Free Axis by Robert Charles Wilson

Hypotheticals.
    But the weather in the Indian Ocean had been reassuringly benign, and because this was Turk’s first passage he had risked the derision of his shipmates by arranging to be on deck when the crossing happened.
    A night crossing of the Arch. He staked out a place aft of the forecastle out of the breeze, made a pillow from a hank of rag stiff with dried paint, stretched out and gazed at the stars. The stars had been scattered by the four billion years of galactic evolution that had transpired while the Earth was enclosed in its Spin membrane, and they remained nameless after thirty years, but they were the only stars Turk had ever known. He had been barely five years old when the Spin ended. His generation had grown up in the post-Spin world, accustomed to the idea that a person could ride an ocean vessel from one planet to another. Unlike some, however, Turk had never been able to make that fact seem prosaic. It was still a wonder to him.
    The Arch of the Hypotheticals was a structure vastly larger than anything human engineering could have produced. By the scale of stars and planets, the scale on which the Hypotheticals were assumed to operate, it was a relatively small thing… but it was the biggest
made
thing Turk imagined he would ever encounter. He had seen it often enough in photographs, on video, in representative diagrams in schoolbooks, but none of those did justice to the real item.
    He had first seen it with his own eyes from the Sumatran port where he joined the
Kestrel.
The Arch’s eastern leg had been visible on clear days and especially at sunset, when the last light climbed that pale thread and burnished it to a fine golden line. But now he was almost directly beneath the apex, a different view entirely. The Arch had been compared to a thousand-mile-wide wedding ring dropped into the Indian Ocean, half of it embedded in the bedrock of the planet and the other half projecting above the atmosphere into naked space. From the deck of the
Kestrel
he couldn’t see either leg where it entered the sea, but he could see the peak of the Arch reflecting the last light of the sun, a brushstroke of silvery-blue fading to dusky red at its eastern and western extremities. It quivered in the heat of the evening air.
    Up close, people said, if you sailed within hailing proximity of either leg, it looked as plain as a pillar of concrete rising from the surface of the sea, except that the enormously wide pillar didn’t
stop
rising, simply vanished from sight. But the Arch wasn’t an inert object no matter how static it appeared. It was a machine. It communicated with a copy of itself—or the other half of itself, perhaps—set in the compatible ocean of the New World, many light-years distant. Maybe it orbited one of the stars Turk could see from the deck of the
Kestrel:
there was a shivery thought. The Arch might appear to be inanimate, but in fact it was watching the near surface of both worlds, conducting two-way traffic. Because that was what it did: that was its function. If a bird, a storm-tossed tree limb, or an ocean current passed beneath the Arch it would continue on its way unmolested. The waters of Earth and the New World never mingled. But if a manned ocean vessel crossed under the Arch it would be picked up and translated across an unimaginable distance. By all reports the transition was so easy as to be almost anticlimactic, but Turk wanted to experience it out here in the open, not down in crew quarters where he wouldn’t even know it had happened until the ship sounded its ritual horn.
    He checked his watch. Almost time. He was still waiting when Tomas stepped out of the shadows into the glare of a deck light, grinning at him.
    “First time, yeah,” Turk said, forestalling the inevitable comment.
    “Fuck,” Tomas said, “you don’t need to explain. I come out every time I pass. Day or night. Like paying respects.”
    Respects to whom? The Hypothetical? But Turk didn’t ask.
    “And, oh