3 Body Problem, explained with the help of an astrophysicist

Netflix’s new adaptation of Liu Cixin’s famous sci-fi series shows us what quantum mechanics might look like in real time.

Netflix’s 3 Body Problem premiered March 21, and there’s a lot of science! The new Netflix series from screenwriter Alexander Woo and Game of Thrones shepherds David Benioff and D.B. Weiss adapts a bestselling sci-fi trilogy by Chinese writer Liu Cixin, an engineer with a high-level understanding of physics. The story that unfolds over 3 Body and its two sequels, also known as the Remembrance of Earth’s Past series, won acclaim for its vision of a future based on a variety of ideas about quantum mechanics and how they might impact a future interstellar existential crisis. In 3 Body’s fictional universe, far-flung theory plays out in real time in the lives of a far-away alien species and its attempts to both interact with and influence humans here on Earth.

Fortunately for audiences who aren’t Einsteins, the Netflix series shifts much of the drama away from the skies and onto humans — it even creates a bunch of entirely new characters to give us people to care about in between all the physics. Liu’s series includes two more books following the first novel; the Netflix series follows the first book, then spins off in its own direction for a while before setting us up for book two. What they both have in common is a zoomed-out view of quantum mechanics and astrophysics underlying all the cool space stuff. Our heroes and villains are all scientists whose decisions and conflicts dictate humanity’s course both now and in the distant future. With the assistance of an actual astrophysicist, let’s go through the basics you need to know to understand what the heck is happening in this show.

What is the three-body problem, and why can’t anyone solve it?

The three-body problem has existed ever since humans began to understand gravity and how it works. You probably know that the Earth rotates around the sun because the sun’s gravitational field is exerting a pull over our planet and all the others in our solar system. We’re able to interact with the sun in that way because as planets, our individual gravitational spheres are all less powerful than the sun, and none are powerful enough to exert a hold on each other. It’s the same with our moon — it’s caught in Earth’s gravitational field, so it floats along hanging out with us.

In other words, two objects whose gravitational fields interact will always form stable orbits along a predictable, unchanging path. Newton figured this out, along with the formula for predicting their orbits, in 1687. It’s sometimes called “the two-body problem.” If you were to introduce another star into the mix, you’d probably wind up with a binary star system — where both stars form stable orbits around a gravitational center. The most common sort of star is one with a stable binary partner, which makes our sun, a solo star, fairly rare. Binary star systems can have stable planets, too, and these types of systems can often be mapped and plotted and predicted by astronomers and physicists.

But that only works with two objects with gravitational forces. When you add a third object into the mix, all bets are off. Instead of stabilizing, the third element creates chaos and causes the objects to fly around and interact in completely unpredictable ways — spinning off into space, crashing into each other, or bouncing off one another’s gravitational spheres and careening in completely different directions.

To explain why this happens, I turned to astrophysicist Dr. Charles J. Horowitz, who told me that the key here is the law of conservation of energy — that’s the one that tells us that energy in a closed system can never be created or destroyed. “Conservation of energy implies that a planet will orbit a single star forever and can never escape to infinity,” Horowitz wrote in an email. In other words, once a planet becomes trapped inside of a star’s gravitational field, it can’t create the additional energy it would need to propel itself out of it.

“Two stars, on the other hand, can exchange energy and possibly eject an orbiting planet,” Horowitz said.

This, then, is “the three-body problem:” How do we stabilize three gravitational objects or predict what their orbits might be?

For centuries, scientists were unable to find any starting point from which the three objects could form stable orbits in relationship to one another. In recent decades, scientists have come closer; increasingly, using computational algorithms and, in at least one instance, modeling their predictions on intoxicated humans, we’ve found multiple solutions to create stability among our three hypothetical objects. But the majority of these solutions are difficult if not impossible to model in reality, so it’s not clear how well they work out of the realm of theory.

What are some possibilities for any civilization unlucky enough to be living on a planet within a three-body star system?

The central conceit of 3 Body Problem is exactly this scenario — an alien species on a distant planet has evolved the capacity to become a technologically advanced civilization — but its planet exists within a solar system with three different suns.

Because of the three-body problem, these suns are constantly exerting gravitational chaos over one another, flinging each other to and fro across the cosmos and in the process wreaking climate havoc on the planet caught in the middle. The alien race, called the Trisolarans, has thus had its civilizations wiped out and destroyed, over and over, for millennia.

I asked Horowitz how likely this scenario would be, and he essentially backed up Three Body’s author, Liu Cixin. “In the short term it might be fine,” Horowitz said. “Over very long times (say, billions of years) many orbits of planets around two stars are thought to be unstable.”

“If life takes billions of years to evolve (as it did on Earth) then such a planet may not provide a suitable environment. However, there may be certain configurations of the three bodies that are stable for long times and could be suitable for life. Or life could develop or colonize the world more quickly,” he added.

