In the animal
kingdom, the most dissatisfied species is humans. They are never content with
their present situation and always seek to change their circumstances. Self-contentment
brings an end to creativity, exploration, and innovation. If humans were
content with their situation collectively, they might still be in the Stone
Age, attempting to ignite fire with stones. Just like the wild animals of that
era still live their lives the same way. Human evolution in terms of lifestyle,
methodology, and practicality always surpasses their past generations due to
their overall dissatisfaction. And this constant advancement fuels new
discoveries and innovations.

Engrossed
scientists in discoveries and innovations all pursue the ultimate goal of
science - simplifying complex mysteries. Scientists from different branches
embrace different approaches to find simple solutions to the complex mysteries
of science. For instance, psychologists attempt to determine the correlation
between the intricate workings of the body and brain with other related
elements, believing that understanding these relationships simplifies complex
problems. Biologists strive to understand how all these processes occur more
straightforwardly by understanding the structure and functioning of cells.
Chemists delve deeper into understanding the chemical structure of molecules
and the process of chemical reactions. If they can unveil the nature of
chemical reactions, their work becomes easier. Understanding the workings of
atoms and subatomic particles further simplifies their tasks. But physicists
refuse to halt anywhere. They delve into the interiors of atoms, even into the
nucleus.

In the world of
science, physics is a kind of imperialism. They believe that all substances and
energies in the universe and all their interactions are encompassed within
physics. Everything that has happened in the universe since its birth and
everything that will happen in the future is included within the realm of
physics. Therefore, the scope of physics extends from the internal structure of
atoms to galaxies. The farther we go in time, the wider the scope of
theoretical physics expands. Satisfaction is nowhere to be found in the visible
universe. Even more countless theories of the invisible hypothetical universe
are being discovered. Amongst all these theories, the ultimate goal of
physicists is to explain everything in the universe correctly - a theory called
the Theory of Everything[1].

Theoretical
physicists believe that the Theory of Everything (TOE) will be such an eternal
theory of physics that it will provide the correct answers to all questions -
how the universe originated, why it originated, how everything in the universe
- from quarks to cosmos - functions. Here, 'everything' means understanding
everything comprehensively or just the fundamental aspects is a matter of
debate.

Having
differences is natural because many claims of physicists seem to be either
wrong or time-consuming to prove in an empirical sense. For example, explaining
how clouds form in the sky can be accurately described by the principles of
physics. However, it may not be possible to create a mathematical model of a
small cloud using the equations of physics that will accurately predict its
formation at a specific time in the sky and match its shape and volume from all
directions - this is not feasible for physics because it requires the use of
highly changeable indices, which cannot be accurately applied even by many
supercomputers.

Examples like
this abound. For instance, if a glass is dropped from the hand, it will surely
fall because of gravity. But whether it will shatter upon hitting the ground or
not requires knowing many more things beforehand - such as what the glass is
made of, how heavy it is, at what distance and with what speed it was dropped,
whether any force was applied during the fall, and countless parameters
starting from the structural elements of the glass. Considering all these
parameters, it can be said that the glass will break. But is it possible to
accurately predict how many pieces the glass will break into after it shatters?
Is it possible to determine the shape and volume of those pieces?

So, no one
expects to provide all details from the theory of everything. However, it is
hoped that this theory can explain the origin of all natural forces in the
universe, calculate energy, how the universe was created, and how it will end.

When we talk
about the theory of everything, what exactly are we trying to understand? What
will happen if this theory is discovered? And what if it's not discovered, or
what are the consequences? Before answering these questions, we need to
understand how physicists conceive such a theory.

In the 5th
century BC, the Greek philosophers Leucippus and Democritus proposed atomism,
or the theory of atoms, as the first theory of everything. With this theory,
they sought to explain that everything in the universe is made up of atoms. For
thousands of years, it was believed that atoms were indivisible.

