By 3100 A.D., Earth's skyrocketing population might require so much energy—to run our virtual-reality consoles, jetpacks, and Hyperloops—that our power plants won't be able to keep up. Physicist Freeman Dyson proposed a solution to just that sort of crisis in 1960: a machine that encircles a star in a shell of solar collectors to harness its energy output. In our solar system, such a sphere could surround the sun at 93 million miles to collect 400 septillion watts per second, trillions of times more energy than the world uses now. The catch is that there are no materials strong enough to craft a solid mega-structure that big. We could, however, deploy tens of thousands of solar panels—a Dyson swarm—to harvest that same energy.