The word “physics” is based on the ancient Greek φύσις, which means “nature”. The study of nature, the explanation of the phenomena observed in it and the prediction of yet unknown events are the goals of physics. Back in the IV century BC, Aristotle laid the foundations of physics as a science in his treatise "Physics". Since then, many secrets of nature have been revealed to us. But the more we learned, the more questions arose. And, perhaps, the main one: how to find a general explanation for all phenomena? To answer it, you need to create a Theory of Everything.

## Standard Model and Her Step Sister

Making their way deep into matter and trying to find the most fundamental, indivisible particles that underlie matter, scientists eventually discovered quarks and leptons. Quarks are used to build hadrons (that's why the Large Hadron Collider is so called), which include, in particular, the protons and neutrons that make up the nucleus of an atom that was previously considered indivisible. The most famous representative of leptons is an electron moving in an electron cloud around this very nucleus.

Today we know that all these particles interact due to four types of forces, four fundamental interactions: gravitational, electromagnetic, strong nuclear and weak nuclear. Although there may be more of them, others are not yet known to us.

Gravity is the force of attraction between objects with mass. Electromagnetism is responsible for the interaction between bodies that have electrical charges. Strong nuclear forces hold together protons and neutrons in the nucleus of an atom, as well as quarks in the protons and neutrons themselves, while weak ones govern processes such as radioactive decay.

The action of each of these forces can be represented as an exchange of particle-quanta of this interaction. The strong interaction is carried out by gluons. They can be said to "glue" quarks, creating protons and neutrons from them. They got their name for a reason: in English, glue means "glue".

The carriers of the weak interaction are the W and Z bosons. For the well-known to us electromagnetic interaction, no less well-known photons are responsible. In addition, all interactions have their own theory describing these very interactions.

At this point, the so-called Standard Model - a theoretical construction in particle physics - is interrupted. What about gravity? Does it have its own quantum particle?

If elementary particles are used to model the basic interactions, then by analogy it can be assumed that some elementary particle should be responsible for the gravitational interaction.

This particle has not yet been found, it is listed as hypothetical, but the name has already been proposed - graviton. A quantum theory of gravity has not yet been built. We use Einstein's General Theory of Relativity to describe gravity today. But it is so different from all other theories that it stands apart. To unite all four interactions in one theoretical model, that is, to create a unified theory of everything, is an old dream of physicists.

## From Newton to Einstein, from apples to black holes

The apple tree in the garden of the house where Isaac Newton lived was a museum exhibit for more than a century, excursions were led to it. But, most likely, the story of how Newton discovered his famous Law of Universal Gravitation after an apple fell on his head is a harmless notion.The great scientist composed the story of a falling apple for his beloved niece in order to explain the essence of the law in an accessible way. Nevertheless, we can say that it was with the postulation of this law that attempts began to systematically explain the world around us in the language of physics.

The very idea of the universal force of gravity was repeatedly expressed before Newton. But earlier, no one could clearly and mathematically convincingly connect the Law of gravitation and the Laws of motion of celestial bodies. Newton's discovery made it possible to unite the celestial and earthly spheres, and after all, they were previously considered incompatible.

For a long time, the Newtonian theory of gravitation was fully confirmed by observations. The law of gravitation was suitable in order to describe the trajectory of the fall of the apple, as well as to predict the orbit of the motion of the planets around the Sun. But, as it turned out, except for one.

The anomalous displacement of the perihelion of Mercury discovered in 1859 put physicists in front of the fact that the planet closest to the Sun did not want to obey the Law of universal gravitation. And even if it was only a barely noticeable deviation in the motion of Mercury, he needed to find an explanation. But this required a new understanding of gravity.

It was only possible to revise the understanding of gravity in 1915, when Albert Einstein presented his General Theory of Relativity to the world. The sun, which has a gigantic mass, bends space and time around itself, which, in particular, affects the orbit of the planet closest to it.

From a practical point of view, general relativity is excellent. It has been repeatedly confirmed by observations and is widely used in practice. Without the Theory of Relativity, there would be no satellite navigation systems, for example.

But nevertheless, it has one significant drawback - the impossibility of constructing a quantum field model for it in a classical way. It is completely different from quantum mechanics. As physicists say, this is a different kingdom with its own laws and inhabitants.

## On the path of unification

Perhaps the first unification of various physical forces was made in 1873, when the British physicist and mathematician James Maxwell in his work "Treatise on Electricity and Magnetism" showed that electricity and magnetism are manifestations of the same force - electromagnetism. Prior to this, it was believed that electricity and magnetism are two separate and independent forces.

Almost 100 years later, in 1967, American physicists Sheldon Lee Glashow and Steven Weinberg, as well as Abdus Salam, a theoretical physicist from Pakistan, will create the Electroweak Interaction Theory, which became a description of two of the four known fundamental interactions - weak and electromagnetic. It postulates that electromagnetic and weak interactions are different manifestations of the same force. At ordinary low energies, we will not find anything in common in them, but at energies above the unification energy (of the order of 100 GeV), they combine into a single electroweak interaction. True, the last time they were united in the first moments after the Big Bang.

In the 70s of the last century, its own theory was proposed for the strong interaction - quantum chromodynamics. So far, no one has been able to convincingly connect this third fundamental interaction with the first two. This theoretical model, which describes the strong, weak and electromagnetic interactions in a unified way, is called the Grand Unification Theory.

