Stephen Baxter - Time

number, ten or a thousand?”

“Nope. Not without evidence.”

“Very wise. Please, pull the lever.”

Malenfant drummed his fingers on the tabletop. Then he pressed the lever.

A small black marble popped into the slot. Malenfant inspected it; it was blank. Emma could see there was easily room for a thousand such balls in the box, if need be.

Malenfant scowled and pressed the lever again.

His name was on the third ball he produced.

“There are ten balls in the box,” Malenfant said immediately.

“Why do you say that?”

“Because if there were a thousand in there it’s not likely I’d reach myself so quickly.”

Cornelius nodded. “Your intuition is sound. This is an example of Bayes’ rule, which is a technique for assigning probabilities to competing hypotheses with only limited information. In fact—” He hesitated, calculating. “ — the probability that you’re right is now two-thirds, on the basis of your ball being third out.”

Emma tried to figure that for herself. But, like most probability problems, the answer was counterintuitive.

“What’s your point, Cornelius?”

“Let’s think about the future.” Cornelius tapped the softscreen embedded in the tabletop before him. The small monitor before Emma lit up, and a schematic graph drew itself elegantly on the screen. It was a simple exponential curve, she recognized, a growth rising slowly at first, steepening up to a point labeled NOW. Cornelius said, “Here is a picture of the growth of the human population over time. You can see the steep rise in recent centuries. It is a remarkable fact that ten percent of all the humans who have ever existed are alive now. More than five percent of all humans, Malenfant, were born after you were.

“But that is the past. Let’s imagine how the future might develop. Here are three possibilities.” The curve continued to climb, steepening as it did so, climbing out of Emma’s frame. “This,” Cornelius said, “is the scenario most of us would like to see. A continued expansion of human numbers. Presumably this would require a move off-planet.

“Another possibility is this.” A second curve extrapolated itself from the NOW point, a smooth tip over to a flat horizontal line. “Perhaps our numbers will stabilize. We may settle for the resources of the Earth, find a way to manage our numbers and our planet indefinitely. A bucolic and unexciting picture, but perhaps it is acceptable.

“But there is a third possibility.” A third curve climbed a little way past the NOW marker — then fell spectacularly to zero.

“Jesus,” said Malenfant. “A crash.”

“Yes. Studies of the population numbers of other creatures, lower animals and insects, often show this sort of shape. Plague, famine, that sort of thing. For us, the end of the world, soon.

“Now. You can see that in the first two cases, the vast majority of humans are yet to be born. Even if we stay on Earth, we estimate we have a billion years ahead of us before changes in the sun will render Earth’s biosphere unviable. Even in this restricted case we would have far more future than past.

“And if we expand off-planet, if we achieve the kind of future you’re working for, Malenfant, the possibilities are much greater. Suppose we — or our engineered descendants — colonize the Galaxy. There are four hundred billion stars in the Galaxy, many of which will provide habitable environments for far longer than a mere billion years. Then the total human population, over time, might reach trillions of times its present number.”

“Oh. And that’s the problem,” Malenfant said heavily.

“You’re starting to see the argument,” Cornelius said, approving.

“I’m not,” Emma said.

Malenfant said, “Remember his game with the balls and the box. Why are we here now? If we really are going on to the stars, you have to believe that you were born in the first one-billionth part of the total human population. And how likely is that? Don’t you get it, Emma? It’s as if I drew out my ball third out of a thousand—”

“Far more unlikely than that, in fact,” said Cornelius.

Malenfant got up and began to pace the room, excited. “Emma, I don’t know statistics from my elbow. But I used to think like this as a kid. Why am I alive now? Suppose we do go on to colonize the Galaxy. Then most of the humans who ever live will be vacuum-sucking cyborgs in some huge interstellar empire. And it’s far more likely that I’d be one of them than what I am. In fact the only pop curve where it’s reasonably likely that we’d find ourselves here, now , is…”

“The crash,” said Emma.

“Yes,” Cornelius said somberly. “If there is a near-future extinction, it is overwhelmingly likely that we find ourselves alive within a few centuries of the present day. Simply because that is the period when most humans who ever lived, or who will ever live, will have been alive. Ourselves among them.”

“I don’t believe this for a second,” Emma said flatly.

“It is impossible to prove, but hard to refute,” said Cornelius. “Put it this way. Suppose I tell you the world will end tomorrow. You might think yourself unlucky that your natural life span has been cut short. But in fact, one in ten of all humans — that is, the people alive now — would be in the same boat as you.” He smiled. “You work in Las Vegas. Ask around. Losing out to one in ten odds is unlucky, but not drastically so.”

“You can’t argue from analogy like this,” Emma said. “There are a fixed number of balls in that box. But the total number of possible humans depends on the undetermined and open-ended future — it might even be infinite. And how can you possibly make predictions about people who don’t even exist yet — whose nature and powers and choices we know absolutely nothing about? You’re reducing the most profound mysteries of human existence to a shell game.”

“You’re right to be skeptical,” Cornelius said patiently. “Nevertheless we have thirty years of these studies behind us now. The methodology was first proposed by a physicist called Brandon Carter in a lecture to the Royal Society in London in the 1980s. And we have built up estimates based on a range of approaches, calling on data from many disciplines—” Malenfant said hoarsely, “When?”

“Not earlier than one hundred and fifty years from now. Not later than two hundred and forty.”

Malenfant cleared his throat. “Cornelius, what’s this all about? Is this an extension of the old eggs-in-one-basket argument? Are you going to push for an off-planet expansion?”

Cornelius was shaking his head. “I’m afraid that’s not going to help.”

Malenfant looked surprised. “Why not? We have centuries. We could spread over the Solar System—”

“But that’s the point,” Cornelius said. “Think about it. My argument wasn’t based on any one threat, or any assumptions about where humans might be Jocate4 or whafJeveJ oftecb- nology we might reach. It was an argument about the continued existence of humanity, come what may. Perhaps we could even reach the stars, Malenfant. But it will do us no good. The Carter catastrophe will reach us anyway.”

“Jesus,” said Malenfant. “What possible catastrophe could obliterate star systems — reach across light years?”

“We don’t know.”

There was a heavy silence in the wood-laden room.

Malenfant said gruffly, “So tell me what you want from me.”

“I’m coming to that,” Cornelius said evenly. He stood up. “May I bring you more drinks?”

Emma got out of her chair and walked to the window. She looked out over Central Park, the children playing. They were engaged in some odd, complex game of shifting patterns. She watched for a while; it looked almost mathematical, like a geometric form of communication. Kids were strange these days. Getting brighter, according to the news media. Maybe they needed to be.