Syukuro Manabe, Klaus Hasselmann and Giorgio Parisi (Illustration: Niklas Elmehed for Nobel Prize Outreach©)

SFI researchers cheered this October when the Nobel Prize in Physics was awarded to Syukuro Manabe, Klauss Hasselmann, and Giorgio Parisi "for groundbreaking contributions to our understanding of complex systems.*”  

Spanning disciplines and notoriously difficult to define, complex systems science has struggled to achieve the mainstream recognition of older, more established fields. But the acknowledgment from the Nobel Committee, arguably the most prestigious award-granting assembly in the world, has emphasized its importance and beauty. 

SFI Professor Jessica Flack hailed the decision as “surprising and entirely overdue” in a Twitter thread that details the deep implications of Parisi’s work in social and biological sciences, from starling flocks to neuroscience to the concept of renormalization in particle physics. Though a physicist by trade, Parisi’s work modeling spin glasses — a kind of disordered magnet — turned out to be applicable to a variety of other fields where understanding disorder is critical.

“Great news today with the @NobelPrize to legendary Italian physicist Giorgio Parisi,” tweeted SFI External Professor Ricard Solé. “His work had a major impact in biology, from molecular evolution to neural networks and the dynamics of complex adaptive systems. Very much deserved.” 

The study of complex systems, to Parisi, was nothing short of a revolution in physics. “[I]t was necessary to change the general philosophy, by introducing probabilistic concepts and probabilistic predictions,” he wrote in 2002. Over the past few centuries, physicists have dismantled the order of Newtonian mechanics, revealing a world that can only be described by equations that account for uncertainty and randomness.  

In an informal, post-Nobel seminar, physicist Dan Stein, an SFI External Professor and Science Board Fellow, explained how Parisi’s work on spin glass fit into that story — with a twist. In a ferromagnet such as a fridge magnet, each atom acts like a tiny bar magnet and points in the same direction. The atoms in spin glass lack a tidy magnetic organization, but in their disorder, Parisi found a “hidden order.” 

Before Parisi, SFI External Faculty Fellow David Sherrington and his collaborator Scott Kirkpatrick published a solvable model of a spin glass in 1975, for which Parisi found the equilibrium solution in 1979. The Sherrington-Kirkpatrick model was itself inspired by previous work by Sam Edwards and SFI co-founder Phil Anderson (also a Nobel Laureate in physics), which had used a mathematical trick of calculating spin-glass properties by creating replicas, effectively enabling the averaging of physical observables over many copies of the system. Parisi realized all these replicas were connected in a highly non-trivial manner, similar to descendants in a family tree: they all came from a single higher-energy state.

“Parisi is one of the leaders of spin glass theory,” wrote SFI Professor Cris Moore. “This [award] joins several people associated with SFI, including Phil Anderson and (more remotely) Dan Stein. Parisi created something called the replica method, which lets us find phase transitions in systems with “glassy” landscapes. A version of this theory is what Lenka Zdeborova, Pan Zhang,** Florent Krzakala, and my other collaborators— including George Cantwell** and Caterina De Bacco** — use to study problems in high-dimensional inference, including community detection in networks.”

Though spin glasses remain esoteric magnetic materials, the mathematical approaches physicists like Anderson and Parisi developed to understand disordered systems spread rapidly to other disciplines, from economics to ecology.

The prize is a reminder that climate, too, is an example of a complex system — one that humanity depends on understanding. Manabe and Hasselmann, who split the prize with Parisi, are climatologists. Parisi also studied the complexity of the climate, a recent comment in Nature pointed out, as do SFI External Professors Dan Schrag (Harvard University) and Elizabeth Bradley (University of Colorado Boulder), and former SFI Postdoctoral Fellow Joshua Garland.

At a press conference, after the prize was awarded, Parisi directly addressed the issue of climate change: “It’s clear that for the future generation, we have to act now in a very fast way.”


* The individual recipients of the 2021 Nobel Prize in Physics are not affiliated with SFI, which is the first research institute dedicated to complex systems science and is generally acknowledged as the global leader in the field.

** Pan Zhang and Caterina De Bacco are former SFI postdoctoral fellows. George Cantwell is a current fellow.

[By Daniel Garisto]