Leonard Susskind, a famous American physicist and one of the founders of string theory, at one time proposed a revolutionary concept of understanding the Universe and the place of man in it. With his research, Susskind inspired a whole galaxy of modern physicists who believed that this theory could uniquely predict the properties of our Universe. Now, in his first book for a wide range of readers, Susskind refines and rethinks his views, arguing that this idea is by no means universal and it will have to give way to a much broader concept of a giant "cosmic landscape."
Studies of the beginning of the XXI century allowed science to rise to a new level in the knowledge of the world, says Susskind. And this fascinating book, which takes the reader to the forefront of battles in modern physics, is a clear confirmation of this.
Elegant supersymmetric Universe?
The real principles underlying string theory are shrouded in great mystery. Almost everything we know about theory includes a special part of the landscape, where mathematics is remarkably simplified thanks to a property called supersymmetry. Supersymmetric areas of the landscape form a perfectly flat plain, located at a height exactly equal to zero, with properties so symmetrical that many things can be calculated without information about the entire landscape. If someone was looking for simplicity and elegance, then the flat plain of supersymmetric string theory, also known as superstring theory, is exactly the place to which they should pay attention. In fact, a couple of years ago this place was the only one that string theorists paid attention to. But some of the physicists have already shaken off their enchanting delusion and are trying to get rid of the elegant simplifications of the super world. The reason is simple: the real world is not supersymmetric.
A world containing a Standard Model and a small non-zero cosmological constant cannot be located on a plane of zero height. It lies somewhere in the uneven region of the Landscape with hills, valleys, high plateaus and steep slopes. But there is reason to believe that our valley is close to the supersymmetric part of the Landscape and that some remnants of the mathematical superchud could help us understand the features of the empirical world. One example that we will examine in this section is the Higgs boson mass. In fact, all the discoveries due to which this book came into being represent the first tentative attempts to move away from the safe supersymmetric plain.
Supersymmetry tells us about the differences and similarities of bosons and fermions. Like so much more in modern physics, the principles of supersymmetry can be traced back to the first works of Einstein. In 2005, we celebrated the centenary of the “anno mirabilis” - the year of the wonders of modern physics. Einstein began two revolutions this year and completed the third. Of course, this was the year of the special theory of relativity. But few know that 1905 was much more than the "year of relativity." He also marked the birth of photons, the beginning of modern quantum mechanics.
Einstein received only one Nobel Prize in physics, although I think that every Nobel Prize awarded after 1905 carried the echoes of Einstein's discoveries. The Nobel Prize was awarded to Einstein not for creating the theory of relativity, but for explaining the photoelectric effect. It was the photoelectric effect theory that was the most radical contribution of Einstein to physics, where he first introduced the concept of photons, energy quanta, of which light consists. Physics was already ready to give birth to a special theory of relativity, its creation was only a matter of time, while the photon theory of light thundered like thunder out of a clear sky. Einstein showed that a ray of light, usually represented as a wave phenomenon, has a discrete structure. If the light has a certain color (wavelength), then all photons march into a leg: each photon is identical to any other. Particles that can simultaneously be in the same quantum state are called bosons in honor of the Indian physicist Shatiendranath Bose.
Almost twenty years later, completing the building laid by Einstein, Louis de Broglie will show that electrons, always perceived as particles, behave at the same time and like waves. Like waves, electrons are able to reflect, refract, diffract and interfere. But there is a fundamental difference between electrons and photons: unlike photons, two electrons cannot simultaneously be in the same quantum state. The Pauli prohibition principle ensures that each electron in an atom has its own quantum state and that no other electron can stick its nose in an already occupied place. Even outside an atom, two identical electrons cannot be in the same place or have the same momentum. Particles of this kind are called fermions by the name of the Italian physicist Enrico Fermi, although in fairness they should be called Paulions. Of all the particles in the Standard Model, about half are fermions (electrons, neutrinos, and quarks), and the other half are bosons (photons, Z and W bosons, gluons, and the Higgs boson).
