Universe

The core theory of cosmic sociology and the trilogy's most influential original concept. Based on two axioms -- survival is the primary need of civilization, and civilizations grow but total matter remains constant -- combined with chains of suspicion and technological explosion, it concludes that every civilization must destroy any other civilization it discovers. This theory provides a chilling answer to the Fermi Paradox and has had profound influence on real-world METI debates.

A discipline whose framework was proposed by Ye Wenjie and ultimately established by Luo Ji. Based on two self-evident axioms — survival is the primary need of civilization, and civilizations grow while the universe's total matter remains constant — combined with the chain of suspicion and technological explosion, it derives the Dark Forest theory as the ultimate law governing relations between cosmic civilizations.

A cosmic-scale safety declaration strategy. By extensively using curvature drives to lower the speed of light around a star system until it falls below escape velocity, a black-hole-like effect is created. Civilizations within a dark domain cannot engage in interstellar travel, thereby proving to the universe that they pose no threat and avoiding dark forest strikes.

A political ideology and social movement that emerged during the Crisis Era. Its core argument was that humanity could never defeat the Trisolaran fleet in direct confrontation, and the only way to preserve the spark of civilization was to build interstellar ships and flee the Solar System. Escapism was declared illegal and treasonous by world governments, because the limited capacity of starships meant only a tiny fraction of humanity could leave — inevitably triggering the ultimate moral crisis of 'who deserves to survive.' Yet the catastrophic defeat at Doomsday Battle ultimately vindicated the Escapists — it was the fleeing ships that carried humanity's legacy into the stars.

A broadcast made by cosmic civilizations to prove their harmlessness and avoid dark forest strikes. Safety notices can take various forms, the most extreme being the dark domain — permanently sealing a civilization within its star system by lowering the speed of light below escape velocity, physically proving its inability to expand outward. The safety notice reveals a third survival strategy for cosmic civilizations: neither offense nor defense, but self-imposed limitation in exchange for peace.

One of the core axioms of cosmic sociology. Due to the vast distances between civilizations, communication delays, and the unpredictability of technological explosions, no two civilizations can truly establish trust. Even if both parties are benevolent, neither can confirm the other's benevolence, nor confirm that the other has confirmed their own benevolence — this suspicion extends infinitely, forming an unbreakable chain that ultimately leads to the Dark Forest state.

One of the core concepts of cosmic sociology. It refers to the possibility that a civilization can achieve a technological leap in a cosmically brief period, jumping from a primitive state to an interstellar civilization. The unpredictability of technological explosions makes reliable threat assessment of other civilizations impossible, and together with the Chain of Suspicion, forms the theoretical foundation of the Dark Forest Law. Human civilization went from an agricultural society to nuclear weapons in less than two hundred years — barely an instant on cosmic timescales.

A time classification system unique to the Trisolaran world. Because the motion of the three suns in the Trisolaran system follows chaotic dynamics and is unpredictable, the Trisolaran planet experiences irregularly alternating Stable Eras (stable climate, suitable for survival) and Chaotic Eras (extreme climate, civilization faces destruction). This unpredictable environmental cycle shaped everything about Trisolaran civilization — the biological mechanism of dehydration for survival, the scorching cultural psychology, and the relentless pursuit of a stable homeworld.

One of the most fundamental biological characteristics of Trisolarans: their thoughts are completely transparent and visible to others of their kind, making it impossible to conceal any thought. Deception, lies, and conspiracy do not exist as concepts in Trisolaran civilization. This trait explains why Trisolarans cannot comprehend the Wallfacer Project (since Wallfacers' thoughts are invisible to Sophons) and why Trisolaran civilization, despite its vastly superior technology, has a fatal blind spot in strategic competition.

Trisolaran civilization used Sophons to interfere with the cosmic microwave background radiation, causing observable flickering. This event was an important demonstration of Trisolaran power to Earth's scientists, designed to psychologically undermine their confidence in fundamental physics. When the entire universe's background radiation appeared to 'blink' for humanity, the sense of insignificance and terror in the face of a transcendent power reached its peak.

The theoretical bedrock of the Dark Forest theory — two deceptively simple axioms that yield the universe's darkest truth. Ye Wenjie passed them to Luo Ji at Yang Dong's grave, and it took Luo Ji two full years to derive the ultimate law governing relations between cosmic civilizations. These two axioms, combined with the chain of suspicion and technological explosion, form the complete logical chain of cosmic sociology and reveal a chilling answer to the Fermi Paradox.