What was the
reason for calling atomism the theory of everything? The reason was that at
that time, whenever the question of what everything is made of arose, the
answer was - atoms. And the internal workings of matter were also explained
through the interactions of atoms with each other. Despite the presence of
different types of atoms in different substances, all substances were
considered to be composed of atoms. Therefore, at that time, the fundamental
elements of matter were atoms.[2]

Within the next
few centuries, there was significant advancement in physics. Natural forces
were discovered.

In 1687, Newton's law of gravitation was published. The discovery of this force was the first indication of natural forces among the natural forces. Newton's discovery revealed the principle of gravitation or gravitational force. Gravity is an extremely weak force. But this weak force has held all the objects in the universe invisibly together. Gravitational force is an attractive force. Its influence is everywhere in the universe. According to the theory of gravity - the force of attraction between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between the two objects. The larger and more massive the objects are, the greater the amount of gravitational force between them. Again, the closer they are, the greater the amount of force. If the distance doubles, the amount of force decreases by four times. According to the theory of gravitational force, no object attracts any other object. Newton's theory of gravity explains how the force of attraction works between objects, but it does not explain why it attracts. However, it is the attraction of the gravitational force that roughly explains why the satellites-planets-stars-galaxies of the universe revolve around each other.

Between 1750
and 1850, scientists from many countries in Europe conducted numerous
experiments in various ways on electricity and magnetism. Among them, the most
important discovery was that - due to the movement of electric charges,
magnetism is created, and moving magnetic charges can generate electricity.
Danish physicist Hans Christian Ørsted was certain in 1820 that there is a
mutual relationship between electricity and magnetism. He named this
relationship electromagnetism.

In 1831,
British physicist Michael Faraday proved in the laboratory that magnetism is
produced from an electric current, and electricity can be generated from a
magnetic field. Fourteen years later, Faraday discovered that there is also a
close relationship between electricity and light.

Then, in 1861,
Scottish physicist James Clerk Maxwell accurately conceptualized - if there is
a change in the electric field, then a magnetic field is created, and if there
is a change in the magnetic field, then an electric field is created.
Furthermore, it can be said more precisely - electricity can be found from
magnetism, and magnetism can be found from electricity. Electric fields and
magnetic fields are always perpendicular to each other. Maxwell mathematically
demonstrated that the speed of light in a vacuum is three hundred thousand kilometres
per second. Later, we have seen that light is a visible part of the
electromagnetic spectrum. But in the vast invisible part, there are less
energetic radio waves, infrared rays, microwaves, and more energetic
ultraviolet rays or non-ionizing radiation, X-rays, gamma rays, etc. In the
20th and 21st centuries, we have seen the extensive use of electromagnetic
waves in technology from Earth to space, from mobile phones to medical science
- everywhere.

Maxwell's
theory led to the discovery of the second fundamental force of nature -
electromagnetic force. Electromagnetic force is much more powerful than
gravitational force. Mathematically, electromagnetic force is very similar to
gravitational force. The amount of electromagnetic force between two charges is
proportional to the product of the charges and inversely proportional to the
square of the distance between them, just like gravitational force. Similarly
to gravitational force, the intensity of this force increases if the amount of
charge increases. If the distance between the charge doubles, the force
decreases by a factor of four.

However, the
key difference between gravitational force and electromagnetic force is that -
this force can be either attractive or repulsive. If the charges are of the
same type (both positive or both negative), the electromagnetic force will be
repulsive. But if the charges are of opposite types (one positive and one
negative), then the force will be attractive. As a result, in the case of large
objects, gravitational force and electromagnetic force neutralize each other.
Therefore, electromagnetic force is not effective in large objects. However,
this force is very active between atoms and molecules. All kinds of chemical
reactions and biological processes occur due to the electromagnetic force.

