Man has learnt predicting since it grew intelligent. Other living
beings also seem to possess this instinct. Given the present scenario one can
say what will happen after sometimes. Say, we throw a stone into air. We can
make a guess where the stone will fall after some time because, we know how much
force was applied while throwing the stone or with what velocity. Mathematics
is an efficient tool in predicting events with pin point accuracy. Many
physicists since a very long time have worked on developing mathematics for
predicting events. The works of Gallileo,
Newton, Lagrange, Euler, De Alembert
known as Classical Mechanics
helped set the foundation of the technologies of the era before 20th century.
Works of Maxwell, Faraday, Kirchhoff, Henry, Ampere and
many other prominent scientists of that time contributed to the development of
the Classical Electromagnetism which
described the behaviour of electricity and magnetism. Given the necessary
parameters at a moment e.g. velocity, position, acceleration of a particle,
using these equations one can predict the future of the event i.e. the
velocity, position, acceleration with pinpoint accuracy. By the end of 19th
century most of the major discoveries had already been taken place and the
scientists were summing up their works. But to their astonishment the beginning
of 20th century proved that it was just the beginning. Soon they
witnessed a dynamic change in their understanding about nature. Some
experiments were carried out which could not be explained using classical
ideas. It was discovered that deep under the atomic world nature behaves in a
completely different way and is governed by completely different set of laws
which are counter intuitive to our daily experience. Those are the laws of Quantum Mechanics. These laws showed that
we cannot predict events in the atomic level with certainty. We can talk only
in terms of probability.
Max Planck’s Quantum Theory in 1900 showed that energy is quantised
i.e. energy is always transferred in discrete amount, not continuously. Neils Bohr incorporated this idea to
his model of atom. Rutherford’s planetary model faced strong criticism and was
rejected. According to Classical Electromagnetism any accelerating charged
particle emits energy in the form of electromagnetic waves. In Rutherford’s
atom electrons are moving in circular path i.e. they are constantly
accelerating every moment. Thus the atom should loose energy in the form of
electromagnetic waves and should spiral into the nucleus. Thus it should show a
continuous spectrum of smeared colors .But it does not justify the stability of
the atom and neither the results of his own experiment. Moreover, he could not
explain why heated gases show only discrete strips of colors and not continuous
spectrum. Bohr’s model (1913) showed
that the electrons can move only in certain orbits of specific radius, not in
any orbit. In those orbits they do not emit energy. The electrons jump from a
lower orbit to higher orbit by gaining some energy (e.g. when heated) and jumps
from a higher orbit to a lower orbit by losing energy in the form of
electromagnetic wave i.e. light of a
specific color. That is why gases when heated show strips of colors in their
spectrum and not a continuous combination of colors. Here it is very strange
idea that while jumping from one orbit to the other the electron is nowhere in-between.
It is as if the electron did not move through space. Bohr was highly successful
in describing the structure of Hydrogen atom or Hydrogen like atoms but failed
to describe many electron atoms or large atoms. Moreover, it could not justify Heisenberg’s Uncertainty Principle. Werner Heisenberg made a very important
observation in the year 1927. He observed that it is impossible to measure the
position and momentum of an electron simultaneously to 100% accuracy. The more
accurately you measure its position, the more uncertain is its momentum and
vice versa. It is because we use light photon to observe the electron. For
better resolution of the microscope the frequency of the light photon used
should be high and we can measure the position of the electron to our desired
accuracy. But, then we have to make a compromise with the momentum .The high
frequency photon imparts a momentum to the electron and thus the uncertainty
about the momentum increases. On the contrary if we choose to measure the
momentum accurately, we have to reduce the frequency of the light and thus the
uncertainty about position increases. Thus we are not able to find the present
position and momentum of the electron. Hence, we cannot predict the position
and momentum of the electron after sometimes.
As a matter of fact one cannot assign a definite path for an electron.
Moreover, Louis De Broglie showed
that every matter has both wave and particle like property. However, with
increase in mass or decrease in velocity the wavelike property becomes less
pertinent. The experiment which gave a blow to the physicists was the Young’s
double slit experiment where the light is replaced by a steam of electrons.
