Because of the speed of light. The known universe is about sixteen billion light-years across, and it’s still expanding. But the speed of light is only three hundred thousand kilometers per second, a snail’s pace. This means that light can never go from one end of the universe to the other. Since nothing can move faster than the speed of light, it follows that no information and motive force can go from one end of the universe to the other. If the universe were a person, his neural signals couldn’t cover his entire body; his brain would not know of the existence of his limbs, and his limbs would not know of the existence of the brain. Isn’t that paraplegia? The image in my mind is even worse: The universe is but a corpse puffing up.” “Interesting, Dr. Guan, very interesting!” “Other than the speed of light, three hundred thousand kilometers per second, there’s another three-based symptom.” “What do you mean?” “The three dimensions. In string theory, excepting time, the universe has ten dimensions. But only three are accessible at the macroscopic scale, and those three form our world. All the others are folded up in the quantum realm.

Let’s turn the kindness we show toward the stars to members of the human race on Earth and build up the trust and understanding between the different peoples and civilizations that make up humanity. But for the universe outside the solar system, we should be ever vigilant, and be ready to attribute the worst of intentions to any Others that might exist in space. For a fragile civilization like ours, this is without a doubt the most responsible path.

It was dark by the time Ye got off work on the eve of Chinese New Year, 1980.