Stephen Baxter: Ark

Zane was already working, paging through notes on his laptop. Holle arranged for the board’s contents to be downloaded to her handheld, and she made notes alongside Liu’s diagrams and equations. All around her the students chatted, argued, joked, and scrolled through what looked like entirely disparate bits of work. This was not like the mostly calm, mostly studious atmosphere she had got used to at the grade schools in Denver.

“The warp bubble as a method of transportation has some paradoxical properties. Because the ship is stationary relative to local landmarks, there are none of the effects we associate with special relativity: no time dilation, no Lorentz-Fitzgerald contraction. Clocks aboard the ship stay synchronized with those at the starting point, and indeed at the destination. And there will be no inertial effects.”

“What does that mean?” Holle whispered.

“You wouldn’t feel any acceleration,” Zane said. “The ship’s not moving compared to the spacetime it’s embedded in. So you don’t get squished against the back wall of your cockpit when you turn on the warp drive.”

“However, there are issues of control, for you run the risk of outrunning any signal sent forward to control your own bubble. Therefore, we think it likely that any piloted mission will have the parameters of bubble formation, propagation, dissipation and so on loaded by a remote station before launch; the crew of the starship inside the bubble will essentially be passengers.”

Joe and Mike burst into gales of laughter over some private joke.

Holle leaned over to Zane. “Is it always like this?”

“Like what?”

“Noisy. Everybody messing around.”

He shrugged. “There are no rules. They just put the material in front of you and expect you to make what you can of it.”

“And if you can’t cope with that,” said the boy next to Kelly, twisting back, “you can go back to the kiddie schools and play with the plastic bricks. There’s always somebody ready to take your place.” He grinned. “Don Meisel. Who the hell are you?”

Holle found herself blushing as she told him her name. A year older than Holle, Kelly was growing into a tall, confident blonde, no great beauty but a leader. And this boy Don, who Holle hadn’t met before, looked that bit older again. His eyes blue, his hair red, a stronger color than Holle’s own pale strawberry, he looked relaxed in his own skin, lively, fearless.

“Your first day?” Don asked.

“Yes,” said Holle. “It’s great.”

“Sure it is. That a Scotch accent?”

“Scottish, I-”

“You up to scratch on relativity?”

Her dad had gone through it with her. “Sure.”

“Special relativity is trivial,” Kelly said. “Nothing more than Pythagoras’s theorem. So how are you on your Christoffel symbols?”

“On my what?”

Kelly and Don just laughed, and turned away.

Zane said, “They’re teasing. They’re talking about tensor calculus. The mathematics of general relativity. Which is what you need to describe how spacetime curves around a warp bubble…” He showed her some of Liu’s equations. She recognized derivatives, but some of the symbols were strewn with superscripts and subscripts. “ That is a tensor,” Zane said. “A kind of multidimensional generalization of a vector, which is a quantity with both magnitude and direction-”

“I’m eleven years old,” she said. “My dad has been cramming me since I was six, when he got attached to Ark One. But how am I supposed to know about tense-”

“Tensors?” He shrugged. “Liu’s actually a good teacher, even if he never looks you in the eye. And if you don’t learn-”

“I’ll be out. I know.”

“I’ll help you.”

“Thanks,” she said sincerely. “So what’s with those two? Kelly and Don. Are they going out?”

Zane just blinked. He didn’t reply. That wasn’t the sort of thing Zane ever took any notice of.

Holle said, “Kelly was always the boss at grade school. Maybe they’ll drive each other on.”

“Or they’ll crash and burn together.”

The lesson didn’t get any easier.

“After five years of intensive study, we do now have a handle on how a warp bubble might be created,” Liu Zheng was saying. He filled up his board with new kinds of diagrams, showing sheets and cylinders. “The expansion or contraction of spacetime locally reflects a change in Einstein’s cosmological constant omega, which, as you know, describes vacuum energy, which is like an antigravity field that permeates spacetime-the engine of universal expansion.

“Now, we believe that our universe has a small extension in higher dimensions-higher, that is, than the three of space and one of time we experience. But those extra dimensions are small. Our universe is like a hosepipe, rolled up around the extra dimensions. The cosmological constant is inversely proportional to the fourth power of the characteristic radius of that hosepipe. Inversely. So the smaller that hosepipe radius, the higher the constant and the greater the expansive effect. Therefore, if you can change that radius locally, you can adjust the cosmological constant, and thus control the expansion of spacetime as you desire. To make a spacetime bubble you pinch the hosepipe.

“But how to pinch that hosepipe? On face value that would seem to require reaching out of the local three-dimensional plane of the universe itself…”

Now he spun off again, into “string theory,” which described space as filled not with point particles like electrons and quarks and neutrinos, but with strings, tiny filaments whose characteristic vibrations determined the properties of the “particles” they defined, such as charge and mass. Holle had heard of these ideas. It was as if the whole universe was a symphony played on tiny violins.

But, Liu said, the strings could interact with those rolled-up extra dimensions of spacetime. In particular the strings could wrap around the extra dimensions, like spiderweb wisps around the hosepipe. That was how the dimensions stayed compact in the first place. And that meant-

“You can squeeze the hosepipe,” Holle burst out, her imagination racing away.

Liu turned to her. “And how would you do that, Ms. Groundwater?”

“With a particle acce-” She stumbled over the word.

“Accelerator?”

“Yes. With an accelerator you’re manipulating matter at its smallest levels. You can yank on the tiny strings.”

Everybody was staring at her. Don and Kelly looked around, Don with amusement, Kelly with something more like resentment.

“I’m sorry,” she said. “I was thinking out loud.”

“Don’t be sorry,” Liu said. “You got it about right; that is what we’re planning. We’re setting up a hadron collider outside the city, based on scavenged components from accelerators in the US and overseas. Though we’re still years away from even a ground test the energies required are ferocious…” He gestured at the board. “And can you see how that basic concept is expressed in my equations?”

“No,” she said frankly.

Kelly laughed. “That figures.”

But Liu was unperturbed. “That’s not important. Intuition is the thing. But though we have a conceptual design for the creation of the warp bubble, we have a fundamental problem. The energy requirement is literally astronomical. A warp bubble is an artifact of curved space-time analogous in some ways to a black hole. Now, suppose we built a bubble a hundred meters in radius. That should be big enough to house a respectably sized spacecraft, shouldn’t it? Give me an order of magnitude estimate of the mass-energy required.”

The students huddled over their computers. Kelly muttered, “The radius of a black hole is twice the mass times the gravitational constant divided by speed of light squared…”