Have you ever noticed that when a
supersonic military aircraft passes overhead at a very high speed, it leaves a thunderous
boom in its wake?
That tremendous cracking sound actually has a formal name - a SONIC BOOM.
This intense phenomenon occurs when an object - in this case, an aircraft - travels
faster than the speed of sound in air.
This might naturally lead to the question does light also have a sonic boom equivalent?
Does anything unusual happen when something travels faster than light?
In 1934, a Soviet physicist named Pavel Cherenkov observed that a water bottle emitted blue
light when bombarded with radiation.
This blue-colored light was also emitted from a number of transparent objects in the proximity
of a radioactive source.
This intrigued Cherenkov, and he conducted a set of experiments to understand the origin
of this strange phenomenon.
Cherenkov irradiated various liquids with gamma rays from a 104 mg radium source.
His initial objective was to study fluorescence emission induced in uranyl salts.
In his early experiments, Cherenkov made naked-eye observations, which was very difficult and
required high levels of patience and experimental skill.
Through his experiments, Cherenkov noted that light was emitted even when a vessel contained
only sulfuric acid - the solvent for the uranyl salt.
He also demonstrated that such a light was observed even when different solvents were used
This led him to conclude that irrespective of the solvent used.
if a particle travelled faster than light in a given solvent, light was emitted.
This light came to be known as CHERENKOV RADIATION.
The next logical question is, where does Cherenkov radiation come from?
You have likely heard that nothing can travel faster than light.
And that is entirely correct, but only when the medium in question is a vacuum.
So, while it's true that nothing can travel faster than light IN A VACUUM , in certain
other mediums, such as water, a particle’s velocity can be greater than the speed of light
because light slows down significantly when it travels through water.
Therefore, when a particle - like a fast-moving electron - travels at a speed higher than
the speed of light, the Cherenkov effect occurs.
As a result of this effect, Cherenkov radiation is produced.
Cherenkov radiation is analogous with the….
well known sonic boom effect we discussed earlier.
The former occurs in the case of light, while the latter…. is observed when sound waves
are involved.
With that in mind, let's take a quick look at how and why a sonic boom occurs.
If you were to visualize sound waves, they would look like rings originating from a sound
source, growing in size as they move away from it.
These rings actually represent the crest of sound waves.
When the source is stationary, the crests emanate 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 frequency of the sound.
That's why an ambulance or police siren sounds different when it approaches you, as opposed
to when it moves away from you.
This change in the frequency of a moving wave in relation to an observer is called the DOPPLER
EFFECT.
The sonic boom caused by supersonic aircraft is 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 causes shock waves to travel away from the aircraft
in a cone at the speed of sound.
This rapid change in pressure waves sounds like 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 light travels 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 an atom breaks open and releases charged particles
- like fast-moving electrons - at extremely high speeds.
When these electrons pass through the water surrounding 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 electrons interact with water molecules, they release
a cascade of light, which collectively take the form of a bluish glow called the Cherenkov
radiation.
Just as an object moving faster through the air 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 nothing can 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 light itself!