Ultima (44 page)

“And then we can proceed in comparative comfort, if we're lucky, all the way to the antistellar,” said the ColU. “For that central locus must be a key node of any transport network.”

Stef had got back to the cart, within which the ColU sat, bundled against the cold. “You want me to put some of this sample in your little analysis lab?”

“Yes, please, Stef Kalinski. Titus Valerius, let us consider. If this length of tunnel is typical, at sixty miles or so, and if we have a journey of less than six thousand Roman miles to complete to the antistellar—”

“We'll need a hundred hops. And if each hop takes us two-thirds of an hour, as you said, that will take, umm . . .”

“Sixty, seventy hours,” Stef said. “I always was good at mental arithmetic. Even allowing for stops, and for hauling the cart between terminals, that's only a few days.”

“It may be hard work,” Titus said. “But we will not freeze to death, or starve, or die of thirst on the way.” He nodded. “Excellent! But you know, Stef, I, Titus Valerius, anticipated that we would find some such fast road as this.”

“You did? How?”

“Because, if not, we would have encountered Ari Guthfrithson and the Inca woman walking back the other way. Would we not? For if we could never have mastered this world of ice on foot, and I suspect that is true,
they
could surely not. Clever fellow, aren't I, for a one-winged legionary? Now then—Clodia, come with me. We will do a little scouting before we return. Let's see if we can find the terminal of the next link, somewhere in the direction of the antistellar . . .” He glanced up at the sky, taking a bearing from Andromeda. “
That
way. Come now! And you, Stef Kalinski, you and your old-lady bones stay put in this cart.”

“With pleasure, legionary.”

As they walked away, she heard father and daughter laughing.

“It's good to hear them happy,” Stef said. “Suddenly a journey that did look impossible has become achievable.”

“You too should be happy,” the ColU whispered.

“I should?”

“For the discovery you have just made.”

“What discovery? The pyramid?”

“It's no pyramid, Stef Kalinski. It's nothing artificial, and nor is it a merely physical phenomenon, as I'm sure you guessed. It is life, Stef Kalinski. Life. An ambassador, perhaps, from a colder world than this . . .”

As they sat huddled together in the cart, the ColU spoke of Titan, moon of Saturn.

•   •   •

Titan was a mere moon, a small world subsidiary to a giant, but a world nevertheless—and a very cold one. Its rocky core was overlaid by a thick shell of water, a super-cold ocean contained by a crust of ice as hard as basalt was on Earth. And over
that
was a thick atmosphere, mostly of nitrogen, but with traces of organics, methane, ethane . . .

“But it is those organic traces that made Titan so interesting,” the ColU said. “On a land of ice rock, where volcanoes belch ammonia-rich water, a rain of methane falls, carving river valleys and filling seas. And in those seas—”

“The probes found life. I remember the reports. Some kind of slow-moving bugs in the methane lakes.”

“Yes, life based—not on carbon, as ours is—but on silicon. Just as carbon-carbon bonds, the backbone of your chemistry, Stef Kalinski, can be made and broken in room-temperature water, so silicon-silicon bonds can be made in the cold methane of Titan's lakes. A form of life not so very unlike ours superficially, but with a different biochemistry entirely—and very slow moving, low in energy, slow to reproduce and evolve. We found nothing but simple bugs on Titan, simpler than most bacteria—not much more complicated than viruses.

“But Titan is not the only cold world. Here at Proxima, while the Earth-like Per Ardua was the planet that caught all the attention—”

“Ah. Proxima d.”

“Yes. It was a Mars-sized world just outside the zone that would have made it habitable for humans, like Per Ardua.”

“So far as I know it was never even given a decent name. Nobody cared about it—or the other Proxima worlds.”

“They did not. But it was very like Titan—another common template for a world, it seems. And room for another kind of life.

“Stef Kalinski, Earthshine has spoken of a panspermia bubble, of worlds like Earth and Per Ardua linked by a common chemistry carried by rocks between the stars, worlds with cousin life-forms. But there could be
other
bubbles, worlds with different kinds of climate, different kinds of biochemistries, yet linked in the same way. Maybe one bubble could even overlap another, you see—for clearly a stellar system may contain more than one kind of world.”

Stef was starting to understand. “You always speak in riddles, ColU, whether you intend to or not. But I think I see. The sample I brought you—”

“The pyramid-beast over there has a silicon-based biochemistry very similar to that recorded on Titan, but not identical. Maybe it is a visitor from Proxima d, do you think? Somehow hardened to withstand what must be for it a ferocious heat, even here on Per Ardua's dark side. As if a human had landed on Venus. But it is here, and surviving. And with more time still . . .”