In 1895, X-rays were discovered, followed by radioactivity in 1896, and the discovery of electrons in 1897. After these discoveries, many physicists came to the idea that everything in physics has been discovered, and everything can be explained by the theories of gravitation and electromagnetism. In 1900, in a lecture at the British Association for the Advancement of Science, Lord Kelvin famously stated, "There is nothing new to be discovered in physics now. All that remains is more and more precise measurement."[3]

But it didn't
take long for Lord Kelvin's words to be proven wrong. From the beginning of the
20th century, there has been a whirlwind of new discoveries in physics. On one
hand, under the guidance of Max Planck, the journey of quantum mechanics began.
On the other hand, Albert Einstein, in 1905 alone, published four
groundbreaking papers that revolutionized our understanding of the entire
universe. The theory of special relativity was formulated, establishing the
relationship between mass, energy, and the speed of light.[4]

The discovery
of the principle of relativity led to challenges in the theory of gravitation.
According to Newton's theory, there is no change in the mass with the motion of
an object, but according to Einstein's theory, there is a change in mass with a
change in motion. Furthermore, in Newton's theory of gravitation, the distance
between objects that are subject to gravitational force is not defined with
respect to any frame of reference. But according to Einstein's theory of
relativity, this distance depends on the reference frame. In the case of slowly
moving objects, this difference in distance is not significant, but to
calculate the gravitational force between extremely fast-moving celestial
bodies, the principle of relativity must be applied to measure distances.

Einstein
realized the need to revise Newton's theory of gravitational force. After a
decade of research, in 1915, Einstein published the general theory of
relativity, which essentially replaced Newton's theory of gravitation.
According to this theory, gravitational force creates curvature in space-time,
which depends on the distribution of mass in space and time. Essentially, the
general theory of relativity transformed physics into geometry. The practical
evidence of the general theory of relativity is regularly found in
astrophysics. Its practical applications include all modern satellites, GPS
navigation, and more.

After the
discovery of special and general relativity theories, Einstein spent the rest
of his life attempting to unify the principles of natural forces. However, he
did not see success. Scientists today believe Einstein failed because he was
not enthusiastic about using quantum mechanics. Furthermore, applying quantum
mechanics to the theory of gravitation remains challenging.

Following the
discovery of the nucleus of the atom, protons, and neutrons, two more natural
forces were discovered from the theory of radioactivity – weak nuclear force
and strong nuclear force. The weak nuclear force is responsible for
radioactivity, which led to the formation of various elements in the universe.
We usually do not experience this force in our daily lives.

The fourth
natural force in nature is the strong nuclear force. This force holds protons
and neutrons tightly within the nucleus. Among the four natural forces, the
strong nuclear force is the most powerful. It is not felt outside the nucleus.
The strong nuclear force is an extremely strong attractive force.

Einstein and
other theoretical physicists attempted to unify these four natural forces into
a grand unified theory or unified theory. Apart from gravitational force, the
other three forces are applicable on the subatomic scale of particles (quarks,
protons, electrons, and neutrinos). To understand the workings of subatomic
particles correctly, quantum mechanics must be applied. Therefore, to properly
understand the workings of the atomic nucleus, whether it's the electromagnetic
force, weak nuclear force, or strong nuclear force, all must be transformed
into quantum mechanics.

Furthermore,
when applying the force of gravity to large celestial bodies like planets,
stars, and galaxies, there is no need to transform it into quantum mechanics.
However, if we want to understand the early universe, we must start from
singularities. At that time, everything was microscopic. So, gravitational
theory also needs to be transformed into quantum mechanics.

Whether it's
Newton's theory of gravitation or Einstein's general theory of relativity, both
are classical theories. None of them can be transformed into quantum mechanics
because quantum mechanics operates under Heisenberg's uncertainty principle.
Applying the uncertainty principle to general relativity will not yield
realistic results. For example, then a black hole will not be entirely black,
and a singularity will not be entirely empty.

To grasp the
principles of everything, all of nature's forces must be unified. To do this,
the quantum transformation of all four forces is necessary. These principles'
quantum forms are called quantum field theory. In quantum field theory,
particles or matter exchange particles are fermions (electrons and quarks), and
force exchange particles among fermions are bosons (photons).

