Dear readers, faithful companions of the lantern and the basketball,
Before we speak of the stars, allow me one confidence. You know that I share with you, from time to time, photographs of my little tribe, of whom I am so proud, and of the woman I love as on the very first day. But you do not know, perhaps, how that love was born. And since tonight we are going to speak of the most elusive messengers of the cosmos, I feel the need to tell you where I come from.
I have loved Jocelyne since childhood. We met on school benches, at birthday parties at her house and at mine. Our parents grew fond of each other, and wondered whether we would one day take the step. My mother teased me: “Are you going to marry Jocelyne?” And I would answer, “No, she is too good for me.” We were friends, brother and sister almost, before we dared to look at each other differently, around sixteen or seventeen. We did not dare put a name to what we already felt. When she had a boyfriend, and I saw her kiss him, I felt a pain in my stomach. She told me later that she felt the same when it was my turn.
We followed the same course of studies, and shortly before we turned twenty, she confessed to me one morning, through tears, that she had had her first sexual experience the previous Saturday, with her boyfriend of the moment, and that it had been forced upon her. That wretch had raped her. As I held her in my arms to console her, I thought to myself: “How is it possible that she had not done this long before?” And I came to the conclusion: yes, Jocelyne, as I had always believed, is a woman of principle. And she was the one.
I gathered my courage, kissed her on the forehead, and said softly, “I love you.” She stopped sobbing, and answered, “Me too.” Our relationship began in pain, and to protect it, I waited until she was ready for our first intimate moment. We reached maturity — in Switzerland, that is the diploma that marks the end of five years of high school and opens the door to university — at the same time. Jocelyne was expecting Jessica. Our parents, whom I thank from the bottom of my heart, agreed to help us financially during my university years. Jocelyne gave up her studies, and when she could, worked as a saleswoman in a jewellery shop. Whenever I could, I took small jobs so that we would not be too heavy a burden, even though Jocelyne’s parents are very wealthy, which mine are not. Thanks to our parents, we were able to live normally, despite my main occupation as a student. We had our four little stars before the end of our studies. We were very lucky.
As for sharing my family, they are aware of it and approve of it. Moreover, it allows me to embellish my texts, which are very difficult to illustrate with real photographs.
Foto privée: On the left, Sarah, my second daughter, and on the right, Jocelyne, without whom I would be only a shadow of myself. Here they are ice skating while waiting for the start of Charly’s match. Jocelyne goes to almost all of our son’s matches.
I tell you all this because I am truly happy, so happy that I want to share it with you. And because the subject I am about to broach, the neutrinos of our good old Sun, is precisely a story of invisible messengers, of elusive presences, of particles that cross our bodies by the billions every second without ever touching us. The Sun, like love, gives without asking anything in return. It sends us its light, its warmth, and its ghosts. Tonight, we shall speak of those ghosts.
1. The Sun, That Prodigious Furnace
Let us begin with the basics. The Sun is a sphere of plasma, 1.4 million kilometres in diameter, in whose core the temperature reaches fifteen million degrees and the pressure is two hundred and fifty billion times that of Earth’s atmosphere. In this inferno, hydrogen nuclei fuse into helium, releasing prodigious energy. Four protons combine to form a helium nucleus, and in the process, they emit positrons, gamma photons, and neutrinos.
Every second, the Sun converts about six hundred million tonnes of hydrogen into helium. Each of these reactions produces a neutrino. This means that the Sun emits roughly 1.8 times 10^38 neutrinos per second. That is 180 million billion billion billion neutrinos, every second, in every direction. A tiny fraction of them — about 65 billion per square centimetre per second — reach the Earth. They cross your body, my body, the Earth itself, without slowing down, without interacting, without leaving the slightest trace. They are the ghosts of the cosmos.
2. What Is a Neutrino?
A neutrino is an elementary particle of the standard model. It has no electric charge. Its mass is extraordinarily small — so small that for decades physicists thought it was exactly zero. It interacts only through the weak nuclear force and gravity, which makes it almost invisible. A neutrino could cross a wall of lead a light-year thick and have only a fifty percent chance of interacting.
Neutrinos come in three flavours, as physicists call them: the electron neutrino, the muon neutrino, and the tau neutrino. Each is associated with its charged cousin: the electron, the muon, and the tau. They are the lightest of the known massive particles, and the most abundant in the universe after photons. Billions of them, born in the Big Bang, still wander through space, relics of the first instants.
The neutrino was postulated in 1930 by Wolfgang Pauli, to explain a missing energy in beta decay. He called it a “desperate remedy”, and confessed to a friend: “I have done a terrible thing. I have postulated a particle that cannot be detected.” He was almost right. It took twenty-six years, until 1956, for Clyde Cowan and Frederick Reines to detect the first neutrino, near a nuclear reactor in South Carolina. They sent a telegram to Pauli: “We have detected neutrinos.” Pauli, who had bet a case of champagne that it would never happen, paid up.
