Alderson disk
An Alderson disk (named after Dan Alderson, its originator) is a hypothetical artificial astronomical megastructure, like Larry Niven's Ringworld and the Dyson sphere.  The disk is a giant platter with a thickness of several thousand miles.  The Sun rests in the hole at the center of the disk.  The outer perimeter of an Alderson disk would be roughly equivalent to the orbit of Mars or Jupiter. According to the proposal, a sufficiently large disk would have a larger mass than its Sun.

The hole would be surrounded by a thousand-mile-high wall to prevent the atmosphere from drifting into the Sun. The outer rim would take care of itself.

The mechanical stresses within the disc would be far beyond what any known material can stand, thus relegating such a structure to the realm of exploratory engineering until materials and construction science become sufficiently advanced. Building a megastructure of this magnitude would require an amount of material that far surpasses the amount of material found in the Solar System.

Life could exist on either side of the disk, though close to the Sun the heat would make life impossible without protection. Conversely, farther away from the Sun living beings would freeze. Therefore, for the entirety of such a structure to be made habitable, it would have to include a vast number of life support systems. Even without such systems, the habitable surface area would be an equivalent of tens to hundreds of millions of Earths.

Because the Sun remains stationary, there is no day/night cycle, only a perpetual twilight. This could be solved by forcing the Sun to bob up and down within the disk, lighting first one side then the other.

In popular culture
In 1974, the science fiction writer Larry Niven suggested that an Alderson disk "would be a wonderful place to stage a gothic or sword and sorcery novel. The atmosphere is right, and there are real monsters." Because the zone habitable by humans is 'relatively narrow' (5% closer in and farther out than Earth's current orbit would provide 50 million times the surface area of Earth), the disc (and the cost of its construction) could be shared with aliens from hotter and colder planets. Over long periods of time, lifeforms would evolve to settle the sparsely-inhabited regions in between. "If civilization should fall, things could get eerie and interesting."

An Alderson disk (the Godwheel) was a prominent feature of Malibu Comics' Ultraverse. The Godwheel was split between two societies, one which used technology and one which used magic (each occupied its own side of the disk).  Larry Niven designed the Godwheel and wrote stories surrounding certain events on it.

Rak Mesba is a partial ancient alien Alderson Disk in Orion's Arm, a multi-authored online science fiction world-building project.

A disk-shaped planet similar to an Alderson disk (though far smaller) served as the homeworld of the fantasy "Aysle" setting (or "cosm") of West End Games' Torg roleplaying game. In contrast with the Alderson disk, the Aysle "diskworld" works according to fantasy physics, including a "gravity plane" that bisects the disk laterally, so that opposite sides "fall" towards the plane.  The diskworld of Aysle had a bobbing Sun and multiple inner layers. Both sides of the disk were inhabited, as were the internal layers.

In Charles Stross's Missile Gap, a copy of the whole Earth (along with copies of many other planets) is placed on an Alderson disk built around a black hole by unknown forces.

Ian McDonald's novel Empress of the Sun features a parallel-universe version of our solar system where creatures evolved from dinosaurs have converted all the mass to an Alderson disk (with a bobbing sun).

In Terry Pratchett's science fiction novel Strata the concept of an Alderson disk is brought up by the protagonist, Kin Arad, as an explanation for the mysterious 'flat earth' that is the focus of the story.

In Gigastructural Engineering & More, a popular modification to the 4X game Stellaris, the Alderson Disk is featured as a buildable megastructure inhabitable by the player.

See also

References

External links
 Diskworlds at Orion's Arm

Exploratory engineering
Megastructures