This is an introductory blog on quantum theory. It explains as in how did the quantum leap start and lay the road to the current quantum regime.

1. April 17, 2020
Quantum Theory – A Theory Which Completely
Changed Our Understanding
atomstalk.com/blogs/quantum-theory/
Sai Teja April 17, 2020
0 Comments
Until the early 20th century, everyone thought that we have pretty clear laws of physics
that govern everything around us, The classical physics and the equations we had till
then predicted everything we say, be it a moving ball, be it ripples, be it planets.
Classical physics explained things with great certainty and applied to all the objects we
knew until then.
But then, there came some experiments and some results which literally shook the
world. These evidences built up to something which is popularly called the Quantum
Theory. Basically, it is a physical theory that describes the properties of nature on an
atomic scale. Now, this theory was so counter-intuitive that even the greatest minds in
physics could not digest it. Einstein himself spent years to prove this theory wrong
(which he could not do), he believed that we were missing something there.
So, What is Quantum Theory? Why is it is so counter-intuitive? How did it all happen?
Let us dive into the quantum world and understand things…
1/9

2. Introduction to Quantum Theory
Quantum theory (otherwise known as quantum physics or quantum mechanics) is
one of the two main pillars of modern physics, along with general relativity. These
two theories claim to explain virtually everything about the universe. General relativity
gives us our picture of the very big (space-time and gravity), while quantum theory gives
us our picture of the very small (atoms and their constituents).
Watch Video At:
https://youtu.be/nEq3hHRCDuQ
This is the video version of this article
Technically, quantum theory is actually the theory of any objects isolated from their
surroundings but, because it is very difficult to isolate large objects from their
environments, it essentially becomes a theory of the microscopic world of atoms and
sub-atomic particles. Now, this theory is so weird, it is so ridiculous and it has so many
counter-intuitive things that you can’t even imagine. So, don’t be afraid we are not
gonna go into much depth and anyhow it is safe to say that no one understands
quantum mechanics completely.
So, before seeing more about quantum theory lets first see the history wherein we will
understand a few important things of quantum mechanics.
How did it all begin?
It all basically started with a quest to know more about the basic building blocks of
matter, i.e the atoms. To understand the beginning of quantum leap, you need to know
about a few events which are
In 1898, J J Thompson discovered electrons using the properties of cathode rays.
2/9

3. In 1900, Max Planck sowed the seeds for quantum theory by stating that
electromagnetic energy could only be emitted in quantized form. His quanta are
now called photons by physicists.
In 1905, Einstein proved the photoelectric effect by suggesting that the light is
made up of packets of energy. With that, he proved that light exists in discrete
packets of energy giving the light a particle behavior.
So, around this time there were a lot of new experiments and a lot of new things
including Einsteins’ Theory of Relativity. A lot of people were trying to explain atoms
and atomic behavior as well. One such behavior was that atoms have a strange property
wherein they emit extremely selective wavelengths when heated (These lines are called
an emission spectrum and it is unique for every atom)
Bohrs’ Atomic Model
This property was explained by Neils Bohr in 1913. He developed an atomic model
named Bohrs Model wherein he said that electrons have fixed orbits and electrons can
exist only in these orbits.
He explained that when heated, electrons get agitated and jump from lower state to a
higher state called electron excitation. When these electrons get back to their
original state, called de-excitation, they emit energy which is very definite. Now here is
the catch, it is like electrons just jump from one state to another, they don’t travel the
space between, it is like mark switching its orbit into Jupiter’s orbit suddenly without
traveling the space between.
Bohrs Atomic Model (Kurzon / CC BY-SA (Source))
Bohr now put together his model with the concepts of Max Planck and he explained
that atoms and electrons have energy which is quantized i.e it has some minimum value
and will only have the multiples of these values and this is why there are only specific
discrete orbits where electrons exist.
Quickly, a lot of evidences came up proving Bohr correct. This was the beginning of the
Quantum era.
3/9