The quantum
transformation of the electromagnetic force has been made possible through
Richard Feynman's discovery of quantum electrodynamics.

Using quantum
field theory, Professor Abdus Salam, Steven Weinberg, and Sheldon Glashow
unified electromagnetism and weak nuclear force.

The weak
nuclear force and the electromagnetic force are fundamentally the same.
However, they appear different because the exchange particles of the weak
nuclear force have mass, whereas the exchange particles of the electromagnetic
force do not have mass. But in both fields, the exchange particles are bosons.
In the field of the electromagnetic force, the exchange particle is the photon,
which has zero rest mass and travels at the speed of light. In the field of the
weak nuclear force, the exchange particle has mass. As a result, the speeds of
these bosons change with distance.

In weak nuclear
interactions, charge changes occur. This means that charge-neutral neutrons
transform into positively charged protons or negatively charged electrons.
Consequently, the current obtained is called a charged current. On the other
hand, in the electromagnetic force field, there is no change in charge.
Therefore, in this case, the current obtained is called neutral current.

Weinberg and
Salam suggested that the only difference between the weak nuclear force and the
electromagnetic force is the mass of the exchange boson. In the field of the
weak nuclear force, the exchange particle, the W boson, is nearly 100 times
heavier than the mass of a proton. Because of this, heavy photons are also
referred to as bosons of the weak nuclear force.

Abdus Salam and
Steven Weinberg proposed that in the weak nuclear force field, both neutral and
charged currents could occur. Both types of exchange particles in the weak
nuclear force are collectively called the W^{+}, W^{-}, and Z^{0}
bosons. +, -, and 0 denote positive, negative, and neutral charged bosons,
respectively. These three are collectively referred to as intermediate vector
bosons. These vector bosons are very heavy inside the nucleus. It is there that
weak interactions occur. Outside the nucleus, electromagnetic interactions
occur.

In 1973, at
CERN's examination, it was possible to demonstrate weak nuclear interactions
without any exchange of charge. The existence of neutral currents was proven.
Similar results were obtained at Fermilab. In 1978, at the Stanford University
Linear Accelerator (SLAC), weak nuclear force and electromagnetic force were
found to be reconcilable by observing the interactions of electrons and
positrons. For this discovery, the 1979 Nobel Prize in Physics was awarded to
scientists Abdus Salam, Steven Weinberg, and Sheldon Glashow.

The possibility
of unifying electromagnetic force and weak nuclear force, observed through the
exchange of W and Z bosons, inspired scientists to further consolidate other
forces. Quantum chromodynamics was developed to incorporate the strong nuclear
force. Within nuclei, protons and neutrons were found to be composed of smaller
particles called quarks. Quarks are categorized into six types: up, down, top,
bottom, charm, and strange. Protons are made up of two up quarks and one down
quark, while neutrons consist of two down quarks and one up quark. The force
between quarks increases as they move farther apart, preventing individual
quarks from being isolated in nature.

In the Standard
Model of particle physics, the inclusion of the Higgs boson completes the
theory. According to the Standard Model, all matter in the universe is composed
of six types of quarks and six types of leptons (electron, electron neutrino,
muon, muon neutrino, tau, and tau neutrino). Among these, only four particles
(up quark, down quark, electron, and electron neutrino) are considered
fundamental constituents of all matter. Forces on the subatomic scale are
mediated by force-carrying bosons (Z, W, photon, gluon, and Higgs).

While the
theory of fundamental particles in the Standard Model can explain phenomena on
both atomic and nuclear scales, it fails to unify interactions on a larger
scale due to the inability to quantize the meaningful gravitational force.

