The great-grandfather of the atomic bomb. Uranium nuclear fission

6. The world of subatomic particles

Splitting the atom

It is often said that there are two types of sciences - big sciences and small ones. Splitting the atom is a big science. It has gigantic experimental facilities, colossal budgets and receives the lion's share of Nobel Prizes.

Why did physicists need to split the atom? The simple answer - to understand how the atom works - contains only part of the truth, but there is a more general reason. It is not entirely correct to speak literally about the splitting of the atom. In reality, we are talking about the collision of high-energy particles. When subatomic particles moving at high speeds collide, a new world of interactions and fields is born. The fragments of matter carrying enormous anergy, scattering after collisions, conceal the secrets of nature, which from the “creation of the world” remained buried in the depths of the atom.

The installations where high-energy particles collide - particle accelerators - are striking in their size and cost. They reach several kilometers across, making even laboratories that study particle collisions seem tiny in comparison. In other areas scientific research the equipment is located in the laboratory; in high-energy physics, laboratories are attached to the accelerator. Recently, the European Center for Nuclear Research (CERN), located near Geneva, allocated several hundred million dollars to build a ring accelerator. The circumference of the tunnel being built for this purpose reaches 27 km. The accelerator, called LEP (Large Electron-Positron ring), is designed to accelerate electrons and their antiparticles (positrons) to speeds that are only a hair's breadth away from the speed of light. To get an idea of the scale of energy, imagine that instead of electrons, a penny coin is accelerated to such speeds. At the end of the acceleration cycle, it would have enough energy to produce $1,000 million worth of electricity! It is not surprising that such experiments are usually classified as “high energy” physics. Moving towards each other inside the ring, beams of electrons and positrons experience head-on collisions, in which the electrons and positrons annihilate, releasing energy sufficient to produce dozens of other particles.

What are these particles? Some of them are the very “building blocks” from which we are built: protons and neutrons that make up atomic nuclei, and electrons orbiting around the nuclei. Other particles are usually not found in the matter around us: their lifespan is extremely short, and after it expires they disintegrate into ordinary particles. The number of varieties of such unstable short-lived particles is amazing: several hundred of them are already known. Like stars, unstable particles are too numerous to be identified by name. Many of them are indicated only by Greek letters, and some by just numbers.

It is important to keep in mind that all these numerous and varied unstable particles are by no means literally components protons, neutrons or electrons. When colliding, high-energy electrons and positrons do not scatter into many subatomic fragments. Even in collisions of high-energy protons, which obviously consist of other objects (quarks), they, as a rule, are not split into their component parts in the usual sense. What happens in such collisions is better viewed as the direct creation of new particles from the energy of the collision.

About twenty years ago, physicists were completely baffled by the number and variety of new subatomic particles, which seemed to have no end. It was impossible to understand For what so many particles. May be, elementary particles are similar to zoo inhabitants with their implicit family affiliation, but without any clear taxonomy. Or perhaps, as some optimists have believed, elementary particles hold the key to the universe? What are the particles observed by physicists: insignificant and random fragments of matter or the outlines of a vaguely perceived order emerging before our eyes, indicating the existence of a rich and complex structure of the subnuclear world? Now there is no doubt about the existence of such a structure. There is a deep and rational order to the microworld, and we begin to understand the meaning of all these particles.

The first step towards understanding the microworld was made as a result of the systematization of all known particles, just as in the 18th century. biologists compiled detailed catalogs of plant and animal species. The most important characteristics of subatomic particles include mass, electric charge, and spin.

Because mass and weight are related, particles with high mass are often called "heavy." Einstein's relation E =mc^ 2 indicates that the mass of a particle depends on its energy and, therefore, on its speed. A moving particle is heavier than a stationary one. When they talk about the mass of a particle, they mean it rest mass, since this mass does not depend on the state of motion. A particle with zero rest mass moves at the speed of light. The most obvious example of a particle with zero rest mass is the photon. It is believed that the electron is the lightest particle with a non-zero rest mass. The proton and neutron are nearly 2,000 times heavier, while the heaviest particle created in the laboratory (the Z particle) is about 200,000 times the mass of the electron.

The electric charge of particles varies in a rather narrow range, but, as we noted, it is always a multiple of the fundamental unit of charge. Some particles, such as photons and neutrinos, have no electrical charge. If the charge of a positively charged proton is taken to be +1, then the charge of the electron is -1.

In ch. 2 we introduced another characteristic of particles - spin. It also always takes values that are multiples of some fundamental unit, which for historical reasons is chosen to be 1 /2. Thus, a proton, neutron and electron have a spin 1/2, and the photon spin is 1. Particles with spin 0, 3/2 and 2 are also known. Fundamental particles with a spin greater than 2 have not been found, and theorists believe that particles with such spins do not exist.

Particle spin - important characteristic, and depending on its size, all particles are divided into two classes. Particles with spins 0, 1 and 2 are called "bosons" - after the Indian physicist Chatyendranath Bose, and particles with half-integer spin (i.e. with spin 1/2 or 3/2 - "fermions" in honor of Enrico Fermi. Belonging to one of these two classes is probably the most important in the list of characteristics of a particle.

Another important characteristic of a particle is its lifetime. Until recently, it was believed that electrons, protons, photons and neutrinos were absolutely stable, i.e. have an infinitely long lifetime. The neutron remains stable while it is "locked" in the nucleus, but a free neutron decays in about 15 minutes. All other known particles are highly unstable, their lifetimes range from a few microseconds to 10-23 s. Such time intervals seem incomprehensibly small, but we should not forget that a particle flying at a speed close to the speed of light (and most particles born in accelerators move at precisely such speeds) manages to fly a distance of 300 m in a microsecond.

Unstable particles undergo decay, which is a quantum process, and therefore there is always an element of unpredictability in the decay. The lifespan of a particular particle cannot be predicted in advance. Based on statistical considerations, only the average lifetime can be predicted. Usually they talk about the half-life of a particle - the time during which the population of identical particles is reduced by half. The experiment shows that the decrease in population size occurs exponentially (see Fig. 6) and the half-life is 0.693 of the average life time.

It is not enough for physicists to know that this or that particle exists - they strive to understand what its role is. The answer to this question depends on the properties of particles listed above, as well as on the nature of the forces acting on the particle from outside and inside it. First of all, the properties of a particle are determined by its ability (or inability) to participate in strong interactions. Particles participating in strong interactions form a special class and are called androns. Particles that participate in weak interactions and do not participate in strong interactions are called leptons, which means "lungs". Let's take a brief look at each of these families.