However, many theoretical physicists believe that it makes no sense to combine these interactions without gravity: they already work together in the form of the Standard Model. The path to the Great Unification lies through the creation of the Theory of Everything.

By the way, scientists believe that at ultrahigh energies, all interactions come together. And, as you might have guessed, the conditions for such a Great Unification could exist in the Universe in the shortest period immediately after the Big Bang.That is, about 13-14 billion years ago, when the age of the newborn Universe was from 10-43 to 10-36 seconds. Then all interactions split and began to live an independent life: first - gravity, then - strong, and then weak and electromagnetic interactions.

## Different kingdoms

However, if the three interactions, united by the Standard Model, are calmly described by quantum mechanics, then the theory of relativity, which describes gravity, is completely different. Quantum mechanics, which tells us how elementary particles behave, and General Relativity are based on different sets of principles. The first is formulated as a theory describing the temporal evolution of physical systems (the same atoms or elementary particles) against the background of external space-time. In the second, there is simply no external space-time - it is itself a dynamic variable of the theory, depending on the characteristics of the classical systems located in it.

Both have limits of applicability, beyond which they stop working. Quantum mechanics works on the microscale and explains the structure and behavior of atoms. General relativity deals with gigantic masses and speeds.

In most cases, they do not intersect and live, in fact, in different worlds. In some situations, quantum effects can be neglected, in others - gravitational. However, there is a place in the Universe where these worlds are forced to intersect - black holes. They are very massive, but at the same time extremely small.

But that's not all, there is one more reason for incompatibility. Thus, General Relativity states that the behavior of an object can be accurately predicted. But in quantum mechanics, everything is different: we can only know the probability of how this or that object will behave.

Einstein spent almost all the last years of his life trying to derive his unifying theory. He did not share the uncertainty principle of quantum mechanics and wanted to create a theory that would combine gravity and the rest of physics, so that all these quantum oddities would be secondary consequences. In one of the letters to the adherent of this principle, Max Born, he wrote: "God does not play dice." To which he received the answer: "Einstein, do not tell God what to do."

The main task of the scientist was to make gravity work with electromagnetism and combine both forces in the Unified Field Theory. To do this, he stretched space-time into five dimensions. The fifth dimension was added to three spatial and one temporal: it had to be so small and curled up that we could not see it. But this approach was unsuccessful.

## Strings, loops and branes

Since then, the idea of creating a unifying theory has dominated the minds of physicists around the world. There are several ideas, some also have several options. And the most serious candidate for the title of Theory of Everything is String Theory.

It is based on a simple assumption. The smallest particles of our world are not at all point objects, as we now imagine them. These are strings, or more precisely, one-dimensional extended objects, the so-called quantum strings. They are very small, their length is about 10–33 centimeters. Like guitar strings, quantum strings are taut and vibrate. The nature of their vibrations determines the properties of matter: thus, the whole variety of elementary particles is reproduced. A string vibrates with one frequency - we get a gluon, vibrates with another - a quark, with a third - a neutrino. Moreover, the strings can be both closed and open.

String theory removes some of the obstacles that previously hindered the construction of a coherent quantum theory of gravity. It allows you to describe a string that looks exactly like a graviton - a hypothetical carrier of gravitational interaction and a quantum of the gravitational field.

At the same time, she also has problems.While Einstein's Unified Field Theory assumed there was one additional hidden dimension, the simplest versions of String Theory require twenty-six.

Pioneered in the 1980s, Superstring Theory comes in five different flavors, dispensing with ten dimensions. But even they are difficult to imagine, because in the world we observe only three spatial dimensions. While physicists suggest it is easy to imagine that only these three dimensions have expanded and become large, others also exist, but have remained fantastically small.

Not everyone shares the ideas of String Theory, so there is another contender - loop quantum gravity.

If String Theory is to take its place above all other theories, then Loop Quantum Gravity is the missing link in quantum mechanics. It is designed to simply bring gravity to a common quantum denominator, and is trying to derive its own quantum theory for it.

The general theory of relativity describes space-time in a classical way, which does not allow to "quantize" gravity in the usual way for elementary particle physics. Loop gravity theory tries to solve this problem. In it, space and time consist of discrete parts - small loops, the dimensions of which are comparable to the Planck length, which is approximately 1.6.10−35 meters.

These loops - small quantum cells of space - are connected to each other in a certain way: so that on small scales of time and distance, they create a discontinuous, discrete structure of space, and on large ones, they smoothly transition into continuous smooth space-time. This is exactly what is described in GR.

However, String Theory is not going to give up. It was noted above that its advanced version already had five options. But another ten years passed, and in the 90s physicists discovered that they could all be transformed into one another. The description methods are different, but the essence is the same. So in 1995, M-theory appeared, ironically called "the mother of all string theories."

It assumes that the world around us has 11 space-time dimensions. It contains spaces of lower dimensions, the so-called branes, and the universe we live in is just one of these branes. It is filled with various quantum particles, which are, in fact, strings.

The ends of the open strings are anchored inside the branes. Such a string cannot leave the brane. Closed strings, on the other hand, are capable of migrating outside the branes. These "free" strings are gravitons, carriers of gravitational forces.

However, M-theory did not lead us to any useful understanding of the universe. It simply does not suggest the existence of one single theory of everything, but implies the existence of many theories. And each of them makes it possible to describe the universe in a convincing way. But at the same time, many universes are assumed. More precisely, their number is equal to 10 to the power of 500. This incredible collection of universes is called the Multiverse, and our universe is just one of them.