Dyson Sphere

Well, personally, i think that the Ringworld is much more plausible conception. At least, there are less problems with it's stability and integrity.

The main problem of Dyson sphere is... for what reason? To "use all the energy of the star"? The civilization, that could do somtehing like that would probably have much more reasonable ways to obtain such energy, or to transport it from one point to another.

The only possibility is some absolutely greedy civilzation, that didn't want to let anyone else use the energy of THEIR star. Honestly, it would be a good point for some Star Trek Ferengi-themed episode.

A dyson sphere is simply a logical extension of the idea that life requires energy to survive. As more and more beings demand higher and higher standards of living, they will expand into space. Once in space they will be able to access more materials (other planets and asteroids, which allows more beings to have even higher standards of living...

The ultimate expression would be most of the actual planets would be demolished for raw materials (no need for shutters to let starlight in or out), and the Dyson sphere (of any sort, either the theoretical solid one or practical ones made of billions of orbiting habitats) would "glow" in the infrared spectrum, as waste heat radiated away.

Since the amount of energy you can receive from stars outside of your solar system is extremely limited (to say the least), building Dyson spheres is hardly "selfish"

An interesting thing to look up is "Server Sky". This is a plan to build trillions of satellites the size and shape of computer chips, which can be used as computers or as solar energy collectors. Interestingly enough, it is calculated that Server Sky would have to be out to the orbit of Uranus to ensure the "blackbody" temperature of the sky would not rise too much and endanger the Earth....