In the distance a bird flapped, shrieking, its colors gaudy against the bland afternoon blue of the skydome.
The bird was flying upside down.
“Spinner-of-Rope.”
The voice was close to her ear. Still chewing her meat, Spinner turned, slowly.
Louise Ye Armonk hovered a few feet away, standing on the squat, neat platform of a zero-gee scooter. Louise grinned. “Did I make you jump? I’m sorry for cheating with this scooter; I’m not sure I would have managed the climb.”
Spinner-of-Rope glared at her. “Louise. Never — never — sneak up on someone at the top of a tree.”
Louise didn’t look too concerned. “Why not? Because you might lose your grip, and drift off the branch a couple of feet? What a disaster.”
Spinner tried to maintain her anger, but she started to feel foolish. “Come on, Louise. I’m trying to make a point.”
Louise, skillfully, brought her scooter in closer to Spinner; without much grace she clambered off the scooter and onto the branch beside Spinner. “Actually,” she said gently, “so am I.” She breathed deeply of the moist forest air, and looked around the sky. “I saw you watching that bird.”
Spinner pushed her spectacles up the bridge of her nose. “So what?”
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…”