Where is the real world of "The Three-Body Problem"?

Recently, the animation of "The Three-Body Problem" was launched, which instantly set off the entire Internet, and the video exceeded 100 million views on the same day;

The reason why "The Three-Body Problem" is so popular is that many of its contents come from the real world. For example, the Three-Body Galaxy has a prototype.

01

The home of the three-body people

It turns out to be so "close" to us

The title of the book "Three Body" comes from a system of three stars - Alpha Centauri, also known as Alpha Centauri. The α in the name means that this star is the brightest star in the Centaur constellation - because on Earth, the human eye cannot distinguish that this bright spot in the night sky is a triple star system.

Centaurus, the largest star in the lower left corner is Alpha Centauri. Image source: Visual China

Alpha Centauri is the closest star system to the Sun, and this neighbor is only 4.3 light-years away from us. How close is this distance? The Oort Cloud at the edge of the solar system, which is the home of many comets, is about 2 light-years away from the Sun itself, which is half the distance from the Sun to Alpha Centauri.

The distance between Alpha Centauri and the Sun. Source: Wikipedia

Because it is so close, Alpha Centauri appears very bright in the night sky. Its brightness ranks third among all stars, after Sirius and Canopus. However, its position in the sky is too "low". In China, only people in several southern provinces can see it.

Alpha Centauri has always been a guest of honor in science fiction works. In addition to "The Three-Body Problem", it is the destination of the Earth in "The Wandering Earth"; Pandora in "Avatar" is also here; even the aliens in "The Hitchhiker's Guide to the Galaxy" said to the earthlings: "You haven't even been to Alpha Centauri? That's too bad!"

Alpha Centauri in the real starry sky (the bright star in the lower right corner of the photo), and the one above the antenna is β. Image source: Wikipedia

In the novel "The Three-Body Problem", the Trisolarans call the period when the planet is affected by the three stars the Age of Chaos. In the Age of Chaos, the seasons and day and night on the Trisolaran planet are in an unpredictable chaotic state. Once, the three stars were directly connected in a line, and the huge gravitational force sucked the Trisolarans directly into the sky.

The opposite of the Chaos Era is the Stable Era. The symbol of the Stable Era is two flying stars (two bright spots in the sky), which means that two stars have flown away and the Three-Body Planet will temporarily only be affected by one star, just like the Earth today.

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However, if the Trisolarans really live in the Alpha Centauri system, they actually don't need to worry about the occurrence of the Chaos Era.

Among the three stars, Alpha Centauri A and Alpha Centauri B are about the same size as our sun. They form a binary star system and revolve around each other. The farthest distance between them is only more than 30 astronomical units (AU), which is about the distance from the sun to Pluto.

An imaginary picture of a binary star system consisting of two stars, Alpha Centauri A and B, and the planets orbiting them, with the sun in the upper right corner. Image source: NASA

The third star, Alpha Centauri C, is a red dwarf star that is smaller and dimmer than the sun. It orbits this two-star system from a distance. This "far away" is very far, about 15,000 astronomical units. What does this distance mean? Voyager 1, the farthest human probe from the solar system, took 41 years to fly less than 150 astronomical units. If you look at the other two stars on Alpha Centauri C, they are just two ordinary small bright spots in the bright starry sky.

Comparison of the sizes of the Sun, Alpha Centauri A, Alpha Centauri B, and the red dwarf Alpha Centauri C (Proxima). Source: Wikipedia

The two-star system can operate stably. They will rotate around a common center of mass. Following them, the Trisolarans can live well. If they follow the distant star C, there is basically no need to worry about the influence of the two distant stars, and life can be just as comfortable.

02

Wandering, to the Three-Body Planet...?

"Look, kids, that's Centaurus, that's Proxima Centauri, that's our new home!" - The Wandering Earth

In the novel "The Wandering Earth", because the sun is about to have a "helium flash", humans have to move "around". The final destination of mankind is called Proxima Centauri, which is Alpha Centauri C, the red dwarf star orbiting far away from its two brothers.