Leptons

The best known of the leptons is the electron. Like all leptons, it appears to be an elementary, point-like object. As far as is known, the electron has no internal structure, i.e. does not consist of any other particles. Although leptons may or may not have an electrical charge, they all have the same spin 1/2, therefore, they are classified as fermions.

Another well-known lepton, but without a charge, is the neutrino. As already mentioned in Chap. 2, neutrinos are as elusive as ghosts. Since neutrinos do not participate in either the strong or electromagnetic interactions, they almost completely ignore matter, penetrating through it as if it were not there at all. The high penetrating ability of neutrinos for a long time made it very difficult to experimentally confirm their existence. It was only almost three decades after the neutrinos were predicted that they were finally discovered in the laboratory. Physicists had to wait for the creation of nuclear reactors, during which a huge number of neutrinos are emitted, and only then were they able to register the head-on collision of one particle with a nucleus and thereby prove that it really exists. Today it is possible to carry out much more experiments with neutrino beams, which arise from the decay of particles in an accelerator and have the necessary characteristics. The vast majority of neutrinos “ignore” the target, but from time to time neutrinos still interact with the target, which makes it possible to obtain useful information about the structure of other particles and the nature of weak interaction. Of course, conducting experiments with neutrinos, unlike experiments with other subatomic particles, does not require the use of special protection. The penetrating power of neutrinos is so great that they are completely harmless and pass through the human body without causing the slightest harm to it.

Despite their intangibility, neutrinos occupy a special position among other known particles because they are the most abundant particles throughout the Universe, outnumbering electrons and protons by a billion to one. The universe is essentially a sea of neutrinos, with occasional inclusions in the form of atoms. It is even possible that the total mass of neutrinos exceeds the total mass of stars, and therefore it is neutrinos that make the main contribution to cosmic gravity. According to a group of Soviet researchers, neutrinos have a tiny, but not zero, rest mass (less than one ten thousandth the mass of an electron); if this is true, then gravitational neutrinos dominate the Universe, which in the future may cause its collapse. Thus, neutrinos, at first glance the most “harmless” and incorporeal particles, are capable of causing the collapse of the entire Universe.

Among other leptons, one should mention the muon, discovered in 1936 in the products of the interaction of cosmic rays; it turned out to be one of the first known unstable subatomic particles. In all respects except stability, the muon resembles an electron: it has the same charge and spin, participates in the same interactions, but has a larger mass. In about two millionths of a second, the muon decays into an electron and two neutrinos. Muons are widespread in nature and account for a significant portion of the background cosmic radiation that is detected on the Earth's surface by a Geiger counter.

For many years, the electron and the muon remained the only known charged leptons. Then, in the late 1970s, a third charged lepton was discovered, called the tau lepton. With a mass of about 3500 electron masses, the tau lepton is obviously the “heavyweight” of the trio of charged leptons, but in all other respects it behaves like an electron and a muon.

This list of known leptons is by no means exhausted. In the 60s it was discovered that there are several types of neutrinos. Neutrinos of one type are born together with an electron during the decay of a neutron, and neutrinos of another type are born during the birth of a muon. Each type of neutrino exists in pairs with its own charged lepton; therefore, there is an "electron neutrino" and a "muon neutrino". In all likelihood, there should also be a third type of neutrino - accompanying the birth of the tau lepton. In this case total number There are three varieties of neutrinos, and the total number of leptons is six (Table 1). Of course, each lepton has its own antiparticle; thus the total number of different leptons is twelve.

Table 1

Six leptons correspond to charged and neutral modifications (antiparticles are not included in the table). Mass and charge are expressed in units of electron mass and charge, respectively. There is evidence that neutrinos may have low mass

Hadrons

In contrast to the handful of known leptons, there are literally hundreds of hadrons. This alone suggests that hadrons are not elementary particles, but are built from smaller components. All hadrons participate in strong, weak and gravitational interactions, but are found in two varieties - electrically charged and neutral. Among hadrons, the most famous and widely distributed are the neutron and the proton. The remaining hadrons are short-lived and decay either in less than one millionth of a second due to the weak interaction, or much faster (in a time of the order of 10-23 s) - due to the strong interaction.

In the 1950s, physicists were extremely puzzled by the number and diversity of hadrons. But little by little, particles were classified according to three important characteristics: mass, charge and spin. Gradually, signs of order began to appear and a clear picture began to emerge. There are hints that there are symmetries hidden behind the apparent chaos of the data. A decisive step in unraveling the mystery of hadrons came in 1963, when Murray Gell-Mann and George Zweig of the California Institute of Technology proposed the theory of quarks.

Fig.10 Hadrons are built from quarks. A proton (top) is made up of two up quarks and one d quark. The lighter pion (bottom) is a meson, consisting of one u-quark and one d-antiquark. Other hadrons are all sorts of combinations of quarks.

The main idea of this theory is very simple. All hadrons are built from more fine particles, called quarks. Quarks can connect to each other in one of two possible ways: either in triplets or in quark-antiquark pairs. Relatively heavy particles are made up of three quarks - baryons, which means "heavy particles". The best known baryons are the neutron and the proton. Lighter quark-antiquark pairs form particles called mesons -"intermediate particles". The choice of this name is explained by the fact that the first discovered mesons occupied an intermediate position in mass between electrons and protons. To take into account all the then known hadrons, Gell-Mann and Zweig introduced three different types (“flavors”) of quarks, which received rather fancy names: And(from up- upper), d(from down - lower) and s (from strange- strange). By allowing for the possibility of various combinations of flavors, the existence of a large number of hadrons can be explained. For example, a proton consists of two And- and one d-quark (Fig. 10), and the neutron is made up of two d-quarks and one u-quark.

For the theory proposed by Gell-Mann and Zweig to be effective, it is necessary to assume that quarks carry a fractional electric charge. In other words, they have a charge whose value is either 1/3 or 2/3 of the fundamental unit - the charge of the electron. A combination of two and three quarks can have a total charge of zero or one. All quarks have spin 1/2. therefore they are classified as fermions. The masses of quarks are not determined as accurately as the masses of other particles, since their binding energy in a hadron is comparable to the masses of the quarks themselves. However, it is known that the s-quark is heavier And- and d-quarks.

Inside hadrons, quarks can be in excited states, much like the excited states of an atom, but with much higher energies. The excess energy contained in an excited hadron increases its mass so much that before the creation of the quark theory, physicists mistakenly took excited hadrons for completely different particles. It has now been established that many of the seemingly different hadrons are in fact only excited states of the same fundamental set of quarks.

