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.
The ColU said, “Given that one simple fact—that the multiverse must be finite—and knowing how old the universe is, or was in the age we came from—it has always been possible to make an estimate of how long the universe was going to last. How long it was likely to be before we hit the multiverse wall. Probabilistic only, but…”
Titus snapped, “How long, then?”
The ColU said, “My latest estimate, based on my inspection of the sky as far back as our time on the Malleus Jesu, is three and a half billion years after the age of mankind.”
Titus shook his head, growling under his breath. “An absurd number.”