Where are the space arks? 

The great and terrible scientific and technological inventions we associate with the present are almost all more than half a century old... During that axial age, humanity dreamed of ecumenopolises, of heavy industry and its by-products removed off-planet, of giant cylindrical worlds in orbit, of towers stretching into the heavens, of Dyson swarms of man-made infrastructure harvesting the full power of the Sun, of a Kardeshev II civilisation capable of re-engineering the solar system. These dreams already seem absurd.