The ultimate tragedy of a universe falling from ten dimensions to three. In the finale of Death's End, Guan Yifan reveals to Cheng Xin the universe's ultimate secret: the universe was born with ten macroscopic dimensions, but through eons of Dark Forest warfare, civilizations weaponized dimensions themselves, causing the universe to collapse again and again. The two-dimensional foil is merely the latest blow in this cosmic-scale dimensional war. What was once a ten-dimensional paradise is now a three-dimensional ruin — and the decay continues.

A superweapon created by the Trisolaran civilization and one of the trilogy's most ingenious sci-fi concepts. A proton is unfolded from higher dimensions into a two-dimensional plane, its surface etched with super-integrated circuits, then folded back to microscopic scale, creating an intelligent particle. Traveling at light speed, the Sophon is used to block Earth's fundamental science and monitor human activity in real time, directly catalyzing the Wallfacer Project. Its creation process includes dramatic scenes of intelligent civilizations within the micro-universe fighting back.

A strong interaction force probe created by the Trisolaran civilization, shaped like a perfectly smooth teardrop. Its surface is made of strong-force material, virtually indestructible and capable of perfectly reflecting all electromagnetic waves. In the Doomsday Battle, a single Droplet destroyed over 2,000 human starships in thirty minutes, shattering humanity's space military power.

A dimensional-reduction weapon used in dark forest strikes. The Dimensional Foil compresses three-dimensional space into a two-dimensional plane. Cast toward the Solar System by the Singer civilization, it caused the entire Solar System to be flattened into two dimensions — one of the cleanest strike methods in the universe, leaving no usable remnants.

Technology that achieves lightspeed travel by altering the curvature of space around a ship. A curvature-drive vessel does not move through space — space itself flows to carry the ship forward. However, the drive leaves a permanent trail, lowering the speed of light in its wake, exposing a civilization.

Independent miniature universes created by super-civilizations. Pocket universes possess their own physical laws and time flow rates, serving as the ultimate refuge for civilizations. As the greater universe approaches heat death, civilizations transfer themselves into pocket universes to survive eternally, but the mass drained from the main universe may prevent it from completing its collapse and rebirth cycle.

A neuroscience technology developed by Wallfacer Bill Hines that can implant unalterable, unquestionable convictions into the human brain. Originally intended to instill the belief that 'humanity will prevail' to boost military morale, the content was secretly altered by his Wall-Breaker wife Keiko Yamasuki to 'humanity will lose,' creating the 'Stamp Clan' — a defeatist faction within the Space Force that triggered a profound crisis of faith in human civilization.

A virtual reality game used by the Earth-Trisolaris Organization (ETO) to recruit members. Players experience the Trisolaran civilization's struggle for survival under chaotic stellar orbits, with great historical figures like Newton, Von Neumann, and Qin Shi Huang appearing as characters attempting to solve the three-body problem. The game serves as both a screening tool for potential ETO members and a microcosm of Trisolaran civilization's cross-generational attempts to predict their chaotic orbits. It is also the central narrative thread driving the plot of the first novel.

A universal broadcast system built by humanity, capable of transmitting stellar coordinates to the entire universe via gravitational waves. The gravitational wave antenna is the core device of the Dark Forest deterrence system — since gravitational waves cannot be blocked or intercepted by Sophons, it became humanity's only effective ultimate deterrent against Trisolaran civilization. The Swordholder can initiate the broadcast by pressing a button, simultaneously exposing the coordinates of both the Trisolaran system and the Solar System to every civilization in the universe, achieving the ultimate threat of mutual assured destruction. When Cheng Xin took over as Swordholder and failed to activate the broadcast in time, the historic transmission was ultimately completed by Blue Space and Gravity from deep space.

A macro-quantum state weapon originating from Liu Cixin's prequel novel of the same name. Ball lightning is essentially a macro-electron in an excited quantum state — a macroscopic quantum object capable of selectively destroying specific materials while leaving others untouched. In the Three-Body trilogy, Tyler (one of the Wallfacers) attempted to use ball lightning to quantize Earth's military forces into 'ghost soldiers' in quantum superposition — both dead and alive — to be deployed as an ultimate weapon against the Trisolaran invasion.