This is precisely the situation the Trisolarans face: From time to time, their three bodies stabilize for long periods, giving their civilizations enough time to rapidly advance and flourish. Inevitably, though, the stable eras give way to “chaotic eras,” when their suns resume their volatility.

The existential problem of the Trisolarans — which a select group of Earthlings eventually devote themselves to solving as well — is how to know and prepare for a chaotic era when you can’t predict one. In essence, they’re living out the three-body problem in real time.

Is it possible for two particles situated across the universe from one another to act as one body and receive/transmit information simultaneously?

This scenario might sound improbable, but it’s actually not — and it’s a crucial part of the plot of 3 Body Problem. In the show, we learn that the Trisolarans are able to essentially spy on Earth through the use of a proton that’s been transmitted to Earth to act as a simultaneous receiver and transmitter for its twin proton, which remains on Trisolaris.

This is possible through a mind-bending phenomenon known as quantum entanglement. Scientists have observed this property in subatomic particles which essentially operate as one entity, even when they’re separated by billions of light years. In fact, notes Dr. Horowitz, “[It’s] perhaps better to say the two entangled particles share the information rather than receive and transmit it.” In other words, they aren’t so much communicating with one another as simultaneously receiving information from both locations — even though they’re on completely different planets.

3 Body posits a scenario where scientists line up a thousand nuclear bombs in space and then set them all off in order, like dominoes. If you tried this in reality, wouldn’t you have to deal with some kind of giant radioactive space cloud?

This may sound like the most unbelievable part of the 3 Body series — even in the show, when our plucky cosmologist, Jin Cheng, presents the idea to her colleagues, they laugh at her and dismiss the idea as a silly game rather than real science.

In fact, Cheng’s idea is based on a real phenomenon known as nuclear thermal propulsion, sometimes called nuclear pulse propulsion. As it turns out, nuclear propulsion produces very little radiation if the engines using it are activated in space instead of on Earth — and the benefits include reduced energy use, reduced exposure to cosmic radiation, and speedier rockets. The Department of Energy even has a web page devoted to touting the benefits of nuclear propulsion.

Although the series presents Cheng’s domino effect idea as far-fetched, the US has a history of experimenting with nuclear thermal propulsion. As Horowitz explained, “Project Orion, early in the Cold War, tried to develop a rocket powered by small atomic bombs.”

However, if you’re wondering about all that radiation, you’re not alone. The first version of Project Orion was ultimately canceled because mid-century scientists were unable to solve the big problem: the near-certainty of deadly nuclear fallout that would result from any attempt to launch a nuclear-powered rocket into space from Earth.

A shame, really. “It would have been a very good rocket,” Horowitz said. Modern iterations of Orion have focused on launching similar rockets from within space and limiting astronauts’ exposure to radiation.

Is it really possible to expand something tiny and point-like, like a proton, into multiple massive dimensions in space?

Perhaps the most difficult aspect of 3 Body to conceptualize involves exactly what the Trisolarans do to the aforementioned proton before they shoot it off into space: They unfold its multiple dimensions into a massive, planet-sized amount of space, inscribe a giant super-computer onto its planes, and then re-fold it back into its original microscopic size.

This is a difficult feat to imagine, much less conceive in reality. Yet this practice exists, at least in theory, as an idea of multidimensional unfolding. Imagine this the way you might imagine creating a simple paper fortune-teller. The paper shape starts out almost fully flat, on a single plane — but it can be uncompressed to reveal more and more layers, until you have a neat schoolyard divination tool.

Now imagine this happening on a grand scale, and with even more dimensions than the three we experience here on Earth. There are multiple processes for how to do it, and multiple ways to try to illustrate what examples might look like in reality. The most famous example is an object that mathematicians and physicists call a hypercube or a tesseract (no, not that one) — a cube equivalent that exists in at least four dimensions. Here’s one attempt to imagine what one might look like:

Humans have devoted considerable time to trying to capture the essence of this; one famous early work of science fiction, Flatland, was published in 1884 by Edwin Abbott Abbot as a satirical attempt to introduce Victorians to the whole idea of higher dimensions by positing the existence of a society of people who existed in two planes only. Today, we can find equivalent thought experiments in places like YouTube:

Of course, none of this fully explains whether it would be possible to unfold a proton into the size of a planet and then inscribe a super-computer onto it. When I asked Horowitz about this, he replied with “??”

And honestly, that might be a fair way to respond to many of the scientific ideas we find in Liu’s expansive series. Ultimately, it’s built less on what’s real, and what we definitely know, than what’s possible given the incredible advances we’ve made in theoretical physics — emphasis on theory.

In other words, 3 Body collides science and fiction like two protons. The result is a wild, unique ride that’s worth suspending a little disbelief.

Bonus: If I dehydrate myself, can I rehydrate myself later and be fine?

No. Do not try this trick at home. Thankfully, some parts of 3 Body remain purely in the realm of the fantastic.