Thanks to the incredible teams at Penguin Press for sending me an advanced copy of In a Flight of Starlings by the Italian theoretical physicist Giorgio Parisi. He is the winner of the 2021 Nobel Prize in Physics jointly with Klaus Hasselmann and Syukuro Manabe.
The Nobel was awarded to the trio for their contributions to the theory of complex systems. It stirred quite an interest during that time because the Nobel press committee and the media outlets touted the implications of these findings in “physical systems from the atomic to planetary scales”. Popular science medium is a double-edged sword that can either simplify or embellish discoveries and lose their focal point in the message. Fortunately, careful digging into a few resources back in 2021 helped to understand spin glasses and related statistical mechanics models that Parisi is known for.
Parisi begins the book by elucidating his work on the flocking of starlings. This phenomenon has a common thread with many other studies in modern physics where the behavior of a system is composed of a large number of interacting components or actors like the starlings themselves. Murmurations are a wondrous thing to observe. But have you ever wondered why individual birds never collide with each other? They seem to maintain a rhythm with their nearest neighbor while swooshing in the air together.
Parisi’s project utilized three-dimensional flight images and applied statistical mechanics techniques to get an insight into collective behavior that emerges from simple rules governing individual entities like the birds. Two years of collecting and collating images, measurements of distances, models, and analyses led to an astonishing finding. The interaction between the birds was not dependent upon the absolute distances between the pairs in any given flock but on relative ratios of the distances. Every time a bird turned others followed suit and this behavior is prompted by the position of their neighbors. Such quantitative analysis of a collective behavior would garner much interest later in ethology groups.
After the tryst with starlings and how collective behavior arises, we enter the author’s journey as a physicist back in 1966 at Sapienza University in Rome. A rich montage that includes figures like Edoardo Amaldi and Giorgio Salvini, the radical Marxist ideas that swooped the Italy of 1968, and the burgeoning Italian universities that promoted academic pursuits in modern physics, thanks to Amaldi’s efforts (he was the first secretary-general of CERN). The author fondly recapitulates a pre-Google era of scientific correspondence and complex mathematical calculations. Appreciate his candor here,
Most simple calculations were done on paper, with the help at most of a slide rule that was frequently carried around in a pocket. The slide rule is now a museum exhibit. At the time it allowed us to quickly perform multiplications to two or three places, and was swept away by the advent of the portable calculator. I remember my astonishment when in 1973 I saw such a calculator for the first time. It took my entire monthly salary to buy one.
Such recollections are present throughout this book. They help paint the collective nature of how science works. There are so many people involved in a scientific process. Most of them remain away from the spotlight of a new discovery. Every year we watch the Nobel prize announcements with excitement and yet few people can grasp the timespan and human effort that went behind every finding. The author compares poetry with science where the arduous work of the process leaves no trace except the result. Precise. Beautiful. Succinct.
Parisi’s work with phase transitions of magnetic systems is the entry point to the sweet spot of this book. At one point he is talking about types of Ising model phases: ferromagnetic and paramagnetic and on the other end he ties it up neatly with a reference to the Platonic view of nature. It’s refreshing to read a book that refrains from simplifying important details (ones that are mathematically complex) and yet is cogent for all readers. Being approachable does not mean trivializing concepts.
Take a jar of honey and turn it upside down. The honey does not fall immediately. Instead, it oozes out slowly. Annoying when you are hurrying at the breakfast table. Spin glasses are metal alloys, made of gold or silver, with a small amount of iron diluted in them. The spin behavior of the metal particles gives rise to magnetic phase transitions similar to the phase transition of glass. Hence, the name spin glasses. At temperatures below a certain value, these alloys behave like glass, wax, or bitumen: the changes get slower and slower and the system never reaches equilibrium.
In spin glasses, a complicated dynamic happens where certain spins try to orient themselves in the opposite direction to that of their neighbor: called an antiferromagnet. This is called ‘frustration’: it is not possible to find an arrangement of spins that satisfies all bonds. In the end, the spins have to find the least troublesome configuration. These frustrated configurations end up being in a huge (exponential!) number of possible states. So, these materials have a very slow response time (decisions to make). Kick them and they stay in their initial configuration for a long time making spin glasses really hard to study.
Here’s where Parisi provided a solution in 1979 that partially solved the problem using replica symmetry breaking. It is a mathematical tool that allows physicists to study and predict the behavior of spin glasses. Indulge your curiosity about how these abstract tools work with a few references I provide below.
Parisi writes about how similar mathematical models such as the ones used for spin glasses can be applied to animal behavior, brain function, or to the economy. He talks more about this in the chapter Metaphors in Science, one of my favorites in this book. Parisi explains that intuitive reasoning and metaphors are important in scientific construction. Hard sciences typically lack the nontechnical reasoning or logical pathway that may precede or accompany a result except in the works of mathematicians like Henri Poincaré. He gives us the example of the Copenhagen school’s interpretation of quantum mechanics which shows a marked similarity to Darwinian selection. In both cases, evolution (both Darwinian and quantum mechanical) passes through various possibilities to a subsequent selection. Although the details of both processes are fundamentally different (biologists and physicists need not be alarmed) their ideation has a similarity. The author duly mentions the lack of references to this comparison.
It is often said that the hard sciences cannot be understood by anyone who has not studied mathematics. But something similar could be said for Chinese poetry, which is an inextricable mixture of literature and painting: the original manuscripts of that poetry are paintings in which the individual ideograms are represented differently each time.
This book is an invitation to see the beauty and value of modern science. Seemingly difficult but not impossible to understand.
References for the curious reader:
Dr. Steven J. Thomson, a theoretical physicist at Freie Universität Berlin wrote an informative Twitter thread on spin glasses back when Parisi won the Nobel prize.
The Quanta Magazine article about the Nobel Prize in Physics for 2021.
This book will be published in July 2023.