How many dimensions does the Three-Body Problem universe actually have?
There is no single number. The Three-Body universe was born with ten macroscopic dimensions and has been collapsing ever since. Today it sits at three, the dual-vector foil is actively flattening parts of it into two, and isolated four-dimensional pockets still drift through space. The three dimensions we live in are not the universe's natural state in Liu Cixin's cosmology — they are the halfway point of a decay that has run for billions of years.
So the honest answer to "how many dimensions" is a falling curve, not a fixed value: ten at birth, three now, and dropping.
What were the ten dimensions, and where did they go?
Near the end of Death's End, Guan Yifan tells Cheng Xin the universe's coldest secret: it began with ten macroscopic dimensions — a "higher-dimensional Eden" whose freedom of movement, information, and energy we cannot really picture.
The dimensions vanished because they were used as weapons. In the endless Dark Forest war, civilizations learned that stripping a dimension away from an enemy's space is the cleanest, most irreversible attack possible. Dimension after dimension was spent as ammunition, and the universe collapsed from ten dimensions down to three. The solid three-dimensional space we trust is the rubble left behind. It even rhymes with real physics: string theory already predicts a universe of ten or eleven spatial dimensions, with the extra ones compactified beyond perception. Liu simply rewrote "compactification" as a war record.
What is the four-dimensional fragment?
It is the only higher-dimensional space humans ever experience directly. The starship Blue Space drifts into a pocket of four-dimensional space and finds a derelict "Ring" left by a dead higher civilization. From four dimensions, the insides of three-dimensional objects are fully exposed — you can see a sealed room's interior and a person's organs at once, and reach any point "from outside."
That scene is the reader's first real lesson in what a dimensional strike means: removing a dimension is not a gimmick, it is subtracting an entire degree of freedom from a region of space. If looking down from four dimensions is that overwhelming, being pushed from three into two is a death sentence.
What does the dual-vector foil do to three dimensions?
The dual-vector foil collapses three-dimensional space into a two-dimensional plane. This dimensional-reduction weapon needs no aiming and no energy duel — once released, the target's three-dimensional space falls spontaneously and irreversibly toward two. Nothing usable survives, which is exactly why a civilization that lives by the dark forest law treats it as an eraser for tidying up loose ends.
How does the Solar System end up two-dimensional?
In the trilogy's climax, the Singer civilization drops a dual-vector foil at the Solar System. The Sun, the planets, and the whole record of human civilization are folded into a two-dimensional plane — an infinitely detailed painting with no thickness left. Cheng Xin and AA escape at lightspeed at the last moment and watch their home system become a flat work of art. Destruction arrives as a masterpiece.
Two is not the floor either. The same logic points to one dimension, then zero, then heat death — unless all the matter hoarded in private mini-universes is returned so the great universe can reset to its ten-dimensional beginning. That return is the final choice the story leaves Cheng Xin holding, and the reader along with her.