When this stream of electrons is passed through two slits and allowed to hit a
detector, they show an unusual pattern of alternative bright and dark fringes.
But, electron is a particle; how could it show such pattern which only waves
can form? So, the physicists of that time were convinced that classical physics
has some limitations. A new framework for describing the atomic world was
inevitable. This was the birth of Quantum
Mechanics. In 1926 an Austrian physicist, Erwin Schrodinger developed a second order partial differential
equation named in his honor as Schrodinger’s
Equation. This equation is the foundation of Quantum Mechanics. The
solution of this equation which is a wave function gives all the information
about the particles in the system concerned. Whereas the wave function has no
physical meaning, the square of its absolute value at any location at a certain
moment gives the probability of finding the electron at that location . The
strangeness about the equation is that the equation depicts particles as three
dimensional waves. It is as if the particle is spread out through space which
means that the electron is at many different locations at the same time. Until
and unless we look for it, it is almost everywhere with varying probability.
Even at infinity there is a finite probability of finding the electron. As soon
as we look for it nature itself compels all the possibilities to collapse and
the electron presents itself at a certain location with certainty. Some people
believe that Quantum Mechanics is not applicable to the macroscopic world. But,
though it has never been observed, the mathematics implies that it is equally applicable
to the macroscopic level. That’s really a weird picture of reality.
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| Fig : Double Slit Experiment Using Electron Beam |
After these developments, further
refinement of the existing mathematics for different types of systems was very
obvious. Classical statistics did not hold good in describing the behaviour of
the systems of subatomic particles like electrons, protons, photons etc. Einstein in collaboration with the
Indian mathematician Satyendranath Bose
came up with the Bose-Einstein
Statistics (BES), one of the two Quantum Statistics. BES explained the
behaviour of the particles like photons, gluons, Higg’s boson etc. Particles
obeying BES are called Bosons (named
in honor of Satyendranath Bose). It led to the discovery of the fifth state of
matter, Bose-Einstein Condensate. Other
of its kind was the Fermi-Dirac
Statistics developed by Paul Dirac
and Enrico Fermi. It described the
behaviour of the particles like electrons, protons, quarks, composite particles
comprising odd number of these particles like many atoms, nucleus etc. These
particles are termed as Fermions
named after Enrico Fermi. Paul Dirac pioneered the relativistic description of
Quantum Mechanical version of Classical Electromagnetism, introduced the
concept of Antiparticles and
predicted the existence of positron, the antiparticle of electron which has
been successfully confirmed. Richard
Feynmann advanced Dirac’s work on electromagnetism and developed Quantum Electrodynamics (QED) and for
the first time Quantum Mechanics was in full agreement with Special Relativity. For this work
Feynmann was awarded the Nobel Prize in physics in 1965.
Quantum Entanglement :
One of the most bizarre phenomena of the
quantum world predicted by the mathematics of Quantum Mechanics is the Quantum
Entanglement. The mathematics shows that if two electrons are brought close to
each other and separated by any distance, then the electrons get entangled as
if some invisible link is connecting them together. If an action is made on any
one of the two, the other gets affected within no time. For example, let us
consider the spin of electron. Say, two electrons are brought close to each
other and then separated by a very large distance (let us take one of the
electron to the surface of moon). Now until and unless we measure, both the
electrons are in a superposition of both up and down spin. But, if upon
measuring we find the spin of the electron on earth to be up, then the spin of
the other one on the moon surface immediately turns to be down even if there
was no connecting link between them. This is something Einstein could not
digest throughout his life. According to Einstein how could the effect reach
the other one instantly as nothing can move faster than light. But, experiment
after experiment Quantum Entanglement has been proved to be right. It has been
used in theorising Teleportation of
particles and even human beings. Teleportation of some particles, based on
Quantum Entanglement has already been carried out successfully in various
laboratories across the world. Einstein termed it as “Spooky action at a distance” and debated against it till his death.