“Yes, ColU?”

“Stef Kalinski, we have seen that, given billions of years, life-forms from across the same panspermia bubble can integrate, grow together.”

“The Earth ants in the Arduan stromatolite.”

“Exactly. Now, is it possible that given
tens
of billions,
hundreds
of billions of years, even different kinds of life could mix and merge? Your fast, quick kind, and the slow-moving Titanian over there? Could that be the next stage in the evolution of the cosmos itself? You already share a world, you see.”

“It's a fantastic thought,” she said slowly. “But it's never going to happen. Is it, ColU? Because this is the End Time, according to you. There will be no tens or hundreds of billions of years—”

“I'm afraid not, Colonel Kalinski. Here on the dark side I have been able to make quite precise assays of the sky: the state of the stars, the proximity of Andromeda—even the background glow of the universe as a whole, which contains warp-bubble clues to its future.”

“Hmm.” She looked up into the dark. “Well, it is marvelous seeing, for an astronomer. And you've come to a conclusion, have you?”

“I have. And a precise estimate of the time remaining.”

Stef felt chilled, as if she'd been given bad news by a doctor. “You're going to have to explain all this to the others, you know. In language they can understand.”

“Yes, Colonel Kalinski. Of course. And the importance of finding Earthshine soon, by the way, is only increased.”

Stef could hear the others returning, father and daughter laughing, full of life and energy. And she looked across at the silicon-life explorer from Proxima d, the ice giant. “I wonder if that thing can
see
us . . . Just tell me,” she said. “How long have we got?”

“A year,” the ColU said flatly. “No more. The data's still chancy.”

And Stef immediately thought of Mardina, and the baby.

She pursed her lips and nodded. “A year, then. For now, not a word. Come on, let's get ready to go on.”

The party gradually penetrated deeper into the cold of the Per Ardua farside.

The forty-minute tunnel hops all felt much the same to Beth, but in the short intervals during which they trekked from one tunnel exit to the entrance of the next, always following trails carefully scouted out by Titus with Clodia or Chu, Beth did get glimpses of parts of her world she had never seen before. After all, during the years she'd spent growing to a young adult on Per Ardua, she had never gone farther than the tall forests that screened the terminator zone.

Stef and the ColU had made such a journey as this once before, with Beth's father and Liu Tao, in a purloined ISF rover. That party had followed a more or less direct course to the antistellar, cutting over the ice surface of a frozen ocean. The gravity tunnels, however, naturally enough, stuck to continental land, detouring around the shores of frozen oceans. As a result the journey was longer than a direct route, and was taking longer than the handful of days that Titus and the ColU had first estimated—but still it would be brief enough.

And while Stef in that earlier party had spent mind-numbing days crossing geometrically perfect ice plains, now Beth saw more interesting features. Eroded mountain ranges from which glaciers spilled like huge, dirty tongues. Places where earthquakes or other geological upheavals had raised and cracked the ice cover, creating frozen cliffs that gleamed a deep blue in the light of their torches.

Yet even these features were probably impressively old, the ColU said. There would always be a lot of weather activity at the terminator, where the warm air and water from the day side spilled into the cold of the night. But here in the dark, weather would always be desperately rare: no clouds, no fresh falls of snow or hail. Even meteor impacts would be infrequent in such an elderly system as this, with much of the primordial debris left over from the planets' formation long since swept up. So they drove across a sculpted but static landscape—and a landscape bathed in the complex, red-tinged light of an aging Andromeda.

Sometimes they saw more “Titanians,” enigmatic, sharp-edged pyramids standing like mute monuments. But the ColU assured them that the Titanians, in their way, on their own timescale, could be exploring just as vigorously as the humans.

Beth noticed, however, that Stef barely glanced at the sky, or the icebound landscape, or even the Titanians. As they traveled, and in the “evenings” as they rested, Stef sat huddled with the ColU at the back of their sled-cart, or in a corner of their shelter, talking softly, Stef making occasional notes on the glowing face of her slate. Everybody knew what they were discussing: the ColU's ideas about the fate of the world. Beth tried to read Stef's expression. There was nothing to be discerned from the ColU's neutral tone.

At last, one evening, after they had cleared away their meal, with them all bundled in their warmest clothes, their feet swathed in layers of socks, gathered around the warmth of the kernel stove, Stef announced that they needed to talk about the End Time.