The journey from the solar system to the Alpha Centauri system. Image source: The Wandering Earth

Humans chose this star not only because it is close, but also because its lifespan is much longer than that of the sun. Proxima Centauri is much weaker than the sun in terms of mass, volume and temperature, so its nuclear fusion is very slow, and its lifespan can even reach hundreds of billions or even trillions of years. It should be noted that the current lifespan of the universe is only 13.8 billion years, so the red dwarf can be said to be "as old as the sky".

In addition, even in its old age, a red dwarf will not turn into a huge red giant like the sun, so it is absolutely safe for the Earth to stay next to it.

An artist's impression of a red dwarf star. Source: Wikipedia

So, has Liu Cixin found a good destination for mankind? In fact, this is not the case.

Because the fusion reaction of a red dwarf is very weak, it radiates very little energy outward, and the habitable zone (the area where water can remain in liquid form) is very close to the star. Proxima b is a planet discovered by humans that is in the habitable zone of Proxima Centauri. It flies almost "close to" Proxima Centauri, and the distance between the two is only one-eighth of the distance between Mercury and the sun.

Proxima Centauri and the planets in its habitable zone: Proxima b (the planet on the right). Image source: NASA

There are two terrible consequences of orbiting so close. First, Proxima b is tidally locked to the star like Mercury - one side of it always faces the star, while the other side never sees the sun. Second, Proxima has a bad temper. In 2017, people observed a strong flare from Proxima: its brightness increased 1,000 times in 10 seconds. The radiation received by Proxima b was 4,000 times stronger than the radiation intensity of solar flares that the Earth normally receives. With such a strong destruction, it is estimated that even the Trisolaran civilization would be directly destroyed.

Image of the flare effect from Proxima Centauri. Source: NASA

It seems that we should go on for a while and settle in the binary star system of Alpha Centauri.

03

"The stars are unsurpassed"

In the novel "The Three-Body Problem", the core reason why the Trisolarans want to occupy the Earth is that they are unable to solve the three-body problem.

The so-called three-body problem is the law of motion of three stars together. The Trisolarans need to use this law to predict the future and prepare for bad weather. Three stars plus three planets is actually a four-body problem, but the mass of planets is very small compared to stars, so they are simplified in the model.

Humans are testing the "vanguard" of the Three-Body Fleet - the flying vehicle water droplets. Image source: "My Three-Body: The Story of Zhang Beihai"

The seemingly simple motion system of three objects is actually very complex. They affect each other all the time during the motion process, and the trajectory changes irregularly. Since the establishment of Newton's mechanics system, the three-body problem has been puzzling all the experts. Until 1899, Poincaré, known as the "last generalist in physics and mathematics", used a paper to characterize the three-body problem: this problem has no solution!

What Poincaré said about no solution does not mean that there is no solution, because the three stars will definitely correspond to a position and a momentum at any time. These are all numerical solutions. People just cannot express these solutions with a formula. However, they can still calculate the mathematical solution at a certain time based on the initial conditions.

If we look at it this way, the Trisolarans can use computers to calculate the positions of the three stars in the next tens of thousands of years, right? But the situation is far from that simple.

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We have all heard of the "butterfly effect" where a butterfly flapping its wings in the United States can cause a storm in Southeast Asia. This is because the weather is a chaotic system. In a chaotic system, the impact of a small change will multiply and multiply, and in a short period of time, this multiple will reach an astronomical figure. Neither humans nor computers can calculate these effects. This is why it is predicted to rain next week, but the sky is actually clear. A slight difference can lead to a huge difference.

The airflow stirred up by the airplane is a chaotic model. Image source: Wikipedia

The three-body problem is such a chaotic system. The disturbance of every planet and even every dust particle in the universe, as well as a breath or a movement of the three-body pedestrians, will have a storm-like and decisive impact on the movement of the three stars in the long run, making predictions meaningless.

The three-body problem is so complicated, but our Earth can run stably in the solar system, which consists of nine planets. It is a miracle! As Poincaré wrote at the beginning of his paper on the three-body problem:

"The stars cannot be surpassed!"

Source: China National Geographic BOOK

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