A scientific analysis of Liu Cixin’s book “The Three-Body Problem”

The novel The Three-Body Problem by Chinese writer Liu Cixin can rightly be called science fiction, as its plot is firmly tied to really complex and interesting problems that scientists have solved in the past or are currently solving. However, there are many moments in the story that look quite controversial from the point of view of modern astronomy and physics.

«Проблема трьох тіл» Лю Цисіня

The Three-Body Problem by Liu Cixin, Ukrainian edition

What is The Three Body Problem?

The fantastic series The Three-Body Problem, released on Netflix on March 21, 2024, drew wide public attention to the novel of the same name by Liu Cixin, on which it is based. The screen version — the story about a computer game, the mysterious suicides of physicists, and the aliens behind it all — turned out to be quite a serious science fiction by itself.

Meanwhile, as an adaptation of a novel, it is rather weak. Most of the scientific mysteries that made up a significant part of the book’s plot have disappeared. The latter is considered to be one of the best examples in the modern world of how serious scientific topics can be told in a work of fiction.

Even the title of the novel refers to the physical problem, which deals with the gravitational interaction of three bodies of approximately the same mass. It is proved that this problem has no general analytical solution. Their behavior can be predicted only for a certain period of time and only by calculating each subsequent moment step by step on a computer.

Демонстрація того, в чому полягає задача трьох тіл у серіалі
Demonstration of the three-body problem in the series. Source: www.tvguide.com

On Earth, this problem has no practical application. However, Liu Cixin has invented a situation where it becomes vital. On Earth, agriculture and the sustainable development of civilization are possible because thousands of years ago people invented the calendar. Our planet orbits the sun in a stable, slightly eccentric orbit, so in the middle of winter we know exactly when summer will arrive. If we know how to add days to leap years correctly, it won’t be a problem to determine this for thousands of years to come.

But what if, instead of a single star, we had the same dynamic system of three stars whose behavior we cannot predict? Then creatures living on such a planet would regularly experience eras of chaos, during which civilization could not develop and even predicting the behavior of the three key celestial bodies would be a tough task. Inventing a calendar becomes a problem that cannot be solved without a computer.

The reader of the book has to solve this scientific riddle together with the characters. It requires some knowledge of the history of science, but it is quite interesting. However, while Liu Cixin did a great job of describing many scientific concepts, he assumed a lot of inaccuracies that need to be addressed.

Анімація, що показує рух трьох тіл у  хаотичній системі

An animation showing the motion of three bodies in a chaotic system. Source: Wikipedia

When will the Sun rise on Trisolaris?

At first glance, the idea of the world of Trisolaris seems quite logical. If there are three separate stars in the sky, sometimes turning into one or two Suns, you are living through an era of chaos. The planet wanders between the luminaries, approaching one or the other, and never stays in a stable orbit. The chaotic alternation of cold and heat makes it a real hell.

If there are two stars in the sky side by side, it means that the planet, along with one of the suns, has moved away from the other two, and there is an almost earthly change of day and night, winter and summer. If three stars are together, the sun will not rise, because the planet has moved away from them all.

All this beautiful scheme is destroyed by the fact that the change of day and night is determined not by the planet’s orbital motion, but by its own rotation. The gravitational fields of the stars have very little effect on it, so the axis around the spinning planet retains its direction for a long time. Approaches to the stars occur at different angles, but this still means that during at least one approach to a star during the chaos era, the Sun should rise and set more or less in the same pattern.

How long one such period should last depends on the brightness of the luminary, about which Liu Cixin says nothing. In the Alpha Centauri system, two of the stars are almost the same size as the Sun, and the third is a tiny red dwarf. If you focus on it, this “apparent stability” will last for several days, and if you focus on the other two, it will last up to a year.

Since the book does not specify what kind of stars make up the triple system, it is difficult to say how long it takes for a planet to travel between them. It’s worth remembering that the luminaries are not really solid balls, but gaseous, and if they come too close to each other, they can deform and exchange material, or even collide.

