 After publication of "Ringworld", Dan Alderson and Ctein, two friends told Niven that the Ringworld was dynamically unstable such that if the center of rotation drifts away from the central sun, gravitational forces will not "re-center" it, thus allowing the ring to eventually contact the sun and be destroyed.Niven used this as a core plot element in the sequel novel, "The Ringworld Engineers".This idea proved influential, serving as an alternative to a full Dyson sphere that required fewer assumptions (such as artificial gravity) and allowed a day/night cycle to be introduced (through the use of a smaller ring of "shadow squares", rotating between the ring and its sun).