String theory. Have you ever thought that the universe is like a cello? That's right - she didn't come. Because the universe is not like a cello. But this does not mean that she has no strings. Let's talk today about String Theory.

Of course, the strings of the universe are hardly similar to those that we imagine. In string theory, these are incredibly small vibrating strands of energy. These threads are rather like tiny "rubber bands" that can twist, stretch and shrink in every way. All this, however, does not mean that it is impossible to "play" the symphony of the Universe on them, because, according to string theorists, all that exists consists of these "threads".

## Physics contradiction

In the second half of the 19th century, physicists thought that nothing serious in their science could no longer be discovered. Classical physics believed that there were no serious problems left in it, and the whole structure of the world looked like a perfectly debugged and predictable machine. The trouble, as usual, happened because of nonsense - one of the small "clouds" that still remained in the clear, understandable sky of science. Namely, when calculating the radiation energy of an absolutely black body (a hypothetical body that at any temperature completely absorbs the radiation incident on it, regardless of the wavelength - NS).

Calculations showed that the total radiation energy of any absolutely black body must be infinitely large. To get away from such an obvious absurdity, the German scientist Max Planck in 1900 suggested that visible light, X-rays and other electromagnetic waves can only be emitted by some discrete portions of energy, which he called quanta. With their help, it was possible to solve the particular problem of an absolutely black body. However, the implications of the quantum hypothesis for determinism were not yet realized. Until another German scientist, Werner Heisenberg, formulated the famous uncertainty principle in 1926.

Its essence boils down to the fact that, contrary to all the prevailing assertions before, nature limits our ability to predict the future based on physical laws. We are, of course, talking about the future and the present of subatomic particles. It turned out that they behave in a completely different way from any things in the macrocosm around us. At the subatomic level, the fabric of space becomes uneven and chaotic. The world of tiny particles is so turbulent and incomprehensible that it defies common sense. Space and time in it are so twisted and intertwined that there are no usual concepts of left and right, top and bottom, and even before and after.

There is no way to say for sure at what point in space this or that particle is at a given moment, and what is the moment of its momentum. There is only a certain probability of finding a particle in a variety of regions of space-time. Particles at the subatomic level seem to be "smeared" in space. Moreover, the very "status" of particles is not defined: in some cases they behave like waves, in others they exhibit the properties of particles. This is what physicists call the wave-particle duality of quantum mechanics.

In the General theory of relativity, as if in a state with opposite laws, the situation is fundamentally different. Space appears to be like a trampoline - a smooth fabric that objects of mass can bend and stretch. They create deformations of space-time - what we experience as gravity.Needless to say, the harmonious, correct and predictable General Theory of Relativity is in an insoluble conflict with the "eccentric hooligan" - quantum mechanics, and, as a result, the macrocosm cannot "make peace" with the microcosm. This is where string theory comes in.

String theory embodies the dream of all physicists to unite two fundamentally contradictory general relativity and quantum mechanics, a dream that until the end of his days haunted the greatest "gypsy and vagabond" Albert Einstein.

Many scientists believe that everything from the exquisite dance of galaxies to the crazy dance of subatomic particles can ultimately be explained by just one fundamental physical principle. Maybe - even a single law that unites all types of energy, particles and interactions in some elegant formula.

General relativity describes one of the most famous forces of the Universe - gravity. Quantum mechanics describes three other forces: the strong nuclear force, which sticks protons and neutrons together in atoms, electromagnetism, and the weak force, which participates in radioactive decay. Any event in the universe, from the ionization of an atom to the birth of a star, is described by the interactions of matter through these four forces.

With the help of sophisticated mathematics, it was possible to show that the electromagnetic and weak interactions have a common nature, combining them into a single electroweak. Subsequently, a strong nuclear interaction was added to them - but gravity does not join them in any way. String theory is one of the most serious candidates for combining all four forces, and, therefore, embracing all phenomena in the Universe - it is not without reason that it is also called the "Theory of Everything."

## In the beginning there was a myth

Until now, not all physicists are enthusiastic about string theory. And at the dawn of her appearance, she did seem to be infinitely far from reality. Her very birth is a legend.

In the late 1960s, the young Italian theoretical physicist Gabriele Veneziano was looking for equations that could explain strong nuclear interactions - the extremely powerful glue that holds the nuclei of atoms together, binding protons and neutrons together. According to legend, he somehow stumbled upon a dusty book on the history of mathematics, in which he found a function two hundred years ago, first written by the Swiss mathematician Leonard Euler. Imagine Veneziano's surprise when he discovered that the Euler function, which for a long time was considered nothing more than a mathematical curiosity, describes this strong interaction.

How was it really? The formula was probably the result of many years of work by Veneziano, and chance only helped to take the first step towards the discovery of string theory. Euler's function, which miraculously explained the strong interaction, took on a new life.

