Mark Littlejohn, of Dunedin, asks :-

I read that Cherenkov radiation occurs when a charged particle moves faster than the speed of light through a transparent medium. But I thought nothing travelled faster than the speed of light?"

Daniel Schumayer, a physicist at Otago University, responded.

What you have read is correct, although it may sound contradictory at first. The main reason of this apparent paradox lies in the loose use of the term "speed of light".

In the 1860s, James Maxwell's theory of electromagnetism predicted that no electromagnetic wave can travel faster than a certain "speed limit". This statement alone was a big surprise at the time, not to mention the fact that the theory also provided a numerical value for this speed. This speed is approximately 300,000 km per second, and this is the value we normally mean by the "speed of light", although a more precise expression would be the "speed of light in vacuum", since any electromagnetic radiation, such as light, radiowave or an X-ray, reaches this speed only in vacuum. The propagation is substantially altered as light enters any material, because it starts interacting with the constituents of that material. One of the results of this interaction is that light appears to slow down. In water, for example, light travels at approximately 225,000 km/s.

In the theory of special relativity, Albert Einstein postulated that nothing can move faster than the speed of light, but this refers to the speed of light in vacuum. Currently no physical principle prohibits travelling at a speed as close to this limiting value as we want. In a cathode-ray television, for example, the electrons can travel with one third of the speed of light in vacuum. In nuclear power stations, where the fuel cells are immersed in water, the emitted particles have high energy, therefore they are very swift and reach a speed which is higher than the speed of light in water. However, their speed is still lower than the speed of light in vacuum.

In summary, the speed of light in vacuum is only an upper bound on the maximum achievable speed, and it is possible for light to travel in a substance at a lower speed. While a particle may travel faster than this lowered speed, it will still be slower than the speed of light in vacuum.