Why is light so special? Light is a very wonderful and
special thing in the universe. The research of light has been from the time
Newton and before to till now modern physics. There have been many developments
in the theory of light and how we understand it.
Light was thought to be a wave just like sound. As you might
know, sound needs a medium through which
it will travel or propagate, therefore if light is a wave then it must also
require some medium to propagate. Sound travels in the atmosphere (air), water
(liquid) and solid substance also, thus it requires some material medium to
travel. It cannot travel in vacuum (where nothing including air is not present
i.e. space). But light can travel in vacuum also but no material medium is
present so on what does it propagate? The
answer to this question that the physicists of around 18th and 19th
century gave was that there was a mysterious substance known as the aether (many where written as ether also)
present everywhere in the universe, even in vacuum. They said light propagated
in this medium. But there was no experimental or observational evidence that
this was true but for many decades this theory was accepted. The change came
when two physicists, Albert Michelson
(1852-1931) and Edward Morley
(1838-1923) performed an experiment to measure the speed of light, which
turned out to be one of the most famous experiments ever and for which they won
the Nobel Prize in Physics in 1907. They build the interferometer which could find the velocity of light but Michelson
took into account that there will be some difference in calculation due to the
presence of aether but there was no such difference as expected. Thus from this
experiment it was confirmed that there was no such thing as the aether or no
such mysterious medium. This experiment also paved way for the formulation of
one of the most revolutionary theory put forward i.e. The Theory of Relativity,
about which I shall mention in the later part.
There is another theory of light. The theory is that it is
composed of particles. Light is a form of energy and it is composed of
different packets or quanta. It is also known as photon. Max Planck
(1858-1947) who is the originator of the Quantum Theory said this thing about energy that it is composed of
distinguished particles. Albert Einstein
(1879-1955) explained the photoelectric
effect through Planck’s quantum theory which won him the Nobel Prize in
Physics in 1921. There he discovered the photon and regarded light as composed
of particles.
But that’s not it, the modern theory says that light has both
wave like and particle like properties thus light has Wave-Particle Duality whose proof can be seen in the double-slit experiment which shows that
an electron or photon gun shows interference that is only possible through wave
like properties. Thus modern quantum mechanics says this theory of light is the
correct one. The wave-particle duality is an amazing phenomenon about which I
shall briefly describe in the next article.
Now coming back to the previous mentioned topic, how did
light helped in the formulation of relativity. Galileo (1564-1642) said that any inertial frame of reference
cannot be distinguished (rom that frame) from any rest frame of reference. In
simple words if we are in a closed room and we are moving at a constant speed
then we will feel that we are at rest. We can perform no experiment to
determine that we are actually moving. If we look out then we can see
everything else is moving but someone from outside will observe that the room
is moving. Both are equally correct in reality. Thus Galileo said that “All laws of physics in any inertial frame
of reference”. In the 19th century James Clerk Maxwell (1831-1879) gave his equations on the light
being an electromagnetic radiation. Those equations told that the speed of light is a constant speed of c
(3 x 108 m/s). This is also a law of physics. Thus it is sure
that the speed of light is same in all inertial frame of reference. But the
speed of anything for two different moving frames is different. Suppose you are
standing and observing that a truck is moving at 40 km/h and a car is moving at
30 km/h in the same direction as the truck. The person in the car observes that
the speed of the truck is 10 km/h from his frame of reference which is a very
obvious thing in reality. But then speed of light will also be different for
two observers but Galileo says that it is not. But both of the principles are
very correct in reality. So which one is wrong? Actually none is. Albert
Einstein used this contradiction to formulate the Special Theory of Relativity (1905) and following the General Theory of Relativity (1915). He
also showed that the speed of light is the speed limit of the universe and no
material object can reach the speed of light. I now again return to the same
question. Why is light so much special?
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