“Well, yes—”

“So we need to know more, before deciding anything.”

She finally let her anger out. “Nonsense! This is a threat! People need to know.” She spread her hands, beseeching him.

“Go and think some more. You are following the right path.” With a wave, he dismissed her.

9.

Catkejen came in from a date, all fancied out in a maroon, bioweb Norfolk jacket with fluorescent yellow spirals down the arms, and found Rachel calculating some ideas. “Actual penciling out! Pushing graphite! You should get outside sometime, y’know.”

Feeling every inch a pedant, Rachel rose, stretched. “I was backtracking those red stars that had hunkered down.”

“You mean the ones that prob’ly knocked out our probes?”

“Yes, plus ones we’ve seen from the 550 AU telescopes. Those that had ruins on them.”

“So you’re running backward their orbits around the galaxy?” A disbelieving frown.

“Yes, it’s a tough many-body problem—”

“Hey, another example of cross-field confusion. We already have that!”

And this was how Rachel learned that astronomers had developed a reverse-history code of extraordinary ability. They had first evolved it to study galactic stellar evolution of spiral arms. Which led to her next audience with the Prefect.

10.

She walked—no, she decided, she skipped with schoolgirl joy in the low grav—out of the Advanced Computational Dome, feeling as if she had returned from a great distance.

She blew past the Prefect’s office staff and marched straight in on the great man, who was staring at a screen. He looked up, not showing any surprise. “You have more.” Not a question.

She flipped on her personal Artilect interface so it projected an image on the office 3-D display. “This shows the dwarf stars our probes and the 550 AU ’scopes found to be defensive or destroyed. No particular correlation between their locations, notice.”

He merely nodded. She had tagged the forty-three cases in bright green. They were scattered through a volume more than a thousand light-years on a side—still a mere bubble in the colossal galactic disk.

“Now let’s run the galaxy backward.”

The green dots arced through their long ellipses. The slow spin of the galaxy itself emerged as the bee swarms of stars glided in stately measure. The sun took a quarter of a billion years to cycle in its slow orbit at about two hundred kilometers a second, taking more than a thousand years to move a light-year. Humanity’s duration was less than a thousandth of one galactic cycle. From SETI messages marking funeral-pyre societies, the Librarians knew that humans were mayflies among sentient cultures, the newest kids on the block.

The Prefect watched the backward-running swarm and raised his eyebrows as the green dots slowly drew nearer each other.

“They follow somewhat different orbits, bobbing up and down in the galactic plane, brushing by nearby stars, suffering small tilts in their courses,” she said, as though this weren’t obvious. Was she making too much of this? She told herself a sharp no.

“I can see some, well, clumps of several green specks forming,” the Prefect said. “They seem to be”—surprise pitched his voice into a tenor note—“occasionally passing within a few light-years of each other. There! And now…” A pause as four dots swooped together. “Another cluster.”