The day after receiving the Nobel Prize, Giorgio Parisi had not yet found the time or the respite to carefully read the long document detailing the scientific reasons for the award. Unlike for prize-winners in other fields, here the Stockholm committee was spoiled for choice: the fields of physics to which Parisi has been able to make decisive contributions are numerous and diverse. This is the advantage of theoretical physicists, who often need only a pen and a sheet of paper to make a discovery or develop a new theory. And, at most, a computer—as well as, obviously, an extraordinary brain.
Giorgio Parisi’s research remained pure theory for a long time: ideas and mathematical models, which were certainly internationally recognized, but at least in the beginning, at the end of the ‘70s, remained merely splendid abstract ideas. But the Nobel Prize rewards discoveries, not theories. It took a few years to understand that those theoretical models on “disordered systems” were able to interpret many real situations. The crisis of the climate, a near-uniquely disordered and complex system, has demonstrated the actuality of these cognitive instruments.
In your career, you have explored very different fields of research. At the beginning, as part of Nicola Cabibbo’s group, you carried out important research in the field of elementary particles, which are still among his most cited research results in the scientific literature. Then you devoted yourself to disordered systems, a field ranging from thermodynamics to neuroscience and economics. Among such an abundance of ideas, what ended up most appreciated by the Stockholm jury?
As I read the motivations, I think that the research on disordered systems and the innovative mathematical solutions with which these topics are being studied counted a great deal. The jurors also mentioned the research on the propagation of light in disordered substances, and the studies on turbulence in chaotic systems carried out with together Benzi, Paladin and Vulpiani. I believe that a role was played by the fact that these are very general problems and they can be found in different aspects of nature: from atoms to the planetary scale, as the official motivation says. The research on elementary particles is now a previous chapter of my career, in comparison to the research that was awarded with the Nobel Prize.
Compared to classical physics as studied in school, disordered systems represent a completely different approach.
In effect, in the physics you study in school you try to remove irregularities and focus on the equilibrium and predictability of bodies. In the research that won the prize, however, the focus was more on disorder and irregularity, setting aside everything else. It is a change of approach similar to what Marcello Cini envisaged when he spoke of a transition from the study of natural laws to that of evolutionary processes.
Cini also spoke of complex systems, and the Nobel jurors also mentioned them. Complexity has become a word used everywhere, and many think that this term is being abused. What exactly does the word “complexity” mean to you?
Over the last thirty years, I have always stressed one aspect that characterizes complex systems in particular. And that is the fact that these systems can have many completely different states of equilibrium. A glass of water has a well-determined pressure and temperature, but a protein can fold in a million different ways. An animal can be in an even greater number of states. And these systems that we call “complex” can pass from one state of equilibrium to another quite easily: just think how quickly an animal can pass from a state of sleep to a state of wakefulness. We physicists talk about “metastability.” It is what also allows the brain to move quickly from one thought to another, but at the same time store memories for a long time.
Did you expect to share the award with two climatologists, Manabe and Hasselmann?
I admit it was a surprise. But in my case, sharing the Nobel Prize with two climatologists makes me a person who now can and should talk about the climate crisis as well, doing my own little part. Mind you, Nobel laureates tend to talk about a little bit of everything, and for some reason people feel that they should be heard out. But this issue is really very important to me, and I’m taking this award as a commitment. For instance, next Friday at the Chamber of Deputies there will be a preparatory meeting for COP26, and I have been asked to deliver a scientific and political speech in the presence of the highest authorities of the state.
Will this be the leitmotif of your public speeches after such an important award?
The climate will be one of the themes, of course. The other issue I will try to address is the lack of funding for public research in Italy. The financial law will have to be presented soon, and I will fight for public research to be given the right amount of weight. While the climate crisis is a global emergency, the underfunding of research is an emergency for Italy.
Which applications of your research are you most excited about today?
To tell the truth, at the moment I am not so much interested in the applications, but in the basic physics of the problems to which I have devoted myself in these decades. In particular, I am now interested in understanding the role of dimensions in these problems: to what extent the physics of these systems depends on the dimensions of the space in which they are located. To tell the truth, in the last two or three years I have only been able to work actively on this research in part. The presidency of the Accademia dei Lincei [Parisi is now vice-president] demanded a lot of effort from me. And then there was Covid, which also occupied me a great deal.
You’ve waged a battle for health data to be shared freely with the scientific community. How did that turn out?
Actually having the full data, which we were able to get from the Istituto Superiore di Sanità, didn’t turn out to be that useful. In any case, the Institute has improved the sharing of data, and now much of it is freely downloadable on the Internet. Today, however, the most interesting thing is to combine the data on the pandemic with that relating to vaccinations, to understand in detail how vaccination coverage is affecting the course of contagion. Right now, we need to understand if and how preschools, primary schools, and early middle schools will sustain the spread of the virus. For now, I note that there has been a slowdown in the drop of the number of cases.
In your first statements, you described the physics department of Sapienza University in Rome, where you studied and spent most of your career, as an extraordinary place, unequaled in the world. What makes it so special?
A university department is made up of the people who work in it. At its inception, the Institute of Physics was founded by Edoardo Amaldi, with great care taken so that it would attract to Rome not only good scientists, but also people who behaved right towards students and colleagues. For Amaldi, it was important that the physics institute was also a pleasant environment. His successors, such as Giorgio Salvini and Giorgio Careri, have done a good job at maintaining this approach, which today is in the DNA of the department.
After the awarding of the Nobel Prize, you were welcomed at the Sapienza University with banners and chanting by the students, who led you on a march through the university campus. It was a moving scene, which does not often happen for a Nobel Prize winner. How do you explain so much affection from colleagues and, above all, from students?
I expected much less of a warm welcome, I must confess. To explain this phenomenon, I will recall what Luciano Maiani once told me about Nicola Cabibbo. Nicola was considered a real hero by everyone, because he had managed to carve out an international space for Italian theoretical physics. I believe that physicists are linked by a common sense of belonging to the same family, and perhaps a similar feeling came up about me as well.
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