The existence of antimatter was conjectured by the English physicist and mathematician Paul Dirac (1902-1984), before any physical experiment had even hinted that such a rare thing would be living with us in the universe.
Dirac, one of the fathers of quantum mechanics, was a convinced Pythagorean and Platonist; one of the pillars on which Dirac based his philosophy of physics is a phrase that he could well have taken directly from Pythagoras: “Every physical law must have mathematical beauty”. And another one is a salute to the unreasonable efficiency of mathematics in explaining nature: “The mathematician plays a game whose rules he has invented himself, while the physicist plays a game in which the rules are determined by nature; yet, as time goes on, it becomes more and more evident that the rules which the mathematician has found interesting are the same as those which nature has chosen”.
In 1928 he proposed an equation to explain the relativistic behaviour of the electron (i.e. when it moves at speeds comparable to those of light and the famous Schrödinger wave equation does not apply). One of the features of the Dirac equation is that it has a beautiful symmetry, a feature of mathematical beauty very much to the taste of its proposer. This symmetry has, however, a disturbing consequence: it implies the existence of a particle “symmetric” to the electron, a particle with the same mass, carrying the same amount of electric charge – positive, in this case – and endowed with negative energy, so that every time an electron encounters its “symmetric” particle, both are destroyed in a mini coven: their bodies disappear in a burst of energy.
Dirac wrote a sentence that one cannot help but feel a lot of sympathy and admiration for: “A good part of my work consists of playing with equations and listening to what they tell me”. And Dirac was consistent with what his equation said, so that he predicted the existence of antimatter: not because he had smelled its smell, or seen its colour, or heard how it sounds when struck with a gong, nor because he had caressed the softness or suffered the roughness of its touch or appreciated its sour or sweet taste. It was not Dirac’s senses that led him to say that antimatter existed, but a mental process guided by a mathematical equation. An equation that is tasteless, odourless and tasteless to the senses as only a mathematical equation can be, but which, by the same token, is to a properly trained brain a prodigy of aromas, flavours, colours, melodies and caresses.
His equation told Dirac that somewhere in the universe there had to be anti-electrons. But this was so revolutionary that Dirac was slow to believe it, and he searched for a while for other, less drastic explanations. But, failing to find them, he ended up relying on the unreasonable ability of mathematics to explain nature, and in 1931, two years or so after he had proposed his equation, he repeated aloud the spectacular prophecy he had heard from the lips of his equation. He did so with due caution – the proper caution, on the other hand, that one would expect from a person of his character.
And the world did not have to wait long to see Dirac’s prediction confirmed: since Henri Becquerel discovered radioactivity in 1896, a search for radioactive sources in nature had begun. It was discovered that, surprisingly, the radiations were ubiquitous and that they increased in the atmosphere with height – an increase reflected by detectors on balloons. In the early 1910s, there was no choice but to conclude that much of it was not of terrestrial but cosmic origin: it came from outside. The study of these cosmic rays was then intensified in an attempt to elucidate their nature, which means investigating what kind of particles they are composed of. One of the laboratories that devoted most effort to this study was owned by the California Institute of Technology, where Carl Anderson (1905-1991), a sorcerer’s apprentice, using powerful magnets, lead filters and fog cameras, succeeded in August 1932 in photographing the trajectory of particles present in cosmic rays: they were identical in every respect to those left by electrons, but had an inverse curvature with respect to the latter, which indicated a positive charge. This newly discovered extraterrestrial inhabitant was christened the positron; it later emerged that it had already left traces detected by other scientists who failed to see the new particle.
And the positron turned out to be the antiparticle that Dirac had listened to in his equation.
That was the first little piece of antimatter discovered; then, in 1955, the antiproton was discovered in Berkeley and, a year later, the antineutron, and in 1965 Soviet scientists discovered antideuterium: the first complex particle made of antimatter; and in 1978 at CERN in Geneva antitritium was created, and in 1996 nine complete atoms of antihydrogen…
Dirac received the Nobel Prize in Physics in 1933 for his contributions to the fundamental equations of quantum mechanics, while Anderson received it in 1936 for the discovery of the positron.
References
A.J. Durán, Pasiones, piojos, dioses… y matemáticas, Destino, Barcelona, 2009.
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