David Brin - Existence

Now?

She and Gavin had made certain to beam a full scan of the wall to Earth, first thing, in case another FACR chose to intervene. Was this the reason for that earlier attack? In order to stop humanity from viewing the chronicle? If so, victory was now complete. The message-the warning-inscribed by little hands so long ago, was on its way.

But there won’t be any flash answers from back home. Not for hours, even days. For a little while, this is ours. And ours alone. A mystery, in the old, exciting and terrifying sense.

Tor had started out viewing the ancient colonists as unsophisticated. How could folk be capable if brewed in test tubes, decanted out of womb tanks, and raised by machines? Baked, modified, and prepared for a planet’s surface, they depended on the mammoth star mother for everything. Might as well view them as fetuses.

Yet clearly, they knew what was going on. And when lethal failure loomed, the creatures figured out a way to preserve one thing. For their story to be read long after all magnetic, optical, or superconducting records decayed. The biologicals found their enduring medium-in a wall of chiseled stone.

“Interpreting the writing will take experts and argument. We can only guess,” Gavin told her as he used a gas jet to blow dust from uneven rows of angular letters. “But with these pictograms to accompany the text, it might just be possible.”

Gavin’s voice was hushed, still adjusting to what they found here. A Rosetta Stone for an entire alien race? Maybe bunches of them.

“You could be right,” Tor commented. The little robot she had been supervising finished a multifrequency radar scan of the southern wall-checking for more layers behind the surface-and then rolled to one side, awaiting further instructions. Tor hopped up to sit cross-legged on another drone, which hummed beneath her patiently. In the feeble gravity Tor’s arms hung before her, like frames encompassing a picture-puzzle.

The creatures must have had time, while battles raged outside their catacombs, for the carvings were extensive, intricate, arrayed in neat rows and columns. Separated by narrow lines of peculiar chiseled text were depictions of suns, planets, and great machines.

And more machines. Above all, pictographs of mighty mechanisms covered the wall.

The first sequence appeared to begin at the lower left, where a two-dimensional starprobe could be seen entering a solar system-presumably this one-its planets’ orbits sketched in thin lines. Next to that initial frame was a portrayal of the same probe, taking hold of a likely planetoid, mining and manufacturing parts, preparing to make self-replicas.

Eight copies departed the system in the following frame. There were four symbols below the set of stylized child probes… Tor could read what must be the binary symbol for eight, and there were eight dots, as well. It didn’t take much imagination to tell that the remaining two symbols also stood for the same numeral.

The wall was meant for self-teaching how to read the rest. They weren’t dopes.

So, translation had begun. Apparently this type of probe was programmed to make eight copies of itself, and no more. It settled a nagging question that had bothered Tor for years. If sophisticated self-replicating probes had been roaming the galaxy for eons, why was there any dead matter left at all? In theory, an advanced enough technology might dismantle not only asteroids but planets and stars. If replicant probes had been simplemindedly voracious, they might gobble the whole galaxy! There’d be nothing left but clouds of uncountable starprobes… preying on each other till the pathological system fell into entropy death.

That fate had been avoided. This Mother Probe showed how. It was programmed to make only a strictly limited number of copies. This type of probe was so programmed, Tor reminded herself.

In the final frame of the first sequence, after the daughter probes had been dispatched to their destinations, the mothership was shown moving next to a round globe-a planet. A thin line linked probe and planet. A vaguely humanoid figure, resembling in caricature the mummies on the floor, stepped across the bridge to its new home.

The first story ended there. Perhaps this was a depiction of the way things were supposed to go. An ideal. Or the way it went for the probe’s own parent, an eon earlier.

But there were other sequences. Other versions of reality. In several, the Mother Probe arrived at this solar system to find others already here. Tor realized that one of these other depictions must represent what really happened, so long ago. But which one? She breathed shallowly while tracing out the next tale, where the Mother Probe arrived to meet predecessors … and all those earlier ones had little circular symbols next to them.

In this case everything proceeded as before. The Mother Probe made and cast out its replicas, and went on to seed a planet with duplicates of the ancient race that had sent out the first version, long ago.

“The little circle means those other probes are benign,” Tor muttered to herself.

Gavin stepped back and looked at the scene she pointed to. “What, the little symbol beside these machines?”

“It represents types that won’t interfere with this probe’s mission.”

Gavin was thoughtful for a moment. Then he reached out and touched a different row. “Then this crosslike symbol…?” He paused, examining the scene, and answered his own question. “It stands for types that would object.”

Tor nodded. That row showed the Mother Probe arriving once again, but this time amidst a crowd of quite different machines, each accompanied by a glyph like a crisscross tong sign. In that sequence the Mother Probe didn’t make replicates. Nor did she seed a planet. Her fuel used up, unable to flee the system, she found a place to hide behind the star, far from the others.

“She’s afraid of them.”

Tor expected Gavin to accuse her of anthropomorphizing, but her partner was silent, thoughtful. Finally, he nodded. “I think you’re right.”

He pointed. “Look how each of the little cross or circle symbols subtly vary.”

“Yeah,” she said, nodding and sitting forward on the gently humming drone. “Let’s assume there were two basic types of Von Neumann probes loose in the galaxy, when this drawing was made. Two contrary philosophies, perhaps. And within each camp there were differences, as well.”

She gestured to the far right end of the wall. That side featured a column of sketches, each depicting a different variety of machine, every one with its own cross or circle symbol. Next to each was a pictograph.

Some of the scenes were chilling.

Gavin shook his head, obviously wishing he could disbelieve. “But why ? Von Neumann probes are supposed to… to…”

“To what?” Tor asked softly, thoughtfully. “For years people assumed that other races would think like us. We figured they would send out probes to gather knowledge, or maybe say hello. There were even a few who suggested that we might someday send out machines like this Mother Probe, to seed planets with human colonies, without forcing biologicals to suffer the impossible rigors of interstellar space. Those were extrapolations we thought of, once we saw the possibilities in John Von Neumann’s great idea. We expected the aliens who preceded us in the galaxy would do the same.

“But that doesn’t exhaust even the list of human motivations, Gavin. There may be concepts other creatures invented which to us would be unimaginable!” She stood up suddenly and drifted above the dusty floor before feeble gravity finally pulled her down in front of the chiseled wall. Her gloved hand touched the outlines of a stone sun.