How far is Trisolaris from Earth?
About four light-years. In the Three-Body Problem trilogy, the Trisolaran homeworld orbits within the Alpha Centauri system — the closest stellar neighbor to our own Sun, roughly 4.2 to 4.4 light-years away.
That choice of location is deliberate. Alpha Centauri is a real triple-star system made of Alpha Centauri A, Alpha Centauri B, and Proxima Centauri. It is close enough that humanity's radio telescopes and ships can plausibly reach it inside the story, yet far enough that every crossing has to be measured in years of light-travel rather than kilometers. The signal Ye Wenjie answered, and the warning she received back, both traveled that same four-light-year gap. The chaos of the three-body problem and its real physics is what makes that nearby system uninhabitable in the first place — three suns whose gravity makes any planet's orbit impossible to predict.
How long does it take to travel four light-years in the story?
That is the real trick: the same distance is crossed three separate times, and each crossing takes a wildly different amount of time. Liu Cixin uses one fixed distance and three different speeds, and the gap between them is the physical engine of the whole crisis.
How do the sophons reach Earth so fast?
The sophons cross four light-years in just over four years — almost at the speed of light itself. A sophon is a proton that has been unfolded into lower dimensions, etched with circuitry, and refolded. Because its mass is effectively nothing, it can be accelerated to near light-speed, so its journey time is essentially light's own travel time.
That is why the first book ends with humanity's physics already locked. The first blow from Trisolaris is not a fleet — it is information, arriving years ahead of anything else. But a near-massless proton can only watch, jam, and relay; it cannot destroy. What the sophons can and cannot actually do is bounded by hard physics, and that boundary becomes the entire space humanity has left to resist.
Why does the droplet probe take two centuries?
The droplet — the mirror-smooth probe made of strong-interaction material — is far slower than a sophon, so it spends close to two hundred years covering the same four light-years. It reaches the solar system during the Deterrence Era and, in the Doomsday Battle, punches through humanity's two-thousand-ship fleet in minutes. Four years for the sophon, two centuries for the droplet, across an identical distance: the gap itself proves a brutal fact, that pushing real mass toward light-speed gets harder the faster you go. The reason the droplet tears through an entire fleet so easily comes down to material strength and raw kinetic energy.
How long does the main Trisolaran fleet take to arrive?
The main invasion fleet — a thousand ships moving at roughly one percent of light-speed — needs about four hundred years. Four light-years divided by one percent of light-speed works out to almost exactly four centuries.
That is why the second book opens not with imminent war but with a war scheduled four and a half centuries out. The timescale is what forces the Wallfacer Project, hibernation technology, and generations preparing for a battle they will never live to see. Why dark forest deterrence works at all depends entirely on that window — the fleet is still in transit, and can be destroyed the instant Earth broadcasts its coordinates.
Line the three journeys up and the distance never changes. It is always four light-years. Only the speed changes, and speed decides everything: the sophons arrive first as information, the droplet arrives mid-way as violence, and the fleet arrives last as time. One distance, split into three timescales — that is the whole game Liu Cixin hides inside the phrase "four light-years."