What If We Built a Really Big Computer?

COMPUTERS HAVE GROWN more and more powerful over the years by pushing the limits of how small their components can be. But just how big can a computer get? Could we turn a planet into a computer, and if so, what would we do with it? • In considering such questions, we go beyond normal technological projections and into the realm of outright speculation. So IEEE Spectrum is making one of its occasional forays into science fiction, with a short story by Karl Schroeder about the unexpected outcomes of building a computer out of planet Mercury. Because we're going much farther afield than in a typical Spectrum article, we've contextualized and annotated Schroeder's story to show how it's still grounded in real science and technology. • This isn't the first work of fiction to consider such possibilities. In “The Hitchhiker's Guide to the Galaxy,” Douglas Adams famously imagined a world constructed to serve as a processor. Real-world scientists are also intrigued by the idea. Jason Wright, director of the Penn State Extraterrestrial Intelligence Center, has given serious thought to how large a computer can get. A planet-scale computer, he notes, might feature in the search for extraterrestrial intelligence. “In SETI, we try to look for generic things any civilization might do, and computation feels pretty generic,” Wright says. “If that's true, then someone's got the biggest computer, and it's interesting to think about how big it could be, and what limits they might hit.” • There are, of course, physical constraints on very large computers. For instance, a planet-scale computer likely could not be a solid object like Earth. “It would just get too hot,” Wright says. Any computation generates waste heat. Today's microchips and data centers “face huge problems with heat management.” • In addition, if too much of a planet-scale computer's mass is concentrated in one place, “it could implode under its own weight,” says Anders Sandberg, a senior research fellow at the University of Oxford's Future of Humanity Institute. “There are materials stronger than steel, but molecular bonds have a limit.” • Instead, converting a planet into a computer will likely involve spreading out its mass. Rather than have a single object that would be subject to all kinds of mechanical stresses, it would be better to break the computer up into a globular flotilla of nodes, known as a Dyson swarm. This strategy would also make it easier to harvest solar energy. • What uses might a planet-scale computer have? Hosting virtual realities for uploaded minds is one possibility, Sandberg notes. Quantum simulation of ecosystems is another, says Seth Lloyd, a quantum physicist at MIT. Which brings us to our story…