As already mentioned in Chap. 5, quarks are held together by strong interaction. But they also participate in weak interactions. The weak interaction can change the flavor of a quark. This is how neutron decay occurs. One of the d-quarks in the neutron turns into a u-quark, and the excess charge carries away the electron that is born at the same time. Similarly, by changing the flavor, the weak interaction leads to the decay of other hadrons.

The existence of s-quarks is necessary for the construction of so-called “strange” particles - heavy hadrons, discovered in the early 50s. The unusual behavior of these particles, which suggested their name, was that they could not decay due to strong interactions, although both themselves and their decay products were hadrons. Physicists have puzzled over why, if both the mother and daughter particles belong to the hadron family, the strong force does not cause them to decay. For some reason, these hadrons "preferred" the much less intense weak interaction. Why? Quark theory naturally solved this mystery. The strong interaction cannot change the flavor of quarks - only the weak interaction can do this. And without a change in flavor, accompanied by the transformation of the s-quark into And- or d-quark, decay is impossible.

In table Figure 2 presents the various possible combinations of three-flavor quarks and their names (usually just a Greek letter). Numerous excited states are not shown. The fact that all known hadrons could be obtained from various combinations of the three fundamental particles symbolized the main triumph of the quark theory. But despite this success, only a few years later it was possible to obtain direct physical evidence of the existence of quarks.

This evidence was obtained in 1969 in a series of historical experiments conducted at the large linear accelerator at Stanford (California, USA) - SLAC. The Stanford experimenters reasoned simply. If there really are quarks in the proton, then collisions with these particles inside the proton can be observed. All that is needed is a subnuclear “projectile” that could be directed directly into the depths of the proton. It is useless to use another hadron for this purpose, since it has the same dimensions as a proton. An ideal projectile would be a lepton, such as an electron. Since the electron does not participate in the strong interaction, it will not “get stuck” in the medium formed by quarks. At the same time, an electron can sense the presence of quarks due to the presence of electric charge.

table 2

The three flavors of quarks, u, d and s, correspond to charges +2/3, -1/3 and -1/3; they combine in threes to form the eight baryons shown in the table. Quark-antiquark pairs form mesons. (Some combinations, such as sss, are omitted.)

In the Stanford experiment, the three-kilometer accelerator essentially acted as a giant electron "microscope" that produced images of the inside of a proton. A conventional electron microscope can distinguish details smaller than one millionth of a centimeter. A proton, on the other hand, is several tens of millions of times smaller, and can only be “probed” by electrons accelerated to an energy of 2.1010 eV. At the time of the Stanford experiments, few physicists adhered to the simplified theory of quarks. Most scientists expected the electrons to be deflected by the electrical charges of the protons, but the charge was assumed to be evenly distributed within the proton. If this were really so, then mainly weak electron scattering would occur, i.e. When passing through protons, electrons would not undergo strong deflections. The experiment showed that the scattering pattern differs sharply from the expected one. Everything happened as if some electrons flew into tiny solid inclusions and bounced off them at the most incredible angles. Now we know that such solid inclusions inside protons are quarks.

In 1974, the simplified version of the theory of quarks, which by that time had gained recognition among theorists, was dealt a sensitive blow. Within a few days of each other, two groups of American physicists - one at Stanford led by Barton Richter, the other at Brookhaven National Laboratory led by Samuel Ting - independently announced the discovery of a new hadron, which was called the psi particle. In itself, the discovery of a new hadron would hardly be particularly noteworthy if not for one circumstance: the fact is that in the scheme proposed by the theory of quarks there was no room for a single new particle. All possible combinations of up, d, and s quarks and their antiquarks have already been “used up.” What does a psi particle consist of?

The problem was solved by turning to an idea that had been in the air for some time: there should be a fourth scent that no one had ever observed before. The new fragrance already had its name - charm (charm), or s. It has been suggested that a psi particle is a meson consisting of a c-quark and a c-antiquark (c), i.e. cc. Since antiquarks are carriers of anti-flavor, the charm of the psi particle is neutralized, and therefore experimental confirmation of the existence of a new flavor (charm) had to wait until mesons were discovered, in which charm quarks were paired with anti-quarkamps of other flavors . A whole string of enchanted particles is now known. They are all very heavy, so the charm quark turns out to be heavier than the strange quark.

The situation described above was repeated in 1977, when the so-called upsilon meson (UPSILON) appeared on the scene. This time, without much hesitation, a fifth flavor was introduced, called b-quark (from bottom - bottom, and more often beauty - beauty, or charm). The upsilon meson is a quark-antiquark pair made up of b quarks and therefore has a hidden beauty; but, as in the previous case, a different combination of quarks made it possible to ultimately discover “beauty.”

The relative masses of quarks can be judged at least by the fact that the lightest of mesons, the pion, consists of pairs And- and d-quarks with antiquarks. The psi meson is about 27 times, and the upsilon meson is at least 75 times heavier than the pion.

The gradual expansion of the list of known flavors occurred in parallel with the increase in the number of leptons; so the obvious question was whether there would ever be an end. Quarks were introduced to simplify the description of the entire variety of hadrons, but even now there is a feeling that the list of particles is again growing too quickly.

Since the time of Democritus, the fundamental idea of atomism has been the recognition that, on a sufficiently small scale, there must exist truly elementary particles, the combinations of which make up the matter around us. Atomism is attractive because indivisible (by definition) fundamental particles must exist in a very limited number. The diversity of nature is due to the large number not of its constituent parts, but of their combinations. When it was discovered that there were many different atomic nuclei, the hope disappeared that what we today call atoms corresponded to the ancient Greeks' idea of \u200b\u200bthe elementary particles of matter. And although, according to tradition, we continue to talk about various chemical “elements,” it is known that atoms are not elementary at all, but consist of protons, neutrons and electrons. And since the number of quarks turns out to be too large, it is tempting to assume that they too are complex systems consisting of smaller particles.

