How Can Anything Travel Faster than Light Leaving a Trail of Blue Light?

We, including scientists often say that nothing or rather no particle can travel ‘faster than light’ and that the ‘speed of light’ is the ultimate limit of speed.

When we say this, we are only telling half the truth, because light actually travels at different speeds through different media. Light travels faster through a vacuum at a speed of   299 792 458 meters (299 792.458 km) per second than if it was through a medium.  It is only through a vacuum which is the final speed of light.

In that case we should say no particle can travel ‘faster than the speed of light in a vacuum’ to be more technical in our claims.

Light travelling through any transparent medium always travels more slowly than it does in a vacuum, in some cases much more slowly. The more slowly it travels in a particular medium, the greater the angle through which it bends at an oblique angle from a vacuum into the medium. We call this as ‘refraction’.  The amount of bending is defined by a quantity termed as the ‘index of refraction.’

If the speed of light in a vacuum is divided by the index of refraction of a particular medium, we can calculate the speed of light in that medium. The index of refraction of air at standard pressure and temperature is about 1.0003.  If   299,792,458 meters per second is divided by 1.0003, this means the speed of light in the air is 299,702,547 metres per second. This is 89,911 metres per second less than the speed of light in a vacuum.

The index of refraction of water is 1.33, for glass it is 1.7, and for diamond it is 2.42. This means the speed of light through water is, 225,407,863 m /s, and at 176,348 ,504 metres per second through glass. But through diamond, its speed is only 123,881,181 metres per second.

 Particles cannot travel faster than 299,792.458 km per second, but they can travel at 225,407,863 km /s through water. When they do this, they are travelling through water faster than the speed of light in water.  Surprisingly to most people, it is possible for particles to travel faster than light in any medium but in a vacuum.

Particles travelling faster than light in some non-vacuum media emit a blue light that trials behind. The angle at which it trails behind depends on how much faster than the speed of light in that medium the particle is going.

The first to observe blue light emitted by faster-than-light particles was a Russian physicist named Parvel A.  Cerenkov, who reported it in 1934. The light is then called ‘Cerenkov radiation’.

Cerenkov is electromagnetic radiation emitted when a charged particle such as an electron passes through a dielectric medium at a speed greater than the phase velocity (speed of propagation of a wavefront in a medium) of light in that medium.

 A classic example of Cherenkov radiation is the characteristic blue glow of an underwater nuclear electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium at a speed greater than the phase velocity (speed of propagation of a wavefront in a medium) of light in that medium.

Its glow is similar to the cause of a sonic boom, the sharp sound heard when faster-than-sound movement occurs.  

 In 1937, two other Russian physicists, Ilya M. Frank and Igor Y. Tamm, explained the existence of this light by relating it to the relative speeds of particles and light in the medium. The result of their discovery and explanation, all three scientists were awarded the Nobel Prize in physics in 1958.

Following their discoveries and observation special instruments called ‘Cerenkov counters’, have been designed to detect such radiation and measure its intensity and the direction in which it was given off.

Cerenkov counters are very useful in particle physics because they are activated only by very fast particles and because from the angle at which the light is emitted, the speed of those particles can easily be estimated.  Very energetic cosmic rays move at a speed so close to light in a vacuum that they will produce Cerenkov radiation even in the air.

Tachyons, which are hypothetical particles that can only move faster than the speed of light in a vacuum, would leave a very brief flash of Cerenkov radiation that physicists hope to prove if tachyons actually exist.