**The popular Netflix series 3 Body Problem, which launched in March 2024, delves into many topics that the researchers at the Faculty of Science deal with on a daily basis. Virtual realities, extra dimensions, and advanced nano fibers weave throughout the storyline. However, the three-body problem is leading. What exactly entails this complex issue, and does it have real-world significance? PhD candidates in theoretical physics, Mick van Vliet and Arno Hoefnagels , shed light on these questions.**

In the 3 Body Problem series, the planet Trisolaris endures periods of extreme climate change, with severe heat and cold making life nearly impossible. The timing of these extreme climate shifts is unpredictable, forcing the inhabitants of Trisolaris to search desperately for a new home: a planet they can colonize.

The extreme conditions on Trisolaris are caused by the unique galaxy in which it resides. The planet orbits within a system of three suns, whose gravitational forces constantly alter each other’s orbits. In a two-body system, predicting the orbits of celestial bodies is straightforward. However, with three celestial bodies, as in the Trisolaris system, the task becomes extraordinarily complex. It is nearly impossible to predict the movement of the suns accurately, making it difficult to determine when Trisolaris will be close to a sun - resulting in extreme heat - or far away, causing massive freezing temperatures. This phenomenon is known as the three-body problem.

While two objects can orbit each other in a stable, predictable manner, three objects behave erratically, flying in all directions

Mick van Vliet, PhD candidate at the Institute for Theoretical Physics

## Erratic behaviour

The three-body problem is not a fabrication; it exists in the real world, too, as a classic physics problem that’s been around for a long time. But why is it so complicated? According to Van Vliet, the complexity arises from the chaotic nature of the orbits of the involved celestial bodies. "While two objects can orbit each other in a stable, predictable manner, three objects behave erratically, flying in all directions", he explains. "Even a minuscule change in the position, velocity, or mass of one of the objects can lead to enormous changes in their eventual paths", Hoefnagels adds.

There is a theoretical solution to the three-body problem, developed by the Finnish mathematician Karl Sundman in the early 20th century. Sundman demonstrated that it is possible to calculate the solution by summing an infinite series of terms. This means the solution is an intricate formula composed of infinitely many smaller formulas. However, Hoefnagels, hesitates to call this a true solution. "To obtain useful results, you would need to calculate an astronomical number of these terms: around 10^{8,000,000}. This is beyond the capability of even the most powerful computers. To put it in perspective, the estimated number of particles in the universe is about 10^{80}."

## Ecosystems and climate

Scientists continue to explore the three-body problem to gain a better understanding of it. For example, researchers are searching for simpler formulas with fewer terms. This quest is particularly important for astronomers who study the motions of celestial bodies. However, there is another compelling reason to investigate this problem further. "A three-body system is one of the simplest examples of a chaotic system, where different parts interact in complex ways", says Van Vliet. "This makes it an ideal environment for studying chaos theory, which can help us better understand other chaotic systems, such as ecosystems, climate, and fluid dynamics."

I appreciate that the scientists in the series are portrayed as ordinary, multi-dimensional characters rather than the stereotypical ’stuffy scientists’ often seen in media

Arno Hoefnagels, PhD candidate at the Institute for Theoretical Physics

## Chaotic versus tame

In a way, Van Vliet’s doctoral research is related to the three-body problem. He investigates the mathematical aspects of physical theories, such as quantum field theory and string theory, focusing on systems that can be described with simple equations. These systems are tame. In contrast, complex systems like three-body systems are chaotic and cannot be captured by a simple formula.

On the other hand, Hoefnagels’ research is not related to the three-body problem at all. He also examines the mathematical aspects of quantum field theory but is currently focusing on understanding the particles and interactions involved in the creation of the universe. "I am developing techniques that make it easier to derive physical results from the theory. This is very complicated, but fortunately, the calculations are tame", he says jokingly.

## Multi-dimensional characters

What do these two young scientists think of the series 3 Body Problem? "I appreciate that the scientists are portrayed as ordinary, multi-dimensional characters rather than the stereotypical ’stuffy scientists’ often seen in media", says Hoefnagels. Van Vliet agrees and adds that he particularly enjoys the blend of physics and creativity. "The plot explores modern scientific concepts while being rooted in an age-old physics problem," he says. "The series uses physical phenomena to tell an engaging story, and it does so exceptionally well."