A particle-scale strike weapon traveling at light speed, one of the methods of Dark Forest strikes. A photoid is essentially a tiny mass accelerated to light speed that, upon striking a star, triggers chain reactions within it, causing the star to explode and be destroyed in an extremely short time. In Death's End, the Trisolaran star system (Alpha Centauri) is destroyed by a photoid — the three suns are destroyed in a chain explosion, and the Trisolaran civilization's home system is completely annihilated.

The core strategic weapon of Wallfacer Manuel Rey Diaz. Rey Diaz led the development of unprecedented stellar-class hydrogen bombs and planned to build a chain reaction system of tens of thousands of giant hydrogen bombs within the Solar System, designed to push Mercury into the Sun and trigger a solar chain nuclear reaction — a mutually assured destruction plan that would leave the Trisolaran fleet arriving to nothing but ruins.

A human cryogenic hibernation technology that runs throughout the entire Three-Body trilogy. By reducing body temperature to extremely low levels and maintaining a special state of suspended animation, hibernation technology allows humans to traverse decades or even centuries of time. Key characters including Luo Ji, Zhang Beihai, and Cheng Xin all use hibernation to travel between different eras, witnessing the rise and fall of human civilization. Hibernation technology is one of the trilogy's most important narrative devices.

A transportation system connecting Earth's surface to space, built using carbon nanotube materials. Space elevators were constructed after the Great Trough, enabling humanity to transport large numbers of people and materials to space at extremely low cost. They served as critical infrastructure for humanity's space fleet construction and large-scale space migration. The completion of space elevators marked humanity's transformation from a planetary civilization to a space civilization.

The Sun's function as an electromagnetic wave amplifier, discovered by Red Coast Base. Ye Wenjie discovered that the Sun can amplify incoming electromagnetic signals by hundreds of millions of times before broadcasting them into the universe. She used this discovery to send the first message from Earth into the cosmos and received a reply from the Trisolaran civilization, thereby initiating the history of contact between Earth and Trisolaris. Solar amplification is the starting point of the entire Three-Body story.

A survival mechanism unique to Trisolarans. Facing the extreme environments of Chaotic Eras, Trisolarans can completely expel water from their bodies, transforming into dried fibrous matter and entering a hibernation-like state. In this dehydrated state, Trisolarans can withstand extreme heat and cold, surviving for decades or longer. When a Stable Era arrives, immersing the dehydrated body in water restores life activity. This mechanism is key to Trisolaran civilization's survival in extreme environments.

One of the most stunning scenes in the Three-Body game. Qin Shi Huang, attempting to predict the motion of the three suns, mobilized thirty million soldiers to form a giant human-powered computer. Each soldier served as a logic gate (AND, OR, NOT), raising and lowering flags to represent binary signals of 0 and 1. The entire army was arrayed across a vast plain, forming the largest 'computer' ever built — yet it still could not solve the three-body problem.

An ultra-strong nanomaterial developed by Wang Miao — an extremely fine nanowire that is nearly invisible but capable of cutting through any known material. Flying Blade played a decisive role in Operation Guzheng — strung parallel across the surface of the Panama Canal, when the ETO's ship 'Judgment Day' passed through, the nanowires sliced the entire vessel and everything aboard into dozens of thin sections like cutting tofu, successfully intercepting Trisolaran communication data.

One of humanity's stellar-class warships and a surviving vessel from the post-Doomsday Battle exodus. Blue Space survived the Dark Battle, ventured into deep space where it encountered a four-dimensional space fragment, and ultimately joined Gravity in transmitting the gravitational wave broadcast that exposed both the Trisolaran system and the Solar System to the entire universe. Carrying forward the escapist philosophy championed by Zhang Beihai, the ship became humanity's 'second seed' beyond the Solar System and a pivotal agent in the turning point of human fate after the collapse of Dark Forest deterrence.

One of the surviving stellar-class warships from the Doomsday Battle and a participant and victor of the Dark Battle. The Bronze Age attacked and destroyed other human ships to seize survival resources, and its crew established an entirely new social order in deep space based on absolute survival priority. When the Bronze Age eventually returned to the Solar System, its entire crew was tried for murder — sparking the most profound moral debate in human history: was the abandonment of civilized principles under extreme conditions an unforgivable crime, or an inevitable choice of the survival instinct? The Bronze Age is essentially a micro-scale Dark Forest experiment.

