Several years after traveling to Wonderland, the English writer Lewis Carroll returned to the character of Alice in the book Through the Looking Glass and What Alice Found on the Other Side, written in 1871. The story begins with Alice sitting on the sofa in her living room, meditating on what she calls the mirror house. Strange as it may seem, Alice is convinced that on the other side of the mirror above the fireplace there is a world as real as the one in her living room, only things are arranged in reverse. The books, for example, look like theirs, but with the letters written backwards. The smoke coming out of the chimney is the same as that coming out of the other side, although Alice cannot see if they also light a fire there in winter.
But what intrigues Alicia most is what she senses when she leaves the living room door open. What if beyond the door of the mirror house was completely different? Would there be a way to go through the mirror and check it?
Suddenly, a fog begins to surround the mirror as if the glass were dissolving. Alice climbs up the fireplace and a moment later she goes through the glass without really knowing how. Alice has crossed over to the other side!
Then he begins to look carefully around him and discovers that inside, in the mirror house, everything is very different…
The Dirac equation
The beginning of Carroll’s book links perfectly with our story, whose plot would have amazed the writer himself. Can you imagine that the Universe we live in looked like Alice’s living room and mirror house?
It all started in the late 1920s. At that time, scientists knew that the atom is made up of electrons, negatively charged particles that revolve around a tiny nucleus where the positively charged protons (the other component of the atom) are concentrated. nucleus, the neutron, without electric charge, had not yet been discovered).
However, it had not been possible to accurately describe the behavior of electrons inside the atom. The problem was that the equations of quantum mechanics, which is responsible for subatomic phenomena, were based on Newtonian mechanics. This is very useful in the case of systems in which the speeds are much lower than that of light, as occurs in our daily lives or with the movement of celestial bodies. But it doesn’t work for particles like the electron, which move almost as fast as light. In such cases we must resort to Einstein’s special theory of relativity, which explains what happens when objects move at speeds close to those of light.
In 1928, English physicist Paul Dirac was the first to combine relativity and quantum mechanics, arriving at an equation that accurately describes the electron. But there was something else. Just as a simple equation such as x2 = 4 has two solutions, x = 2 and x = -2, Dirac’s equation also predicts the existence of a particle with the same properties as the electron, but with a positive charge: an antiparticle. And there had to be an antiparticle not only for the electron, but also for the rest of the known particles.
The discovery in 1932 of the positron, the antiparticle of the electron, confirmed Dirac’s theory and left the door open to a fascinating possibility. In the same way that ordinary particles form the matter around us, their corresponding antiparticles could also form antimatter atoms; that is, positrons that circle around a nucleus composed of antiprotons and antineutrons. Antimatter would be the reflection of matter, as the mirror house was in Alice’s living room before going through the mirror.
Antimatter, source of energy?
When antimatter comes into contact with matter, an enormous amount of energy is generated, more than in any other reaction known in physics. To give you an idea, matter-antimatter annihilation is 2 billion times more energetic than the combustion of gasoline and 100 times more energetic than nuclear fission (the energy of nuclear reactors and atomic bombs). One gram of antihydrogen would be enough to produce more energy than the Hiroshima bomb.
But that doesn’t mean antimatter is a useful energy source. The main drawback is that the antimatter that exists in the Universe is very scarce, so the first thing to do is manufacture it. And it turns out that this is a very inefficient process, because millions of antiprotons and positrons need to be created to produce a few atoms of antihydrogen. In short, the energy provided to create antimatter is much greater than that obtained later when it is annihilated. It seems almost impossible that in the future there will be, for example, an antimatter engine like the one that propels spacecraft. Star Trek Enterprise.
But imagine that our purpose is not to use antimatter as a source of energy, but for other, more dubious purposes. Like the terrorists in the novel Angels and Demons by Dan Brown, who steal antimatter from CERN (European Organization for Nuclear Research) with the intention of creating a devastating bomb. After all, a gram of antimatter is a very small amount, right?
