What is the three body problem in the “The Three-body Problem” novel?

This post contains spoiler!

Liu Ci Xin’s The Three-Body Problem (Chinese: 三体) is a Chinese novel that won many book awards. The leader of the free world read it on his buzy schedule. It is the first novel of the trilogy, Remembrance of Earth’s Past (Chinese: 地球往事). I like this book because it incorporates many science concepts in the story. It is a hard science fiction novel in the likes of Arthur C. Clarke’s novels. I read it in the Chinese version.

As seen from the title of the novel itself, the three body problem is a central problem in the novel. The three body problem is actually a real physics problem. It studies the interaction of three bodies and tries to predict their movements. Imagine three things floating in the emptiness of space, it can be anything. They interacts through their own gravity. To predict their future movement, we need to know their starting velocities and masses. A small change in initial condition may affects future movements a lot. This is a very hard problem, as three body interaction is very complex. The calculation is beyond what we know or knowing at all. Three body interaction is very chaotic.

Three-body problem simulation (2D)
Three-body problem simulation (2D). Source: Reddit

The three body problem is only revealed half way through the book. There is a star system with alien life. The aliens are called Trisolarans (三体人). They are so-called because the planet they live in, Trisolaris revolves around three stars. Instead of only one star (the sun) like our solar system, they have three stars. The interaction of the three stars is unpredictable. Sometimes, the way they move causes destruction of lives on the planet. Civilizations come and go. There is a period of stability then followed by a period of destruction. The period is unpredictable, and can lasts only few days, or few millennium.

Civilizations of Trisolaris have tried to find patterns. They tried to predict the periods of peace and destruction and found no pattern. So far, the longest surviving civilization has reached beyond the space age. They are not able to predict the movements of the three stars, despite their advanced technologies. Staying on the three star system is very dangerous. The civilization faces extinction any time. This is the three body problem of the Trisolarans.

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