For the system described above to exist, provided that all three luminaries are similar to the Sun, the distance between them must be measured in astronomical units most of the time. This means that an Earth-like planet can travel between them for years. And all this time, it will be very cold and dark there, even though the stars that the suns have turned into will fly alone. And if the “era of order” in a novel can last for decades, so can the icy flight between the stars.

Світанок на планеті з трьома сонцями
Dawn on a planet with three suns. Source: phys.org

What is a “stable orbit”?

According to Liu Cixin, when Trisolaris orbits one of the stars in a stable orbit, it is always a signal that a civilization can be developed. In fact, in the situation described, there is no guarantee that the usual day and night changes will even occur there.

The reason is that the planet’s axis maintains a constant orientation and when entering orbit it can be positioned relative to its plane in any way. If the angle between them is close to zero, the change of day and night there will be like on Uranus — once in half a year one of the hemispheres will be completely immersed in darkness, and the other will have eternal day. If this world receives the same amount of energy as the Earth from the Sun, the fluctuations in heat and cold will not be much less than in the “era of chaos,” with the only difference being that they will occur regularly.

But this is not the worst case. Because a “stable orbit” does not mean that it will have almost zero eccentricity, i.e., it will be practically circular like the Earth’s. It is much more likely that it will be elongated, and this will have its consequences.

Рух Урана орбітою
Uranus’ orbital motion. Source: www.pbs.org

According to the laws of celestial mechanics, the planet will pass the most distant part of its orbit at the minimum speed, and the closest part at the maximum speed. Therefore, for it, a short summer will be followed by a long winter, and this is no better for the development of civilization than the same era of chaos.

In addition, no one can guarantee that a “stable orbit” will not be too close or too far from the luminary. In this case, life on it must die again due to overheating or freezing.

All three factors can be combined with each other in any way. It follows that, in fact, in most cases, entering a “stable orbit” would not bring anything good to the inhabitants of the Trisolaris.

Орбіти в «зоні життя»
Orbits in the “habitable zone”. Source: static.wikia.nocookie.net

Is Alpha Centauri the birthplace of the Trisolarians?

However, most of the questions to Liu Cixin arise because he indicated that the trisolar system where the aliens come from is Alpha Centauri. Again, at first glance, everything is logical — it is indeed a triple system, and at least one planet has even been discovered there. The only problem is that our closest neighbor is definitely not an example of a physical three-body problem.

In order for an analytical solution to be impossible, all three bodies must be approximately the same size and be at approximately the same distance from each other. Otherwise, two of them will interact with each other as a pair, which in turn will interact with the third as a single entity.

A hierarchical ternary system will emerge, which is very well described mathematically. Alpha Centauri is an example of such a system. In it, two large stars orbit each other in a close pair, and a third, much smaller one makes one revolution around them in hundreds of thousands of years. The situation with the traveling planets, which is the basis of the plot of the novel, simply cannot occur there.

Система Альфа Центавра
The Alpha Centauri system

So maybe we should just replace Alpha Centauri with some other star system that more closely matches what is described in the book, and everything will be fine? No, because in fact, astronomers have studied the Sun’s neighborhood encompassing dozens of light-years around and there is nothing like it. Moreover, scientists do not know of any star system among the billions of cataloged star systems exemplifying the “three-body problem”.

This is because this situation is not only unpredictable, but also unstable. A system in a state of dynamic chaos would have to disintegrate over a period measured in thousands of years, if not years. Either two stars would collide and merge, or a third would be ejected. And all of this would have happened before life, let alone civilization, could have emerged on the hypothetical planet.

So, in fact, Liu Cixin’s novel is quite ambiguous. On the one hand, it does deal with real scientific problems and ideas, and in this respect it surpasses most other examples of contemporary literature. On the other hand, it is definitely a purely fantastic work.

We know for sure that there are no chaotic triple star systems in reality. And if an Earth-like planet does appear in them, it will be uninhabitable most of the time. And certainly no trisolarians are coming to us from Alpha Centauri. However, all of this does not make the book less exciting. It’s definitely worth reading and figuring it all out for yourself.