# Planetarium

## "Ok, so, I know that nothing can go faster than the speed of light. The other day, though, I heard that particles which travel faster than light emit a strange kind of blue radiation. But, I didn't think anything could go faster than light?"

It is true, at least according to Einstein's Theory of Special Relativity, that light (denoted by 'c,' because it is a constant in all inertial reference frames) is the highest attainable velocity. However, 'c' refers to the speed of light propagating through a vacuum, which equals 299,792,458 metres per second.* Light slows down when it passes through other media, such as air or water. The presence of particles impedes the photons through scattering. As a vacuum is devoid of such particles, light can attain its maximum velocity, which, as far as we know, cannot be surpassed.

However, light travels at about 0.75c (75% light speed) through water. Some charged particles can move faster than 0.75c in water and therefore travel faster than light. These particles will not, of course, exceed the actual speed of light (c). This charged particle will excite the water molecules, causing them to emit a bluish light. As this particle is moving faster than light (in water), it will generate copious light photons that are in phase with each other. The result is a pervasive glow called "Cherenkov radiation," after Soviet radiation physicist Pavel Cherenkov who first observed this glow in 1934.

One can think of this radiation as the light equivalent of a "sonic boom," that one hears when objects move faster than sound. So, it is possible for objects to move faster than light in media other than a vaccum.

We hope this response proves helpful.

*This speed is exact, as the length of a meter is defined as the distance a beam of light traverses in 1/299,792,458 of a second.