In the end, it caught the eye of the young American theoretical physicist Leonard Susskind, who saw that, first of all, the formula described particles that had no internal structure and could vibrate. These particles behaved in such a way that they could not be just point particles. Susskind realized that the formula describes a thread that is like an elastic band. She could not only stretch and shrink, but also hesitate, wriggle. After describing his discovery, Susskind introduced the revolutionary idea of strings.

Unfortunately, the overwhelming majority of his colleagues greeted the theory rather coldly.

## Standard model

At the time, mainstream science represented particles as dots, not strings. For years, physicists have studied the behavior of subatomic particles, colliding them at high speeds and studying the consequences of these collisions. It turned out that the universe is much richer than one could imagine. It was a real "population explosion" of elementary particles. Graduate students from physics universities ran along the corridors shouting that they had discovered a new particle - there were not even enough letters to designate them.But, alas, in the "maternity hospital" of new particles, scientists have not been able to find the answer to the question - why are there so many of them and where do they come from?

This prompted physicists to make an unusual and startling prediction - they realized that the forces acting in nature can also be explained using particles. That is, there are particles of matter, and there are particles that carry interactions. Such, for example, is a photon - a particle of light. The more of these carrier particles - the same photons that matter particles exchange - the brighter the light. Scientists predicted that this particular exchange of carrier particles is nothing more than what we perceive as a force. This was confirmed by experiments. So physicists managed to get closer to Einstein's dream of uniting forces.

Scientists believe that if we fast forward to the moment immediately after the Big Bang, when the universe was trillions of degrees hotter, the particles-carriers of electromagnetism and weak interaction will become indistinguishable and combine into a single force called electroweak. And if we go back even further in time, then the electroweak interaction would combine with the strong one into one total “superpower”.

While all of this is still awaiting proof, quantum mechanics has suddenly explained how three of the four forces interact at the subatomic level. And she explained beautifully and consistently. This neat picture of interactions ultimately became known as the Standard Model. But, alas, this perfect theory had one big problem - it did not include the most famous macro-level force - gravity.

For the string theory, which did not have time to "blossom", the "autumn" has come, it contained too many problems from the very birth. For example, the calculations of the theory predicted the existence of particles, which, as it was quickly established, did not exist. This is the so-called tachyon - a particle that moves faster than light in a vacuum. Among other things, it turned out that the theory requires as many as 10 measurements. Unsurprisingly, this was very confusing for physicists, because it is obviously more than what we see.

By 1973, only a few young physicists were still grappling with the mysterious computations of string theory. One of them was the American theoretical physicist John Schwartz. For four years, Schwartz tried to tame the naughty equations, but to no avail. Among other problems, one of these equations persisted in describing a mysterious particle that had no mass and was not observed in nature.

The scientist had already decided to abandon his bad job, and then it dawned on him - maybe the equations of string theory describe, among other things, gravity? However, this implied a revision of the size of the main "heroes" of the theory - strings. Assuming that strings are billions and billions of times smaller than an atom, the stringmen turned the theory's flaw into its merit. The mysterious particle that John Schwartz had tried so hard to get rid of was now acting as a graviton - a particle that had been long sought for and which would allow gravity to be transferred to a quantum level. This is how string theory supplemented the puzzle with gravity not found in the Standard Model. But, alas, the scientific community did not react even to this discovery. String theory remained on the brink of survival. But this did not stop Schwartz. Only one scientist willing to risk his career for the sake of mysterious strings wanted to join his search - Michael Green.

## Subatomic nesting dolls

In spite of everything, in the early 1980s, string theory still had insoluble contradictions, called anomalies in science. Schwartz and Green set about eliminating them. And their efforts were not in vain: scientists were able to eliminate some of the contradictions of the theory. Imagine the amazement of these two, already accustomed to the fact that their theory was ignored, when the reaction of the scientific community blew up the scientific world. In less than a year, the number of string theorists jumped to hundreds.It was then that string theory was awarded the title of Theory of Everything. The new theory seemed to be able to describe all the components of the universe. And these are the components.

Each atom, as you know, consists of even smaller particles - electrons, which circle around a nucleus, consisting of protons and neutrons. Protons and neutrons, in turn, are composed of even smaller particles - quarks. But string theory says that quarks don't end there. Quarks are made up of tiny twisting strands of energy that resemble strings. Each of these strings is incredibly small.

Small enough that if the atom were enlarged to the size of the solar system, the string would be the size of a tree. Just as different vibrations of a cello string create what we hear, like different musical notes, different modes (modes) of vibration of the string give particles their unique properties - mass, charge, and so on. Do you know how, relatively speaking, the protons in the tip of your nail differ from the still unopened graviton? Just the set of tiny strings that make them up and the way those strings vibrate.

