Free Ark by Stephen Baxter

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