Black holes can release enormous energy through hot disks, surrounding plasma and powerful jets, enough to support a civilization far beyond ours. Any giant collector would release unused energy as heat, creating a glow that current telescopes could potentially detect. Astronomers could search existing black hole catalogs for unusual ultraviolet, visible or infrared signals that normal stars and disks cannot explain. Black holes appear built to hide energy, not advertise it. Yet a civilization wrapping one in vast collectors could create a detectable glow, turning one of the universe’s darkest objects into a beacon of technology. The clue would come from heat. An international team led by researchers at National Tsing Hua University in Taiwan examined whether an advanced civilization could build a Dyson sphere around a black hole. Their analysis also asked whether existing telescopes could detect it. A Dyson sphere is a hypothetical system that surrounds an energy source and captures its output. A complete shell would be mechanically unstable, so the team also considered swarms, rings, bubbles and light sails. Six ways …