As we know, we live in a world of ever-growing entropy: things that have fallen are shattered and regretfully, the fragments that have formed will never come together in a way that can be seen when a film is rewound. But how has it happened that stars, planets, wildlife and man have been born out of the ancient chaos after the Big Bang? Giorgio Parisi is one of those who laid the foundations for the physics of complex systems, a field that helps us to understand why, against the background of the growth of entropy, there is a seemingly opposite trend, the self-organization: a process without which neither the solar system nor humanity would exist.
Girogio Parisi is not the only pioneer in this field. His epoch-making paper from 1986, "Dynamic scaling of growing interfaces" in Physical Review Letters, was co-authored with Mehran Kardar and Yi-Cheng Zhang; however, their subsequent contributions remained somewhat smaller. In this context, definitely the most prominent name which must be mentioned is the Danish physicist Per Bak whose role in founding the physics of complex systems is certainly no less than that of Giorgio Parisi. The paper by Per Bak, Chao Tang, and Kurt Wiesenfeld from 1987, "Self-organized criticality: An explanation of the 1/f noise," is in fact almost twice as referenced as the paper by Giorgio Parisi. Unfortunately, Per Bak left us prematurely.
Complex systems are systems that consist of many small components - the "building blocks" - which may themselves be very simple, but whose interaction leads to the complex behavior of the system as a whole. "Building blocks" can be sand grains in a sand pile, biological species involved in the evolutionary process, stockbrokers whose trades move the stock prices, protoplanetary gas cloud particles whose collisions and gravity lead to the formation of planets, and so on. The most important component in complex systems is the interaction of "building blocks": a change in the state of one "building block" leads to a change in the state of its "neighboring stones".
The Earth's atmosphere is also a complex system, and this fact links Giorgio Parisi to the other two recent Nobel laureates, Syukuro Manabe and Klaus Hasselmann. Many physicists will probably find their research less elegant than that of Giorgio Parisi as it focuses only on a single problem - modeling the Earth' s climate - while the methods of the physics of complex systems are applicable in a very wide range of interdisciplinary fields. Yet it is the work of Syukuro Manabe and Klaus Hasselmann that gives humanity a chance to avoid a global catastrophe by warning us and giving us a tiny bit of time to apply the brakes on the climate change. It might very well be this Nobel Prize that will help politicians to understand how minute the time given us to act is and how rapidly it ticks while politicians are playing their games.
In the case of Syukuro Manabe and Klaus Hasselmann, it is worth mentioning that in a sense, they brought the atmospheric physics to a new circle. With his work, Newton laid the foundation for determinism: if we know the initial data accurately enough, we can predict the future behavior of the system. More than a hundred years ago, Poincaré showed that this is not quite the case: already three bodies in gravitational interaction begin to behave chaotically, and tiny changes in the initial state will lead to exponentially growing changes in the future. This is known as the butterfly effect, thanks to Edward Lorenz's work from 60 years ago: one butterfly's wing strike can lead to the formation of a hurricane a week later. Thanks to the research of Syukuro Manabe and Klaus Hasselmann, we know now that weather can still be predicted to some extent over a longer time horizon - in the chaos, there are regularities!