The only physical space where humans could achieve true information security under total Sophon surveillance. Built from strong-interaction materials that block Sophon penetration, these shielded rooms allowed genuinely private conversations and strategic planning — the last physical refuge of free thought in an age of total transparency.

The most historically significant of humanity's stellar-class warships — carrier of the gravitational wave broadcast system and the secondary failsafe of the Dark Forest deterrence system beyond the Swordholder. Gravity was equipped with humanity's only gravitational wave transmitter, theoretically capable of broadcasting any star system's coordinates to the universe. In the late Deterrence Era, Gravity pursued the fleeing Blue Space, but after a transformative encounter with a four-dimensional space fragment, the two ships together executed the gravitational wave broadcast that sealed the Solar System's fate.

A colossal ring-shaped particle accelerator orbiting the Sun, built by Star Ring Corporation under Thomas Wade's leadership. This was the critical infrastructure for humanity's curvature drive (lightspeed ship) research, with a circumference comparable to Earth's orbital path. The circumsolar accelerator represented humanity's highest achievement in fundamental physics, yet it sparked immense political controversy — lightspeed ships were seen as tools of 'escapism.' Cheng Xin's fateful decision to halt Wade's project indirectly left the Solar System without sufficient means of escape when the dimensional strike came.

The highest class of military spacecraft built by humanity to counter the Trisolaran invasion. Powered by nuclear fusion and capable of interstellar travel, stellar-class warships formed the backbone of humanity's three space fleets. Approximately 2,000 vessels were constructed, nearly all destroyed by the Droplet in the Doomsday Battle. The handful of survivors — Natural Selection, Blue Space, Gravity, and others — played pivotal roles in subsequent history. The rise and fall of the stellar-class warship fleet mirrors humanity's arc from confidence to despair, from unity to fragmentation.

The most fundamental survival technology of Trisolaran civilization. When Chaotic Eras arrive, Trisolarans dehydrate into dried fibrous forms to endure extreme conditions; when Stable Eras return, they rehydrate through immersion to restore life functions. This mechanism is not merely a biological marvel — it profoundly shaped the Trisolaran understanding of death, individuality, and collectivism, producing a civilization radically unlike humanity's.

The ultimate propulsion technology achieving lightspeed travel through curvature drive. The ship does not move through space — it warps space behind it, letting space itself carry the vessel forward. Yet lightspeed propulsion permanently lowers the speed of light along its trail, leaving indelible 'death lines.' This technology is simultaneously the only escape from dimensional strikes, a tool for creating black domain safety declarations, and the hope that Wade staked everything on — and that Cheng Xin twice killed with her own hands.

A stellar-class flagship warship of the Asian Fleet, hijacked by Zhang Beihai and driven into deep space. The Natural Selection was among humanity's most advanced warships. Under Zhang Beihai's 'Ahead Four' command, it broke formation, triggering a pursuit by four other vessels. This chase ultimately devolved into the darkest event in interstellar history — the Dark Battle. The Natural Selection's story vindicated Zhang Beihai's vision spanning two centuries: escape was humanity's only path to survival.

The Three-Body game is a virtual reality platform used by the ETO to recruit members, where players experience the cyclical rise and fall of Trisolaran civilization. Built around the alternation of Stable and Chaotic Eras, and featuring historical figures like Copernicus, Newton, and Von Neumann as characters, the game reveals the mathematical essence of the three-body problem. It functions simultaneously as a precise recruitment tool and a profound philosophical metaphor for civilizational survival.

A light tomb is a region where the speed of light has been reduced to zero, forming an absolute physical trap. Any matter or energy entering a light tomb can never escape — even light itself is frozen. In Death's End, New World Three is identified as a light tomb, representing the most extreme form of safety declaration under the Dark Forest doctrine — trading eternal stillness for absolute security.

The Four-Dimensional Fragment is a pocket of four-dimensional space that the Blue Space accidentally enters during its interstellar voyage. It is one of the most imaginative scenes in the entire trilogy — humanity's first direct experience of the grandeur and terror of higher-dimensional space. Within this fragment, the internal structures of three-dimensional objects are fully exposed, and the crew discovers a mysterious 'Ring,' gaining crucial insight into the true meaning of dimensional strikes.

