A lot of readers finish The Three-Body Problem with one very concrete question: if the Trisolarans want to wipe out Earth, why not just attack right away? Why make humanity wait for centuries?
The answer hides in one number — four light-years — and one colder fact of physics: in this universe, no civilization, however advanced, can outrun light.
How long does the Trisolaran fleet take to reach Earth?
Roughly four centuries. Trisolaris orbits in the Alpha Centauri system, about four light-years from the Solar System, and the Trisolaran First Fleet travels at around one percent of the speed of light. Divide the distance by the speed and you get the countdown that runs through the entire second book: about 400 years.
That is the real origin of the "Crisis Era." It is not Trisolaran mercy that gives humanity time to prepare. It is the speed of light. Liu Cixin picked the nearest real star system rather than an invented distance, and he made the fleet take the full four centuries instead of hand-waving the trip away. That fidelity is part of what separates the trilogy from ordinary space opera.
Is one percent of light speed actually slow?
No — it is staggeringly fast by any real-world standard. The phrasing "only one percent of light speed" is misleading. Kinetic energy scales with the square of velocity, so accelerating a warship to one percent of light speed already demands an astronomical amount of energy. And the fleet has to decelerate at the other end, or it would simply slam into the Solar System. The hard part is not flying across — it is flying across and still being able to stop.
This is where Liu Cixin hands hard-SF readers a gift: he refuses to paper over the cost of interstellar travel with a magic warp drive. The Trisolaran ships burn and push and grind their way across for centuries. Only very late in the story does humanity even reach the threshold of curvature-drive travel near light speed, and that respect for the real limits of physics is the backbone of the whole series.
Why does the droplet arrive before the main fleet?
Because the droplet is not the main fleet. The water-drop probe is a small craft sent ahead at far higher speed, so it reaches the Solar System roughly two centuries early — well before the main fleet. It is a single droplet that annihilates humanity's entire combined space fleet at the Doomsday Battle, while the bulk of the Trisolaran armada is still grinding along behind it on the four-century timeline.
That layered design — fast scouts first, slow main force later — lets the Trisolarans break humanity's will to resist long before the real fleet arrives. It also explains why the human fleet is destroyed so completely: they were not facing an armada, just one advance probe.
Did the Trisolaran fleet ever actually reach Earth?
No — and that is the irony of the whole journey. The fleet that flew for centuries never carried out its planned occupation. Luo Ji established dark forest deterrence by broadcasting the coordinates of a star, freezing the invasion in place and forcing the armada to change course. Physics gave humanity 400 years, and humanity spent those years deducing the dark forest law, turning its one piece of leverage into a stalemate.
So "how long does the Trisolaran fleet take" looks like a physics question, but it is really the dramatic engine of the trilogy. Precisely because the speed of light is an unbreakable wall that no one can outrun, you get the four-century standoff, the game theory, and the last-ditch gamble that defines the entire story.