“In a way,” Stef began cautiously, “the idea that the world will have an end—that the universe
itself
will end, and relatively soon—ought to feel natural to us.

“We have no direct experience of infinity, of eternity. Our own lives are short. And the scientists in my Culture proved quite definitively that eternity doesn't lie
behind
us, that our universe had a beginning, a birth in a cataclysmic outpouring of energy. Why, then, should we imagine that eternity lies ahead of us, an unending arena for life and mind?”

Beth was sitting beside her pregnant daughter. Now, under a blanket, she took her daughter's hand, and Mardina squeezed back. Mardina's eyes were wide in the firelight, her expression blank. This was
not
a conversation either of them wanted to be part of, Beth was sure.

The ColU was on Chu's lap, next to Stef. Titus Valerius sat beside the slave boy, listening intently.

And Titus was skeptical. “Well, we Romans had no trouble imagining eternity. Or at least, we failed to anticipate an end. Because we never anticipated the
Empire
to end—do you see? Unbounded and eternal . . .”

That sounded magnificent in the legionary's guttural soldier's Latin, Stef thought.
Imperium sine fine.

The ColU said, “Our own Culture, mine and Stef's and Beth's, had its own account of an undying empire—but an empire of scientific logic. We thought we could know the future by looking out at the universe, working out the physical laws that govern it—and then projecting forward the consequences of those laws.

“The universe only has so much hydrogen—the stuff that stars are made out of. The hydrogen will, or
would have
, run out when the universe is ten thousand times as old as it is now. No more stars. After the stars there would be an age of black holes and degenerate matter—the compressed, cooling remnants of stars—and the galaxies, huge and dim, would begin to break up. There would be a major transition when protons began to decay—that is, the very stuff of which matter is made . . . In the end everything would dissolve, and there would be nothing left but a kind of sparse mist, of particles called electrons and positrons—a stuff called positronium—filling an expanding, empty universe. Even so, it was possible that minds could survive. Minds more like mine than yours, perhaps. Thoughts carried on the slow wash of electrons—thoughts that might take a million years to complete.”

“That sounds horrible,” Mardina said, and Beth could feel the grip of her hand tighten. “It doesn't even make any sense. How could a single thought last a million years? I can't imagine it.”

“But experiences of time can differ,” the ColU said. “In my Culture there was a Christian scholar called Thomas Aquinas—I wasn't able to trace him in your history, Titus. He distinguished three kinds of time, or perhaps perceptions of time.
Tempus
was human time, which we measure by changes in the world around us—the swing of a pendulum, the passage of a season. A Titanian ice giant would experience a slower
tempus
than a human.
Aevus
was angel time, measured by internal changes—by the development of thoughts, understanding, moods. For the angels, you see, stood outside the human world. And then there was
aeternitas
, God's time, for God and only God could apprehend all of eternity at once. The electron-positron minds would not be God, but in the timeless twilight of the universe they might have been like angels . . .”

“Might have been,” Mardina said, almost bitterly.
“Might have been.”

The ColU said, “The positronium angels
will never exist
. Our universe won't last long enough for that. And the reason our universe is not eternal is because of the existence of
other
universes. And we know they exist because we, all of us, have visited several of them.”

“Aye, and fought in them,” Titus said, stirring from his space and pushing back blankets. “But in
this
universe my bladder's full. Anybody want more tea? Chu, maybe you could put another pot of ice on the fire . . .”

•   •   •

It took an hour before they were settled again.

When they took their places Beth thought they seemed calmer, more attentive—more ready to take in this strange news from the sky. The break had been a smart bit of people management by Titus Valerius, she thought. Who in the end hadn't really needed a piss at all.

“So,” Titus said now, slurping the last of his tea, “as if the fate of this universe wasn't bad enough, you have to talk about all the other ones.”

Stef smiled. “All right, Titus, I know we are leading you on a march you'd rather not be following . . . It's all about logic, though. When all else fails, ask a philosopher. Sorry. Old physicist's joke.

“Look,
we
all know from personal experience that other universes exist, with histories more or less similar to this one—or to the one into which each of us was born. And in my Culture our philosophers had predicted the existence of those universes. Our laws of nature were well founded, you see, but they did not prescribe how the universe had to be. Many universes were possible—an infinite number. It is just as our science would have predicted the sixfold symmetry of a snowflake, which comes from the underlying geometry of ice crystals, but within that sixfold rule set, many individual snowflakes are possible, all different from each other.”