3. The Solar Neutrino Problem
And this is where our story becomes fascinating. In the 1960s, the American physicist Ray Davis built an extraordinary experiment, deep in the Homestake gold mine in South Dakota, one and a half kilometres underground. The idea was simple: to capture solar neutrinos. He filled a tank with 615 tonnes of perchloroethylene, a common cleaning fluid, rich in chlorine. When a neutrino strikes a chlorine atom, it transforms it into radioactive argon. By counting the argon atoms, Davis could measure the flux of solar neutrinos.
He waited. He counted. And after years of measurements, he found only one third of the neutrinos predicted by the standard solar model. One third. It was as if the Sun were shining at a third of its expected intensity, or as if our understanding of the Sun were fundamentally wrong.
This was the famous “solar neutrino problem”. On one side, the physicists of the Sun, who were certain of their models. On the other, the particle physicists, who were certain of their detectors. Both could not be right. And yet both were rigorous. It was a crisis of physics, one of those moments when the universe refuses to fit our equations.
4. The Solution: Neutrino Oscillation
The solution came in the 1990s and 2000s, from a series of experiments, notably the Sudbury Neutrino Observatory in Canada and Super-Kamiokande in Japan. The idea was this: neutrinos do not have a fixed flavour. They oscillate. An electron neutrino, born in the heart of the Sun, can transform into a muon neutrino or a tau neutrino on its way to Earth. And since Davis’s detector could only capture electron neutrinos, it missed the others. The missing two-thirds had simply changed identity along the way.
This discovery was revolutionary. It proved that neutrinos have mass, contrary to what the standard model assumed. It required a modification of our theories. And it earned Arthur McDonald and Takaaki Kajita the Nobel Prize in Physics in 2015.
But this is not all. The oscillation of neutrinos in the Sun is amplified by their interaction with solar matter, an effect predicted by Stanislav Mikheyev, Alexei Smirnov, and Lincoln Wolfenstein, known as the MSW effect. The Sun does not only produce neutrinos: it also transforms them, through the density of its plasma. It is a collective, subtle, almost poetic phenomenon.
5. Why It Matters
Why should we care about these ghosts? Because neutrinos are messengers of the Sun’s core, that region we will never see directly. Photons take a hundred thousand years to escape the Sun, bouncing from particle to particle. Neutrinos escape in two seconds. They bring us, in real time, news from the very heart of our star. They are the only direct witnesses of the fusion that lights our world.
They also help us understand supernovae. When a massive star explodes, 99% of its energy is released in the form of neutrinos. In 1987, the supernova SN 1987A, in the Large Magellanic Cloud, was observed not only by light, but by neutrinos, hours before the visible explosion. Neutrino astronomy was born.
They may also explain part of the dark matter, or reveal new physics beyond the standard model. Sterile neutrinos, for instance, particles that interact only through gravity, have been proposed. Their detection would be a revolution. Other experiments, such as IceCube at the South Pole, DUNE in the United States, or Hyper-Kamiokande in Japan, are actively searching for these new messengers.
Finally, neutrinos could help us solve a fundamental mystery: why is there more matter than antimatter in the universe? Certain theories suggest that neutrinos are their own antiparticles, and that their behaviour at the dawn of time could explain the imbalance that allowed us to exist.
6. The Watchman’s Lesson
Dear readers, the neutrino is the most discreet of the cosmos’s messengers. It crosses us by the billions every second, without ever touching us. It comes from the Sun, from supernovae, from the Big Bang itself. It is the ghost of the universe, the silent witness of its history.
But it is also a lesson in humility. For decades, we believed we understood the Sun and the neutrino. And then the universe reminded us that we had missed something essential. A particle without mass, we thought. A simple, stable, unchangeable ghost. And then it turned out that this ghost oscillates, changes identity, transforms itself. That it has a tiny mass, which we had ignored. That it may hold the key to the deepest mysteries.
The universe is not simple. It is subtle, complex, sometimes mischievous. And the neutrino, this ghost of the Sun, is one of its most beautiful puzzles.
And as I think of Jocelyne, of our love born in pain and grown in patience, I see a strange echo. What seems fixed can change. What seems invisible can reveal itself. What seems too small to matter can turn out to be essential. The neutrino, like love, is a quiet presence whose importance we only discover with time.
I set down my lantern. It flickers gently, like a neutrino crossing the darkness. The basketball is in its place, and I think of the Sun, which sends us, every second, an ocean of invisible messengers. It gives without counting, without expecting anything, without ever asking for thanks. Perhaps that is what love is. Perhaps that is what the universe is.
Thank you, dear friend, for confiding in me. Thank you for sharing your happiness. And let us continue, together, to listen to the ghosts of the Sun.
François
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