What are the "complex systems" that earned Giorgio Parisi the Nobel Prize in Physics 2021

After 37 years, the highest scientific recognition returns to Italy. The Roman physicist focused on the study of spin glasses

Nobel Prize to Giorgio Parisi (Jonathan Nackstrand / Afp via Getty Images) “It doesn't happen, but if it happens…”. And it happened: the 2021 Nobel Prize in Physics, after the advances of Clarivate Analytics, half went to the Italian Giorgio Parisi, of Sapienza University of Rome, for "the discovery of the interactions of disorder and fluctuation in physical systems from atomic to planetary ones ”, returning to Italy after 37 years (the last completely Italian Nobel Prize for physics was Carlo Rubbia, to whom he was awarded in 1984). The other half of the award was jointly awarded to colleagues Syukuro Manabe, of Princeton University in the US, and Klaus Hasselmann, of the Max Planck Institute for Meteorology in Hamburg, Germany, "for the physical modeling of the earth's climate, which quantifies its variability and reliably predicts global warming ".

So the Royal Swedish Academy of Sciences decided this year to award the studies of so-called complex physical systems, those characterized, to explain it as simply as possible, by chance and disorder, the understanding of which - as the name suggests - is rather difficult: "This year's award", they explain from the Swedish academy, "recognizes the new methods developed to describe [complex systems, ed.] and to predict their long-term behavior ". Let's put enthusiasm aside for a moment and try to better understand what it means.

What is a complex system

In principle, all complex systems are made up of different "parts" that interact with each other: physicists try to study and understand them from at least two centuries, clashing with their mathematical complexity (sic!) due to the fact that they can have an enormous number of components and that their behavior is often linked to random phenomena.

Let's think, for example, of a tumultuous waterfall of water: it is a system made up of billions and billions of liquid molecules that interact with each other in a way that is almost impossible to predict. As if that were not enough, complex systems can also be chaotic: translated, it means that small variations in their initial conditions evolve into large differences in successive stages. The climate is a perfect example of this: "Could the flapping of a butterfly's wings in Brazil cause a tornado in Texas?" , the American mathematician Edward Lorenz asked provocatively in 1972.

All three scientists awarded this year have made important contributions in the field of complex systems: Manabe and Hasselmann, in particular, have developed physical models and mathematicians that allow us to reliably predict the evolution of the earth's climate. Parisi, among (many) other things, has worked on the so-called spin glasses, complex and disordered systems whose understanding also finds application in apparently distant phenomena and fields, including biology, neuroscience and machine learning.

Giorgio Parisi and order in disorder

Parisi was born in 1948 in Rome, he obtained his PhD in Sapienza in 1970 and is currently professor of theoretical physics at the Roman university . In his long career, in addition to complex systems, he has also dealt with particle physics, condensed matter and much more, obtaining decisive results in each of these areas.

Up to now, apart from the Nobel Prize, the all the most prestigious awards in the industry had been awarded: Wolf Prize, Max Planck Medal and Boltzmann Medal. Around 1980, he presented some of his most important works in the field of complex systems, which showed how apparently random phenomena were governed by "hidden rules", and which are now considered among the fundamental contributions of the sector.

Lo "modern" study of complex systems began in the second half of the nineteenth century, in particular with the works of James Clerk Maxwell, Ludwig Boltzmann and Willard Gibbs, who developed the methods of so-called statistical mechanics, the branch of theoretical physics that describes systems as gases or liquids, made up of a large number of particles: to do this, statistical mechanics tries to "mediate" the random behavior of the particles rather than studying them one by one. An example is the temperature of a gas, which in the statistical interpretation can be considered as the average value of the energy of the particles that compose it.

Let's look more deeply, relying on the in-depth analysis provided by the experts of the Swedish academy (the more curious - or brave - can take a look at this document, even more technical): the particles of a gas, to a first approximation, can be seen as tiny balls, which wander around in the container that contains the gas a speeds higher and higher as the temperature increases. What happens, however, when the temperature decreases (or when the pressure increases)? The balls tend to slow down, until they "condense" into a liquid and finally solidify, often in a crystalline structure, ie in which the balls are arranged according to regular patterns.

And here (re) comes into play the complexity: if the temperature change occurs fast enough, the balls no longer arrange themselves in a crystalline structure, but according to an irregular pattern that does not change when the liquid is further cooled or compressed. Even more curiously, if the experiment is repeated under the same conditions, the balls are arranged in a different and apparently unpredictable pattern. How is this possible?

Luckily there is Parisi. The Roman physicist focused on the study of spin glasses, a particular type of metal alloys in which iron atoms, for example, are randomly inserted into a grid of copper atoms. Even if the number of iron atoms is relatively low, these are enough to change the magnetic properties of the entire material in a complex way. In particular, each iron atom behaves like a small magnet (a spin, to be precise) whose orientation is linked to that of neighboring atoms. In a traditional magnet, all spins point in the same direction, while in a spin glass some point in one direction and others in the opposite.

It is a problem that has hidden analogies with that of irregular gas patterns cooled down. And trying to solve it, according to Parisi himself, "is a bit like watching the human tragedies represented by Shakespeare". In any case, the physicist has devised an ingenious system (the so-called replication system) which allows not only to solve the problem of spin glasses, but which can be applied to many other disordered systems, and which has become a stone. milestone in the theory of complex systems.

Manabe and Hasselmann: modeling the Earth's climate

The other half of the prize went to Syukuro Manabe and Klaus Hasselmann: the former is a meteorologist and a climatologist at Princeton University, the latter works at the Max Planck Institute for Meteorology in Hamburg, Germany. Both have dealt with complex systems, and in particular with climate. Manabe, in particular, studied carbon dioxide, showing how the increase in the concentration of the substance in the atmosphere leads to an increase in temperature on the earth's surface (by virtue of the infamous greenhouse effect). In the sixties, in particular, Manabe worked on the development of physical models of the Earth's climate system, and was the first scientist to study the interaction between radiation and vertical transport of masses of air.

For his part , Hasselmann, about a decade later, created a model that ties together meteorological phenomena and climate (we generally speak of weather, the British have two more appropriate words: weather and climate), helping us to understand why climate models can be reliable. despite the meteorological phenomena are extremely variable and chaotic. In addition, Hasselmann has developed methods to identify the characteristic footprints that signal, respectively, the impact of natural phenomena and anthropogenic activities on the climate. Thanks to which we now know with certainty that climate change is, alas, our own fault.


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