Nobel Prize in Physics 2021

Though nothing can bring back the hour
Of splendour in the grass, of glory in the flower;
We will grieve not, rather find
Strength in what remains behind.

William Wordsworth

Nobel Prize in Physics 2021. Syukuro Manabe, Klaus Hasselmann and Giorgio Parisi

The Royal Swedish Academy of Sciences has awarded the 2021 Nobel Prize in Physics “for their innovative contributions to our understanding of complex physical systems with one half jointly to Syukuro Manabe (Princeton University, USA) and Klaus Hasselmann (Max Planck Institute for Meteorology, Hamburg, Germany) and the other half to Giorgio Parisi (Sapienza University of Rome, Italy).

The three laureates, the Swedish Academy announces, share this year’s Nobel Prize in Physics for their studies of chaotic and seemingly random phenomena.

What exactly is meant by chaotic, random phenomena?
And, complex systems?

We will try to shed some light on this exciting chapter of current physics-mathematics.

Chaos Theory and the Butterfly Effect

We can agree that the great power of science lies in the ability to relate cause and effect. From the laws of gravity, for example, eclipses can be predicted thousands of years in advance.

But there are other natural phenomena that are not so easy to predict. Although the motions of the atmosphere obey the laws of physics to the same extent as the movements of the planets, weather predictions are made in terms of probabilities. Contrary to what has long been thought, the fact that a system is governed by completely deterministic laws, does not guarantee that its behaviour can be predicted in the future.

These types of phenomena are included in the so-called chaos theory and have therefore come to be called chaotic systems. They are characterised in that tiny variations in the initial conditions soon lead to huge differences in the evolution of the system.

This has a very important consequence: in the chaotic regime it is impossible to make long-term predictions, since the initial conditions of the system can never be known with infinite precision.

A now very popular way of referring to the above phenomenon is the term butterfly effect, which comes from the title of Edward N. Lorenz’s 1972 lecture at the American Society for the Advancement of Science: “Can the flapping of a butterfly in Brazil trigger a tornado in Texas?” Lorenz wanted to emphasise, with a provocative image, the extreme dependence on initial conditions of a chaotic system par excellence such as meteorological weather.

Chaotic systems: weather and the Earth’s climate

Thus, weather is a typical example of a chaotic system under this definition and not many scientists have ventured to find behavioural laws or patterns and draw conclusions from this phenomenon.

It is precisely for daring to do this that Manabe and Hasselmann, two climatologists who have defined the foundations of our knowledge of the Earth’s climate and how mankind influences it, have been honoured.

The Japanese scientist Syukuro Manabe has, throughout his 90 years of life, led the development of physical models of the Earth’s climate (as early as the 1960s and 1970s), largely based on the interaction between the atmospheric circulation and its heat transport. These studies have led to the foundations of today’s climate models, and it is this that has earned him this year’s top prize, the Nobel Prize in Physics.

A few years later, the German researcher Hasselmann, founder of the Max Planck Institute for Meteorology, developed a model linking meteorological weather (chaotic and, as such, unpredictable in the long term) and climate. In particular, Hasselmann showed that chaotic weather dynamics underlie long-term climate variability. Among other applications, its methods would later be used to demonstrate that the global rise in the earth’s temperature is due to human-generated carbon dioxide emissions.

Complex systems. What could climate, ants’ nests, rainforests, the human brain, language and the stock market have in common?

One of the immediate consequences of chaos theory is the idea that the whole is more than the sum of its parts.

In an influential 1972 article entitled “More is different”, physicist and Nobel laureate Philip W. Anderson emphasised that there is a profound conceptual difference between the properties of individual constituents and the emergent characteristics of a system or aggregate.

At each level of organisation, completely new phenomena and laws emerge that bear no obvious relation to those governing the previous level. Anderson argued that these emergent laws were just as fundamental in character as the first, so that a large number of interacting constituents give rise not only to a larger system, but to a fundamentally different one.
More is different.

Thus, we could say that complex systems are those that, consisting of a large number of elements, exhibit “emergent” properties. These are characterised by the fact that, although they appear as a consequence of the interaction between the individual components of the system, they cannot be explained by the simple “sum” of these components. The whole is much more than the sum of its parts.

The reason why these systems are called “complex” is because it is very difficult to model their interaction and predict their future evolution.

Thus, the climate, ants’ nests, rainforests, the human brain, language and the stock market world are just a few examples of complex systems endowed with special properties halfway between order and disorder

Parisi and the flocks of starlings

In this context, the research of the Italian theoretical physicist Giorgio Parisi led him to develop new techniques for understanding complex systems. As early as the 1980s, he discovered hidden patterns in complex disordered materials. These discoveries are among the world’s most important contributions to complex systems theory.

Not surprisingly, the Swedish Academy does not spare any praise when expressing Parisi‘s immense and extensive activity: “he receives the Nobel Prize for the discovery of the interplay of disorder and fluctuations in physical systems from the atomic to the planetary scale“.

His main contributions in physics have been in quantum field theory, statistical physics and complex systems: from spin glasses to neural networks and artificial intelligence……. via flocks of starlings.

Let’s look at this last complex system: a flock of starlings.

Hundreds of birds that move in waves, in a mixture of order and chaos. The flock is an example of a complex system: individual birds do not know how to fly in waves. The movement of the flock is an emergent property. It only takes place when the group or system exists.

Computer simulations have been able to create these movement patterns by making each individual in the group follow only three simple rules: it must not move away from the group, it must maintain a certain distance from its neighbours and it must fly in line with the others. It is believed that the birds in the flock only really interact with about seven or so close neighbours, but because they form a network, they all end up connecting with each other.
In Parisi’s own words: “The change in behaviour of one animal affects and is affected by all the animals in the group, no matter how large it is.

Complex systems. Human language

To conclude, let us try to succinctly present another interesting complex system: human language.

If we take a realistic and natural view of language, i.e. try to explain its actual use by speakers, we find that its behaviour is very similar to that of complex systems.

We could say that language is complex for two reasons: because it is made up of different subsystems (phonological, morphological, syntactic, semantic, etc.) and, what is more, because these subsystems are interdependent, that is, a change in any one of them produces modifications, either directly or indirectly, in the others.
In short, language is complex because its overall behaviour emerges from the interaction of subsystems, it is not a mere sum or product of them.

We hope that with this blog we have aroused the reader’s curiosity and we are grateful that the Swedish Academy has recognised a field that is of enormous help for the future development of mankind.

In conclusion, I leave it to the reader to savour the emergent property that emerges from just twenty-eight words of the poem by W. Wordsworth’s poem “Intimations of Immortality” which heads this article.

Author: Manuela Maza Ruiz.