In 1976,
scientists envisioned a possibility: the concept of supergravity emerged as a
potential theory for incorporating gravity into the framework of Einstein's
General Relativity by introducing some theoretical quantum particles. The idea
of supergravity originated mainly from the concept of supersymmetry. Symmetry
or symmetry breaking is widely used in particle physics for the mathematical
needs of quantum mechanics. A system is called symmetric if it remains the same
under transformations of space-time or quantum states. For example, a perfect
circle. The concept of supersymmetry is somewhat more complex. According to the
principles of supersymmetry, every particle will have a bosonic component and a
fermionic component. That is, if a quark carries a certain mass, there will be
another particle with the same mass that carries force. On the other hand, a
photon carries force naturally. In supersymmetry, there will be another partner
photon that carries mass. After the mathematical processes of the entire system,
if one of these hypothetical particles becomes positive, the other becomes
negative and cancels out.[5]

Supergravity
theory assumes that the carrier of the gravitational force will be the
"graviton," which has a spin number of 2. It is a supersymmetric
particle. Particles with spins of 3/2, 1, 1/2, and 0 can also be added to the
graviton, where their spins are 5 (2s + 1 = 2 x 2 + 1 = 5), 3/2, 1/2, and 0,
respectively. The energy of particles with spins of 0, 1, and 2 will be
positive, while particles with spins of 3/2 and 1/2 will be negative. Thus, the
sum of energies of these hypothetical particles will be the sum of positive and
negative energies. However, calculating this infinite series of hypothetical
particles can be time-consuming even for powerful computers, taking years after
years. And if there's an error in the calculation, there's no going back—everything
must start from scratch. So, while supergravity or supersymmetry (SUSY) may be
successful mathematically, it's not very realistic. Super symmetry has not yet
been proven in the Large Hadron Collider.

General
relativity is a classical theory. When scientists attempted to quantize it,
they pursued another method. We know gravity exists everywhere in the universe,
and it is continuous. However, quantum systems are discrete. Scientists thought
of quantum gravity as loops, where space-time would be quantized – meaning
divided into small loops or spin networks. Just as in quantum mechanics, where
energy levels are calculated in discrete steps, gravity would also be
calculated in loop quantum gravity. This would allow for quantization of both
two-dimensional area and three-dimensional volume. However, loop quantum
gravity does not support singularities. This means that loop quantum gravity
cannot provide answers to how the universe began. If it cannot answer this
question, it cannot lay claim to the entire theory.

At this time,
scientists introduced another theory of different concept – called string
theory. More probable than supergravity theory, string theory posits that
everything in the universe is made up of extremely small strings or filaments.
According to the standard model of particle physics, where fundamental
particles like electrons and quarks are assumed to be the end point, string
theory suggests that these are not the ultimate particles; they are also made
up of even smaller string-shaped waves. String theory is more complex than any
other theory because it operates in a ten-dimensional world instead of the
conventional four-dimensional space-time.[6]

Imagining
ourselves living in a ten-dimensional space rather than the familiar
three-dimensional space is not easy. But even on a smaller scale than electrons
and quarks, space-time can be divided in such a way that there are ten
dimensions. The concept of a ten-dimensional space is necessary in string
theory for mathematical purposes. String theorists have attempted to
mathematically prove that this theory is extremely successful and effective as
a unified theory of everything. However, various research groups have
discovered several more forms of string theory using different mathematical
approaches. It has been observed that in the realm of multiple-dimensional
space, one can split the five primary forms of string theory, each representing
the conventional four-dimensional space-time, in various ways. All these forms
were united into the extended version of string theory called M-theory. Stephen
Hawking wrote in his book "The Grand Design", "Perhaps M stands
for 'Master', 'Miracle', 'Mystery' - or all of them."[7] However, John Gribbin
explained that the M in M-theory stands for "Membrane" instead of
"String" or "Superstring."[8] It is believed that
M-theory can explain everything in the universe. To work with M-theory, one
needs eleven-dimensional space. It is said that in the ten-dimensional string
theory, one dimension was dropped.