Although for this reason there is some dissatisfaction with the quark scheme, most physicists consider quarks to be truly elementary particles - point-like, indivisible and without internal structure. In this respect they resemble peptones, and it has long been assumed that there must be a deep relationship between these two distinct but structurally similar families. The basis for this point of view arises from a comparison of the properties of leptons and quarks (Table 3). Leptons can be grouped in pairs by associating each charged lepton with a corresponding neutrino. Quarks can also be grouped in pairs. Table 3 is composed in such a way that the structure of each cell repeats the one located directly in front of it. For example, in the second cell the muon is represented as a "heavy electron" and the charm and strange quarks are represented as heavy variants And- and d-quarks. From the next box you can see that the tau lepton is an even heavier "electron", and the b quark is a heavier version of the d quark. For a complete analogy, we need one more (tau-leptonium) neutrino and a sixth flavor of quarks, which has already received the name true (truth, t). At the time this book was being written, the experimental evidence for the existence of top quarks was not yet convincing enough, and some physicists doubted that top quarks existed at all.

Table 3

Leptons and quarks naturally pair up. as shown in the table. The world around us consists of the first four particles. But the following groups, apparently, repeat the upper one and consist, in the crown of neutrinos, of extremely unstable particles.

Can there be a fourth, fifth, etc. vapors containing even heavier particles? If so, the next generation of accelerators will likely give physicists the opportunity to detect such particles. However, an interesting consideration is expressed, from which it follows that there are no other pairs except the three named. This consideration is based on the number of neutrino types. We will soon learn that at the moment of the Big Bang, which marked the emergence of the Universe, there was an intense creation of neutrinos. A kind of democracy guarantees each type of particle the same share of energy as the others; therefore, the more different types of neutrinos, the more energy is contained in the sea of neutrinos filling outer space. Calculations show that if there were more than three varieties of neutrinos, then the gravity created by all of them would have a strong disturbing effect on nuclear processes, occurring in the first few minutes of the life of the Universe. Consequently, from these indirect considerations a very plausible conclusion follows that the three pairs shown in table. 3, all quarks and leptons that exist in nature are exhausted.

It is interesting to note that all ordinary matter in the Universe consists of only two lightest leptons (electron and electron neutrino) and two lightest quarks ( And And d). If all the other leptons and quarks suddenly ceased to exist, then very little would probably change in the world around us.

Perhaps heavier quarks and leptons play the role of a kind of backup for the lightest quarks and leptons. All of them are unstable and quickly disintegrate into particles located in the upper cell. For example, the tau lepton and the muon decay into electrons, while the strange, charmed, and beautiful particles decay quite quickly into either neutrons or protons (in the case of baryons) or leptons (in the case of mesons). The question arises: For what Are there all these second and third generation particles? Why did nature need them?

Particles are carriers of interactions

The list of known particles is by no means exhausted by six pairs of leptons and quarks, which form the building material of matter. Some of them, such as the photon, are not included in the quark circuit. The particles “left overboard” are not “building blocks of the universe”, but form a kind of “glue” that does not allow the world to fall apart, i.e. they are associated with four fundamental interactions.

I remember being told as a child that the moon causes the oceans to rise and fall during the daily tides. It has always been a mystery to me how the ocean knows where the Moon is and follows its movement in the sky. When I learned about gravity at school, my bewilderment only intensified. How does the Moon, having overcome a quarter of a million kilometers of empty space, manage to “reach” the ocean? The standard answer - the Moon creates a gravitational field in this empty space, the action of which reaches the ocean, setting it in motion - of course, made some sense, but still did not completely satisfy me. After all, we cannot see the gravitational field of the Moon. Maybe that's just what they say? Does this really explain anything? It always seemed to me that the moon must somehow tell the ocean where it is. There must be some kind of signal exchange between the moon and the ocean so that the water knows where to move.

Over time, it turned out that the idea of force transmitted through space in the form of a signal is not so far from the modern approach to this problem. To understand how this idea arises, we must consider in more detail the nature of the force field. As an example, let's choose not ocean tides, but a simpler phenomenon: two electrons approach each other, and then, under the influence of electrostatic repulsion, fly apart in different directions. Physicists call this process the scattering problem. Of course, electrons don't literally push each other. They interact at a distance, through the electromagnetic field generated by each electron.

Fig. 11. Scattering of two charged particles. The trajectories of particles are bent as they approach each other due to the action of electrical repulsion.

It is not difficult to imagine the picture of electron-on-electron scattering. Initially, the electrons are separated by a large distance and have little effect on each other. Each electron moves almost rectilinearly (Fig. 11). Then, as repulsive forces come into play, the electron trajectories begin to bend until the particles are as close as possible; after this, the trajectories diverge, and the electrons fly apart, again beginning to move along rectilinear, but already diverging trajectories. A model of this kind can easily be demonstrated in the laboratory using electrically charged balls instead of electrons. And again the question arises: how does a particle “know” where another particle is, and accordingly changes its movement.

Although the picture of curved electron trajectories is quite visual, it is completely unsuitable in a number of respects. The fact is that electrons are quantum particles and their behavior obeys the specific laws of quantum physics. First of all, electrons do not move in space along well-defined trajectories. We can still determine in one way or another the starting and ending points of the path - before and after scattering, but the path itself in the interval between the beginning and end of the movement remains unknown and uncertain. In addition, the intuitive idea of a continuous exchange of energy and momentum between the electron and the field, as if accelerating the electron, contradicts the existence of photons. Energy and momentum can be transferred field only in portions, or quanta. We will obtain a more accurate picture of the disturbance introduced by the field into the motion of the electron by assuming that the electron, absorbing a photon from the field, seems to experience a sudden push. Therefore, on quantum level The act of electron-on-electron scattering can be depicted as shown in Fig. 12. The wavy line connecting the trajectories of two electrons corresponds to a photon emitted by one electron and absorbed by the other. Now the act of scattering appears as a sudden change in the direction of movement of each electron

Fig. 12. Quantum description of the scattering of charged particles. The interaction of particles is due to the exchange of an interaction carrier, or virtual photon (wavy line).

Diagrams of this kind were first used by Richard Feynman to visually represent the various terms of an equation, and initially they had a purely symbolic meaning. But then Feynman diagrams began to be used to diagrammatically depict particle interactions. Such pictures seem to complement the physicist’s intuition, but they should be interpreted with a certain amount of caution. For example, there is never a sharp break in the electron trajectory. Since we only know the initial and final positions of the electrons, we do not know exactly when the photon is exchanged and which particle emits and which absorbs the photon. All these details are hidden by a veil of quantum uncertainty.

Despite this caveat, Feynman diagrams have proven to be an effective means of describing quantum interactions. The photon exchanged between electrons can be thought of as a kind of messenger from one of the electrons telling the other: “I’m here, so get moving!” Of course, all quantum processes are probabilistic in nature, so such an exchange occurs only with a certain probability. It may happen that electrons exchange two or more photons (Fig. 13), although this is less likely.

