What really mattered to Alia nowadays was Skimming.

She met her sister in the Engine Room, the deepest bowel of the Nord, in steel-gray light, where hulking, anonymous machinery loomed. The sisters faced each other and laughed at the delicious prospect of what was to come.

Like Alia, Drea was naked, the best way to Skim. Drea’s body, coated with golden hair, was neatly proportioned, with her arms only a little shorter than her legs, and she had long toes, not as long as her fingers but capable of grasping and manipulation. It was a body built for zero gravity, of course, and for hard vacuum, the natural environment of mankind, but it was believed that this body plan was pretty much the same as that of the original human stock of old Earth.

Drea was ten years older than Alia. The sisters were very alike, but there was more gravity to Drea, a little more levity to Alia. As the light shifted, multiple lids slid across Drea’s eyes.

Drea leaned close, and Alia could smell the sweetness of her breath. “Ready?”

“Ready.”

Drea grasped Alia’s hands. “Three, two, one—”

Suddenly they were in the Nord’s Farm deck.

This was a high, misty hall, where immense ducts and pipes snaked down through the ceiling, lamps shed a cool blue-white glow, and green plants burgeoned in clear-walled hydroponic tanks. The Nord was a starship, a closed ecology. The big pipes delivered sewage and stale air from the human levels above, and carried back food, air, and clean water.

Alia breathed deep. After the cold, static austerity of the Engine Room she was suddenly immersed in the Farm’s vibrant warmth, and the deck plates thrummed in response to the huge volumes of liquid and air being pumped to and fro. Even the quality of the gravity felt subtly different here. Alia had felt nothing of the Skimming: no time passed during a Skim, so there was no time for sensation. But the transition itself was delicious, a rush of newness, like plunging from cold air into a hot pool.

And this was just the start.

Drea’s eyes were bright. “Jump this time. Three, two, one—” Flexing their long toes the sisters sailed up into the air, and at the apex of their coordinated jump they popped out of existence.

On the sisters fled, to all the Nord’s many decks, shimmering into existence in parks, schools, museums, gymnasiums, theaters. In each place they stayed only a few seconds, just long enough to lock eyes, agree the next move, and jump or pirouette or somersault into it. It was really a kind of dance, the challenge being to control the accuracy of each Skim and the mirror-image precision of their positions and movements at each emergence.

Skimming, voluntary teleporting, was so easy small children learned to do it long before they walked. Alia’s body was made up of atoms bound into molecules, of fields of electricity and quantum uncertainty. Alia’s body was her. But one atom of carbon, say, was identical to another — absolutely identical in its quantum description — and so it could be replaced without her even knowing. She was just an expression of a temporary assemblage of matter and energy, as music is an expression of its score regardless of the medium in which it is written. It made no difference to her.

And once you knew that, it was easy to see that she, Alia, could just as easily be expressed by a heap of atoms over there as one over here. It was just a question of will, really, of choice, along with a little help from the nanomachines in her bones and blood. And very little Alia willed was denied her.

Most children Skimmed as soon as they found out they could. Adults found it harder, or gave it up as they gave up running and climbing. But few of any age Skimmed as skillfully as Alia and Drea. As the sisters passed, scattering startled birds, young people watched them with envy, and older folk smiled indulgently, trying to mask their regret that they could never dance so gracefully again.

And at each step, in the instant after the girls had vanished, two clouds of silvery dust could be seen suspended in the air, pale and transparent, still showing the forms of the two sisters. But in the ship’s artificial breezes these chimeras of abandoned matter quickly dispersed.

In one last mighty Skim the girls leapt all the way out of the Nord itself.

Alia felt the tautness of the vacuum in her chest, the sting of hard radiation on her face as delicious as a shower of ice water on bare skin. With her lungs locked tight, and the Mist of biomolecules and nanomachinery that suffused her body eagerly scouring for damage, she was in no danger.

There were stars all around the sisters, above, below, to all sides; they were suspended in three-dimensional space. In one direction a harder, richer light came pushing through the thick veil of stars. That was the Core, the center of the Galaxy. The Nord was some fifteen thousand light-years from the center, about half the distance of Sol, Earth’s sun. Only ragged clouds of dust and gas lay before that bulging mass of light, and if you looked carefully you could make out shadows a thousand light-years long.

Alia looked down at the Nord, her home.

The ship beneath her feet was a complex sculpture of ice and metal and ceramic, turning slowly in pale Galaxy light. You could just make out the vessel’s original design, a fat torus about a kilometer across. But that basic frame had been built on, gouged into, spun out, until its lines were masked by a forest of dish antennae, manipulator arms, and peering sensor pods. A cloud of semi-autonomous dwellings, glowing green and blue, swam languidly around the ship: they were the homes of the rich and powerful, trailing the Nord like a school of fish.

Their hands locked, the sisters spun slowly around each other, their residual momentum expressing itself as a slow orbit. Complex starlight played on Drea’s smiling face, but her eyes were masked by the multiple membranes that slid protectively over their moist surfaces. Alia savored the moment. When they were younger the sisters had been the most important people on the Nord for each other. But Alia was growing up. This was a cusp of her life, a time of change — and the thought that there might not be too many more moments like this made this all the sweeter.

But Alia was distracted by a gentle voice, a whisper in her ear.

Her mother was calling her. Come home. You have a visitor…

A visitor? Alia frowned. Who would visit her that could be important enough for her mother to call? None of her friends; any of them could wait. But there had been a gravity about her mother’s tone. Something had changed, Alia thought, even as she had danced through the Nord. Drea clung to Alia’s hands. Alia felt a surge of love for this sister, companion of her childhood. But Drea’s expression was complex, concerned. She knew something, Alia realized. There was suddenly a subtle barrier between them.

They swam toward each other, and they Skimmed one last time.

Like a clash of cymbals their bodies overlapped, the atoms and electrons, fields and quantum blurrings overlapping. Of course this merging was frowned on; it was a dangerous stunt. But for Alia it was delicious to be immersed in her sister’s essence, to become heavy with her, everything about the two of them merged into a single cloudy mass, everything but some relic trace of separateness in their souls. It was closer even than sex.

But it lasted only a second. With a gasp they Skimmed apart, and drifted side by side. And with that moment of oceanic closeness over, Alia’s niggling worry returned.

Let’s go home, Drea said.

The sisters spiraled down toward the Nord’s bright, complicated lights.