A sort of uber-world envisioned by
Larry Niven. The concept is of a gigantic ring encircling a star, with the living space on the inside. It would have a
radius of 1
AU, and be spinning at 770 mi/sec to give it 1 Earth gravity. This would cause approximately Earth-like conditions on the entire inner surface of the ring. Niven estimates that one could be built of this size out of roughly the mass of
Jupiter. This would give us roughly three million times the usable surface area of
Earth. Or, as he put it, by the time we need one we'll know how to build it.
Larry Niven has fleshed out this concept to an amazing degree. For example, you'd put
shadow squares in an orbit closer to the star, which would allow for day-night cycles. You produce energy by lining the sunward side of the squares with solar
collectors, and beam it back to recieving dishes on the ringworld as
microwaves. A set of 1000-mile high walls on the edges hold the
atmosphere in. Any oceans and mountains would be curved directly into the material the ringworld was made out of- you'd get a
bas relief by looking at the other side. You could put a series of electromagnetic rings on the rim to create an em catapult for launching ships with. A series of
Bussard ramjets would keep the Ringworld on-center. Oceans would have to have a plumbing system to keep all the soil from winding up at the sea bottom. A meteor defense system would be necessary (in the book this was accomplished by using the star as an
astronomically sized
gas laser).
In other words, it's a gigantic concept to get your head around. It's probably a more practical concept than the
Dyson Sphere, but vastly more efficient than these lame ball-shaped worlds we use now.
StrawberryFrog's write-up is good, but is perhaps a little short-sighted about solving some of the technical issues involved in building one of these suckers- for example, spinning up the ringworld is an easy problem to solve. All you have to do is take the ramscoop motors mounted on the rim walls and point their exhausts
tangentially- and wait. I suspect anyone who's building a ringworld can wait long enough for the world to spin itself up. If not, well, you could always use the flare devices in the mirror squares to increase the star's output.
As for finding the matter, that's a relatively simple
enterprise (heh- sci-fi pun!) as well. The easy way to do is to find a really large star and mine it. If you spin up a star (left as an exercise to the reader), a bunch of the matter will escape, and all you have to do is catch it (and if you've already got ramscoop drives, that's obviously not difficult).