Ringworld is a Nebula and Hugo-award winning 1970 science fiction novel by Larry Niven. It is considered a classic novel of the genre. In the years since the publication of the novel, the term "ringworld" has come to refer to its namesake in the novel: a huge ring-shaped colony equal in length to the entirety of Earth's orbit. Its width was 1 million miles, 125 times wider than the Earth's diameter. Its circumference is stated as 6×108 miles, for a surface area was about 300 million times greater than the Earth. Artificial gravity was provided by rotating the ring quickly. In the story, every planet in the solar system had been disassembled and used to construct the ringworld.
A ringworld is a perfect example of a megastructure in science fiction; informally, a artificial construct exceeding a hundred kilometers in at least one dimension. Megastructures are not completely science fiction — at least two already exist on early 21st century Earth — the Great Wall of China, at 3,947 mi (6,352 km) long, and the Rice Terraces of the Philippine Cordilleras, a 10,360 square kilometer (4,000 square miles) agricultural complex. In science fiction, the word often refers to something suspended in outer space.
Running the physics numbers on the ringworld structure has long been an interest of science fiction geeks and extreme engineering enthusiasts. In the novel, it is stated that 20 Jupiter masses of energy has to be consumed just to get the ring spinning fast enough to simulate 1 g of gravity through centrifugal force. Calculations by physicists found this is about right. In order to prevent the ring from flying apart due to centrifugal force, a foundation material with a tensile strength about equal to the atomic nucleus would be necessary. In the novel, this material is called scrith. The only known material with a tensile stretch equal to scrith is the neutronium that makes up neutron stars. If neutronium could somehow be formed into a ring, it would still be likely to collapse until its own gravity.
Larry Niven said his ringworld was an intermediary step between a planet and a Dyson sphere; an artificial sphere completely enclosing a star. Building one might take hundreds of thousands of years, if it could be constructed at all.
Most people who discuss the ringworld object consider it a fascinating thought experiment and an interesting science fiction plot devices. Scaled-down ringworlds are featured in the computer game series Halo.