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Giorgio Parisi, a physicist from Sapienza Università di Roma and Istituto Nazionale di Fisica Nucleare, and current vice-president of Accademia dei Lincei, has been awarded the 2021 Nobel Prize in Physics, along with Syukuro Manabe from Princeton University in the USA and Klaus Hasselmann from the Max Planck Institute for Meteorology in Hamburg, Germany.

Parisi is an eclectic physicist whose research work has covered areas such as fundamental particles, condensed matter, statistical physics and disordered materials. The Nobel committee has highlighted his contributions to “the discovery of the interplay of disorder and fluctuations in physical systems from atomic to planetary scales”.

The study of complex physical systems has applications ranging from neuroscience to biology and machine learning. In making the announcement, the Nobel Committee for Physics highlighted one complex system in particular: Earth’s climate. “The discoveries being recognised this year demonstrate that our knowledge about the climate rests on a solid scientific foundation, based on a rigorous analysis of observations,” said Thors Hans Hansson, committee chair.

This is the first time that complex systems are explicitly mentioned in the Nobel Prize ruling, and is a cause for celebration in Italy and worldwide. “Giorgio's work has been a source of deep inspiration for many scientists and its elegant formalism has added beauty to the science, and to the life, of many of us,” said Alessia Annibale, a graduate of Sapienza and frequent co-author of Parisi, now at King’s College London.

“Parisi has developed a paradigm that goes beyond condensed matter” said Enzo Marinari, a physicist at Sapienza who has co-authored over 100 papers with the new laureate. “His theories can be applied at all scales: glass, the brain, finance, the flight of birds, glaciers and much more”.

Born in Rome in 1948, Parisi graduated in physics from Sapienza University in 1970, joining the ranks of a school that dates back to Enrico Fermi. “It was an extraordinary environment, providing a level of education that was not comparable with any other university [even] abroad. It was at the forefront,” Parisi told an audience at Sapienza the day he was awarded the Nobel Prize.

He initially focused on high-energy physics, under Nicola Cabibbo, another celebrated Sapienza physicist. Parisi contributed to the Altarelli-Parisi equation, used to track particles emerging from collisions in accelerators. He also worked at the APE project, a series of supercomputers designed to carry out quantum theory calculations.

Between 1979 and 1983, Parisi developed what is probably his most impactful idea: Replica Symmetry Breaking (RSB). His curiosity had been caught by a class of metal alloys with strange magnetic properties, called spin glasses.

In normal magnetic substances, each atom can be imagined as a tiny compass (or ‘spin’) that tends to align along the same direction as its neighbors. But in spin glasses, there are some atom pairs aligning in the same direction, and others that go in opposite directions. Globally, it’s hard for these systems to find an optimal alignment – and for physicists to understand them. But, Parisi managed it by using the “replica trick”, a mathematical technique based on handling many different copies of the system at the same time. His breakthrough was finding a way to actually do the maths of replicas, which among other things, implied working with a ‘zero-dimensional’ geometry. In the following years, spin glasses became the benchmark of complex systems and RSB a cornerstone of their understanding.

Complex systems encompass a wide variety of phenomena, in which a large number of units are connected through disordered interactions. These units can be atoms, but also neurons, genes, proteins, species, agents, etc. In recent years Parisi studied even the flight of large flocks of starlings. Replicas have been used also in neural networks, with important applications in machine learning.

Proving that the Parisi solution was mathematically correct took many years. This effort, led by the French mathematician Michel Talagrand “has created a whole field of research in probability”, said Yan Fyodorov, a mathematical physicist at King’s College London.

Beyond replicas, Parisi has introduced some of the most important theoretical tools in recent condensed matter physics: among others, the Kardar-Parisi-Zhang (KPZ) model for rough surfaces; stochastic resonance, a counter-intuitive process found in glaciers’ evolution; and the Parisi-Sourlas application of supersymmetry to condensed matter.

“He is a person who thinks very fast”, says Juan Jesús Ruiz-Lorenzo, another long-time collaborator of Parisi, at the University of Extremadura. “Sometimes he does not write the end of an equation on the blackboard. You cannot see it, but he can,” he said.

“I have been impressed by how kind and approachable he has been with my students” points out Lisa Manning, a condensed matter physicist at Syracuse University “He is generous with his time and he makes my students feel they ask interesting questions”.

“He is a delightful person, but not a naïve one”, says Marinari, referring to the political commitment of Parisi, who has put efforts in advocating for Italian science, including during his recent position as director of Lincei Academy.

“The acknowledgement of [the importance of] science has changed recently in Italy and I hope this will be reflected in the next budget law” said Parisi in his Nobel-day speech at Sapienza, in front of Italy’s research minister Maria Cristina Messa. “Italy should become a welcoming country for researchers. Now it is not, which is why there are many Italian scientists abroad and few foreign ones in Italy”, he concluded.

Parisi, a father of two, is the sixth Italian scientist to win the Physics Nobel Prize, after Guglielmo Marconi (1909), Enrico Fermi (1938), Emilio Segré (1959), Carlo Rubbia (1984) and Riccardo Giacconi (2002).