The basic concept of the Dyson Sphere is straightforward: rather than squander stellar output by permitting it to radiate into space, one can surround the star with a shell to intercept light, thus permitting the energy to be put to use. From the viewpoint of the star system’s inhabitants, this would be a useful source of energy (although any civilization able to turn its planets into a Dyson Sphere would be at most twenty years from commercial controlled fusion). From our Earth perspective (as was Dyson’s), the Dyson Sphere’s infrared emissions, visible across the lightyears, would be evidence of an alien civilization of tremendous achievement.