3 Body Problem: What is the Three-Body Problem in Physics?

This article contains 3 Body Problem spoilers.

The orbit of the Earth around the sun is quite stable, and there are equations that can describe how the two masses pull on each other to keep our planet’s seasons and annual revolutions predictable. Such is the nature of two celestial bodies in a gravitational dance. But what if a planet, like that of the aliens in Netflix’s 3 Body Problem, is being pulled around by three stars orbiting each other? The math then gets complex very quickly, and a multitude of variables make conditions almost impossible to predict.

Thankfully, all that the viewers of the Netflix series based on the Cixin Liu novel really need to know about the astrophysics concept referred to by the show’s title is that the home planet of the San Ti is the victim of the aforementioned unpredictability. Like weather systems or fluid dynamics here on Earth, the orbits in a triple star system are chaotic, and no civilization can thrive under the constant threat of suns that could come too close or retreat too far away relative to the planet’s position.

In 3 Body Problem, Ye Wenjie’s original message to the San Ti takes eight years to receive an answer, which means that their tortured planet is four light years away from Earth. There’s only one star that fits that description: our nearest neighbor Alpha Centauri, which is actually a three-star system made up of Alpha, Beta, and Proxima Centauri, the third of which has a planet in its goldilocks zone wherein liquid water — and thus life — can exist. So does that mean the three-body problem actually exists in nature?

It may, but in reality most triple star systems like Alpha Centauri are hierarchical in nature, with two stars revolving around each other in a tight, stable orbit and the third orbiting the pair from far away, feeling the binary’s pull as though it were a single object. Proxima Centauri’s planet, whether it houses a San Ti-like civilization or not, is actually in a quite stable orbit, if you’ll pardon the digression.

Nevertheless, the game that Jin and Jack play in 3 Body Problem seems to be focused on this physics hypothetical. The super-advanced VR headsets that illustrate the San Ti’s dilemma through a series of puzzles encountered by familiar ancient civilizations of Earth isn’t merely meant as a search for a solution to a chaos theory math problem. The game is more geared towards garnering sympathy for a people that need a new home. In other words, it’s propaganda meant to recruit human spies in a future war of conquest.

Interestingly, this recruitment method only lasts as long as the San Ti believe they need human allies. Once they realize that Earthlings, even those that supposedly serve them, can lie (an ability that they lack), the game presumably becomes obsolete. Notably, the headsets eventually serve as communication interfaces between humans and the sophon AI that acts as a proxy for the San Ti, but the virtual setting on the ruined planet is preserved, a fitting reminder about what’s at stake.

In the end, the three-body problem of 3 Body Problem provides a strong narrative motivation for an alien race to conquer Earth. Sure, witnessing a tri-star syzygy as it pulls people up into the sky with its tripled gravity is pretty cool, whether you’re a physics enthusiast or not. But knowing that an impending invasion is all about the survival of a race that can’t lie really puts into perspective how some humans could decide that Earth might be better off with someone else in charge.

All eight episodes of 3 Body Problem are available to stream on Netflix now.