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its density averages out at the same as Earth’s. Surface gravity is the same as our original planet, and since we’ve been transplanted here and survived we have learned that it’s a remarkably hospitable environment for our kind of life; only on the large scale does it seem different.”
    The astronomer sits up. “Do any of you gentlemen have any idea just how preposterously powerful whoever built this structure is?”
    “How do you mean, ‘preposterously powerful’?” asks Brundle, looking more interested than annoyed.
    “A colleague of mine, Dan Alderson, did the first analysis. I think you might have done better to pull him in, frankly. Anyway, let me itemize: item number one is escape velocity.” Sagan holds up a bony finger. “Gravity on a disk does not diminish in accordance with the inverse square law, the way it does on a spherical object like the planet we came from. We have roughly Earth-like gravity, but to escape, or to reach orbit, takes tremendously more speed. Roughly two hundred times more, in fact. Rockets that from Earth could reach the moon just fall out of the sky after running out of fuel. Next item:” another finger. “The area and mass of the disk. If it’s double-sided, it has a surface area equal to billions and billions of Earths. We’re stuck in the middle of an ocean full of alien continents, but we have no guarantee that this hospitable environment is anything other than a tiny oasis in a world of strangeness.”
    The astronomer pauses to pour himself a glass of water, then glances round the table. “To put it in perspective, gentlemen, this world is so big that, if one in every hundred stars had an Earth-like planet, this single structure could support the population of our entire home galaxy . As for the mass—this structure is as massive as fifty thousand suns. It is, quite bluntly, impossible: as-yet-unknown physical forces must be at work to keep it from rapidly collapsing in on itself and creating a black hole. The repulsive force, whatever it is, is strong enough to hold the weight of fifty thousand suns: think about that for a moment, gentlemen.”
    At that point Sagan looks around and notices the blank stares. He chuckles ruefully.
    “What I mean to say is, this structure is not permitted by the laws of physics as we understand them. Because it clearly does exist, we can draw some conclusions, starting with the fact that our understanding of physics is incomplete. Well, that isn’t news: we know we don’t have a unified theory of everything. Einstein spent thirty years looking for one, and didn’t come up with it.
    “But, secondly.” He looks tired for a moment, aged beyond his years. “We used to think that any extraterrestrial beings we might communicate with would be fundamentally comprehensible: folks like us, albeit with better technology. I think that’s the frame of mind you’re still working in. Back in ’sixty-one we had a brainstorming session at a conference, trying to work out just how big an engineering project a spacefaring civilization might come up with. Freeman Dyson, from Princeton, came up with about the biggest thing any of us could imagine: something that required us to imagine dismantling Jupiter and turning it into habitable real estate.
    “This disk is about a hundred million times bigger than Dyson’s sphere. And that’s before we take into account the time factor.”
    “Time?” Echoes Fox from Langley, sounding confused.
    “Time.” Sagan smiles in a vaguely disconnected way. “We’re nowhere near our original galactic neighborhood and whoever moved us here, they didn’t bend the laws of physics far enough to violate the speed limit. It takes light about 160,000 years to cross the distance between where we used to live, and our new stellar neighborhood, the Lesser Magellanic Cloud. Which we have fixed, incidentally, by measuring the distance to known Cepheid variables, once we were able to take into account the measurable blue