Fermions and bosons play different roles in the picture of the world. Usually we represent matter consisting of atoms, that is, of electrons and nuclei. In the first approximation, nuclei consist of protons and neutrons held together by nuclear forces, but at a deeper level, protons and neutrons are assembled from small building blocks — quarks. All these particles - electrons, protons, neutrons and quarks - are fermions. Matter consists of fermions. But without bosons, atoms, nuclei, protons and neutrons will simply fall apart. These bosons, primarily photons and gluons, jumping back and forth between fermions, create gravitational forces that hold everything together. Although fermions and bosons are critically important for the world to be as it is, they have always been considered "animals of a different breed."
But around the early 1970s, theorists, inspired by the first successes of string theory, began to play with new mathematical ideas, according to which fermions and bosons are actually not so different. One idea was that all particles form ideal pairs of identical twins, identical in all respects, except that one of them is a fermion and the other is a boson. It was a completely wild conjecture. Its validity for the real world would mean that physicists somehow managed to lose half of all elementary particles, failing to detect them in their laboratories. For example, according to this hypothesis, there must be a particle with exactly the same mass, charge, and other properties as that of an electron, just being not a fermion, but a boson. How could one not notice such a particle at the Stanford or CERN accelerators? Supersymmetry assumes the existence of a massless neutral fermion in a photon, as well as boson twins in electrons and quarks. That is, the hypothesis predicted a whole world of mysteriously missing "opposites". In fact, all this work was only a mathematical game, a purely theoretical study of a new kind of symmetry - a world that does not exist, but that could exist.
Identical twin particles do not exist. Physicists did not bother and did not miss the whole parallel world. What interest then is this mathematical speculation, and why has this interest suddenly intensified in the last 30 years? Physicists have always been interested in all sorts of mathematical symmetries, even if the only reasonable question that could be asked: “Why is this symmetry not found in nature?” But both the real world and its physical description are full of various symmetries. Symmetry is one of the most long-range and powerful tools in the arsenal of theoretical physics. It permeates all sections of modern physics, and especially those related to quantum mechanics. In many cases, the type of symmetry is all we know about the physical system, but the analysis of symmetry is such a powerful method that it often tells us almost everything we want to know. Symmetries are often the garden in which physicists find aesthetic satisfaction from their theories. But what is symmetry?
Let's start with a snowflake. Every child knows that there are no two identical snowflakes, but at the same time they all have a common feature, namely symmetry. The symmetry of the snowflake immediately catches the eye. If you take a snowflake and rotate it to an arbitrary angle, then it will look different from its original form - rotated. But if you turn the snowflake exactly 60 °, then it will coincide with itself. The physicist might say that turning a snowflake through 60 ° is a symmetry.
Symmetries are associated with operations or transformations that can be performed on a system without affecting the result of the experiment. In the case of a snowflake, such an operation is a rotation of 60 °. Here is another example: suppose that we set up an experiment aimed at measuring the acceleration of free fall on the surface of the Earth. The simplest option would be to drop a stone from a known height and measure the time of its fall. Answer: about 10 meters per second per second. Please note that I am not worried about telling you where I dropped a stone: in California or in Calcutta. In a very good approximation, the answer will be the same anywhere on the surface of the Earth: the result of the experiment will not change if you move with all the experimental equipment from one place of the earth's surface to another. In physical jargon, shifting or moving something from one point to another is called translation. Therefore, on the gravitational field of the Earth, we can say that it has “translational symmetry”. Of course, some side effects can introduce disturbances in the results of our experiment and spoil the symmetry. For example, conducting an experiment on very large and massive mineral deposits, we will get a little more importance than in other places. In this case, we would say that symmetry is only approximate. Approximate symmetry is also called broken symmetry. The presence of individual deposits of heavy minerals "breaks the translational symmetry."
Can the symmetry of a snowflake be broken? No doubt some snowflakes are imperfect. If a snowflake is formed in non-ideal conditions, then one side may differ from the other. It will still have a shape close to hexagonal, but this hexagon will be imperfect, that is, its symmetry will be broken.
In outer space, far from any disturbing influences, we could measure the gravitational force between the two masses and get the Newtonian law of the world. Regardless of where the experiment was conducted, we, in theory, should receive the same answer. Thus, the Newtonian law of the world has a translational invariance.
