Megastructure name: Alderson disk
Named after: The late Jet Propulsion Laboratory scientist Dan Alderson, who proposed the idea in the early 1970s Selected science-fiction portrayals: As the "Godwheel," it was featured in Malibu Comics' Ultraverse series, and in Missile Gap, a 2006 novella by Charles Stross.
Think of the Alderson disk as a DVD so big that an entire star fits in the hole in the center. In sci-fi, the disk serves a dual purpose as a rich narrative setting and an extreme-engineering thought experiment.
The Alderson disk would have a diameter of 150 million miles or so—the size of the inner solar system out past Mars. It would be several thousand miles thick, large enough that there would be a vertical gravitational pull on both side. Civilizations could live on either side, their citizens' feet pointing toward each other through the disk. Light from a sun, or suns, shining in the inner, empty core of the Alderson disk strikes the length of the disk edgewise, bathing disk residents in a permanent twilight.
"An Alderson disk is a wonderful site for horror stories," said Larry Niven, who wrote about an Alderson disk–like behemoth, dubbed the Godwheel, in Malibu Comics' 1990s Ultraverse series.
So the setup is simple. The execution presents immense, if not insurmountable, engineering challenges. Compared to other megastructures Popular Mechanics has covered, "the Alderson disk is more purely for fun and less likely to ever happen," Niven says. Nevertheless, "you can do some wonderful things" with the architecture, he says.
(A schematic of an Alderson disk, indicating an approximately Earthly habitable zone where liquid water would neither completely freeze nor boil away. Credit: Cryonic07/Wikipedia)
A Gargantuan Space Platter
An initial showstopper: Alderson disks contain ludicrous amounts of mass. In our solar system, the sun possesses some 99.8 percent of the total mass. Alderson disks upend this conventional distribution of matter. "You're supposed to have a disk that is more massive than the Sun," said Niven, who also wrote a 1974 essay discussing Alderson disks that appeared in the magazine Analog Science Fiction and Fact.
Robert Frietas, now a senior research fellow at the Institute for Molecular Manufacturing, a nonprofit organization based in California, once ran the ginormous numbers for an Alderson disk. In his 1979 book, Xenology, Frietas assumed an annulus breadth of about 370 million miles—that is, from inner to outer edge—and a thickness around 3,000 miles. (For comparison, Earth's radius is about 4,000 miles.)
Frietas then assumed an average material density of water, which is 62.4 pounds per cubic foot. "Water is a good 'typical' density to use when you're doing gross order-of-magnitude estimates involving solid hunks of ordinary physical matter," Freitas said. Based on these figures, this particular Alderson disk would possess on the order of 13 decillion pounds of mass—that's 13 with 33 zeroes behind it. It's also 3,000 times the mass of the sun.
There Goes the Neighborhood
Gathering anywhere close to that much material would require collecting all the available, usable mass (read: everything excluding stars' unworkably superhot plasma) for perhaps hundreds of light years around. "You would find an Alderson disk surrounded by a lot of empty space," Niven says.
A better bet would be to mine a so-called giant molecular cloud—the matter-rich birthplaces of stars where tens of thousands of sun-masses can be had within mere scores of light-years (though most of the stuff is hydrogen gas—hardly a useful building material on its own).
Vacuuming up the cosmic neighborhood is admittedly something an Alderson disk's engineers would want to do anyway, as the disk would make a fat target for asteroids or even rogue planets.
Thorny Issues of Stability
Should engineers figure out the logistics of gathering all that mass together, there's this problem: Pooling a few thousand suns of mass into a plane would not give you a stable configuration, according to today's physics. Anders Sandberg, a research fellow at Oxford University's Future of Humanity Institute, who has studied megastructure concepts, said the middle portions of the disk would exert a net gravitational pull its edge regions. That pull would conspire to warp the disk into a donut.
"I think an Alderson disk will fail and will implode in a gigantic torus," Sandberg says. The mass concentration might even be sufficient to collapse the whole thing into a black hole.
And that's not all. Scientist would also have to prevent the disk and its star from straying too close to one another. Because the disk is more massive than its star, the star should settle comfortably into the central hole and stay put. But Alderson disk residents would want some kind of safeguard in case instabilities arose due to passage through the galaxy (say, while traversing a molecular cloud).
Frietas, in xenology, suggested placing high-powered gamma-ray lasers—an active area of current research—on the disk's inner edge. These "grasers" could blast the star at its periphery, overheating a region to produce thrust and nudge a star in an intended direction. This same mechanism could be used to move the sun up and down relative to the plane of the disk to provide the residents of one side at a time an occasional, nighttime-like respite from sunlight.
Let's talk about some more mundane problems, like this: A retaining wall encircling the central hole, measuring hundreds of miles high, would be needed to keep the sun's gravity from stealing the atmosphere. Farther out on the disk, gases in the atmosphere—for instance, water—would freeze to the disk, while closer in they'd boil off; keeping the different atmospheric zones separate could be mitigated by a series of retaining walls. Niven imagined aliens that prefer different climates could populate regions of the disk, with the cold-worlders taking up residence in the outer disk and thermophiles occupying interior real estate.
Humans and our terrestrial species, meanwhile, could live in a habitable zone just like those referred to in studies of exoplanets. A relatively tiny band, extending from 5 percent closer in and farther out from Earth's average orbital distance from the sun, would present colonists with about 50 million times the surface area of our planet.
What could be done with all that room? Holes in the disk could allow for oceans—giant blobs of water gravitationally suspended in the plane of the disk—to be "bottomless," swum in by residents from both sides, Niven suggests.
Engineers could play with the plane of the Alderson disk to further suit its occupants. Disk thickness could vary, altering local gravitational strength. Gravity, however, would get weird at the disk's edges, since the pull is toward the central portions of the disk.
"As you get close to edge, you would start to tilt a little," Sandberg says. "It would be a bit like going uphill as you walk toward the end of the world."