IN THE BEGINNING, THERE WAS AN IDEA
It all started with the idea of antigravity. In the 1880s, British physicist and mathematician William Hicks (don’t mix him up with Peter Higgs!) was considering the vortex theory of gravity. According to this theory, the attraction between bodies arises from vortices of aether which penetrate space. Hicks proposed that a substance with negative gravity exists. Another British physicist, Sir Arthur Schuster, reduced this idea to the term “antimatter,” which he used in a letter published in the journal Nature in 1898. He also conjectured that when matter and antimatter meet, their equivalent quantities should disappear, turn into nothing, and annihilate one another (from the Latin nihil, for “nothing”). The idea was not entirely correct. It should be noted that, at that time, the scientific community had not yet fully adopted the concept of atoms and molecules. Although, in 1897, yet another British physicist, J.J. Thomson, discovered the electron. Fast-forwarding, we can now say that antimatter does not possess antigravity, but it does know how to annihilate.
Physicists decidedly liked these terms, so they coined them as a way to describe these hypothetical phenomena. The beginning of the 20th century was a time of rapid development in physics: quantum mechanics, wave-particle duality, and other daunting ideas came into the world.
When solving the Schrödinger wave equation in 1928, one of the greatest physicists in history, Englishman PAUL DIRAC, predicted that an electron should have a counterpart similar in mass but opposite in charge.
An Above-Ground Discovery
The discovery of antimatter did not take place entirely on Earth. In 1932, a young American physicist, Carl Anderson, set up a series of experiments on the study of cosmic rays in a cloud chamber.
Based on the deviation of the particle trajectory in the chamber, it was clear that this particle was the carrier of a single positive charge. At first, everyone thought about the good old proton. But the proton has a much larger ionization potential (the ratio of the energy needed to detach the particle from its charge); that is, a much larger mass. This value, by magnitude, coincided with the value of an electron. From all appearances, the mass of the unusual particle was approximately equal to that of an electron.
But there was still the possibility that these were not positively charged particles but only electrons that changed direction as a result of scattering. To rule out this possibility and trace the direction of the particle’s motion, Anderson developed an elegant experiment: he placed a 1⁄4-inch lead plate parallel to the Earth in a chamber. Now the particle, passing through it, should have reduced its energy and changed accordingly with the curvature of the trajectory. There was nothing left to do but to recognize that “positively-charged electrons” originate in space. The scientist called this particle a “positron” (from “positive electron”) and proposed changing the name of the electron to “negatron,” by analogy with the positron. Anderson’s article was published on March 15, 1933, in the journal Physical Review. However, six months earlier, the physicist had announced the first antiparticle in a short message in Science magazine.
From Antiparticles to Antimatter
And that’s how antiparticles were discovered. Well, more precisely, how one of them was discovered. In 1955, Emilio Gino Segrè and Owen Chamberlain discovered the antiproton, winning a Nobel Prize for their work along the way. A year later, Bruce Cork and his colleagues discovered the antineutron, and it thus became possible to study antimatter.
In principle, the antiproton was already the core of antihydrogen, but something was missing. In 1965, an antideuteron—a hybrid of an antiproton and an antineutron—was obtained. In the 1970s, antitritium and antihelium nuclei were discovered in Serpukhov, USSR (present-day Russia). Only in 1995 CERN physicists were able to “assemble” an antihydrogen atom. But at that time, they were separate atoms.
A real breakthrough in the study of antimatter was achieved in the 2010s. Thirty-thousand antiprotons, cooled down to – 99.4°F, and 2 million positrons, chilled to –387.4° F, yielded 38 antihydrogen atoms that lived for 172 milliseconds. A year later, 309 atoms were created, and they existed for 1000 seconds!
THE NATURE OF ANTIMATTER
Physicists were able to make use of much smaller quantities of antimatter. Now we know for sure that antimatter interacts gravitationally in the same way as matter. The spectrum of anti-atoms is identical to that of ordinary atoms. Therefore, we do not distinguish between anti- and normal matter through a telescope.
But the main mystery still remains: why does the world around us most likely consist of matter, while there is very, very little antimatter?
In accelerator experiments, particles are produced only in pairs: an antiparticle and a particle. The same thing probably happened at the time of the Big Bang — an equal number of particles and antiparticles appeared. But in that case, all matter would have been annihilated and the universe would consist of radiation alone. The paradox of the so-called Baryon asymmetry (cases in which there is matter but no antimatter) remains unresolved.
One of the most prominent versions of the paradox is the violation of CP symmetry. What is this? For a long time, it was believed that if all particles were simultaneously replaced by antiparticles and a mirror image of the physical system was made, then the laws of physics would not “notice” the inversion; everything would remain the same. This is CP invariance. What emerged from it is that there is no difference between particles and antiparticles, including the way they come into existence.
However, in the 1960s, two American physicists proved that invariance is violated in some instances. James Cronin and Val Fitch won the Nobel Prize for demonstrating that invariance is violated during the disintegration of neutral kaon. Their experiments exhibited that this violation is not sufficient to obtain the required amount of a substance, because reactions going backward in time are not identical to those going forward.
It has also been suspected that dark matter consists of antimatter. But, in that case, gamma-telescopes should register signals from its annihilation. The results of a 413-week observation session of the Fermi Gamma-ray Space Telescope were recently published, but there was nothing suspicious!
On a daily basis, the same telescope observes the birth of large amounts of antimatter on Earth.
ANTIMATTER AT THE SERVICE OF HUMANKIND
But most interestingly, antimatter has been at humankind’s service for four decades. During certain medical research on the human body, annihilation takes place.
One example of this stems from our need to see the activity of brain regions in real-time. What is required to accomplish this? One cyclotron, one chemical laboratory, and a positron emission tomography, or PET scanner, the first versions of which were created back in the 1970s.
Our brain, although it only weighs 1.5–2% of our total body weight, is voracious and consumes 20% of all of our energy, generally in the form of glucose. In locations where the brain is more active at a given moment, there is more glucose in the blood. But how can we see it? It’s simple. In a cyclotron, you develop an isotope of fluorine 18F, the half-life of which is 109.8 minutes. This isotope exhibits beta plus decay, which emits a positron. Next, a typical glucose molecule is taken and one of the hydroxyl groups in it is replaced by a fluorine atom, which is close in size.
This makes 18-fluorodeoxyglucose, and you drink this solution before undergoing the test. Glucose enters the brain and the fluorine atoms continue to disintegrate.
That is to say, positrons are being born right inside our brains. They don’t get very far, though: after moving just a few fractions of an inch, annihilation occurs. The positron disappears, and two gamma quanta shoot out in opposite directions, before being “caught” by the sensors in the ring of the scanner.