DX3906 is the star system that Yun Tianming gifted to Cheng Xin, containing the planets Blue Star and Gray Star. This distant star system becomes the setting for the trilogy's epilogue — where Cheng Xin and Guan Yifan discover the entrance to Pocket Universe 647, witness the eve of universal reset, and make the final choice to return their mass to the grand universe.

A strategic plan created by the UN Planetary Defense Council. Exploiting the one blind spot of the Sophons — their inability to read human thoughts — four Wallfacers were granted nearly unlimited authority to devise secret strategies against the Trisolaran invasion. The four Wallfacers were Taylor, Rey Diaz, Hines, and Luo Ji.

A covert plan to send Yun Tianming's brain into the Trisolaran fleet. Using sequential nuclear detonations to propel a solar sail, the craft carrying only a brain was accelerated to one percent of light speed. After capture by the Trisolarans, Yun Tianming received a new body and became the only human embedded within Trisolaran civilization.

The person who holds the gravitational wave broadcast switch in the Dark Forest deterrence system. The Swordholder has the power to broadcast the Trisolaran star system's coordinates to the universe — pressing the button means both Trisolaris and Earth face destruction by other civilizations. The first Swordholder, Luo Ji, maintained deterrence for half a century; the second, Cheng Xin, failed to execute deterrence within minutes of assuming the role.

The decisive battle between humanity's space fleet and the Trisolaran probe known as the Droplet. Humanity assembled nearly 2,000 stellar-class warships to confidently intercept the Trisolaran fleet's advance probe, only to be almost entirely annihilated within thirty minutes. The Doomsday Battle is one of the most impactful turning points in the Three-Body series, completely shattering humanity's optimistic illusions about the Trisolaran crisis.

A defensive plan devised by humanity in the late Deterrence Era to survive a dark forest strike. The core idea was to build space cities behind the Solar System's gas giants — Jupiter, Saturn, Uranus, and Neptune — using them as shields against a photoid attack on the Sun. Humanity spent decades constructing numerous space cities and relocating most of the population, but the actual strike came as a dimensional foil rather than a photoid, rendering the Bunker Project utterly useless and exposing humanity's fatal limitation of thinking within known frameworks.

The brutal conflict among fleeing human warships after the Doomsday Battle, as they turned on each other to compete for limited resources. The Dark Battle is a microcosm verification of the Dark Forest theory within a single species — when survival becomes the sole imperative, the moral veneer of civilization is stripped away entirely. Human ships enacted the exact same chains of suspicion and technological explosion logic that governs relations between cosmic civilizations. It remains one of the most disturbing episodes in the entire Three-Body series, proving that the Dark Forest principle is not exclusive to alien civilizations but an inevitable fate of intelligent life under resource scarcity.

After the failure of Dark Forest deterrence, Trisolaran civilization forced all 4+ billion surviving humans to relocate to the Australian continent as a managed reservation zone. This represented the darkest period in human history — loss of dignity, severe food shortages, and living conditions resembling concentration camps. Trisolaran civilization administered the reservation through Sophon's humanoid avatar with cold politeness, compressing what was once a proud human civilization into a species reservation awaiting extinction. This experience profoundly revealed the fragility of civilization when stripped of technological advantage, and was ultimately ended by the gravitational wave broadcast from Blue Space and Gravity.

After being sent into the Trisolaran world, Yun Tianming uses his single permitted meeting with Cheng Xin to transmit critical strategic intelligence through three elaborately crafted fairy tales — 'The New Painter of the King,' 'The Glutton Sea,' and 'The Prince of the Deep Water.' These stories encode vital information about curvature drive (lightspeed ships), black domains (lightspeed black holes), and defense against dimensional strikes. Humanity successfully decodes some of the intelligence, but the most critical element — the 'safety declaration' (black domain solution) — is overlooked, an omission that indirectly seals the Solar System's fate.

The 54-year period of terrifying peace between humanity and Trisolaran civilization. As Swordholder, Luo Ji held the gravitational wave broadcast switch, maintaining a 'mutually assured destruction' deterrence balance with the Trisolaran world. During this half-century, the two civilizations experienced unprecedented cultural exchange and technological reciprocity, while human society basked in a false sense of security. When Cheng Xin replaced Luo Ji as the new Swordholder, the deterrence collapsed in just 15 minutes.

