released that, when crossing the reactor's water, produce a blue glow. Obviously, a large part of Cherenkov radiation is invisible to us, as it occurs at a wavelength our eyes cannot perceive. The effect that intrigued a Russian physicist for many years now helps us analyze the origin of high-energy particles generated millions of light-years away. Isn't that fascinating?
