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Already in Ancient Greece, ancient thinkers wondered about the fundamental structure of matter. In the scientific fashion of those years, the Greeks were looking for primary elements. As a result of these searches, the Greeks had several completely different sets of primary elements and even the concept of atoms as an extravagant appendage. But the Greeks could not make a choice between different sets, because the choice was not enough only logical arguments, and to the idea of staging a decisive experiment was almost 2000 years.
Only at the turn of the XVII—XVIII centuries physics was formed as a science, the main driving force of which is the experiment, and remained it until the first quarter of the XX century. It was the unexpected experimental results that gave rise to classical electrodynamics, special relativity and quantum mechanics.
However, in 1928 everything changed. The outstanding English theoretical physicist, one of the founders of quantum mechanics Paul Dirac wrote a relativistic quantum equation for particles with half-integer spin. This equation had one important feature, which Dirac did not put in it: if this equation had a solution for particles with a negative electric charge, then inevitably there was an additional solution for particles with a positive charge. In the early 1930s, there was only one known particle with half-integer spin and a negative charge — it was an electron — and one particle with half-integer spin and a positive charge, and that was a proton. At first, physicists thought that the two solutions to the Dirac equation corresponded to these two particles. But very soon the German mathematician Hermann Weil proved that particles from the Dirac equation with positive and negative charges must have the same mass. And there was a problem, because the proton is about 2,000 times heavier than the electron.
That is, Dirac's theory predicted a fundamentally new fact. In modern terms, Paul Dirac predicted antiparticles. At first no one believed, and the Dirac criticized for the allegedly erroneous equation. And for good reason. It's been a year since antiparticles were discovered. Just had no idea about this even their discoverer, a talented Soviet physicist-experimenter Dmitry skobeltzyn. The fact that he was fascinated by the actual problem of the time: the study of the composition of cosmic rays, that is, particles that fall to Earth from space. To measure the momentum of cosmic ray particles and their charge, Skobeltsyn placed the Wilson camera — the newest device for the 1930s that recorded tracks of charged particles-in a constant magnetic field. In such a chamber, positively charged particles coming from space should twist in one direction, and negative particles — in the other. Skobeltsyn observed several tracks similar to the tracks of electrons, but twisting in the opposite direction. From the height of modern knowledge we understand that such tracks left positrons. But the scientist assumed that these tracks leave electrons that fly from the Earth's surface, where they are formed as a result of natural radioactivity, and ceased to be interested in these tracks.
Therefore, the discoverer of positrons in the world is Carl Anderson. This brilliant American experimenter knew about Dirac's theory and wanted to test experimentally the existence of "electrons with a different charge." Anderson used Skobeltsyn's technique with a small addition that made the American experimenter a Nobel laureate: he placed a lead plate in Wilson's chamber. The charged particle, getting into the plate, loses some of its energy, its momentum decreases, the curvature of the track in the magnetic field changes. Therefore, by changing the curvature of the track, it is possible to understand from which side of the lead plate the particle flew into the chamber. This was the information Skobeltsyn did not have to discover the positron. It turned out that particles whose tracks are similar to the tracks of electrons, but twisted in the other direction, fly from space in the same way as ordinary electrons. Anderson set up his experiment in 1932. This year is considered the year of the discovery of antiparticles and the year from which theory in particle physics began to outpace experiment. Neutrinos, Higgs boson, top quark were first predicted by theorists. Sometimes experiments confirmed the theory after half a century, as it was, for example, with the Higgs boson.
We can say that on a new level we have returned to the situation that was in Ancient Greece: theorists offer a lot of new fundamental concepts, as the Greeks once offered different sets of primary elements. Only now are experimenters trying to test these concepts, if there is such a technological possibility.
What about antiproton? This is the second antiparticle that has been discovered by physicists. It was discovered in 1955 at a proton accelerator by a group of talented Italian physicist Emilio Segre, who fled the Nazis to America. The discovery was awarded the Nobel prize for 1959. Almost simultaneously with the antiproton was opened antineutron.
Hundreds of antiparticles have now been detected. Any charged particle, not necessarily with a half-integer spin, has its antiparticle. The discovery of antiparticles is no longer awarded the Nobel prize. And Anderson's discovery of the property of particles and antiparticles in the interaction to turn into photons — annihilate — gave rise to one of the fundamental mysteries of modern physics — the baryon asymmetry of the Universe. The Dirac equation has long been recognized by all physicists and formed the basis of quantum field theory.
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