One of the questions I get most often in my physics and astronomy courses is "can anything travel faster than the speed of light?" The problem is, this is an incomplete question. The question should state "Can anything travel faster than the speed of light in a vacuum?" The answer to that question is "no". (A theoretical particle known as a tachyon is purported to travel faster than light speed, though such a particle has never been detected.)
However, the speed of light slows down in different media. For instance, the speed of light is slower in air than it is in a vacuum; and it is slower yet in glass or in water. In certain cases it is possible for particles to travel faster than the speed of light for a given medium.
What is Cherenkov Radiation?
When a very high energy gamma-ray -- that is, a very short wavelength photon -- travels from a vacuum into a denser medium like air or water it will interact with the material. As a result the photon actually ceases to exist, and instead turns its considerable energy into mass in the form of an electron and a positron (sometimes called an anti-electron since it is the electron's anti-particle).
The electron and positron are created with such high energy, however, that they are actually traveling faster than the speed of light for the given medium they are in. As they slow down they create what amounts to an optical boom. It is similar to the sonic boom that a jet aircraft creates when it exceeds the speed of sound in air. The difference is that instead of making an audible noise, a faint blue light is created. This light, known as Cherenkov radiation spreads out through the medium in a cone shape along the trajectory of the charged particle.
Applications to Astronomy
The most well known occurrence of Cherenkov radiation is in nuclear reactors. During the nuclear fission process very high energy gamma-rays are emitted. As these gamma-rays propagate from the reactor rods, they pass through a pool of water causing the water to glow blue. This is actually useful, as the intensity of the Cherenkov radiation is an indication to the rate of fission in the reactor. Also, very high energy protons can be created in the reactor, which will also produce Cherenkov radiation as they pass through the water.
But you don't need to peer into the core of a nuclear reactor to find Cherenkov radiation. Rather, it is created all the time in the Earth's atmosphere. Very high energy gamma-rays (and high energy protons known as cosmic rays) from stellar explosions, neutron stars and even other galaxies stream toward Earth and interact with the molecules in our atmosphere. This leads to Cherenkov radiation beaming down to the surface of the Earth. But because of the height in the atmosphere at which these reactions occur, as well as how faint the light is to begin with, it is difficult if not impossible to detect the radiation with the naked eye. (I have met people who claim to be able to see the faint blue light on particularly dark evenings, but have never been able to do so myself. I'm not convinced it's possible.)
Since detecting gamma-rays and cosmic rays are important to understanding the nature of stars, black holes and galaxies, we need a way of detecting them. Since conventional, space-based, detectors can only do so much, a more novel approach must be made. Ground-based optical reflectors can be used to collect the Cherenkov radiation and use it to determine where and with what energy the original gamma-ray or cosmic ray came from. Instruments, known as Atmospheric Cherenkov Telescopes, such as VERITAS, HESS, MAGIC II and CANGAROO III use this technique to search the Universe for gamma-rays so that we may better understand the physics of the cosmos.