Of course, all this is more than amazing. Since the days of Ancient Greece, physicists have become accustomed to the fact that everything in this world consists of something like balls, tiny particles. And now, not having time to get used to the illogical behavior of these balls, arising from quantum mechanics, they are invited to completely abandon the paradigm and operate with some scraps of spaghetti …

## Fifth Dimension

Although many scientists call string theory a triumph of mathematics, it still has some problems - first of all, the lack of any opportunity to test it experimentally in the near future. Not a single instrument in the world, neither existing nor capable of appearing in perspective, is capable of "seeing" the strings. Therefore, some scientists, by the way, even ask the question: is string theory a theory of physics or philosophy?.. True, it is not at all necessary to see the strings "with your own eyes". Rather, proof of string theory requires something else - what sounds like science fiction - confirmation of the existence of extra dimensions of space.

What is it about? We are all accustomed to three dimensions of space and one - time. But string theory predicts other - extra - dimensions as well. But let's start in order.

In fact, the idea of the existence of other dimensions originated almost a hundred years ago. It occurred to the then unknown German mathematician Theodor Kaluza in 1919. He suggested the possibility of the presence of another dimension in our Universe, which we do not see. Albert Einstein found out about this idea, and at first he liked it very much. Later, however, he doubted its correctness, and delayed the publication of Kaluza for two whole years. Ultimately, however, the article was still published, and the additional dimension became a kind of hobby of the genius of physics.

As you know, Einstein showed that gravity is nothing more than a deformation of the dimensions of space-time. Kaluza theorized that electromagnetism could also be ripples. Why don't we see it? Kaluza found the answer to this question - the ripples of electromagnetism can exist in an additional, hidden dimension.

## But where is it?

The answer to this question was given by the Swedish physicist Oskar Klein, who suggested that the fifth dimension of Kaluza is curled billions of times stronger than the dimensions of one atom, so we cannot see it. The idea of the existence of this tiny dimension that is all around us is at the heart of string theory.

But in fact, the equations of string theory require not even one, but six additional dimensions (in total, with the four we know, there are exactly 10 of them). All of them have a very curved and curved complex shape. And all are unimaginably small.

How can these tiny dimensions affect our large world? According to string theory, the decisive factor is that shape determines everything.When you press different keys on a saxophone, you get different sounds. This is because when you press one or another key or a combination of them, you change the shape of the space in the musical instrument where air circulates. Thanks to this, different sounds are born.

String theory believes that the extra curved and twisted dimensions of space appear in a similar way. The shapes of these extra dimensions are complex and varied, and each vibrates the string that is within such dimensions in different ways precisely because of its shapes. After all, if we assume, for example, that one string vibrates inside a jug, and the other - inside a curved post horn, these will be completely different vibrations. However, if you believe string theory, in fact, the shapes of extra dimensions look much more complex than a jug.

## How the world works

Science today knows a set of numbers that are fundamental constants of the universe. They determine the properties and characteristics of everything around us. Among such constants, for example, the electron charge, the gravitational constant, the speed of light in vacuum … And if we change these numbers even a small number of times, the consequences will be catastrophic. Suppose we have increased the strength of the electromagnetic force. What happened? We may suddenly find that the ions have become more repelled from each other, and the thermonuclear fusion, which makes the stars shine and radiate heat, suddenly malfunctioned. All the stars will go out.

But what does string theory with its extra dimensions have to do with it? The fact is that, according to her, it is the additional dimensions that determine the exact value of the fundamental constants. Some forms of measurement cause one string to vibrate in a specific way, and generate what we see as a photon. In other forms, the strings vibrate differently and generate an electron. Truly, God is hidden in the "little things" - it is these tiny forms that determine all the fundamental constants of this world.

## Superstring theory

In the mid-1980s, string theory took on a majestic and slender appearance, but confusion reigned within this monument. In just a few years, as many as five versions of string theory have emerged. And although each of them is built on strings and extra dimensions (all five versions are combined into a general superstring theory - NS), the details of these versions diverged significantly.

So, in some versions, the strings had open ends, in others they resembled rings. And in some versions, the theory even required not 10, but as many as 26 measurements. The paradox is that all five versions today can be called equally true. But which one really describes our universe? This is another mystery in string theory. That is why many physicists again gave up on the "crazy" theory.

But the main problem with strings, as already mentioned, is the impossibility (at least for now) to prove their existence experimentally.

Some scientists, however, still say that in the next generation of accelerators there is a very minimal, but still, opportunity to test the hypothesis of extra dimensions. Although the majority, of course, are sure that if this is possible, then, alas, it should not happen very soon - at least in decades, at most - even in a hundred years.

The article was published in Naked Science magazine # 14.