It is important to realize that in reality we do not see photons scurrying from one electron to another. Interaction carriers are the “internal matter” of two electrons. They exist solely to tell electrons how to move, and although they carry energy and momentum, the corresponding conservation laws of classical physics do not apply to them. Photons in this case can be likened to a ball that tennis players exchange on the court. Just as a tennis ball determines the behavior of tennis players on the playground, a photon influences the behavior of electrons.

The successful description of interaction using a carrier particle was accompanied by an expansion of the concept of a photon: a photon turns out to be not only a particle of light visible to us, but also a ghostly particle that is “seen” only by charged particles undergoing scattering. Sometimes the photons we observe are called real, and photons carrying the interaction are virtual, which reminds us of their fleeting, almost ghostly existence. The distinction between real and virtual photons is somewhat arbitrary, but nevertheless these concepts have become widespread.

The description of electromagnetic interaction using the concept of virtual photons - its carriers - in its significance goes beyond just illustrations of a quantum nature. In reality, we are talking about a theory thought out to the smallest detail and equipped with a perfect mathematical apparatus, known as quantum electrodynamics, Abbreviated as QED. When QED was first formulated (this happened shortly after the Second World War), physicists had at their disposal a theory that satisfied the basic principles of both quantum theory, and the theory of relativity. This is a wonderful opportunity to see the combined manifestations of two important aspects of new physics and. check them experimentally.

Theoretically, the creation of QED was an outstanding achievement. Earlier studies of the interaction of photons and electrons had very limited success due to mathematical difficulties. But as soon as the theorists learned to carry out calculations correctly, everything else fell into place. QED proposed a procedure for obtaining the results of any no matter how complex process involving photons and electrons.

Fig. 13. Electron scattering is caused by the exchange of two virtual photons. Such processes constitute a small amendment to the main process shown in Fig. eleven

To test how well the theory matched reality, physicists focused on two effects that were of particular interest. The first concerned the energy levels of the hydrogen atom, the simplest atom. QED predicted that the levels should be slightly shifted from the position they would occupy if virtual photons did not exist. The theory predicted the magnitude of this shift very accurately. The experiment to detect and measure displacement with extreme accuracy was carried out by Willis Lamb from the University of State. Arizona. To everyone's delight, the calculation results perfectly coincided with the experimental data.

The second decisive test of QED concerned the extremely small correction to the electron's own magnetic moment. And again, the results of theoretical calculations and experiment completely coincided. Theorists began to refine their calculations, and experimenters began to improve their instruments. But, although the accuracy of both theoretical predictions and experimental results has continuously improved, the agreement between QED and experiment has remained impeccable. Nowadays, the theoretical and experimental results still agree within the limits of the achieved accuracy, which means a coincidence of more than nine decimal places. Such a striking correspondence gives the right to consider QED the most advanced of the existing natural science theories.

Needless to say, after such a triumph, QED was adopted as a model for the quantum description of the other three fundamental interactions. Of course, fields associated with other interactions must correspond to other carrier particles. To describe gravity it was introduced graviton, playing the same role as a photon. During the gravitational interaction of two particles, gravitons are exchanged between them. This interaction can be visualized using diagrams similar to those shown in Fig. 12 and 13. It is gravitons that carry signals from the Moon to the oceans, following which they rise during high tides and fall during low tides. Gravitons scurrying between the Earth and the Sun keep our planet in orbit. Gravitons firmly chain us to the Earth.

Like photons, gravitons travel at the speed of light, hence gravitons are particles with “zero rest mass.” But this is where the similarities between gravitons and photons end. While a photon has a spin of 1, a graviton has a spin of 2.

Table 4

Particles that carry four fundamental interactions. Mass is expressed in proton mass units.

This is an important difference because it determines the direction of the force: in electromagnetic interaction, similarly charged particles, such as electrons, repel, while in gravitational interaction, all particles are attracted to each other.

Gravitons can be real or virtual. A real graviton is nothing more than a quantum gravitational wave, just as a real photon is a quantum of an electromagnetic wave. In principle, real gravitons can be “observed”. But because the gravitational interaction is incredibly weak, gravitons cannot be detected directly. The interaction of gravitons with other quantum particles is so weak that the probability of scattering or absorption of a graviton, for example, by a proton is infinitely small.

The basic idea of the exchange of carrier particles also applies to other interactions (Table 4) - weak and strong. However, there are important differences in detail. Let us recall that the strong interaction provides the connection between quarks. Such a connection can be created by a force field similar to an electromagnetic one, but more complex. Electric forces lead to the formation of a bound state of two particles with charges of opposite signs. In the case of quarks, bound states of three particles arise, which indicates a more complex nature of the force field, to which three types of “charge” correspond. Particles - carriers of interaction between quarks, connecting them in pairs or triplets, are called gluons.

In the case of weak interaction the situation is somewhat different. The radius of this interaction is extremely small. Therefore, the carriers of the weak interaction must be particles with large rest masses. The energy contained in such a mass has to be “borrowed” in accordance with the Heisenberg uncertainty principle, which has already been discussed on p. 50. But since the "borrowed" mass (and therefore energy) is so large, the uncertainty principle requires that the repayment period of such a loan be extremely short - only about 10^-28s. Such short-lived particles do not have time to move very far, and the radius of interaction they carry is very small.

There are actually two types of weak force transporters. One of them is similar to a photon in everything except rest mass. These particles are called Z particles. Z particles are essentially a new kind of light. Another type of weak force carrier, W particles, differ from Z particles by the presence of an electric charge. In ch. 7 we will discuss in more detail the properties of Z and W particles, which were discovered only in 1983.

The classification of particles into quarks, leptons and carriers of interactions completes the list of known subatomic particles. Each of these particles plays its own, but decisive role in the formation of the Universe. If there were no carrier particles, there would be no interactions, and each particle would remain in the dark about its partners. Complex systems could not arise, any activity would be impossible. Without quarks there would be no atomic nuclei or sunlight. Without leptons, atoms could not exist, chemical structures and life itself would not arise.

What are the goals of particle physics?

The influential British newspaper The Guardian once published an editorial questioning the wisdom of developing particle physics, an expensive undertaking that consumes not only a significant share of the national science budget, but also the lion's share the best minds. “Do physicists know what they’re doing?” asked the Guardian. “Even if they do, what’s the use of it? Who, other than physicists, needs all these particles?”

