Ring

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,” Spi

Louise reached out and took her hand; the old engineer’s skin was cold, leathery. “Spi

Spi

Louise nodded, studying her.

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

“Spi

Spi

“No.” Louise wouldn’t meet her eyes.

Spi

“Spi

“But, without the hyperdrive, journeys like that would take mille

Spi

Louise hesitated. “Spi

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, Spi

“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, Spi

“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.

“Spi

“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, Spi

Spi

“Spi

“Think of a sheet of paper, Spi

Spi

Louise hesitated. “Not really. Wormholes are defects in our three-plus-one dimensional spacetime, Spi

Spi

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

Spi

Louise leaned closer so that Spi

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

Spi

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