A team of underwater archaeologists has just discovered a wreck off the Galician Coast of Death. After the first explorations, the members of the group agree that it may be one of the 20 vessels of Martín Padilla’s fleet that were lost in a terrible storm at the entrance to the Corcubión estuary in 1596. Cannons can be seen , anchors? In 2001 the Civil Guard recovered a piece of artillery that had been collected from the coast of Carnota (La Coruña) by a resident of the town of Boiro. Perhaps it belonged to this ship? Recovery work is slow and painstaking. When all the material is already safe on land and is being classified, a mysterious character calls the person in charge of the excavation by phone. He identifies himself as a professor of nuclear physics and is very interested in certain metallic pieces, such as anchor counterweights or others that have practically no historical value. The negotiations are tough but in the end the physicist manages to get hold of a few. He already has enough to be able to start his experiment: to determine the nature of an unknown type of matter that occupies 25% of the total matter and energy content of the Universe . It is the so-called dark matter of which we only know that it exists because we have no idea what it is. Only one thing is certain: it is not like the one we know, the one we are made of.
Although the previous scene is a fiction, it reveals something that has happened and is happening: What interest can old pieces from sunken ships have for a physicist specialized in astroparticles? Or train and tram tracks from the early 20th century? The answer is very simple: you need your experiment not to be affected by the slightest radioactive contamination. Why? Because it’s on the hunt for some ghostly particles: the elusive neutrinos and the even more elusive and mysterious dark matter. Both are of a type of particles that interact very weakly with ordinary matter; so little that, for example, a neutrino is capable of passing through a wall of lead (attention!) several hundred billion kilometers thick as if it were air. Or to put it another way: of the neutrinos coming from the nuclear reactions that take place inside the Sun , we only detect one in 5 billion of them… once they have passed through the Earth . That is why some physicists define the neutrino as a very sharp knife without a handle and without a blade.
Astronomy under tons of rock
Such difficulty in detecting them has forced physicists to make two decisions. The first is to use large detectors: the more massive it is, the more likely it is that an unsuspecting neutrino will collide with an atom. Thus, in Japan there is Super-Kamiokande, a detector that contains 50,000 tons of water. The second is to bury the detector under tons of rock – like the one already mentioned, which is found in an old mine, Kamioka Mozuni.The objective pursued is to shield it so that nothing disturbs it, since cosmic rays reach us from outer space., high-energy radiation that produces background noise in the detector and masks the detection of neutrinos. It is the same situation that occurs when we want to chat with a hoarse friend: the noise of other conversations in a crowded bar prevents us from hearing his voice; to hear the whisper of neutrinos we must go to a quiet place. In astroparticle physics, these places are deep mines –Kamioka in Japan, Homestake in the US or Sudbury in Canada– or tunnels under mountains –Gran Sasso in Italy or Canfranc in Spain–. Now, we might think that this is enough. But it’s not like that. Everything around us is radioactive. This means thatthe rock of the mountain, and even the material with which the experiment is built, produce a radiation background. Hence the interest in old tracks or lead remains of sunken ships. In fact, for the most delicate parts, the iron and lead used are carefully chosen, so thatthey had to have been melted before the first nuclear explosions in the atmosphere and thus avoid that additional human contamination. Furthermore, the older it is, the better. For example, the lead used contains an isotope –a word used to describe the atomic variants of a chemical element–, Lead 210, which has a half-life of 22 years. This means that if we have a kilo of this lead, half of it will have disintegrated after just over two decades and, as in the famous paradox of the tortoise and Achilles, after another 22 years, half of what remains… and so on. The consequence is obvious:a lead cast in Roman times is “cleaner” than one cast in 1900. This explains something that is shocking at first: to observe the neutrinos that come from the sun or from other astronomical phenomena, such as supernova explosions, we must bury the telescopes under tons of rock; observe the sky from a hole: this is what astroparticle physicists do.
The so-called underground physics aims to understand processes that are too weak to be detected on the surface due to the large number of disturbances to which they are exposed. In essence, there are three major topics that it deals with: neutrinos, dark matter and gravitational waves, one of the most surprising predictions of Einstein’s general relativity. These waves are ripples in the space-time fabric of the universe due to the movement of stars and galaxies or violent explosive processes, such as when a supernova explodes or the core of a galaxy bursts. The effect is the same as if we throw a stone into a pond.
Ferris, T. (1997) The whole shebang, Simon & Schuster
Wheeler, J. G. (2000) Cosmic catastrophes, Cambridge University Press