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| Fig:Two Entangled Particles |
Schrodinger’s Cat :
Schrodinger’s Cat experiment is
a paradox based on the laws of Quantum Mechanics presented by Erwin Schrodinger.
The experiment follows as below:
A cat, a flask of poison, a
radioactive source, a hammer tied to a spring and a detector are placed inside
a closed box. Now we switch on the radioactive source. If the detector detects
any radiation, the hammer is released and it shatters the flask of poison and
the cat dies. But, until and unless we open the box we cannot say certainly
whether the source radiated or not. That is both the cases have equal
probability. It radiated and did not radiate at the same time. Thus, the cat is
also both dead and alive at the same time. But as we open the box, immediately we find that the cat is either dead or alive. How is it possible? How the cat
which was both dead and alive at the same time suddenly turned to be either
dead or alive? This paradox has puzzled even the most brilliant minds of the
world and no one has come up with a satisfactory answer till now.
 |
| Fig: Schrodinger's Cat Experiment |
The Tunnelling Effect :
Consider the case of rolling a ball up a
hill. If we do not provide enough velocity to the ball it fails to roll over
the hill. A certain amount of minimum kinetic energy is a must for the ball to
reach the other side of the hill. It is the classical picture of the story. The
very same case in the atomic level behaves in a counter intuitive way. A
particle moving against a potential wall can reach the other side of the wall
even if it does not possess the minimum energy. It is because particles behave
like waves in the atomic level. The particle wave extends to the other side of
the wall and there is a finite probability of finding the particle on the other
side of the wall too. It is as if the particle tunnelled through the wall to the
other side. It’s really a bizarre fact and we cannot deny it. Because, this property
was used in designing the electronic devices like Tunnel Diode, Tunnel Field-Effect
Transistor and the Scanning Tunnelling Microscope.
 |
| Fig: Particle Tunnelling Through The Wall |
Bohr-Einstein Conflict :
After the development of Quantum Mechanics
the world of physics started to move in a direction that Einstein did not want
to go. Though he contributed to the development of Bose-Einstein Statistics,
the probabilistic interpretation of reality in Quantum Mechanics greatly
dissatisfied him. Quantum Mechanics says
that until and unless you look at the sun, it is everywhere in the universe
with different probabilities. But as we look for the sun, all the probabilities
collapse and we find the sun at a certain position. But, Einstein was very much
discontented on this explanation. He said the sun is exactly at a certain
position even if we do not look for it. One of the famous quote of the debate was
: Einstein to Bohr, “God does not play
dice”. Bohr replied on it, “Stop
telling God what to do”. Einstein believed that, though the mathematics of
Quantum Mechanics is highly successful in predicting the bizarre picture of
nature, its interpretation is incomplete. Einstein sensed that something was
missing in Quantum Mechanics. Einstein stood firm in his words and did not
believe in Quantum Mechanics till his death. On April 18, 1955 Einstein breathed
his last with his work left incomplete.
Quantum Mechanics is the most
elegant achievement of human kind. It has large contribution to the development
of 21st century. Erwin Schrodinger and Paul Dirac were awarded the
Nobel Prize in physics in 1933 for their contribution to the development of
Quantum Mechanics. Erwin Schrodinger is called the father of Quantum Mechanics.
But, even after almost nine decades of its development we have not succeeded in
answering the basic question, “What does it mean about reality?”. Even the
brilliant minds of the world put their hands up. We cannot suspect the consistency
of these laws. Because, the whole world of electronics is governed by the laws
of Quantum Mechanics. These laws were used to design the diodes, transistors,
integrated circuits etc. If Quantum Mechanics goes for a strike for a while,
then all our electronic instruments like cellphones, computers, televisions
will go off. What is not clear is its interpretation. Falling into this trap
Feynmann once made the famous quote, ”If
you say that you understand Quantum
Mechanics, you actually don’t understand Quantum Mechanics”. Whatever be
the case we must be very thankful to those genius minds who introduced us to
the strange world, the Quantum Realm. Most of the credit for the technological
advancement of the present world goes to them.