A few months after this publication, I had the opportunity to attend a lecture in Baltimore by George Keyworth, the US President's adviser on science. Keyworth also addressed particle physics, but his lecture took a completely different tone. American physicists were impressed by a recent report from CERN, Europe's leading particle physics laboratory, about the discovery of fundamental W and Z particles, which were finally obtained at a large proton-antiproton colliding beam collider. Americans are accustomed to the fact that all sensational discoveries are made in their high-energy physics laboratories. Isn't the fact that they lost the palm a sign of scientific and even national decline?

Keyworth had no doubt that for the United States in general and the American economy in particular to prosper, the country needed to be at the forefront of scientific research. Main projects basic research, Keyworth said, are at the forefront of progress. The United States must regain its supremacy in particle physics,

That same week, news channels circulated reports about an American project for a giant accelerator designed to conduct a new generation of experiments in particle physics. The main cost was estimated at $2 billion, making this accelerator the most expensive machine ever built by man. This Uncle Sam giant, which would dwarf even CERN's new LEP accelerator, is so large that the entire state of Luxembourg could fit inside its ring! Giant superconducting magnets are designed to create intense magnetic fields that will curl a beam of particles, directing it along a ring-shaped chamber; it is such a huge structure that the new accelerator is supposed to be located in the desert. I would like to know what the editor of the Guardian newspaper thinks about this.

Known as the Superconducting Super Collider (SSC), but more often referred to as "de-zertron" (from the English. desert - desert. - Ed.), this monstrous machine will be able to accelerate protons to energies approximately 20 thousand times higher than the rest energy (mass). These numbers can be interpreted in different ways. At maximum acceleration, particles will move at a speed of only 1 km/h less than the speed of light - the maximum speed in the Universe. The relativistic effects are so great that the mass of each particle is 20 thousand times greater than at rest. In the system associated with such a particle, time is stretched so much that 1 s corresponds to 5.5 hours in our frame of reference. Each kilometer of the chamber through which the particle sweeps will “seem” to be compressed to only 5.0 cm.

What kind of extreme need forces states to expend such enormous resources on the ever more destructive fission of the atom? Is there any practical benefit to such research?

Any great science, of course, is not alien to the spirit of struggle for national priority. Here, just like in art or sports, it’s nice to win prizes and world recognition. Particle physics has become a kind of symbol of state power. If it develops successfully and produces tangible results, then this indicates that science, technology, as well as the country’s economy as a whole, are basically at the proper level. This supports confidence in the high quality of products from other more general technology branches. To create an accelerator and all related equipment, very high level professionalism. The valuable experience gained from developing new technologies can have unexpected and beneficial effects on other areas of scientific research. For example, research and development on superconducting magnets needed for the “desertron” has been carried out in the USA for twenty years. However, they do not provide direct benefits and are therefore difficult to value. Are there any more tangible results?

One sometimes hears another argument in support of fundamental research. Physics tends to be about fifty years ahead of technology. Practical application of one or another scientific discovery Although not obvious at first, few of the significant achievements of fundamental physics have not found practical applications over time. Let's remember Maxwell's theory of electromagnetism: could its creator have foreseen the creation and success of modern telecommunications and electronics? And Rutherford's words that nuclear energy is unlikely to ever find practical use? Is it possible to predict what the development of elementary particle physics can lead to, what new forces and new principles will be discovered that will expand our understanding of the world around us and give us power over a wider range of people? physical phenomena. And this could lead to the development of technologies no less revolutionary in nature than radio or nuclear energy.

Most branches of science eventually found some military application. In this respect, particle physics (as opposed to nuclear physics) has so far remained untouchable. By coincidence, Keyworth's lecture coincided with the publicity hype around President Reagan's controversial project to create an anti-missile, so-called beam, weapon (this project is part of a program called the Strategic Defense Initiative, SDI). The essence of this project is to use high-energy particle beams against enemy missiles. This application of particle physics is truly sinister.

The prevailing opinion is that the creation of such devices is not feasible. Most scientists working in the field of elementary particle physics consider these ideas absurd and unnatural and speak out sharply against the president's proposal. Condemning the scientists, Keyworth urged them to "consider what role they might play" in the beam weapon project. Keyworth's appeal to physicists (purely by chance, of course) followed his words regarding the financing of high-energy physics.

It is my firm belief that high-energy physicists do not need to justify the need for fundamental research by citing applications (especially military ones), historical analogues, or vague promises of possible technical miracles. Physicists conduct these studies primarily in the name of their ineradicable desire to find out how our world works, the desire to understand nature in more detail. Particle physics is unparalleled among other disciplines human activity. For two and a half millennia, humanity has sought to find the original “building blocks” of the universe, and now we are close to ultimate goal. Giant installations will help us penetrate into the very heart of matter and wrest from nature its deepest secrets. Humanity can expect unexpected applications of new discoveries, previously unknown technologies, but it may turn out that high-energy physics will not give anything for practice. But even a majestic cathedral or concert hall has little practical use. In this regard, one cannot help but recall the words of Faraday, who once remarked: “What good is a newborn baby?” Types of human activity that are far from practice, which include the physics of elementary particles, serve as evidence of the manifestation of the human spirit, without which we would be doomed in our overly material and pragmatic world.

The installations where high-energy particles collide - particle accelerators - are striking in their size and cost. They reach several kilometers across, making even laboratories that study particle collisions seem tiny in comparison. In other areas of scientific research, the equipment is located in a laboratory; in high-energy physics, laboratories are attached to an accelerator. Recently, the European Center for Nuclear Research (CERN), located near Geneva, allocated several hundred million dollars to build a ring accelerator. The circumference of the tunnel being built for this purpose reaches 27 km. The accelerator, called LEP (Large Electron-Positron ring), is designed to accelerate electrons and their antiparticles (positrons) to speeds that are only “a hair’s breadth” different from the speed of light. To get an idea of the scale of energy, imagine that instead of electrons, a penny coin is accelerated to such speeds. At the end of the acceleration cycle, it would have enough energy to produce $1,000 million worth of electricity! It is not surprising that such experiments are usually classified as “high energy” physics. Moving towards each other inside the ring, beams of electrons and positrons experience head-on collisions, in which the electrons and positrons annihilate, releasing energy sufficient to produce dozens of other particles.

