Oceanic by Greg Egan Page B

far-side planets,” he said. “ The physical worlds .”
    He’d bought himself some time on a geographically scattered set of processor clusters. He was no longer repeating his dangerous incursions, of course, but by playing around in the natural ebb and flow at the border, he’d made some extraordinary discoveries.
    Alison and I had realized long ago that random “proofs” in the natural world would influence what happened at the border, but Campbell’s theory made that notion more precise. By looking at the exact timing of changes to propositions at the border, measured in a dozen different computers world-wide, he had set up a kind of ... radar? CT machine? Whatever you called it, it allowed him to deduce the locations where the relevant natural processes were occurring, and his model allowed him to distinguish between both near-side and far-side processes, and processes in matter and those in vacuum. He could measure the density of far-side matter, out to a distance of several light-hours, and crudely image nearby planets.
    “Not just on the far side,” he said. “I validated the technique by imaging our own planets.” He sent me a data log, with comparisons to an online almanac. For Jupiter, the farthest of the planets he’d located, the positions were out by as much as a hundred thousand kilometers; not exactly GPS quality, but that was a bit like complaining that your abacus couldn’t tell north from north-west.
    “Maybe that’s how Sam found us in Shanghai?” I wondered. “The same kind of thing, only more refined?”
    Campbell said, “Possibly.”
    “So what about the far-side planets?”
    “Well, here’s the first interesting thing. None of the planets coincide with ours. Nor does their sun with our sun.” He sent me an image of the far-side system, one star and its six planets, overlaid on our own.
    “But Sam’s time lags,” I protested, “when we communicate—”
    “Make no sense if he’s too far away. Exactly. So he is not living on any of these planets, and he’s not even in a natural orbit around their star. He’s in powered flight, moving with the Earth. Which suggests to me that they’ve known about us for much longer than Shanghai.”
    “Known about us,” I said, “but maybe they still didn’t anticipate anything like Shanghai.” When we’d set Luminous on to the task of eliminating the defect – not knowing that we were threatening anyone – it had taken several minutes before the far side had responded. Computers on board a spacecraft moving with the Earth would have detected the assault quickly, but it might have taken the recruitment of larger, planet-bound machines, minutes away at lightspeed, to repel it.
    Until I’d encountered Campbell’s theories, my working assumption had been that Sam’s world was like a hidden message encoded in the Earth, with the different arithmetic giving different meanings to all the air, water and rock around us. But their matter was not bound to our matter; they didn’t need our specks of dust or molecules of air to represent the dark integers. The two worlds split apart at a much lower level; vacuum could be rock, and rock, vacuum.
    I said, “So do you want the Nobel for physics, or peace?”
    Campbell smiled modestly. “Can I hold out for both?”
    “That’s the answer I was looking for.” I couldn’t get the stupid Cold War metaphors out of my brain: what would Sam’s hot-headed colleagues think, if they knew that we were now flying spy planes over their territory? Saying “screw them, they were doing it first!” might have been a fair response, but it was not a particularly helpful one.
    I said, “We’re never going to match their Sputnik, unless you happen to know a trustworthy billionaire who wants to help us launch a space probe on a very strange trajectory. Everything we want to do has to work from Earth.”
    “I’ll tear up my letter to Richard Branson then, shall I?”
    I stared at the map of the far-side