A catastrophic economic and social collapse in the early Crisis Era. When global resources were massively redirected toward space defense programs, Earth's economy buckled and disintegrated. Billions died from famine, war, and societal breakdown. The Great Ravine lasted nearly half a century and stands as one of the darkest periods in human history, yet it was this very catastrophe that catalyzed the subsequent technological explosion and civilizational renaissance.

The Human-Formation Computer is one of the most visually striking scenes in the Three-Body VR game. Qin Shi Huang mobilizes 30 million soldiers to form a giant computer on a vast plain, using flag signals in place of electronic signals to perform computations. Von Neumann designs the architecture, while Mozi and Aristotle provide the logic gate principles. This scene not only demonstrates the essence of computation but serves as a desperate metaphor for the Trisolaran civilization's approach of building civilization through sheer expenditure of life.

A Universal Safety Declaration is a survival strategy in which a civilization permanently seals itself off to prove to the entire universe that it poses no threat. The core logic: if a civilization is physically incapable of outward expansion, there is no motive to destroy it. This concept is hidden within Yun Tianming's three fairy tales and represents a more fundamental cosmic survival strategy than Dark Forest deterrence.

The PIA (Planetary Intelligence Agency) is the intelligence arm of the United Nations Planetary Defense Council, led by Thomas Wade. The PIA was responsible for executing the Staircase Program — the audacious intelligence operation to launch Yun Tianming's brain toward the Trisolaran fleet. Known for its ruthlessly pragmatic operational style, the PIA was one of the most critical forces on humanity's covert front against the Trisolaran invasion.

A clandestine organization founded (or indirectly enabled) by Ye Wenjie, dedicated to assisting the Trisolaran civilization's invasion of Earth. It contains three factions: the Adventists led by Evans, who seek humanity's total annihilation; the Redemptionists, who hope Trisolaran civilization will reform human society; and the Survivalists, who aim to secure survival privileges for themselves after the invasion.

An advanced cosmic civilization appearing in Death's End. The 'Singer' is a low-ranking worker responsible for 'cleansing' — monitoring star systems that have exposed their coordinates and hurling cleansing tools at them. It was the Singer who cast a dimensional foil (二向箔) at the Solar System, triggering the dimensional reduction that collapsed it from three dimensions to two. The Singer's perspective transforms the Dark Forest theory from abstract principle into visceral, horrifying reality.

A mysterious cosmic-scale force and movement appearing at the end of Death's End. The Returners believe that the universe has been severely damaged by wars between civilizations -- dimensions have been continuously reduced, physical constants tampered with, and the speed of light limited. The once beautiful ten-dimensional garden with infinite light speed has been reduced to a broken three-dimensional world. They broadcast to the entire universe, demanding that all civilizations sheltering in pocket universes return the mass taken from the main universe, so it can collapse and restart from a new Big Bang rather than expand forever into heat death. The Returners represent the trilogy's ultimate philosophical proposition: wars between civilizations destroy not only each other, but the universe itself.

A military facility located in the Greater Khingan Mountains of Inner Mongolia, China, established during the Cultural Revolution era, originally designed for military communications and radar research. Ye Wenjie used the base's equipment and the sun's amplification effect to send humanity's first interstellar message into the cosmos, and received the warning reply from Trisolaris — 'Do not answer! Do not answer! Do not answer!' — yet she chose to respond anyway. Red Coast Base is the origin point of the entire Trisolaran crisis, a place that altered the fate of two civilizations. The base was later decommissioned and demolished, its ruins becoming a spiritual pilgrimage site within the Three-Body story.

An elite organization in The Three-Body Problem that ostensibly promotes interdisciplinary academic exchange but secretly serves as a front for the Earth-Trisolaris Organization (ETO) to infiltrate and recruit top scientists. Frontiers of Science gathered the most brilliant scientists from around the world, building relationships through academic discussions and social events, screening for potential members who harbored doubts or despair about human civilization and channeling them into the ETO. Wang Miao entered the Frontiers of Science circle through his investigation of the scientist suicide wave, gradually uncovering the vast network the Trisolaran organization had hidden within the scientific community.

A massive oil tanker purchased and converted by Mike Evans, serving as the Earth-Trisolaris Organization's (ETO) maritime headquarters and core communication facility with the Trisolaran civilization. Judgment Day was equipped with communication equipment for contacting the Trisolaran world, making it the only channel on Earth capable of direct information exchange with Trisolaris (the 'Second Red Coast'). It was ultimately destroyed in Operation Guzheng by nano-material cutting, and its stored Trisolaran communication records were captured.

