In his 1970 science fiction novel Ringworld, author Larry Niven describes the ep
ID: 1381098 • Letter: I
Question
In his 1970 science fiction novel Ringworld, author Larry Niven describes the eponymous Ringworld, a gigantic structure shaped as a ring with a radius of around 1 AU, rotating around a star in the center of the ring. This system is described as physically stable; however many readers have complained that it is actually unstable and the structure will drift away in time.
My first thought reading this is that the Ringworld's center of mass is identical to the star's center of mass, so the system should be stable. Why is it unstable?
Some additional details of the structure:
Radius: ~1 AU
Mass: ~1 Solar Mass
Year duration: ~220 hours
The Ringworld is also made of material strong enough to withstand the stresses affecting it in such a system.
Explanation / Answer
It is only for a spherically symmetric shape that you can treat an extended body as if it were a point mass at the CoM.
The Ringworld is
stable against axial displacements after which it will gently bob back and forth around the star.
unstable against transverse ones because the gravitational attraction of the near-side is large than that of the far-side.