The Nobel, the COP26 and the physics of complexity

Flames. Truth be told, this year’s Nobel Prize in Physics is surprising. Placed under the common banner of complexity, the two rewarded fields are in reality very different, one half welcoming essential contributions on climate physics (Syukuro Manabe and Klaus Hasselmann), the other half of the work of ‘an extraordinary physicist, Giorgio Parisi. The latter was greeted “For the discovery of the interaction between disorder and fluctuations in physical systems”, a formulation which, to be concise and exact, must have left more than a non-specialist reader unsatisfied …

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To try to shed light on what this expression covers, let us take the example of Strasbourg in 1399, when it was a question of choosing on which side of the facade of the cathedral to build the spire. Let us suppose that the City then proceeded to a preposterous survey by asking the inhabitants to place in front of their homes a flag representing the tower, either to the left or to the right of the facade. For several weeks, everyone could change their mind and replace the pennant in front of their house. No one had a preconceived opinion and could, in a fairly versatile way, change their mind spontaneously. On the other hand, everyone going about their business saw the choice of their neighbors and was influenced by it, either positively (by imitating friendly neighbors), or negatively (by wanting to have a choice opposed to neighbors considered painful). If this interaction had been guided exclusively by imitation, this rather peculiar election would have looked a lot like what happens in a magnet. A large majority vote for one side would have emerged after some time, just as magnetization results from the concerted interaction of tiny magnets carried by atoms.

Spin glasses

Parisi’s work has focused on more complex but more realistic cases, when the nature of interactions between neighbors is “frozen” in a random position, each citizen having a certain proportion of friendly neighbors and annoying neighbors. The “disorder” of which the Nobel committee speaks refers to this frozen randomness, which introduces indeterminacy and frustration into the choice of each and everyone and prevents the emergence of a clear collective behavior. For example, the City of 1399 would not have seen in this case a collective opinion manifesting itself spontaneously, the numbers of votes of the two camps differing only by very slight fluctuations. For the physicist, this situation is a challenge: because if nothing emerges macroscopically, extremely complex correlations to describe nevertheless exist inside such a system and render its responses to external requests (a classic way of exploring the material in physics) slow and difficult to decipher. In the 1980s, Parisi was the first, on the one hand to show which concepts are relevant to analyze them, and on the other hand to describe the immense complexity behind their very slow evolution in time, another signature of these so-called “spin-glass” states.

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The Nobel, the COP26 and the physics of complexity

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