Immediately following the Battle of Darkness, Luo Ji proclaims that he knew it all along and that he holds the key to human victory. While Beijing descends into chaos, he travels a great distance away from the city, and waits for the droplet to kill him in the middle of the desert.
Luo Ji’s demise is not meant to be as the droplet suddenly changes course and starts an electromagnetic wave towards the sun, effectively blocking humanity from using it to send any more messages into deep space the way that Ye Wenjie had done centuries ago and how Luo Ji had sent his spell.
Upon returning to the city, Da Shi and Luo Ji see another mob has grown, but this one has a much different makeup, they are there in reverence for Luo Ji. His spell has worked and the distant star 187J3X1 has been destroyed by a small object travelling at near light speed called a ‘photoid.’ He is now worshiped as the savior of humanity and has his Wallfacer status restored by Hines and Ben Jonathan,
Da Shi is finally able to get Luo Ji to tell him the secret of the spell in confidence. The basis of the spell is rooted in the cosmic axioms imparted to him by Ye Wenjie all those years earlier (even though he doesn’t reveal it was her who told him.)
From those axioms he extrapolated the theory of the “chain of suspicion”, such that a civilization can be considered to be either benevolent or malicious, however, due to the nature of space and the inability to communicate, the true nature of distant civilizations cannot be determined. And even if it could, there’s no way to determine what that civilization considers you to be. This is coupled with the other principle of “technological explosion”, the fact that in the cosmic timescale, civilizations can make great progress in a short time.
The Battle of Darkness was a microcosm of these axioms, and among cosmic civilizations, like a hunter in a dark forest, if a society makes itself known to another, it is simply safer for that civilization to fire upon them rather than make themselves known to the other hunters in the dark forest or risk being found themselves.
Luo Ji used the sun to broadcast the coordinates of a distant star to the universe, and like he predicted, it was destroyed by another cosmic civilization not willing to take the chance and decided that destruction was the safer alternative.
Unable to awaken his family from hibernation and unable to send another “spell” since the droplet has blocked the sun, Luo Ji immerses himself in a relatively inconsequential project known as The Snow Project, which is to encircle the solar system in stellar bombs covered in oil from Neptune that will help detect the location of the 9 other droplets on the way to Earth.
After 3 years of working on the project, Luo Ji falls out of favor with the public and is once again considered to be a fraud. Disheveled, he packs a shovel and heads to the cemetery where Ye Wenjie and Yang Dong are buried to dig his own grave.
It is there where he has his final confrontation with the Trisolarans. Similar to Ray Diaz’s threat, he has rigged a dead man’s switch to himself that when triggered will signal the bombs encircling the solar system to detonate in a very specific pattern that will broadcast the coordinates of Trisolaris to the universe. Realizing that Luo Ji’s well-being is now critical to the survival of Trisolaris, a trio of sophons unfold and negotiate with him. They agree to divert the droplets, Trisolaran fleet, lift the sophon lock, and help humanity build gravitational wave antennas.
We end with Luo Ji and his family going to observe the gravitational antenna five years later where he once again encounters an unfolded sophon. This time it is the Trisolaran pacifist who sent the warning to Ye Wenjie all those years ago, and Luo Ji concludes that he hopes that one day a brilliant sunlight will illuminate the dark forest