4. The idea of electrons jumping their fixed orbits without traveling space in between itself
is a weird sounding thing, right? But more and more weird things were yet to come
which literally shook everyone and seta battle ground between some of the greatest
physicists for the stakes of nature of the reality itself.
The next big thing was hidden in an experiment called Youngs Double Slit experiment.
Double Slit Experiment
Basically, this double-slit experiment was performed by Thomas Young in 1801 which
proved the wave nature of light and this result was opposite to what Einstein proved
with the photoelectric effect. Scientists were not able to understand that how can light
show one kind of behavior in some experiments and completely different behavior in
some other experiments.
Watch Video At:
https://youtu.be/A9tKncAdlHQ
What is the Double Slit Experiment?
In this experiment, there is a barrier which has two slits and behind the barrier, there is
a detector. Now, if it is a particle moving from the barrier, it would hit the detector only
at two spots which are right behind the slits. But if it is a wave, the two slits would split
the wave and then the waves would join and hit the detector at various points creating
an interference pattern.
It was in the 1920s, there was another surprise when two physicists named Davisson
and Germer performed the same Double Slit Experiment with electrons. It resulted in
electrons showing wave nature. What??? Yes, that was the reaction of everyone. An
electron that was considered a particle till then showed the properties of a wave.
Physicists argued that somehow while traveling electron spread up like waves. A lot of
4/9

5. Heisenberg’s Equation where x is the position, p is the momentum and h cross is the Plancks
constant divided by 2*pi
people could not digest it. But a physicist called Erwin Schrödinger came up with an
equation called Schrödinger wave equation that described these electron waves
with pretty good accuracy.
Schrödinger wave equation
Though we got an equation which described the wave, how can an electron be a wave?
The answer came from another physicist named Max Born. Born suggested that this
wave was not a normal wave rather it was a probability wave wherein the probability of
an electron existing at one place is dependant on the wave size. This again was
ridiculous. This is when Einstein gave his famous quote…
'God does not play dice with the universe.' - Albert Einstein Click To Tweet
Einstein argued that there was something missing in it and he believed that we should
be able to predict everything with certainty. But Bohr believed that it is all uncertain, a
particle-like electron can be in any state and at any location until and unless observed.
Quantum theory suggested that an electron can be a particle as well as a wave at same
time. All this was and is still really confusing to any common person.
Adding to all the uncertainty, Heisenberg stated his Uncertainty Principle, that it is
impossible to exactly measure the position and momentum of a particle at the same
time.
Adding to these mysteries there came another ridiculous, mysterious, absurd and most
crazy prediction the equations of Quantum mechanics made…
Quantum Entanglement
If you don’t know about this till now, be ready to hear something which will blow your
mind.
5/9

6. John Stewart Bell
Quantum Mechanics predict that two particles can be linked together when they are
close enough and they become entangled and their properties get connected. Now, the
weird thing here is that these particles will be connected even if we take them long away
in opposite directions, without any physical connection, What??? Yes, that is true, we
will see its possible applications later in the article.
This was first found by Schrodinger and was
studied by Einstein. Einstein showed this bizarre
prediction of Quantum mechanics to say that
something is surely missing and particle cannot get
connected just like that as if there was no space in
between. Einstein called this as the particle getting
connected by some spooky actions.
No one could understand whether the particle really
get connected or it is decided beforehand only that
these particles will have particular properties. It was
in 1967 when young astrophysicist John Clauser.
He set up an experiment using the research of an
Irish physicist John Bell and he proved that entanglement is real and the particle gets
connected. Don’t ask how does it happen? or Why does it happen? As Professor Walter
Lewin says “It is an illegal question and you are not supposed to ask that, it is
supposedly how world ticks“
After this, there were a lot of improvements in quantum theory and all that gave rise to
a field of physics called quantum mechanics. It was widely accepted and confirmed
again and again by several experiments.
Quantum mechanics was accepted as a superior set of rules which govern everything
around us. It was also explained how classical mechanics come from quantum
mechanics only.
The current problem which is eating away all the physicists on the planet is to get a
unified theory that can explain all the four fundamental forces os nature. Though some
theories like the M-theory (a theory in physics that unifies all consistent versions of
superstring theory) and the loop quantum gravity gave some hope, they did not really
prove that effective. In simple words, we need a theory which unifies two of the best
theories of the present time namely Quantum Mechanics and the General Relativity.
Let us wait for the grand unification. In the meantime, let us look at the other side of the
world where these theories gave rise to a whole new segment of technology.
Applications of Quantum Theory
6/9