“Universes as numerous as snowflakes,” Beth said. “That's wonderful. Scary.”

Stef said, “But what are these universes?
Where
are they? You know that the science of my Culture was more advanced than in any other we've yet encountered—”

The ColU said, “And Earthshine would say that was because we had been the least deflected into efforts to build Hatches for his Dreamers.”

“We did have some models of the
multiverse
—I mean, of a super-universe that is a collection of universes. After centuries of study we never came to a definitive answer. We probably never got far out enough into our
own
universe to be able to map the truth.

“Still, we believed our universe had expanded from a single point, out of a Big Bang. Expanded, cooled, awash with light at first, atoms and stars and planets and people condensing out later. But our universe was like a single bubble in a bowl of boiling water, like a pot we put on the fire.” She gestured at the clay pot, within which water was languidly bubbling. “You see? There is a substrate, something like the water in the pot. And out of that heated-up substrate emerges, not just one bubble, but a whole swarm of them, expanding, popping . . .
They
are the other universes we've been visiting.

“And what's inside those universes is going to be different, one universe to the next—a little or a lot. Some could differ wildly from the others, not just in historical details. Suppose gravity were stronger—I mean, the force that gives us weight. Then stars would be smaller, and would burn out more quickly.
Everything
would be different. And if gravity were weaker, there might be no stars at all. And of course some universes are going to be more similar than others.”

It seemed to be Chu who understood most readily. Not for the first time, Beth wondered what kind of scholar he might have become, given the chance. “All the universes we have seen
are
similar. They all have planets, suns, people. They even have the
same
people, up to a point.”

“Yes,” Stef said eagerly. “You've got it. When you think about it the differences are pretty small. I mean, whether Rome falls or not would be a big deal for us,” and she smiled as Titus scowled ferociously, “but from Per Ardua, say, you wouldn't even notice it.”

The ColU said, “We believe that the Dreamers can somehow reach out to other universes that are—
nearby
. There is no good term for it. What is nearness in a multiverse? Beginning in one universe, they reach out into another that is similar, yet that contains a human Culture that is more—conducive—to Hatch-building. And we, our small lives, are swept along in the process.”

Beth found herself frowning. “But why? Why would they do that?”

Stef said, “We need to find that out. In fact I suspect Earthshine may already be learning that secret. What's important now is that we know the multiverse exists. OK? We've
been
there. Now, the multiverse is big. Surely that's true. But it can't be infinite.”

Titus scratched his head. “Here we go again . . . Dare I ask, why not?”

“The trouble is, Titus,” the ColU said, “some scholars have always believed that nature does not contain infinities. Infinities are just a useful mathematical toy invented by humans, with no correspondence to reality. Unlike the number
three
, say, which maps on to collections of three objects: three people, three potatoes . . .”

Stef said, “Infinities can make sensible questions meaningless. Titus, start with the number one.”

“I think I can grasp that.”

“Add another one.”

“I have two.”

“Subtract one.”

“I have one again.”

“Add one.”

“Two.”

“Subtract one.”

“One!”

“Add one!”

“Two!”

“Subtract one!”

“One!”

She held up her hands. “OK, that's enough. You get the idea. Now if I asked you to stop doing that after some finite number of steps—twelve or twenty-three or five hundred and seventy-eight—what answer would you get?”

“That's easy. Either two or one.”

“Definitely one or the other?”

“Of course.”

“But if I asked you to go on
forever
, what answer would you end up with?”

“I—ah . . . Oh.”

“You see?” Stef said. “The answer can't be determined. The question becomes absurd, once you bring infinity into it.”

Titus said, “I can feel my brain boiling like the water in that pot.”

“Physics—my philosophy—is about asking sensible questions and expecting sensible answers. About being able to predict the future from the past. When you bring in infinities, sensible questions have dumb answers. The whole system breaks down.”

The ColU said, “So the point is, the multiverse—the collection of the universes we visit—
must
be finite. Because nature won't allow infinities.”

Mardina scowled. “Well, so what? What do I care if there is one reality, or ten or twenty or a million?”

Stef said, gently but persistently, “It matters because a finite multiverse
has an edge
. And if one of the member universes should encounter that edge . . .” She looked into the pot of water, and pointed out one largish bubble slowly migrating from the boiling center toward the side of the clay pot. “Watch.” When the bubble reached the edge, it popped, vanishing as if it had never existed.