One of the most significant events of the Year of Science should be the launch of a giant neutrino telescope on Lake Baikal. It took place on March 12. Dmitry Naumov, Deputy Director of the Laboratory of Nuclear Problems of the Joint Institute for Nuclear Research, Doctor of Physical and Mathematical Sciences, spoke to RG about the significance of this facility for Russian and world science.
- Dmitry Vadimovich, so that the taxpayer understands and agrees to spend almost 10 billion dollars on the creation of the famous Large Hadron Collider, scientists packed the Higgs boson in a beautiful wrapper - the image of a "divine particle". She closed the famous Standard Model, which is recognized as the most outstanding achievement of theoretical physics of the 20th century, and explained where mass comes from. And how to convince us that we need to let millions go hunting for some incomprehensible neutrino. This particle even has almost no mass. In short, why are they catching neutrinos in the depths of Lake Baikal?
Dmitry Naumov: I would give this analogy. Archaeologists are conducting excavations to understand the evolution of mankind, to understand our distant history. So the neutrino will allow a glimpse into the history of the Universe. Find out what happened in it millions and even billions of years ago. How galaxies were born and developed. It is neutrinos that can become a tool for reconstructing these long-standing events.
- Isn't it possible for giant ground-based telescopes and observatories located in space?
Dmitry Naumov: Firstly, such telescopes cannot see everything. The fact is that light may not get out of the dense and hot regions of the Universe, or the signal may change beyond recognition. Secondly, in order for optical telescopes to have a place to look, they need to specify the exact address. After all, the sky is huge, telescopes cannot afford to rummage through the endless sky in search of interesting objects. They need the most accurate addresses possible in order to concentrate there as much as possible and to conduct observations from day to day, from month to month. This is a long and painstaking process. So, neutrinos are the spotters for space addresses. In fact, before our eyes, a new science is being born - neutrino astronomy. Until quite recently it seemed like fantasy, but now it is already a reality.
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Dmitry Naumov: The game is based on the main feature of the neutrino - it interacts very weakly with matter, which is almost transparent for this particle. How to catch her if she avoids any contact? Let's say, from the Sun, these particles reach the Earth, and trillions of neutrinos per second pass through each square centimeter. But we do not notice them at all. We are like an empty space for them. For example, to catch half of the neutrinos emitted by the Sun, it would be necessary to fill the entire region of outer space with lead from us to the nearest star Alpha Centauri.
- Neutrino will allow you to look into the history of the Universe, find out what happened in it millions and even billions of years ago, how galaxies were born and developed
Dmitry Naumov: Such weakness of interaction puzzled physicists - how to see it, how to work with it? The German physicist, Nobel Prize laureate Wolfgang Pauli, who invented the neutrino purely theoretically, generally believed that we would never be able to see this particle. But don't underestimate the ingenuity of the experimenters! They learned to catch this amazing particle and reconstruct the history of the Universe. Moreover, the weakness of the neutrino interaction turned out to be very useful!
- How did you manage to turn this disadvantage into dignity?
Dmitry Naumov: Here we need to go back billions of years, when the first galaxies were just emerging in our Universe. Then each star fought desperately for its existence. The luckier ones devoured their little neighbors and got bigger. This happened until the insatiable star turned into a black hole, which almost does not glow anymore. But already invisible, it continues to devour its neighbors, increasing its mass to millions and even billions of solar masses. Moreover, the material falling on the hole heats up and glows very strongly. This wonder of the world is called "active galactic nucleus".
What is important to emphasize? Neither electromagnetic waves, nor protons, nor electrons, or anything else can get out of such a hell without losing the initial energy and direction of motion. Only neutrinos. This is their phenomenon. This is why the weakness of their interaction is a huge advantage. The most important thing is that neutrinos fly to Earth unchanged, which means they carry valuable information about the events in the Universe that took place billions of years ago, as well as their addresses.
- For almost ten years in Antarctica, the American telescope IceCube has been catching neutrinos. Over the years, the catch, frankly, is not rich, about 100 particles. What can be reconstructed using them?
Dmitry Naumov: The South Pole experiment managed to make a remarkable discovery. Scientists have discovered that neutrinos with enormous energies exceeding the energies of solar neutrinos by hundreds of millions and even billions of times do exist. This means that somewhere in the Universe there are natural accelerators capable of accelerating particles to such energies that we on Earth with our accelerators are completely incapable of. Is this an important discovery?
