What exactly is the three-body problem in '3 Body Problem'?

If you're the type of person who yells "wey-oh!" if someone in a TV show says the name of the TV show, you're in good company.

In Netflix's 3 Body Problem, based on Liu Cixin's novels, the actual title of the series plays a major part in the narrative. But what exactly is the three-body problem? It's a centuries-old scientific quandary that's famously difficult to solve, with the world's top mathematicians labouring over it — in fact, just last year they found 12,000 new solutions to it. In the 17th century, revolutionary research by Sir Isaac Newton would show the world how gravity holds the universe together, and in doing so, he unlocked the scientific dilemma that would perplex scientists to this day — and delight science-fiction writers like Cixin.

In a nutshell, it's been beautifully described as "the orbital dance of three celestial objects."

For those who don't study science or regularly read physics journals, showrunners David Benioff, D.B. Weiss, and Alexander Woo include some easy to follow explanations in the series — and it's all down to one of the best characters in the show: physicist Dr Jin Cheng (Jess Hong).

Where does the three-body problem come up in the Netflix show?

The concept arises in the Netflix series from the mysterious, high-tech, alien-built VR game that Jin and her millionaire friend Jack Rooney (John Bradley) spend hours trying to solve. Jin pores over charts, measurements, and data on her whiteboard, trying to figure out what initially appears to be the main mission of the game: to predict the movements of the sun to determine whether a civilisation is about to enter into an era defined by Chaos or Stability.

In episode 3, Jin explains her theory to Pope Gregory, having measured the sun's size and luminosity, the sun's position relative to the planet, and the fact that she's seen multiple celestial bodies in the sky deemed "too bright to be a planet."

"The only explanation for these observations is that this planet is part of a three-body star system." she says.

In a fittingly Sir Isaac Newton move, Jin gets out an apple and demonstrates by walking around three standing candelabras. "If our planet revolves around one of the suns in a stable orbit, that's a Stable era. However, if one of the other suns snatches our planet away, we wander through the gravitational fields of all three suns. That's a Chaotic era."

Proving that Jin has solved it, the child Follower (the one that shows up in every level) screams, "Three suns in the sky! The world is ending!" (it's basically the TL;DR of the whole thing, as subtle as Legolas elvesplaining Aragorn's whole Return of the King plan with "a diversion".) Then, with the world turned to red hot lava, the San-Ti warrior confirms this too, congratulating Jin for solving the basic structure of the star system.

How does the three-body problem work in the alien game?

Solving the three-body problem might seem like the most pressing way to move through the game, but it's not the ultimate mission — nor is it even really possible. Jack and Jin advance to Level 3, set in Shangdu aka Xanadu, where they meet Kublai Khan, the fifth emperor of China's Yuan dynasty. Discussing the three-body problem, Jack notes there's "famously no general solution for it." Despite this, fellow players attempt to do so with a human computer made from an army holding changeable flags. But Jin refutes their claim, saying they'd "still need the starting parameters of all three bodies" to solve it.

Jin speeds up time, to see the three suns line up in what's known as syzygy, basically a three-way eclipse, which means the planet comes under the gravitational impulses of all three suns — and everything lifts into the air, horses, soldiers, even the Great Khan. It's described by the San-Ti warrior as "destroyed by the stacked gravitational attractions of a tri-solar syzygy." 

But the big turning point? Jin figures out their goal isn't actually to solve this three-body problem at all, it's about saving the people of the planet. "If one of us survives, we all survive," is a mantra that keeps being repeated throughout the series. "That's the object of the game, to help them survive," Jin says. But it's not just a game.

The three-body problem is an apocalyptic situation for the San-Ti.

The three-body problem exists beyond the VR game — it's the very real fate of the San-Ti themselves. The San-Ti hail from an unstable three-body star system four light years from Earth, where the fate of their people lies in flux. In fact, the aliens have been dubbed the San-Ti by the human followers of the Earth-Trisolaris Organization (ETO) for their very predicament; as Tatiana explains, the term stems from the translation of "three-body people" in Mandarin. You can even see the ETO have adapted the moment of syzygy for their organisation logo:

When Jin and Jack visit the wasteland civilisation of Level Four, the warrior says, "You were right. There is no solution to the three-body problem. No computer can predict the behaviour of three bodies indefinitely. With three suns in the sky, every civilisation ends in chaos. There will eventually be a cataclysm from which we cannot recover. Our planet will be ripped in half. Or pulled into one of the suns. Or expelled into space forever." After a sped-up montage of a civilisation rising and falling multiple times, it's clear Jack and Jin have just witnessed the doomed fate of the San-Ti's very real home world.

"And when you know your planet is doomed, what is the solution?" Jack and Jin are united in their answer: to flee and find a new home. The San-Ti have built a 1000-ship-strong interstellar fleet to travel to Earth, and Follower explains that some of the San-Ti have had to stay behind on their doomed home world, which is where the "if one of us survives, we all survive'" mantra comes from. 

"Our only chance of survival lies elsewhere. The moment we received the invitation to your world, we knew our destination."

How to watch: 3 Body Problem is now streaming on Netflix.