Louise picked at the tree bark. “Well, the bird seems to be doing its best to get by, in zero-gee.”

“Maybe. Not everyone here is doing so well,” Spinner said heavily. The loss of gravity was, slowly but surely, devastating the forest biota. “The higher birds and animals seem to be adapting okay… The monkeys quickly learned to adjust the way they climb and jump. But otherwise, things are falling apart, in a hundred tiny ways.” She thought of spiders which could no longer spin webs, of tree-dwelling frogs which found their tiny leaf-bound ponds floating away into the air. “We’re doing our best to keep things working — to save whatever we can,” she said. “But, damn it, even the rain doesn’t fall right any more.”

Louise reached out and took her hand; the old engineer’s skin was cold, leathery. “Spinner, we have to reestablish all of this. Permanently.” Louise lifted her face; the diffuse light of the dome softened the etched-in age lines. “I designed this forest Deck, remember. And this is the only fragment of Earth that’s survived, anywhere in the Universe — as far as we know.”

Spinner-of-Rope pulled her hand away. “I know what your little parable about the bird was about, Louise. I should adapt, just like the plucky little bird. Right? You want me to come back to the nightfighter.”

Louise nodded, studying her.

“Well, it was a dumb parable. The bird is the exception, not the rule. And — ”

“Spinner, I know you needed a break. But you’ve been climbing around these trees for a long time, now. I need you to come back — we all do. I know it’s difficult for you, but you’re the only person I have who can do the job.”

Spinner watched her face, skeptically. “But we’re not talking about mere discontinuity-drive jaunts around the Solar System now. Are we, Louise?”

“No.” Louise wouldn’t meet her eyes.

Spinner felt a hollowness in her chest — as if it had expanded, leaving her heart fluttering like a bird in some huge cavity. Hyperdrive…

“Spinner, we need the hyperdrive. You understand that, don’t you? The Sun is dying. Perhaps we could attempt to establish some sort of colony here, in the Solar System. But we need to find out what’s happening beyond the System. Are there any people left, anywhere? Maybe we can join them — find a better place than the Solar System has become.

“But, without the hyperdrive, journeys like that would take millennia, more even with the discontinuity drive. And I don’t think we have millennia…”

Spinner took a deep breath. “Yes, but… Louise, what will happen when I throw the switch? How will it feel?”

Louise hesitated. “Spinner, I don’t know. That’s the truth; that’s what we want to find out from the first flight. We aren’t going to know for sure until we try it in anger. Mark and I have only just begun to put together theories on how the damn hyperdrive works… Spinner, all we know is it’s something to do with dimensionality.”

A conventional craft (Louise said) worked in a “three-plus-one” dimensional spacetime — three spatial dimensions, plus one of time. And within those dimensions nature was described by a series of fundamental constants — the charge on the electron, the speed of light, the gravitational constant, Planck’s constant, and others.

But — humans believed — physics was governed by the Spin (10) theory, which described symmetries among the forces of nature. And the symmetries needed to be expressed in higher dimensions than four.

“So, Spinner-of-Rope, there are more than three spatial dimensions,” Louise said. “But the ‘extra dimensions’ are compactified—”

“They’re what?”

“Collapsed down to the smallest possible scale — to the Planck scale, below which quantum physics and gravitation merge.”

Once — just after the initial singularity — the forces of physics were one, and the Universe was fully multi-dimensional. Then the great expansion started.

“Three of the spatial dimensions expanded, rapidly, to the scales we see today. The other dimensions remained compactified.”

“Why did three dimensions expand? Why not four, or two, or one — or none at all?”

Louise laughed. “That’s a good question, Spinner. I wish I had a good answer.

“Geometrically, three-dimensional spaces have some unique attributes. For instance, only in three dimensions is it possible for planets to have stable orbits governed by the central forces exerted by stars. Did you know that? Planets in a four-dimensional cosmos would drift into space, or spiral into their suns. So if life needs billions of years of a stable planetary environment, three dimensions are the only possibility. Matter isn’t stable in higher dimensions, even: the Schrödinger wave equation would have no bound solutions… And waves can propagate without distortion, only in three dimensions. So if we need high-fidelity acoustic or electromagnetic signals to be able to make sense of the world, then again, three dimensions is the only possibility.

“Spinner, maybe there are alternate universes, out there somewhere, where more than three dimensions ballooned up after the initial singularity. But as far as we can see, life — our kind of life — couldn’t have evolved there; the fundamental geometry of spacetime wouldn’t have allowed it…

“Remember, though, the extra dimensions are here, still, but they’re rolled up very tightly, into high-curvature tubes a Planck length across.”

“So we can’t see them.”

“No. But — and here’s the trick we think the Xeelee have exploited, Spinner — the extra dimensions do have an impact on our Universe. The curvature of these Planck tubes determines the value of the fundamental constants of physics. So the way the tubes are folded up determines things like the charge of an electron, or the strength of gravity.”

Spinner nodded slowly. “All right. But what has this to do with the hyperdrive?”

“Spinner-of-Rope, we think the Xeelee found a way to adjust some of those universal numbers. By changing the constants of physics — in a small region of space around itself — the hyperdrive can make spacetime unfurl, just a little.” Louise lifted her face. “Then the nightfighter can move, a short distance, through one of the higher dimensions.

“Think of a sheet of paper, Spinner. If you’re confined to two dimensions — to crawling over the paper — then it will take you a long time to get from one side to the other. But if you could move through the third dimension — through the paper — then you could move with huge apparent speed from one place to another…”

Spinner frowned. “I think I see that. Is this something like wormhole travel?”

Louise hesitated. “Not really. Wormholes are defects in our three-plus-one dimensional spacetime, Spinner; they don’t involve the higher collapsed dimensions. And worm-holes are fixed. With a wormhole you can travel only from one place to another, unless you drag the termini around with you. With the Xeelee drive — we think — you can travel anywhere, almost at will. It’s like the difference between a fixed rail route and a flitter.”

Spinner thought it over. “It sounds simple.”

Louise laughed. “Believe me, it’s not.” She turned, distracted. “Hey. Look,” she said, pointing to the skydome.

Spinner looked up, squinting through her spectacles against the glare of the dome. “What?”

Louise leaned closer so that Spinner could sight along her outstretched arm. “See? Those shadows against the dome, over there…”

The shadows, ten or a dozen forms, clambered across a small corner of the skydome, busy, active.

Spinner smiled. “Howler monkeys. They’ve colonized the skydome. I wonder how they got up there.”

“The point is,” Louise said gently, “they’ve adapted, too. Just like that parrot.”