- I think yes.
Dmitry Naumov: So even one neutrino is enough for him, and 100 is just a gift from nature. But where are these natural accelerators located? What physical mechanisms govern them? While there are different hypotheses. And we hope that the neutrinos we have caught will be able to precisely point the direction in which ordinary telescopes should look.
The South Pole experiment uses ice as the medium with which neutrinos interact. But ice strongly rescatter light, so it is still difficult to specify with high precision the address where the neutrino was born. This is where the Baikal neutrino telescope comes into play. Its address determination accuracy is several times better than in an ice telescope. And there is hope to find sources of neutrinos!
- How did our telescope looks next to the American IceCube?
Dmitry Naumov: It looks decent. We started building the telescope in 2015, and IceCube started working in 2010. Therefore, we are still smaller, but quite a bit. The Baikal neutrino telescope is already the largest in the Northern Hemisphere with an effective volume of 0.35 cubic kilometers. This year we will catch up with the "southerner" by this indicator, bringing the volume up to 0.4 cubic kilometers. In the future, this figure will be about one cubic kilometer. At the same time, as I said, the accuracy of determining the direction of the Baikal telescope is many times better.
I want to emphasize a fundamental point. Although competition always exists, this is how the modern world works, scientists understand that it is much more effective to work together. Therefore, our Baikal telescope and the American one, as well as the telescope under construction in the Mediterranean Sea KM3NeT, are all doing a common cause. We are united into a single Global Neutrino Network.
- The American telescope costs $ 270 million, and ours is several times less. Why?
Dmitry Naumov: We were just lucky. On Baikal, two months a year, the surface of the lake is covered with a meter layer of ice. This allows us to install the telescope cheaply and simply and even repair broken parts. At the South Pole, colleagues have to heat holes in the ice with a diameter of about a meter and a depth of almost three kilometers in order to immerse their detectors there. It is very expensive. Also, the delivery of detectors to Baikal with a developed railway infrastructure is much easier and cheaper than special operations for the delivery of equipment to the South Pole.
- Who participated in the creation of our telescope?
Dmitry Naumov: The pioneers in our country and in the world were scientists from the Moscow Institute for Nuclear Research of the Russian Academy of Sciences. They have been building this line of research since the 1980s. And now, together with the Joint Institute for Nuclear Research from Dubna, they play a leading role in the project. In addition, scientists and engineers from Irkutsk State University, Nizhny Novgorod State Technical University, St. Petersburg State Marine Technical University, Institute of Experimental and Applied Physics of the Czech Technical University (Prague, Czech Republic), the Faculty of Mathematics, Physics and Informatics of the University named after Ya.A. Komensky (Bratislava, Slovakia), Institute of Nuclear Physics of the Polish Academy of Sciences (Krakow, Poland), EvoLogics GmbH (Berlin, Germany).
How a telescope catches neutrinos
To catch a neutrino, you need a large volume of the most transparent substance with which it interacts. In addition, the telescope must be protected from various background processes. For this, the installation is submerged to a depth of 750 m to 1.4 km. A string of 36 optical modules (photomultiplier and electronics) is anchored to the bottom of the lake. When particles pass through the water column, part of the neutrino will "stumble" on the nucleus of the water molecule. As a result of this interaction, new particles are born, which will glow with bluish Cherenkov radiation. It is registered by the optical modules of the telescope. Now the effective volume of water of the installation, which is involved in the search for neutrinos, amounted to 0.35 cubic km, and in the future it will grow to one cubic km. The total number of optical modules will exceed 2300 pieces.
Serious passions are raging around neutrinos in the scientific world. The fact is that physicists for more than ten years could not understand why the law of conservation of energy is not fulfilled in one of the most fundamental physical phenomena. The question was so acute that in 1931 the famous Danish physicist Niels Bohr came up with the revolutionary idea of non-conservation of energy. However, there was another explanation - the "lost" energy is carried away by some unknown and imperceptible particle. The hypothesis of its existence was put forward in 1930 by the German theorist Wolfgang Pauli. But it will never be discovered, since it does not interact with anything. About this, the scientist made a bet on a box of champagne with his friend. And on June 15, 1956, he received a telegram from American physicists Reines and Cowen that they had discovered a new particle - neutrino.
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