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in person. Just how often Yuen wangled free time for his students — whatever their nationality — was another question, but if he believed it was the best cover for us, I was in no position to argue. I only hoped he'd planted a seamless trail of reassuring lies in the university records and beyond, in case the Institute administration decided to check up on us in any detail.
    We stopped in at the operations room, and Yuen chatted with the technicians. Banks offlatscreens covered one wall, displaying status histograms and engineering schematics. It looked like the control center for a small particle accelerator — which wasn't far from the truth.
    Luminous was, literally, a computer made of light. It came into existence when a vacuum chamber, a cube five meters wide, was filled with an elaborate standing wave created by three vast arrays of high-powered lasers. A coherent electron beam was fed into the chamber — and just as a finely machined grating built of solid matter could diffract a beam of light, a sufficiently ordered (and sufficiently intense) configuration of light could diffract a beam of matter.
    The electrons were redirected from layer to layer of the light cube, recombining and interfering at each stage, every change in their phase and intensity performing an appropriate computation — and the whole system could be reconfigured, nanosecond by nanosecond, into complex new "hardware" optimized for the calculations at hand. The auxilliary supercomputers controlling the laser arrays could design, and then instantly build, the perfect machine of light to carry out each particular stage of any program.
    It was, of course, fiendishly difficult technology, incredibly expensive and temperamental. The chance of ever putting it on the desktops of Tetris-playing accountants was zero, so nobody in the West had bothered to pursue it.
    And this cumbersome, unwieldy, impractical machine ran faster than every piece of silicon hanging off the Internet, combined.
    We continued on to the programming room. At first glance, it might have been the computing center in a small primary school, with half a dozen perfectly ordinary work stations sitting on white formica tables. They just happened to be the only six in the world that were hooked up to Luminous.
    We were alone with Yuen now — and Alison cut the protocol and just glanced briefly in his direction for approval, before hurriedly linking her notepad to one of the work stations and uploading the encrypted map. As she typed in the instructions to decode the file, all the images running through my head of what would have happened if I'd poisoned the soldier at the gate receded into insignificance. We now had half an hour to banish the defect — and we still had no idea how far it extended.
    Yuen turned to me; the tension on his face betrayed his own anxieties, but he mused philosophically, "If our arithmetic seems to fail for these large numbers — does it mean the mathematics, the ideal, is really flawed and mutable — or only that the behavior of matter always falls short of the ideal?"
    I replied, "If every class of physical objects 'falls short' in exactly the same way — whether it's boulders or electrons or abacus beads . . . what is it that their common behavior is obeying — or defining — if not the mathematics?"
    He smiled, puzzled. "Alison seemed to think you were a Platonist."
    "Lapsed. Or ... defeated. I don't see what it can mean to talk about standard number theory still being true for these statements — in some vague Platonic sense — if no real objects can ever reflect that truth."
    "We can still imagine it. We can still contemplate the abstraction. It's only the physical act of validation that must fall through. Think of transfinite arithmetic: no one can physically test the properties of Cantor's infinities, can they? We can only reason about them from afar. ''
    I didn't reply. Since the revelations in Hanoi, I'd pretty much lost faith in my power