About twenty years ago, physicists were completely baffled by the number and variety of new subatomic particles, which seemed to have no end. It was impossible to understand For what so many particles. Perhaps elementary particles are like the inhabitants of a zoo, with their implicit family affiliation, but without any clear taxonomy. Or perhaps, as some optimists have believed, elementary particles hold the key to the universe? What are the particles observed by physicists: insignificant and random fragments of matter or the outlines of a vaguely perceived order emerging before our eyes, indicating the existence of a rich and complex structure of the subnuclear world? Now there is no doubt about the existence of such a structure. There is a deep and rational order to the microworld, and we begin to understand the meaning of all these particles.

Because mass and weight are related, particles with high mass are often called “heavy.” Einstein's relation E =mc^ 2 indicates that the mass of a particle depends on its energy and, therefore, on its speed. A moving particle is heavier than a stationary one. When they talk about the mass of a particle, they mean it rest mass, since this mass does not depend on the state of motion. A particle with zero rest mass moves at the speed of light. The most obvious example of a particle with zero rest mass is the photon. It is believed that the electron is the lightest particle with a non-zero rest mass. The proton and neutron are nearly 2,000 times heavier, while the heaviest particle created in the laboratory (the Z particle) is about 200,000 times the mass of the electron.

In ch. 2 we introduced another characteristic of particles – spin. It also always takes values that are multiples of some fundamental unit, which for historical reasons is chosen to be 1 /2. Thus, a proton, neutron and electron have a spin 1/2, and the photon's spin is 1. Particles with spin 0, 3/2 and 2 are also known. Fundamental particles with spin greater than 2 have not been discovered, and theorists believe that particles with such spins do not exist.

The spin of a particle is an important characteristic, and depending on its value, all particles are divided into two classes. Particles with spins 0, 1 and 2 are called “bosons” - after the Indian physicist Chatyendranath Bose, and particles with half-integer spin (i.e. with spin 1/2 or 3/2 - “fermions” in honor of Enrico Fermi. Belonging to one of these two classes is probably the most important in the list of characteristics of a particle.

Another important characteristic of a particle is its lifetime. Until recently, it was believed that electrons, protons, photons and neutrinos were absolutely stable, i.e. have an infinitely long lifetime. A neutron remains stable while it is “locked” in the nucleus, but a free neutron decays in about 15 minutes. All other known particles are highly unstable, with lifetimes ranging from a few microseconds to 10-23 seconds. Such time intervals seem incomprehensible small, but we should not forget that a particle flying at a speed close to the speed of light (and most particles born at accelerators move at precisely such speeds) manages to fly a distance of 300 m in a microsecond.

It is not enough for physicists to know that this or that particle exists; they strive to understand what its role is. The answer to this question depends on the properties of particles listed above, as well as on the nature of the forces acting on the particle from outside and inside it. First of all, the properties of a particle are determined by its ability (or inability) to participate in strong interactions. Particles participating in strong interactions form a special class and are called androns. Particles that participate in weak interactions and do not participate in strong interactions are called leptons, which means “lungs”. Let's take a brief look at each of these families.

Splitting the nuclei of atoms of various elements is currently used quite widely. All nuclear power plants operate on the fission reaction; the operating principle of everything is based on this reaction. nuclear weapons. In the case of a controlled or chain reaction, the atom, having split into parts, can no longer join back and return to its original state. But, using the principles and laws of quantum mechanics, scientists managed to split the atom into two halves and connect them again without violating the integrity of the atom itself.

Scientists from the University of Bonn used the principle of quantum uncertainty, which allows objects to exist in several states at once. In the experiment, with the help of some physical tricks, scientists forced a single atom to exist in two places at once, the distance between them was a little more than one hundredth of a millimeter, which on the atomic scale is simply a huge distance.

Such quantum effects can only appear at extremely low temperatures. A cesium atom was cooled using laser light to a temperature of one tenth of one millionth of a degree higher absolute zero. The cooled atom was then optically trapped by a beam of light from another laser.

It is known that the nucleus of an atom can rotate in one of two directions; depending on the direction of rotation, the laser light pushes the nucleus to the right or to the left. “But an atom, in a certain quantum state, can have a “split personality”, one half of it rotates in one direction, the other in the opposite direction. But, at the same time, the atom is still a whole object,” says physicist Andreas Steffen. Thus, the nucleus of an atom, the parts of which rotate in opposite directions, can be split into two parts by a laser beam and these parts of the atom can be separated over a considerable distance, which is what scientists managed to achieve during their experiment.

Scientists claim that using a similar method, it is possible to create so-called “quantum bridges”, which are conductors of quantum information. An atom of a substance is divided into halves, which are moved apart until they come into contact with adjacent atoms. Something like a road surface is formed, a span connecting two pillars of a bridge, along which information can be transmitted. This is possible due to the fact that an atom divided in this way continues to remain a single whole at the quantum level due to the fact that the parts of the atom are entangled at the quantum level.

Scientists at the University of Bonn intend to use such technology to simulate and create complex quantum systems. “For us, the atom is like a well-oiled gear,” says Dr Andrea Alberti, the team leader. “Using many of these gears, you can create a quantum computing device with characteristics that far exceed those of the most advanced computers. You just need to be able to correctly position and connect these gears.”

In 1939Albert Einsteinappealed to President Roosevelt with a proposal to make every effort to master the energy of atomic decay before the Nazis. By that time, he had emigrated from fascist ItalyEnrico Fermiwas already working on this problem at Columbia University.

(In the accelerator chamber of the European Particle Physics Laboratory (CERN), the largest center of its kind in Europe. Paradoxically, giant structures are needed to study the smallest particles.)

Introduction

In 1854 a German Heinrich Geisler. (1814-79) invented a vacuum glass tube with electrodes, called a Heusler tube, and a mercury pump, which made it possible to obtain a high vacuum. By connecting a high-voltage induction coil to the electrodes of the tube, he received a green glow on the glass opposite the negative electrode. In 1876, a German physicist Evgeniy Goldstein(1850-1931) suggested that this glow was caused by rays emitted by the cathode, and called these rays cathode rays.

(New Zealand physicist Ernest Rutherford (1871-1937) at the Cavendish Laboratory at the University of Cambridge, which he headed in 1919.)

