A long journey in the development of physics connects the two Roman scientists who were awarded the Nobel Prize in Physics: Enrico Fermi in 1938 and Giorgio Parisi in 2021. Both had the opportunity to develop their exceptional talents in the wake of two different revolutionary periods in physics.

Enrico Fermi was born in 1901, four years before one of the epochal changes in classical physics brought about by Einstein’s theory of Special Relativity. This theory, contrary to common sense, postulates that the speed of light is the same when measured by observers in uniform linear motion relative to fixed stars. This, measured with enormous precision, implies that velocities do not add up in a classical manner and, most importantly, that time passes differently for two observers in relative motion to each other. One of the many consequences is the fascinating insight that mass (m) and energy (E) are two aspects of the same entity, making it possible to convert one into the other and vice versa. The famous equation E=mc² has become the most famous formula in physics.

The second revolution during Enrico Fermi’s formative years was the development of quantum mechanics, which challenged another cornerstone of common sense: the notion of the trajectory of a moving body. When considering the motion of elementary particles like protons and electrons, one must abandon the classical view that they move along a simple continuous line where each instant corresponds to a single position in space. Instead, their motion must be described through waves that do not have an obvious physical reality, like electromagnetic or sound waves, but are mathematical tools to understand physics at the atomic scale. These are interpreted as probability waves and satisfy equations verified with incredible precision over the last century, allowing unprecedented scientific and technological development. Enrico Fermi and his colleagues, the “Via Panisperna boys,” embarked on their work based on the two revolutions of relativity and quantum physics to understand nuclear physics and elementary particles. Fermi was a key figure in the exceptional feat of liberating the tremendous amounts of energy contained in subnuclear matter, as predicted by Einstein’s Theory of Relativity. Moreover, these studies laid the foundation for modern research on the fundamental constituents of matter, for which Enrico Fermi was awarded the Nobel Prize in 1938.

For the scientific and technological importance of nuclear physics and the military interest in these researches, they were considered the only true intellectual challenge. This approach focuses on the constituents of matter, believing that understanding their functioning would make it possible to comprehend nature through the application of the fundamental laws discovered in the microscopic world. Despite the enormous leap forward in the understanding of nature that resulted from this, this reductionist view proved incapable of explaining collective behaviors observed in many natural phenomena. Therefore, it was complemented by an approach that developed from the 1970s onwards: the revolution that refers to chaotic phenomena, systems composed of many interacting elements, and the set of research activities now known as the physics of complex systems. If chaos challenged another cornerstone of common sense, that of the predictability of a system when its fundamental dynamic laws are known, the physics of complex systems has incorporated chaotic phenomena into a more general conceptual framework that transcends the boundaries of physics itself.

As soon as the complexity of structures and systems increases, especially when they are composed of many interacting elements, leading to collective behaviors on a large scale, one encounters new situations in which the knowledge of the properties of individual elements (particles, atoms, molecules, etc.) is no longer sufficient to describe the overall system. The point is that in such systems, interactions between the elements are such that they determine complex structures that cannot be directly derived from the properties of individual isolated elements. For this reason, the behavior of the whole is fundamentally different from any of its elementary subparts. It is precisely in this context that Giorgio Parisi’s work fits, which has also made important contributions to the field of elementary particles, quantum chromodynamics, and which has received the Nobel Prize for “innovative contributions to our understanding of complex physical systems.” Neural networks in neuroscience, the collective behavior of flocks of birds, protein folding, are examples of complex systems to which Parisi has made a decisive contribution. Parisi’s work fits into the framework of an intense activity developed since the 1970s in various academies worldwide, with a central role played by the Department of Physics at the Sapienza University of Rome, where many illustrious colleagues and collaborators have worked, and others continue their activities.

The Historical Museum of Physics, housed in the building on Via Panisperna, now assigned to the new research institution named after Enrico Fermi (the Enrico Fermi Research Center – CREF), which will open soon, offers a path focused on the work and figure of Enrico Fermi, concluding with the physics of complexity, thus outlining an imaginary line that connects the two Roman Nobel laureates in physics. This museum route aims to connect historical events of the past to the ongoing scientific research at CREF. The CREF also aims to develop original and cutting-edge research at the frontier of various disciplines, precisely at the interface of the field of complex systems. The museum itinerary aims to ideally connect past historical events to ongoing scientific research.

(Published on Il Fatto Quotidiano)