Imagine a distant future when humans reach beyond our pale blue dot, forge cities on planets  thousands of light-years away, and maintain a galactic web  of trade and transport. What would it take for our civilization to make that leap? There are many things to consider— how would we communicate? What might a galactic government look like? And one of the most fundamental  of all: where would we get enough energy to power that civilization— its industry, its terraforming operations, and its starships? An astronomer named Nikolai Kardashev proposed a scale to quantify an evolving civilization’s  increasing energy needs. In the first evolutionary stage,  which we’re currently in, planet-based fuel sources  like fossil fuels, solar panels and nuclear power plants are probably enough to settle other  planets inside our own solar system, but not much beyond that. For a civilization on the third  and final stage, expansion on a galactic scale  would require about 100 billion times more energy than the full 385 yotta joules our sun releases every second. Barring a breakthrough in exotic physics, there’s only one energy source  that could suffice: a supermassive black hole. It’s counterintuitive to think  of black holes as energy sources, but that’s exactly what they are,  thanks to their accretion disks: circular, flat structures formed  by matter falling into the event horizon. Because of conservation  of angular momentum, particles there don’t just plummet  straight into the black hole. Instead, they slowly spiral. Due to the intense gravitational field  of the black hole, these particles convert their potential energy to kinetic energy as they inch closer to the event horizon. Particle interactions allow for this kinetic energy to be radiated out into space at an astonishing matter-to-energy  efficiency: 6% for non-rotating black holes,  and up to 32% for rotating ones. This drastically outshines nuclear fission, currently the most efficient  widely available mechanism to extract energy from mass. Fission converts just 0.08%  of a Uranium atom into energy. The key to harnessing this power  may lie in a structure devised by physicist Freeman Dyson, known as the Dyson sphere. In the 1960s, Dyson proposed  that an advanced planetary civilization could engineer an artificial sphere  around their main star, capturing all of its radiated energy  to satisfy their needs. A similar, though vastly  more complicated design could theoretically be applied  to black holes. In order to produce energy,  black holes need to be continuously fed— so we wouldn’t want to fully cover  it with a sphere. Even if we did, the plasma jets  that shoot from the poles of many supermassive black holes would blow any structure  in their way to smithereens. So instead, we might design  a sort of Dyson ring, made of massive,  remotely controlled collectors. They’d swarm in an orbit  around a black hole, perhaps on the plane  of its accretion disk, but farther out. These devices could use  mirror-like panels to transmit the collected energy to a powerplant, or a battery for storage. We’d need to ensure that these collectors are built at just the right radius: too close and they’d melt  from the radiated energy. Too far, and they’d only collect  a tiny fraction of the available energy and might be disrupted by stars orbiting the black hole. We would likely need several Earths worth of highly reflective material like hematite to construct the full system— plus a few more dismantled planets to make a legion of construction robots. Once built, the Dyson ring  would be a technological masterpiece, powering a civilization spread across every arm of a galaxy. This all may seem like wild speculation. But even now,  in our current energy crisis, we’re confronted  by the limited resources of our planet. New ways of sustainable energy production will always be needed, especially as humanity works  towards the survival and technological progress of our species. Perhaps there’s already a civilization out there that has conquered  these astronomical giants. We may even be able to tell by seeing the light  from their black hole periodically dim as pieces of the Dyson ring pass  between us and them. Or maybe these superstructures are fated  to remain in the realm of theory. Only time— and our scientific ingenuity—  will tell.