She did. The cylinder transmitted a gentle throb to her flat palm, a thrum-thrum-thrum so faint that it felt like the tingle of a weak electric discharge.

“Tuned as well as we could do them,” Paul said. “Ninety-nine point nine-eight efficiency. One hundred percent isn’t possible, even in theory.”

“What’s going on inside? If they’re called Omnivores, they ought to be eating something.”

“It’s probably not the best name for them.” Paul patted the bulbous cylinder, then left his hand to lie alongside Jan’s. “If you were inside — which thank heaven you can’t ever be — you’d find that nuclear fusion is taking place right here, inside this section. At the moment we are fusing hydrogen to helium to power the drive. We can do that with an internal temperature as low as ten million degrees. But if we ever ran short of hydrogen we could fuse helium to make carbon, or anything all the way up to iron. That’s why these are called Omnivores, because they can fuse lots of different elements. But most fusion reactions need at least a hundred million degrees before they start to produce useful net power. We try to avoid it, because the higher temperatures are harder on the engines.”

Jan pressed her hand down on the cylinder. It was quite cool, but her fingers were just a few centimeters from a roaring fusion furnace. Paul might speak casually of “as low as ten million degrees,” but that sounded more than enough to her.

Paul was watching closely. “Scary?”

“No, not at all. Kind of exciting.” It was, too. So much pent-up power, vibrating under her fingers and responding to human control — it gave her a definite lift, an odd kind of turn-on.

“I hoped you would like it.” Paul again patted the blue cylinder. “I think of this as a kind of test of people. A visit to the engine room produces one of two reactions. Some are terrified at being close to so much raw power — they don’t seem to realize that if the engines ever did blow, they’d be no safer at the other end of the ship than they are standing here. Other people are stirred by what they see as the power that humans have gained over nature. We are doing things inside the Diabellis that once took place only in the middle of stars. I find that impressive and exciting.” He turned away from the Omnivore cylinder. “Let me know if you’d like to come here again. Meanwhile, we’d better be getting back forward. Mars orbit rendezvous in an hour or two. Dr. Bloom will be sitting there itching to get at you.”

“I think she wants Sebastian more than she wants me.”

“Even so, it doesn’t sound like much fun for either of you. But I hope you enjoyed this visit.”

“Very much.”

That was quite true, and it left in Jan’s mind one question: Why had Paul Marr singled her out, from all the passengers, for the guided tour? Or maybe there was a second question, too: Had Paul Marr singled her out, or was she one on a list of a dozen?

Jan preferred not to ask. Something told her that she would find out in due course. And if she did, and the answer proved to be that he was interested in Jan alone, there was one other question that she still had to ask herself.

12

The control room was cold, and Alex was sweating. In one hour, he and Kate — which in practice probably meant he alone — had to give the most important briefing of his life. It was also likely to be the most difficult. He had insisted that the computing and data resources of the Seine were all that he needed to make his model into a practical prediction tool. Kate had told him that was bullshit, because the model was producing nonsense. He wasn’t sure he believed her. He was sure that he had no idea what might be going wrong.

Calm, stay calm.

First, run the model for the decades preceding the Great War. As before, it predicted the occurrence of the war to the year and to the month. Beyond the war that run of the model offered no prediction, but that was as it should be. Such a traumatic event was a singularity of the timeline, beyond which prediction was impossible.

So what about the runs that Kate had completed while he was, in her words, “diddling little Lucy”? He wasn’t sure it had been Lucy-Maria, but it wouldn’t help with Kate to explain that uncertainty.

The model automatically stored every parameter of every run. Alex called on the Seine to perform in parallel all the runs that Kate had tried, one after another, the previous night. It would take months to track every variable, so for the moment he wouldn’t try. He settled for gross aggregates. The crucial number for the moment was total solar system population. He asked for that value, averaged over all the runs that he and Kate had performed, to be displayed as a function of time.

And here it came, the number of humans in the whole solar system, for every year in the coming century and a half.

The starting value, for the year 2097, was today’s actual count of 5.2 billion. The number was rounded, to two significant figures, but Alex had demonstrated, over and over, that his results were not sensitive to small errors in inputs or minor changes in initial conditions. The value for 2098, 5.3 billion, came five seconds later than he expected. The amount of computation that Alex’s model required was enormous, but nowhere near enough to tax the capacity of the Seine. However, he did not command the system’s highest priority. That went to emergency real-time missions, and to the often-meaningless (in Alex’s humble opinion) computational demands of other government programs.

2099: the rounded average over all runs remained at 5.3 billion. Alex spot-checked the exact value, which showed an increase. 2100: sure enough, the number was up slightly again, to 5.4. Alex was aware of Kate at his shoulder, watching not the display but Alex himself. The years moved on steadily, the population count crept higher.

Maybe nothing was wrong. Maybe Kate had screwed up. Alex’s models were different from all earlier predictive models. He was still struggling to find a way to explain those differences in a way that Kate’s chosen test subject, moronic Macanelly, could understand, but every conceivable model had certain things in common. It had endogenous variables, computed within the model and used to produce future values of those same variables; and it had exogenous variables, values that must be fed into the model from some external source.

Any model needed both.

2105: 5.6 billion; 2106: 5.7 billion; 2107: 5.7 billion…

Endogenous variables were easy, you simply provided their present-day values and the model ran with them to estimate their future values. The big question was always, where do you get values for exogenous variables? In a model designed to predict the development of the solar system for a century or more, the one thing you knew for certain was that there would be surprises.

2110: 5.9 billion; 2111: 6.0 billion; 2112: 6.1 billion…

The increase in solar system population, averaged over all model runs, was accelerating.

By definition, a surprise was something that no one could hope to predict. And since surprises were inevitable, in this sense Alex’s model runs were all bound to be wrong. Cheap faster-than-light travel, true immortality as opposed to today’s longevity treatments, the arrival of aliens in the solar system; any of these might happen, in any future year. Alex’s model could assume any one of them, and still make predictions. However, none of the runs that Kate had made assumed any such thing.

2117: 6.5 billion; 2118: 6.6 billion; 2119: 6.7 billion…

What Alex had learned, over years of experience, was that futures in which no surprise exogenous variables were introduced tended to be conservative futures. They had slower growth, and better stability.

But what about war, all-out combat like the Great War that had torn the solar system apart thirty years ago? That produced huge effects, but it was not an exogenous variable, introduced from outside. It arose directly from a steady series of changes in human activities. It was a prediction of the model — the main prediction, in fact, that motivated all of Alex’s work. If your model could predict that a war was on the way, you had a chance to explore changes in exogenous variables to make the war disappear.