Ringworlds

After Professor Johnson's very interesting plug for Professor Jason Wright's posts on Dyson spheres, I was reminded by a science fiction novel I started (and unfortunately, never got around to finishing) a long time ago: Ringworld, by Larry Niven.

When Freeman Dyson published his idea of a Dyson sphere many years ago, he postulated that any sufficiently advanced civilization would have to harness the energy of its host star to keep on growing. The reasons for constructing a Dyson sphere are simple: they provide immense amounts of energy, and the sphere itself provides a huge amount of room to live on. Imagine a surface 1 AU in radius, all available for habitation!

Artists' rendition of a Dyson sphere. Note that, unless there's another star in the system, the sphere probably won't appear lit (since it's enveloping the host star).

The problem with Dyson spheres is that it's very, very costly to construct a sphere with a radius of 1 AU! Think of all the raw material you would have to collect.

In Ringworld, author Larry Niven poses the question of a "partial" Dyson sphere - that is, taking only an equatorial slice of a Dyson sphere, a ring, and setting it in gravitational equilibrium around the star. The amount of living space would still be much greater than that of the Earth, due to the huge radius of the Ringworld, and the ring could be set to rotate to simulate gravity. Sufficiently efficient solar collectors could be placed throughout the Ringworld to power the entire system. An atmosphere could even be created, if the Ringworld could spin fast enough to prevent the atmosphere from escaping!

Artist's rendition of a Ringworld.

The novel itself is actually pretty good, and hopefully I'll get around to finishing it one day. Ringworld was the first novel to pose the idea of humanity (or any species), living on a ring rotating around the Sun. Could humanity one day build its own Ringworld to replace Earth, and live on free solar energy?

It turns out that this isn't the case, unfortunately. Although Dyson spheres, i.e. spherical shells, are gravitationally stable, the two-dimensional ring structure of a Ringworld makes it an inherently unstable equilibrium. Back to the drawing board, then.