7. Quantum computer based on superconducting qubits developed by IBM
Research.
All these arguments, proofs, evidences were going on one side but on the other side the
accepted things were used in technology advancement. Due to these quantum
mechanics, the equations and all the theories, we understood electrons better and it
paved a way to lot of new devices like the diode, the transistor and the integrated
circuits.
Quantum mechanics is the base of all the modern-day electronic gadgets. So, besides
that let us discuss some applications which are already being used and some
applications which might become true in the future.
Quantum Cryptography
For the people who don’t know, cryptography is basically the encoding and decoding
thing used to securely transfer data. Classical cryptography basically has two keys that
are used to encode and decode the information. Though this classical method is safe,
there is always a risk of someone cracking the key and decoding the information.
Researchers used quantum properties to make something called quantum keys. Here
the communicator sends information using the polarized photons. Quantum rules
dictate that “reading” the polarized photons will always change their states, and any
attempt at trying to read the data will alert the communicators to a security breach. This
makes Quantum cryptography pretty secure.
Quantum Computers
We all
know the
modern-
day
computers
works on
binary code
that is 0s
and 1s. The
bits in the
current
circuits
basically
have two
states
where it
can be
either 0 or
1 and if you
can have
both 0 and
1 at the same time, you get something called as Quantum Bits or qubits.
7/9

8. Quantum bits are basically superconducting circuits that can run in two directions at
once. The resulting computers would be so powerful that our brains can’t even imagine.
We are yet to build quantum computers which can be used anywhere like normal
computers. The major problem we face is that these qubits work only at low
temperatures as they use superconducting materials.
A lot of researches are going on and the operating temperature is slowly coming up.
Here is one such recent research.
We don’t really feel a need for something so fast as our daily needs are not that much
but some scientific works are so heavy that they need too much power which can be only
achieved by quantum computers. For example, to do a simulation of two black
holes merging together as shown in the post below, NASA has to spend 46 days on a
13.3 petaFLOPS computer called Blue waters. Believe me, it is too powerful and NASA
could only do partial simulation and some parts of the model were neglected as the
processing power was insufficient. It was just two black holes, imagine the power one
would need to simulate a whole universe…
So, in these kinds of cases, we would need quantum computers and it would help us a
lot in understanding the cosmos.
Teleportation
Teleportation has been one of the biggest dreams. It basically means that we should be
able to send objects from one place to another without the object traveling the space in
between.
You would ask, is it even possible? Yes, in theory, it is possible to teleport
objects. By using the property of quantum entanglement, we can have a lot of
entangled particle at two far places and we can make an object interact with one set of
entangled particles and by this we can transfer the exact quantum states of the object to
another set of entangled particles which would result in teleportation.
Now, this might sound like science fiction but this has been already done with photons
and other elementary particles wherein scientists have teleported these particles from
one place to another without them traveling the space in between.
Probably, we might also be able to teleport humans as shown in star trek using this so-
called spooky quantum entanglement.
Let us wait and see
In Conclusion
8/9

9. So, that was a brief about quantum theory, how it all happened and what is the
importance. Quantum mechanics still have a lot of scope for development and once we
achieve a whole unified theory we would have a lot better understanding of the universe
and no one knows we might find something more and more mysterious in this grand
journey.
9/9