An Alderson disk (named after Dan Alderson, its originator) is a hypothetical artificial astronomical megastructure, like Niven's Ringworld, or a 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 radius of an Alderson disk would be roughly equivalent to the orbit of Mars or Jupiter. According to the proposal, a sufficiently massive disk would have a larger mass than its sun.
Close to its surface, the gravity of the disk would closely approximate that of an infinite flat plate, for which gravity is perpendicular to the surface. Near the inner and outer edges of the plate, edge effects would become significant.
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 our solar system. Another option in the way of heliocentric habitats is the Dyson Sphere proposed by Freeman Dyson, it could be scaled up to use roughly the same amount of usable material in the solar system if the sphere were 8–20 cm thick and had a radius of 1 AU to keep the edge of the habitat in the habitable zone.
Life could exist on either side of the disk, though close to the sun the heat would make life impossible without protection. Conversely, further 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.
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
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). Author Larry Niven designed the Godwheel and wrote stories surrounding certain events on it.
Rak Mesba is a partial ancient alien Alderson Disk in the Orion's Arm setting.
A disk-shaped planet similar to an Alderson disk (though far smaller) served as the home world 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.