Suppose we could trap all the antiprotons produced at CERN and use them to form antihydrogen, something unthinkable at the moment. That would give us about 1 billion atoms of antihydrogen per second. Or what is the same, about 30,000 billion (3 x 1016) per year. It seems like a lot, but if you want to form one gram of antihydrogen, you need 6 x 1023 atoms of antihydrogen. At that rate, it would take millions of years to gather one gram, something unfeasible no matter how much the techniques are perfected. In reality, terrorists who want to make antimatter weapons are going to have a harder time than in fiction. So we can forget about antimatter as a source of energy or destructive weapon and focus on the true interest of scientists, which is understanding the laws that govern nature.
The origin of antimatter
Our Universe is basically made up of ordinary matter, so where did antimatter come from and why don’t we see it around us? To answer this question we must go back in time more than 13 billion years, when all the energy in the Universe was concentrated in a single tiny point. At an uncertain moment, what is now called big Bang, a catastrophic explosion that began the expansion of the Universe. As it grew, the Universe cooled and part of the energy generated in the explosion began to transform into particles and antiparticles. That must have happened when not even a billionth of a second had passed since the big Bang.
It was the only time when matter and antimatter coexisted naturally. When a particle meets its antiparticle, the two annihilate and become radiation. In principle, the big Bang it should have generated the same number of particles and antiparticles, which would have destroyed each other until turning the Universe into nothing more than pure radiation. We can be sure that that did not happen, because in that case we would not be here to tell about it. For some unknown reason, the balance between matter and antimatter tipped in favor of matter. It is estimated that for every 1 billion antiparticles, 1 billion plus one particle was formed. That is, for every 1 billion particle-antiparticle pairs that were annihilated, there was one lucky particle that was saved. The difference may seem insignificant, but the Universe as we know it today began to form there: these surviving particles then joined together to form the first atoms, which would later constitute the first stars and galaxies.
Scientists suspect that the cause of this imbalance between matter and antimatter is that both behave differently and, therefore, the physical laws for both are not exactly the same. This would be something extraordinary, as surprising as it was for Alice to discover that her parlor and her mirror house were different.
How to check it? One way would be to create an antihydrogen atom – which is the simplest of all, made up of an antiproton and a positron –, study its physical properties and then compare the results with those of the hydrogen atom, which we know so well.
Antimatter production
The production of antiparticles such as positrons and antiprotons has become routine in particle accelerators. But bringing these antiparticles together and forming antimatter atoms is much more difficult, since any contact with ordinary matter has disastrous consequences. The first attempts date back to the early 1990s. The method consisted of passing a very fast antiproton close to a heavy atomic nucleus, for example xenon, which from time to time created an electron-positron pair. In this hypothetical case, the antiproton could join with the positron and form an antihydrogen atom, although this was even less frequent than the previous one. It was an enormous achievement when researchers at CERN (European Organization for Nuclear Research) in Geneva managed to create the first nine atoms of antihydrogen in 1995. The problem with this technique, in addition to being very inefficient, is that antihydrogen is manufactured at speeds close to those of light, so there was no possibility of studying its properties before antiatoms disappeared.
In the early 2000s, CERN’s ATHENA experiment separately produced positrons – from various radioactive substances, such as fluorine or sodium – and antiprotons – in particle accelerators. Since antihydrogen atoms cannot be cooled in the conventional way—for example, with liquid helium—because they would annihilate upon contact with it, they must already be created with low energy, or as physicists say, cold. And its energy depends mainly on the energy of the incident antiprotons, since they are much more massive than positrons. The colder the antiprotons are (i.e. the slower they are), the easier it is to then create and capture antihydrogen atoms.
With this idea, the so-called Antiproton Decelerator (AD) was built, a ring in which various sheets called degraders were placed. The antiprotons were made to rotate around the ring and collide with the atoms in the sheets; those that were not annihilated were slowed to 10% of their initial speed.
The antiprotons leaving the AD mixed with the positrons in electromagnetic “bottles” called Penning traps. Are…