For any known elementary particle, there is a probability of finding an antiparticle – that is, a particle with the same mass, but opposite other physical characteristics.
In the 1920s – after the introduction of the principles of quantum mechanics – the subatomic world seemed extremely simple. Only two types of elementary particles – protons and neutrons – made up the nucleus of the atom (although the existence of neutrons was experimentally confirmed only in the 1930s), and one type of particles – electrons – existed outside the nucleus, revolving around it in orbits. It seemed that all the diversity of the Universe was built from these three particles.
Alas, such a simple picture of the world was not destined to last long. Scientists, having equipped high-altitude laboratories around the world, began to study the composition of cosmic rays bombarding our planet (cm. Elementary particles), and soon began to discover all kinds of particles that have nothing to do with the above idyllic triad. In particular, completely unthinkable in nature were discovered antiparticles…
The world of antiparticles is a kind of mirror image of the familiar world. The mass of the antiparticle is exactly equal to the mass of the particle, to which it seems to correspond, but all its other characteristics are opposite to the prototype. For example, an electron carries a negative electric charge, and its paired antiparticle – “positron” (derived from “positive electron”) – positive. The proton has a positive charge, while the antiproton has a negative charge. Etc. During the interaction of a particle and its paired antiparticle, their mutual annihilation occurs – both particles cease to exist, and their mass is converted into energy, which is scattered in space in the form of a flash of photons and other ultra-light particles.
The existence of antiparticles was first predicted by Paul Dirac in an article he published in 1930. To understand how particles and antiparticles behave when interacting according to Dirac, imagine an even field. If you take a shovel and dig a hole in it, two objects will appear in the field – the hole itself and a pile of soil next to it. Now imagine that a pile of soil is an ordinary particle, and a hole, or “the absence of a pile of soil,” is an antiparticle. Fill the hole with the soil previously extracted from it – and there will be no hole or pile left (analogous to the annihilation process). And again, there is a flat field in front of you.
While theorizing around antiparticles was going on, a young experimental physicist at the California Institute of Technology, Carl David Anderson (1905–91), was assembling equipment from an astrophysics laboratory at Pike Summit in Colorado, intending to study cosmic rays. Working under the direction of Robert Millikan (cm. Millikan’s experiment), he invented a device for registering cosmic rays, consisting of a target placed in a powerful magnetic field. By bombarding the target, the particles left tracks of condensate droplets in the chamber around the target, which could be photographed and the trajectories of the particles could be studied from the photographs obtained.
With this apparatus, called condensation chamber, Anderson was able to register particles resulting from the collision of cosmic rays with a target. By the intensity of the track left by the particle, he could judge its mass, and by the nature of the deviation of its trajectory in the magnetic field, he could determine the electric charge of the particle. By 1932, he managed to register a series of collisions, as a result of which particles with a mass equal to the mass of an electron were formed, but they were deflected under the influence of a magnetic field in the opposite direction compared to an electron and, therefore, had a positive electric charge. This is how the antiparticle, the positron, was experimentally detected for the first time. In 1932, Anderson published his results, and in 1936 he was awarded half of the Nobel Prize in physics for them. (The second half of the prize went to the Austrian experimental physicist Victor Franz Hess (1883-1964), who was the first to experimentally confirm the existence of cosmic rays. Approx. translator.) This was the first (and, so far, the last) case of awarding the Nobel Prize to a scientist who was not even officially listed at that time in the staff of research assistants of his university!
While the above example would seem to be a perfect illustration of the prediction-test scenario within the scientific method described in the Introduction, historical reality is not as simple as it seems. The fact is that Anderson, apparently, knew absolutely nothing about Dirac’s publication before his experimental discovery. So in this case we are talking, rather, about the simultaneous theoretical and experimental discovery of the positron.
All antiparticles following the positron were experimentally discovered already in laboratory conditions – at accelerators. Today, experimental physicists have the ability to literally churn them out in the right quantities for ongoing experiments, and antiparticles have not been considered out of the ordinary for a long time.
Nuclear decay and fusion