Physics, the oldest branch of natural sciences has evolved for centuries with lots of theories. We tried to understand the nature of reality with those theories but time changed our way of thinking. In the 20^{th} century and later on, physicists want one theory to explain all phenomena in the universe. It is called the theory of everything in physics.

## Einstein’s efforts to find the theory of everything in physics

Einstein published special relativity in 1905 as well as general relativity in 1915. The theory of relativity encompasses both of them. Quantum mechanics was founded by Niels Bohr and Max Planck. These two theories are only right alone. They fail when applying to each other.

Einstein wanted a unifying theory of the two. He tried to build this theory of everything in physics in 1920. One of his speeches for his students proves his desire of finding TOE. “I’m not interested in this or that phenomenon, in the spectrum of this or that element. I want to know His (God) thoughts; the rest are just details.”.

He never accepted weird paradoxes in quantum mechanics. But he needs a theory of everything until he died. One of his beliefs was that electromagnetism and gravity can be mathematically described in one framework.

## The theory of almost everything

The standard model or theory of almost everything is a work by mid-20th century physicists. It is unifying all known subatomic particles (Quarks, Leptons, and Bosons) and three of the four fundamental forces excluding gravity.

Gluon(g) is responsible for strong force while photon(ϒ) is responsible for electromagnetism. Both W and Z bosons are responsible for weak nuclear force.

The reason behind including electromagnetism and the strong and weak nuclear forces but not gravity is general relativity. In general relativity, gravity is not considered a force. A force is an influence that can change the motion of an object. But gravity is the warping of space and time because of mass.

## Can string theory be the theory of everything in physics

String theory is one framework of physics. It describes reality is made of infinitesimal vibrating strings. String theory replaced the mathematical model of a point particle with a vibrating string.

It was once believed to be the theory of everything. Different vibrations create different particles including gravity. The string theory can unify all fundamental forces and particles in the universe. So many people were excited that they finally found the theory of everything. But it was only a theoretical solution.

We live in a three spatial dimensional universe in which we can see length, width, and deep. Time is a temporal dimension in our universe. String theory is not a three-dimensional model. The string theory requires 10 dimensions in total. Anyone who wants to study string theory must do calculations with model universes and then apply them to ours by getting rid of 6 dimensions.

No one could succeed in any attempts. String theory or its prediction is not proven by any experiment. So, we cannot take it as the theory of everything in physics. But we should not forget mathematics in string theory worked out. Since physics is based on mathematics string theory still holds some value.

## Evolution of gravity theories

Newton’s law of universal gravitation introduces a long-distance force between masses. The gravitational force increases as the mass of two objects increases and the distance between them decrease. It was able to explain many phenomena in the universe including planetary movements. But Newton’s law of universal gravitation had some holes.

Things that cannot be explained by the law of universal gravitation include the peculiar orbit of Mercury around the sun, Planet orbits shifting over time. Also, Newton’s predictions are wrong about Mercury’s orbit shift.

The general theory of relativity was a better theory of explaining how gravity works.

The left side of the equation is about the curvature of spacetime, and the right side is about mass-energy content. Stress-energy-momentum tensor represented by T is a source of curvature. We must not forget gravity does not create the curvature it is the curvature. American theoretical physicist John Wheeler said “Space-time tells matter how to move, matter tells space-time how to curve”

But it also has limitations. It couldn’t explain the singularity inside a black hole. The widely accepted theory about the origin of our universe big bang is another thing that cannot be explained by the general theory of relativity. Physicists wanted a more accurate explanation of how gravity works.

## Quantum mechanics and gravity

Quantum mechanics is a more accurate concept. General relativity can not explain gravity in quantum scales. According to quantum mechanics, the electron in a hydrogen atom is in a superposed state. The simple meaning is it is in multiple positions that distance varies from the nucleus at the same time. Put that into general relativity, can space-time curve differently at the same time?

Steven Weinberg, a Nobel laureate (1933) said “No one should take general relativity seriously for distances shorter than 10^{-35} meters”. Information loss is another paradox that general relativity failed to explain. Loop quantum gravity and string theory come close to explaining gravity at very small sizes.

## Loop quantum gravity

The loop quantum gravity is another theory that can put quantum mechanics and general relativity together to find a theory of everything in physics. So, we know the true nature of reality. Loop quantum gravity introduced a quantized background space-time instead of classic continuous infinity fabric.

In LQG minimum distance between two areas in space-time is close to 10^{-35}m. The minimum area is close to 10^{-75}m^{2} and the minimum volume should be 10^{-105}m^{3}. Time also has a minimum quantity, and it is close to plank time (10^{-44} seconds). Imagine it like a TV screen. When you enjoy a movie, you see a continuous picture. But if you zoom it with the help of some optical instrument, you can see separate LEDs represent individual pixels that make a frame by combining millions of them.

## Quantum loops

Like individual LEDs are creating a picture, quantum loops and nodes create spin networks which is the geometry of itself is space, and movement of itself if time. Spin net and movement combined make spin foam. Time is not continuous like a flowing river in loop quantum gravity theory. It is like a ticking clock. Each tick (quanta movement) is equal to 10^{-43} seconds. When mass and energy are added to the spin network it gets distorted. So, time and space get distorted as a result. And that is what we called gravity in loop quantum gravity.

The LQG left us a way to make it proven. According to the theory photons of high energy should travel slower than those with low energy. We can test this by detecting high-frequency and low-frequency ϒ-rays from a billion-year-old burst. If LQG is right they should arrive on earth at a slightly different time.

But still, we could not detect such a thing. That makes us think about whether the theory is wrong or our instruments are not enough to do it.

## Big bang brings it closer to reality

New major big bang discoveries include the speed of expansion of the universe, bringing the theory of everything closer to reality. They found it could be many times the speed of light immediately after the big bang happened.

How does this connect to the theory of everything in physics? We did mention that general relativity had some failures in explaining the big bang. One failure is explaining the first instances after the big bang. This discovery confirms the theory of inflation. It says the cosmos exploded from mere quantum fluctuations into something of macroscopic size in no more than fractions of seconds.

## Grand unification

Gauge field theories explain a simple gauge group describing strong force, weak force, and electromagnetic force with quarks and leptons and their antiparticles combined. We refer gauge field theories by grand unification.

Maxwell’s theory unify electricity and magnetism. GUT works similarly. It foresees proton and bound neutron decay, though with an extremely long lifetime (More than 10^{30} years). A lifetime that has not been observed ever.

## Quantum field theory

fundamental Quantum fields are fields that fill the universe and decide what can and cannot happen. Quantum field theory was first introduced in 1927 in “The quantum theory of the emission and absorption of radiation” paper.

A wave in a quantum field represents a particle. A wave in an electromagnetic field represents a photon and a wave in a gluon field represents a gluon. Particles interact with one another by exchanging force carriers. Photon is the force carrier of the electromagnetic field and likewise, other fields have their respective force carriers.

## We may never find it

The idea of a theory that can explain every phenomenon in the universe unveiling the true nature of reality is an ultimate goal of humanity. But such a thing can be never found as the descriptions of the reality we perceive and live in, are always incomplete.

Whether it is gravity, black holes, or dark energy, physics discoveries help humans to develop as a species. Stephen Hawking said, “I’m now glad that our search for understanding will never come to an end and that we will always have the challenge of discovery”.

## Conclusion

Finding a theory to explain everything is in the mind of physicists for a long time ago. If we can somehow unify the general theory of relativity and quantum mechanics, we can find the theory of everything in physics. But such attempts didn’t succeed yet.

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