Schild’s Ladder

Riding her ion scooter the million kilometers to the Quietener, Cass found herself reveling in the view for the first time in years. The scooter was doing one-and-a-quarter gees, but the couch pressed against her back so gently that she might have been floating. Floating in dark water, beneath an alien sky. Even at half a light-year, Mimosa punched a dazzling violet hole in the blackness, a pinprick ten times as bright as a full moon. Away from its glare, the stars were far too plentiful to suggest constellations; any stick-figure object that she began to sketch between them was soon undermined by an equally compelling alternative, then a third, then a fourth — like a superposition of graphs, each with a different choice of edges between the same nodes. When she’d first arrived, she’d homed in on her own star, watching with a mixture of fear and exaltation as it hovered at the edge of visibility to her thousandth-scale eyes. Now, she’d forgotten all the cues she’d need to find it, and she felt no urge to ask her navigation software to remind her. The sun was no beacon of reassurance, and she’d be seeing it close-up again soon enough.

Each time one of Livia’s staged targets had been achieved, Cass had dispatched a small army of digital couriers to pass on the news to seven generations of her ancestors and descendants, as well as all her friends in Chalmers. She’d received dozens of messengers herself, mostly from Lisa and Tomek, full of inconsequential gossip, but very welcome. It must have grown strange for her friends as the years had passed, and they no longer knew whether or not there was any point continuing to shout into the void. If she had traveled embodied, as a handful of ancients still did, she could have caught up with centuries of mail on the return voyage. Reduced to a timeless signal en route, though, she’d have no choice but to step unprepared into the future. Her homecoming was going to be the hardest thing she’d ever faced, but she was almost certain now that her time here would prove to have been worth it.

Half an hour before arrival, Cass rolled onto her stomach and poked her head over the edge of the couch. Her engine’s exhaust was a barely perceptible flicker, fainter than a methanol flame by daylight, but she knew that if she reached down and placed her hand in the stream of plasma, she’d rapidly lose any delusion that her Mimosan body was indestructible.

She watched the Quietener growing beneath her, the silvery sphere glinting Mimosa-blue. Surrounding it was a swarm of smaller, twinned spheres, unevenly colored and far less lustrous. Tethers, invisibly slender, allowed the twins to orbit each other, while ion jets balanced the slight tug of the Quietener’s gravity, keeping each pair’s center of mass fixed against the stars.

The Quietener made it possible to perform experiments that could never be carried out elsewhere. The right distribution of matter and energy could curve space-time in any manner that Einstein’s equations allowed, but creating a chosen state of quantum geometry was a very different proposition. Rather than simply bending space-time in bulk, like a slab of metal in a foundry, it had to be controlled with the same kind of precision as the particles in a two-slit interference experiment. But the "particles" of geometry were twenty-five orders of magnitude smaller than atoms, and they could never be vaporized, ionized, or otherwise coaxed apart to be handled one by one. So the same degree of delicacy had to be achieved with the equivalent of a ten-tonne lump of iron.

Refining the starting material helped, and the Quietener did its best to screen out every form of impurity. Ordinary matter and magnetic fields absorbed or deflected charged particles, while a shell of exotic nuclei, trapped by gamma-ray lasers in states from which they could not decay without absorbing neutrinos, mopped up a greater fraction of the billions wandering by than would have been stopped by a galaxy’s-worth of lead.

Gravitational waves passed through anything, so the only antidote was a second train of waves, tailored to cancel out the first. There was nothing to be done about sporadic cataclysms — supernovae, or black holes gorging on star clusters in the centers of distant galaxies — but the most persistent gravitational waves, coming from local binary stars, were cyclic, predictable, and faint. So the Quietener was ringed with countersources, their orbits timed to stretch space at the center of the device when the bodies they mimicked squeezed it, and vice versa.