“Tell me about that approach.”

“The key information is the relative position of stars. If you specify a region of space in the Milky Way that contains a sufficient number of stars—perhaps a few dozen would be sufficient—their relative arrangement in three-dimensional space would be totally unique, like a fingerprint.”

“I’m starting to understand. We send out a message containing the position of the star we wish to point out, relative to the surrounding stars, and the recipient compares the data to its star map to determine the star’s location.”

“Right. But things aren’t that simple. The recipient must possess a three-dimensional model of the entire galaxy that precisely indicates the relative position of every one of a hundred billion stars. Then, after receiving our message, they would have to search through that enormous database to find an area of space that matches the pattern of positions we sent out.”

“No, it’s not simple at all. It’s like recording the relative position of every grain of sand in the desert.”

“Even harder than that. The Milky Way, unlike the desert, is in motion, and the relative positions of its stars are constantly changing. The later the position information is received, the greater the error caused by these changes. This means the database has to be able to predict the changes in position of each of those hundred billion stars. In theory, it’s not a problem, but to actually do it … God…”

“Would it be hard for us to send that positional information?”

“No, because we would only need to have a position pattern for a limited number of stars. And now that I’ve had time to think about it, given the average stellar density of the outer arm of the galaxy, a position pattern with no more than thirty stars should be sufficient. That’s a small amount of information.”

“Good. Now I’ll ask a third question: Outside the Solar System, there are other stars with planets. You’ve discovered several hundred, right?”

“More than a thousand to date.”

“And the closest to the sun?”

“244J2E1, sixteen light-years from the sun.”

“As I remember it, the serial numbers are set like this: the prefix digits represent the order of discovery; the letters J, E, and X stand for Jupiter-type planets, Earth-type planets, and other planets, respectively; and the digits following the letter indicate the number of that type of planet in the system.”

“That’s right. 244J2E1 is a star with three planets, two of them Jupiter-type and one Earth-type.”

Luo Ji thought for a moment, then shook his head. “That’s too close. How about a little farther, like … around fifty light-years?”

“187J3X1, 49.5 light-years from the sun.”

“That one’s fine. Can you draw up a position pattern for that star?”

“Of course I can.”

“How long would it take? Would you need help?”