The powerful
claim of M-theory is that it can explain everything in terms of its principles.
Although no concrete evidence for it has been found yet, mathematically,
M-theory can explain the workings of everything from one-dimensional strings,
two-dimensional membranes, to three-dimensional objects. Mathematically,
M-theory is so potent that it supports the idea of countless new universes.
Scientists are exploring the theory of everything to express everything in a
single universe. However, if M-theory is indeed that theory, then it gives rise
to a new problem - the birth of even more countless universes. So, there is
doubt about how successful it will be as a theory of everything.

Now the
question arises, why do we need the theory of everything? There is no other
branch of science, apart from physics, that is so concerned with this. But why
do we need this theory in physics?

Einstein spent
more than half of his life searching for the theory of everything - the Grand
Unified Theory. He hoped to find a mathematical formula within which everything
in the universe could be encapsulated. He was not successful, but physics did
not suffer any loss from it.[9]

Later,
theoretical physicists set their sights on a theory even more ambitious than
Grand Unification, one that could provide answers to all the fundamental
questions of the universe. Behind popularizing this theory, the contributions
of scientists like Stephen Hawking were among the most significant. In 1981, in
his first lecture as the Lucasian Professor, Stephen Hawking expressed hope
that in the next twenty years, the Theory of Everything (TOE) would be
discovered. Then, once the theoretical physicists had it in their hands, there
would be no more work left to do. Stephen Hawking concluded his Brief History
of Time in this manner: if we can discover a complete theory, it will not only
be understandable to scientists, but to everyone. Then we will all be able to
discuss why and how we and our universe came into existence.

Stephen
Hawking's life partner, Jane Hawking, in her remarkable memoir "Music to
Move the Stars: A Life with Stephen," recounts an anecdote where James
Marsh, when making a film, contemplated naming it after one of Stephen
Hawking's popular books, "The Theory of Everything."[10] Released in 2014, this
film gained widespread popularity - for it was a reflection of Stephen
Hawking's life. The book "The Theory of Everything" is a collection
of seven popular science lectures by Stephen Hawking, initially published in
1996 as "The Cambridge Lectures." Later, in 2005, the book was
published under the title "The Theory of Everything." The seventh and
final chapter of the book is titled "The Theory of Everything." Five
years later, in 2010, Stephen Hawking collaborated with Leonard Mlodinow to
publish another popular book, "The Grand Design." In this book as
well, there is a chapter titled "The Theory of Everything." Stephen
Hawking considered the Theory of Everything to be the Grand Design of the
universe.

But Stephen
Hawking had sufficient doubt that the entire mystery of the universe could be
solved by the sole theory of physics. In his 2002 lecture titled "Gödel
and the End of Physics," Stephen Hawking said, "Many will be
disappointed if we do not find one ultimate theory that can explain everything.
I used to be one of them, but I have changed my mind. I am now glad that our
search for understanding will never come to an end and that we will always have
new discoveries to look forward to."

[1] P.C.W.
Davies and J. Brown (Eds.), "Superstrings: A Theory of Everything?"
Cambridge University Press, Cantor Edition, Victoria, 1992.

[2] James
R. Johnson, "Does a Theory of Everything Exist?" Philosophy and
Cosmology, Issue 26, 2021.

[3] "A
Theory of Everything?" Nature, Issue 433, January 2005.

[4] Don
Lincoln, "Einstein's Unfinished Dream: Practical Progress Towards a Theory
of Everything," Oxford University Press, 2023.

[5] Moataz
H. Imam (Ed.), "Are We There Yet? The Search for a Theory of
Everything," Bentham Science Publishers, 2011.

[6] Brian
Greene, "The Elegant Universe," Vintage Books, New York, 2003.

[7] Stephen
Hawking and Leonard Mlodinow, "The Grand Design," Bantam Books,
London, 2010.

[8] John
Gribbin, "In Search of Superstrings," Second Edition, Icon Books,
London, 2007.

[9] Michio
Kaku, "The God Equation: The Quest for a Theory of Everything,"
Doubleday, New York, 2021.

[10] Stephen
Hawking, "The Theory of Everything," Phoenix Books, California, 2005.

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