To measure the force of attraction between two objects, it is necessary to arrange them at some distance from each other. For example, we can arrange two objects in such a way that the line connecting them will be parallel to the x axis in some given coordinate system. With equal success, we can arrange objects on a straight line parallel to the y axis. Will the force of attraction measured by us depend on the direction of the straight line connecting these objects? In principle, yes, but only if the laws of nature differ from those that we have. In nature, the law of universal perception claims that the force of attraction is proportional to the product of the masses and inversely proportional to the square of the distance between them and it does not depend on the orientation of one object relative to another. Directional independence is called rotational symmetry. Translational and rotational symmetries are the most important fundamental properties of the world in which we live.
Look in the mirror. Your reflection is like two peas in a pod like you. The mirror image of your pants is no different from the pants themselves. The reflection of the left glove exactly repeats the left glove.
Stop. Something is wrong here. Let's look again carefully. The mirror image of the left glove is not all identical to the left glove. It is identical to the right glove! And the mirror image of the right glove is identical to the left glove.
Now look more attentively at your own reflection. It's not you. The mole you have on your left cheek, your reflection on your right. And if you opened your own chest, you would find that the heart of your reflection is not on the left, as in all normal people, but on the right. Let's call the mirror man - man.
Suppose that we have a futuristic technology that allows us to collect any object we want from individual atoms. We will build with the help of this technology a person whose mirror image will exactly repeat you: the heart on the left, the freckle on the left, etc. Then the original, which we build, will be a man.
Will a person function normally? Will he breathe? Will his heart beat? If you give him candy, will he assimilate the sugar that is part of it? The answers to most of these questions are yes. Basically, the person will function in the same way as the person. But with his metabolism will have problems. He will not be able to assimilate ordinary sugar. The reason is that sugar exists in two mirror forms, like right and left gloves. A person is able to assimilate only one of the mirror forms of sugar. A person is able to absorb only sugar. Molecules - sugar and sugar - differ from each other in the same way as the right and left glove. Chemists call ordinary sugars that people can assimilate, D-isomers (from Latin dextra - right), and mirror them, which only people can absorb, L-isomers (from Latin lævum - left).
Replacing something with its mirror image is called mirror symmetry, or parity. The consequences of mirror reflection are, in principle, obvious, but let's repeat one important thing again: if everything in the world is replaced with its mirror reflection, then the behavior of this world will in no way change and will not differ from the behavior of our world.
In fact, mirror symmetry is not exact. It is a good example of broken symmetry. Something leads to the fact that the mirror image of a neutrino is many times heavier than the original. This applies to all other particles, although to a much lesser extent. It seems that the great world mirror is slightly crooked, it distorts the reflection a little. But this distortion is so insignificant that it practically does not affect ordinary matter. But in the behavior of high-energy particles in the mirror world can occur very significant changes. However, for the time being, let's pretend that mirror symmetry in nature is accurate.
What do we mean when we say that there is a symmetry relation between particles? In a nutshell, this means that each type of particle has a partner or a twin with very similar properties. For mirror symmetry, this means that if the laws of nature allow for the existence of a left glove, then the existence of a right glove is also possible. Establishing the existence of D-glucose means that L-glucose must exist. And if the mirror symmetry is not broken, the same should apply to all elementary particles. Each particle must have a twin, identical to it up to specular reflection. When a person is mirrored, each elementary particle that makes up his body is replaced by its mirror twin.
Antimatter is another type of symmetry, called charge conjugation symmetry. Since symmetry implies the replacement of everything with its symmetrical analogue, the symmetry of charge conjugation involves the replacement of each particle by its antiparticle. It changes the positive electric charges, for example, protons, into negative ones, in this case antiprotons. Similarly, negatively charged electrons are replaced by positively charged positrons. Hydrogen atoms are replaced by antihydrogen atoms consisting of positrons and antiprotons. Such atoms are actually obtained in laboratories, however, in very small quantities, insufficient even to build antimolecules from them. But no one doubts that anti-molecules are possible. , , . . , , . , , , .
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