Why write this book now?
In recent years, the field of astrophysics has experienced a major revolution, “multimessenger” observation. The idea is that we should not limit ourselves to light – the electromagnetic spectrum – to study what is happening in the Universe. We can also use gravitational waves and cosmic particles, made up of atomic nuclei and neutrinos. One of the first examples of this approach is the supernova of 1987 (SN 1987A), whose explosion was followed in light, but also in neutrinos. Several experiments around the world have collected around twenty of these particles associated with this event.
But the event that really demonstrated that the multi-messenger approach was essential if we want to solve certain enigmas of the Universe was the merger of two neutron stars observed in 2017 and named GW170817. This coalescence produced gravitational waves detected by the network of giant interferometers Ligo et Virgo. And, unlike the merger of two black holes, it also emitted a lot of light, in the entire spectral range, which was scrutinized by numerous observatories on the ground and in space. The combination of information from gravitational waves and electromagnetic emissions has allowed us to better understand what happens in an event as violent as the merger of two compact bodies like neutron stars.
Is this violence what inspired the title of your book?
Yes, the Universe hosts numerous phenomena that can be described as “violent” because the quantities of energy involved are gigantic. We see stars merging, others exploding in supernovae, black holes at the centers of galaxies which accrete large quantities of matter and produce jets of light and particles which extend over millions of years – light, particles that bombard the earth’s atmosphere, etc.
But there’s nothing scary about it. As I tell in my book throughout the chapters, we have a privileged place to witness this spectacle of stars which pulse, explode, spring, sparkle, etc. Thanks to the multi-messenger approach, we can, in a certain way and with a little poetry, appeal to almost all our senses to appreciate the diversity of these joyful stars: sight with light, hearing with waves gravitational forces, touch with cosmic rays, and taste with neutrinos.
Neutrinos are the common thread of your book with the project Grand. What is it about?
Neutrinos are quite special particles. First of all, they come in three flavors, which is why I like to talk about the meaning of “taste”. But they interact very little with matter; very large experiments are needed that collect data over years to detect them. Some are interested in low-energy neutrinos produced in the heart of the Sun and in nuclear reactors. Others, like IceCubein Antarctica, or KM3Netin the Mediterranean, also relate to very high energy neutrinos originating from astrophysical processes.
Neutrinos are interesting for astrophysics because, unlike atomic nuclei, they do not carry an electric charge. They are therefore not deflected by the magnetic fields which bathe the intergalactic and galactic environments. In the case of atomic nuclei, these “cosmic rays” have quite complex trajectories which make it impossible to trace back to the source of these particles. Neutrinos ignore these magnetic fields and move in a straight line. It is possible to determine their origin. In 2023, the teamIceCube collected several thousand neutrinos which they showed came from the Milky Way, thus drawing a unique map of the galactic plane.
Experiences like IceCube cannot, however, see ultra-high energy neutrinos (more than 1017 electronvolts). However, we know that these neutrinos exist because they are produced when ultra-high energy cosmic rays interact with photons from the cosmic microwave background. These ultra-high energy cosmic rays are detected and studied, for example, by experiment Pierre Augerin Argentina.
But an even larger experiment, dedicated to ultra-high energy neutrinos, is missing. The idea of Grand is the following. Some “tau” flavor neutrinos interact with an atom while passing through the Earth. They then produce a tau particle, a cousin of the electron, heavier and unstable, which quickly disintegrates into a cascade of particles. These will interact with the earth’s magnetic field and emit electromagnetic radiation in the radio frequency range. It is this signal that we want to collect with antennas distributed over an area of several tens of thousands of square kilometers. With these data, it is possible to determine the properties (energy and direction) of the initial neutrino.
The difficulty is that we must be able to distinguish this radio signal from all emissions linked to human activity. Many physicists were reserved about our chances of success.
Where is the project?
-We currently have a prototype with 49 antennas operating in the Gobi Desert, China. The results are very encouraging, we isolate stray signals very well: for example, we can follow the passage of a plane in the sky above the site! This data will be valuable for refining the configuration of the experiment in its final version. We are also discussing with American and Argentinian colleagues to use antenna arrays which exploit different techniques and will improve the sensitivity of the experiment. If all goes well – the Covid-19 epidemic caused a four-year delay on the project –, Grand should see its first astrophysical neutrinos around 2030.
A particularity of your book is its very embodied, very personal tone, why this choice?
My initial intention when I started writing this book was to tell the human adventure of research. Behind the discoveries, the theories, the experiences, there are people who meet to discuss, exchange, who build friendships, who inspire each other. For me, this is what makes the world of research so exciting. Obviously, there is science that motivates us to get up every morning, we all work towards better understanding the world around us, but that would be impossible without building this community of people.
I told this adventure by feeding it with my own experience. I wanted to show that, far from the clichés that we see in films, science advances through discussions totally disconnected from our research work, cups of coffee that are spilled, the balance that we find between this profession which requires a significant investment and personal life.
Among all these meetings, which were the most important in your career?
There are many, but some have played a crucial role. I especially think of Angela Olinto, whom I met during my postdoc in Chicago in 2009. She was the first woman professor in the physics department at the University of Chicago. She inspired me a lot on how to do science, but also how to cultivate human qualities to lead a team, work with international collaborations and defend your place as a woman in the research environment.
James Cronin, who won the Nobel Prize in physics for his discovery of the violation of certain symmetries in the decay of particles, is also a major figure in my career. He supported me a lot, especially to launch the project Grand.
Today, I find myself in the position where young researchers come to me to ask for advice. This reversal of roles in the cycle of transmissions is confusing, reassuring and very rewarding. Moreover, in terms of meetings, students and doctoral students are an incredible asset, they bring a lot of energy to my work and to the project. Grand.
And finally, I must obviously mention, Olivier Martineau, my colleague and accomplice on the project Grand. There were two of us at the beginning and now there are a hundred of us on this experience. This probably would not have been possible without our great complementarity.
At the turn of a chapter on black holes in your book, you have an unexpected encounter, who is it?
Yes, while writing the book, I documented a lot about Karl Schwarzschild, who was the first, in 1916, to find an exact solution to Albert Einstein’s equations of general relativity. It was an opportunity for me to bring out my master’s courses, but I also immersed myself in his notebooks. The joyful spirit of this character really moved me. While in the trenches on the Eastern Front, in appalling conditions, he continued to do physics as a mental escape. He continued epistolary exchanges with Albert Einstein and other scientific friends. It was these bonds that allowed him not to succumb to the darkness of war (he would, however, die of an autoimmune disease in 1916). We always come back to the same thing: science is above all a human adventure.