An aerospace technology corporation co-founded by Cheng Xin and Ai AA, primarily engaged in research and development of curvature drive engines. Star Ring Corporation became one of humanity's most important research institutions during the Bunker Era, dedicated to achieving light-speed travel technology. However, due to societal fears that light-speed ships could trigger escapism, Star Ring's research was heavily restricted. The corporation ultimately succeeded in developing the light-speed ship 'Star Ring,' but could not save the Solar System's fate.

People who received the Mental Seal with the unshakable conviction 'humanity will lose' implanted in their brains. Originally Space Force personnel, they became defeatists after undergoing the Mental Seal technology developed by Wallfacer Bill Hines. The emergence of the Stamp Clan was one of the greatest 'side effects' of the Wallfacer Project, creating a profound internal crisis during humanity's confrontation with Trisolaran civilization and undermining the will to resist.

The highest planetary defense coordination body established by the United Nations after the Trisolaran Crisis. Responsible for coordinating global military, technological, and strategic resources, the PDC oversaw the Wallfacer Project, Staircase Program, fleet construction, and other key defense initiatives throughout the Crisis Era and Deterrence Era.

The unified space military force established by humanity to counter the Trisolaran crisis. Comprising three major fleets — the Asian Fleet, European Fleet, and North American Fleet — with nearly 2,000 stellar-class warships, the Space Force marked humanity's pivotal transition from a planetary to an interstellar civilization. Yet its near-total annihilation in the Doomsday Battle exposed humanity's fatal vulnerability against a higher-dimensional civilization. The rise and fall of the Space Force encapsulates humanity's struggle for survival in the cosmos.

The Cultural Revolution is not merely the starting point of Three-Body's story but the emotional and philosophical undertone of the entire trilogy. Ye Wenjie witnessed her father Ye Zhetai beaten to death by Red Guards at a denunciation rally, her mother Shao Lin's betrayal, and her sister Ye Wenxue's fanaticism — traumas that directly shaped her despair toward human civilization and led her to invite the Trisolaran world to Earth. Through the Cultural Revolution, Liu Cixin anchors his grand cosmic narrative in the real abyss of human nature.

Not all individuals within Trisolaran civilization supported the invasion of Earth. Listener 1379 represents the peace faction of the Trisolaran world — after intercepting Ye Wenjie's signal, he sent the warning 'Do not answer!' The pacifists believed that different civilizations could coexist peacefully, but they were a tiny minority in Trisolaran society, suppressed by the dominant faction. Ye Wenjie's decision to ignore the warning and reply ultimately sealed the tragic fate of both civilizations.

The Earth-Trisolaris Organization (ETO) was far from a unified ideological body. It comprised three major factions: the Adventists (led by Mike Evans, advocating human extinction), the Redemptionists (hoping Trisolaran civilization would guide human evolution), and the Survivors (serving Trisolaris for personal survival). The ideological conflicts and power struggles among the three factions, along with the devastating Operation Guzheng that dismantled the ETO, form the most compelling underground organization narrative in The Three-Body Problem.

Descendants of the Blue Space and Gravity crews who established a new civilization in deep space, becoming a fundamentally different branch of humanity unbound from the Solar System. Their encounter with Cheng Xin on the 'Blue Planet,' represented by figures like Guan Yifan, revealed a cosmic worldview radically different from Earth-bound thinking. Galactic Humans represent humanity's extension on a universal scale and Liu Cixin's most optimistic vision of what humanity could become.

From early chemical rockets to stellar-class warships, humanity's space fleet evolved over centuries. Facing the existential threat of Trisolaran invasion, the United Nations established the Planetary Defense Council, and three great fleets — Asian, European, and North American — built a massive space military force across the Solar System. Yet the catastrophic Doomsday Battle proved this fleet's fragility, while the fugitive ships ultimately became arks carrying the seeds of human civilization to the stars.

After the collapse of Dark Forest deterrence, Trisolaran civilization imposed total occupation on Earth. The Sophon android announced the forced relocation of humanity to an Australian reservation and established the 'Earth Security Force' — composed of human collaborators — to maintain order. Meanwhile, a human resistance movement led by figures like Shi Qiang launched a secret struggle amid desperation. This dark period profoundly reveals the fracturing and resilience of human nature under extreme oppression.