Electrons

English scientist William Crooks(1832-1919) improved the Heusler tube and showed the possibility of deflecting cathode rays by a magnetic field. In 1897, another English researcher, Joseph Thomson, suggested that the rays were negatively charged particles and determined their mass, which turned out to be about 2000 times less than the mass of a hydrogen atom. He called these particles electrons, taking a name suggested several years earlier by an Irish physicist George Stoney(1826-1911), who theoretically calculated the magnitude of their charge. This is how the divisibility of the atom became obvious. Thomson proposed a model in which electrons were interspersed throughout the atom, like raisins in a cupcake. And soon other particles included in the atom were discovered. In 1895 he began working at the Cavendish Laboratory Ernest Rutherford(1871-1937), who, together with Thomson, began researching the radioactivity of uranium and discovered two types of particles emitted by the atoms of this element. He called particles with the charge and mass of an electron beta particles, and others, positively charged, with a mass equal to the mass of 4 hydrogen atoms, alpha particles. In addition, uranium atoms were a source of high-frequency electromagnetic radiation - gamma rays.

(Otto Hahn and Lise Meitner. In 1945, Gan wasinterned by the Allies in England and only there did he learn that he had been awarded the Nobel Prize in Chemistry for 1944 “for the discovery of the fission of heavy nuclei.”)

Protons

In 1886, Goldstein discovered another radiation propagating in the direction opposite to the cathode rays, which he called cathode rays. Later it was proven that they consist of atomic ions. Rutherford proposed to call the positive hydrogen ion protone (from Greekproton- the first), because he considered the hydrogen nucleus to be an integral part of the atomic nuclei of all other elements. Thus, at the beginning of the 20th century. The existence of three subatomic particles was established: the electron, the proton and the alpha particle. IN1907 Mr. Rutherford became a professor at the University of Manchester. Here, trying to figure out the structure of the atom, he conducted his famous experiments on alpha particle scattering. By studying the passage of these particles through a thin metal foil, he came to the conclusion that at the center of the atom there is a small dense nucleus capable of reflecting alpha particles. Rutherford's assistant at the time was a young Danish physicist.Niels Bohr(1885-1962), which in1913 g., in accordance with the recently created quantum theory, proposed a model of the structure of the atom known asRutherford-Bohr model. In it, electrons revolved around the nucleus like planets around the Sun.

( Enrico Fermi (1901-54) received the Nobel Prize in 1938 for his work on neutron irradiation of matter. In 1942, he first carried out a self-sustaining chain reaction of the decay of atomic nuclei.)

Atom models

In this first model, the nucleus consisted of positively charged protons and a number of electrons that partially neutralized their charge; in addition, additional electrons moved around the nucleus, the total charge of which was equal to the positive charge of the nucleus.Alpha particles, like the nuclei of helium atoms, should have consisted of4 protons and2 electrons.It's been over10 years before this model was revised. IN1930 Mr. German Walter Bothe(1891-1957) announced the discovery of a new type of radioactive radiation produced when beryllium is irradiated with alpha particles. EnglishmanJames Chadwick(1891-1974) repeated these experiments and came to the conclusion that this radiation consists of particles equal in mass to protons, but without an electric charge. They were called neutrons. Then the GermanWerner Heisenberg(1901-76) proposed a model of an atom whose nucleus consisted only of protons and neutrons.A group of researchers with one of the first subatomic particle accelerators -cyclotron(1932). This device is designed to accelerate particles and then bombard special targets with them.

(A group of researchers with one of the first subatomic particle accelerators - the cyclotron (1932). This device is designed to accelerate particles and then bombard special targets with them.)

Splitting the atom

Physicists around the world immediately saw in neutrons an ideal tool for influencing atoms - these heavy, chargeless particles easily penetrated atomic nuclei. IN1934-36 Italy Enrico Fermi(1901-54) received their help37 radioactive isotopes of various elements. By absorbing a neutron, the atomic nucleus became unstable and emitted energy in the form of gamma rays. Fermi irradiated uranium with neutrons, hopingpreturn it into a new element - “uranium”. In the same direction of work in Berlin, the German Otto Hahn(1879-1 Sand an AustrianLise Meitner(1878 - 1968). IN1938 Ms. Meitner, fleeing the Nazis, went to Stockholm and continued to work together withFriedrich Strassmann(1902-80). Soon Hahn and Meitner, continuing the experiment and comparing the results by correspondence, discovered the formation of radioactive barium in neutron-irradiated uranium. Meitner suggested that I am a uranium atom (atomic number92) racesplits into two nuclei: barium (atomic number of element with number43 later namedtechnetium). Thus, the possibility of splitting the atomic nucleus was discovered. It was also found that when the nucleus of a uranium atom is destroyed,2-3 neutrons, each of which, in turn, is capable of initiating the decay of uranium atoms, causing a chain reaction with the release of a huge amount of energy...

The installations where high-energy particles collide - particle accelerators - are striking in their size and cost. They reach several kilometers across, making even laboratories that study particle collisions seem tiny in comparison. In other areas of scientific research, the equipment is located in a laboratory; in high-energy physics, laboratories are attached to an accelerator. Recently, the European Center for Nuclear Research (CERN), located near Geneva, allocated several hundred million dollars to build a ring accelerator. The circumference of the tunnel being built for this purpose reaches 27 km. The accelerator, called LEP (Large Electron-Positron ring), is designed to accelerate electrons and their antiparticles (positrons) to speeds that are only “a hair’s breadth” different from the speed of light. To get an idea of the scale of energy, imagine that instead of electrons, a penny coin is accelerated to such speeds. At the end of the acceleration cycle, it would have enough energy to produce $1,000 million worth of electricity! It is not surprising that such experiments are usually classified as “high energy” physics. Moving towards each other inside the ring, beams of electrons and positrons experience head-on collisions, in which the electrons and positrons annihilate, releasing energy sufficient to produce dozens of other particles.

It is important to keep in mind that all these numerous and varied unstable particles are not literally components of protons, neutrons or electrons. When colliding, high-energy electrons and positrons do not scatter into many subatomic fragments. Even in collisions of high-energy protons, which obviously consist of other objects (quarks), they, as a rule, are not split into their component parts in the usual sense. What happens in such collisions is better viewed as the direct creation of new particles from the energy of the collision.

About twenty years ago, physicists were completely baffled by the number and variety of new subatomic particles, which seemed to have no end. It was impossible to understand why there were so many particles. Perhaps elementary particles are like the inhabitants of a zoo, with their implicit family affiliation, but without any clear taxonomy. Or perhaps, as some optimists have believed, elementary particles hold the key to the universe? What are the particles observed by physicists: insignificant and random fragments of matter or the outlines of a vaguely perceived order emerging before our eyes, indicating the existence of a rich and complex structure of the subnuclear world? Now there is no doubt about the existence of such a structure. There is a deep and rational order to the microworld, and we begin to understand the meaning of all these particles.