Have you ever noticed that when a supersonic military aircraft passes overheadat a very high speed, it leaves a thunderous boom in its wake? That tremendous cracking sound actually hasa formal name – a SONIC BOOM.
This intense phenomenon occurs whenan object – in this case, an aircraft – travels faster than the speed of sound in air.
This might naturally lead to the questiondoes light also have a sonic boom equivalent? Does anything unusual happen when somethingtravels faster than light? In 1934, a Soviet physicist named Pavel Cherenkovobserved that a water bottle emitted blue light when bombarded with radiation.
This blue-colored light was also emitted froma number of transparent objects in the proximity of a radioactive source.
This intrigued Cherenkov, and he conducteda set of experiments to understand the origin of this strange phenomenon.
Cherenkov irradiated various liquids withgamma rays from a 104 mg radium source.
His initial objective was to study fluorescenceemission induced in uranyl salts.
In his early experiments, Cherenkov made naked-eyeobservations, which was very difficult and required high levels of patience and experimentalskill.
Through his experiments, Cherenkov noted thatlight was emitted even when a vessel contained only sulfuric acid – the solvent for the uranylsalt.
He also demonstrated that such a light wasobserved even when different solvents were used This led him to conclude that irrespectiveof the solvent used.
if a particle travelled faster than lightin a given solvent, light was emitted.
This light came to be known as CHERENKOV RADIATION.
The next logical question is, where does Cherenkovradiation come from? You have likely heard that nothing can travelfaster than light.
And that is entirely correct, but only whenthe medium in question is a vacuum.
So, while it's true that nothing can travelfaster than light IN A VACUUM , in certain other mediums, such as water, a particle’svelocity can be greater than the speed of light because light slows down significantlywhen it travels through water.
Therefore, when a particle – like a fast-movingelectron – travels at a speed higher than the speed of light, the Cherenkov effect occurs.
As a result of this effect, Cherenkov radiationis produced.
Cherenkov radiation is analogous with the….
well known sonic boom effect we discussedearlier.
The former occurs in the case of light, whilethe latter….
is observed when sound waves are involved.
With that in mind, let's take a quick lookat how and why a sonic boom occurs.
If you were to visualize sound waves, theywould look like rings originating from a sound source, growing in size as they move awayfrom it.
These rings actually represent the crest ofsound waves.
When the source is stationary, the crestsemanate and grow bigger in a symmetrical pattern.
However, when the sound source starts to move, the crests that travel in the same direction as the sound source get squashed together, causing each wave to reach the observer swifter than its predecessor.
As a result, there is an increase in the frequencyof the sound.
That's why an ambulance or police siren soundsdifferent when it approaches you, as opposed to when it moves away from you.
This change in the frequency of a moving wavein relation to an observer is called the DOPPLER EFFECT.
The sonic boom caused by supersonic aircraftis actually a special case of the Doppler Effect As the aircraft breaks the sound barrier, that is, exceeds the speed of sound in air, a sudden pressure change occurs, which causesshock waves to travel away from the aircraft in a cone at the speed of sound.
This rapid change in pressure waves soundslike a thunderous crash – the “boom” that you hear when a supersonic jet passes overhead.
It's pretty similar in the case of light.
Remember that light travels at different speeds in different mediums, depending on how photons interact with the atoms and molecules of the medium in question.
For instance, light travels fastest through a vacuum like space, but when it travels through water, it moves at only 75% of its speed in a vacuum.
In glass, light travels even slower, and while traveling through diamond, light travels the slowest among all these mediums.
Therefore, in any of the mediums where lighttravels slower than its speed in a vacuum, it's possible for some faster-moving particles to move faster than light in that particular medium.
This is exactly what happens in nuclear reactors.
During nuclear fission, the nucleus of anatom breaks open and releases charged particles – like fast-moving electrons – at extremelyhigh speeds.
When these electrons pass through the watersurrounding the nuclear reactor, they interact with various water molecules along their path.
This causes them to release a bunch of photons.
Thus, when a ton of fast-moving electronsinteract with water molecules, they release a cascade of light, which collectively takethe form of a bluish glow called the Cherenkov radiation.
Just as an object moving faster through theair than the speed of sound produces a sonic boom Cherenkov radiation is produced when a particle moves faster than the speed of light IN THAT particular medium.
So, the next time anyone claims that nothingcan travel faster than light, consider reminding them that's only true when light travels through a vacuum.
In other mediums, particles do exist that can travel faster than lightitself!.