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In the long-running TV show Doctor Who, aliens known as time lords derived their power from the captured heart of a black hole, which provided energy for their planet and time travel technology. The idea has merit, according to a new study. Researchers have shown that highly advanced alien civilizations could theoretically build megastructures called Dyson spheres around black holes to harness their energy, which can be 100,000 times that of our Sun. The work could even give us a way to detect the existence of these extraterrestrial societies.

“I like these speculations about what advanced civilizations might do,” says Tomáš Opatrný, a physicist at Palacký University Olomouc, who was not involved with the work but agrees that a Dyson sphere around a black hole would provide its builders with lots of power.

If humanity’s energy demands continue to grow, a point will come when our power consumption approaches, or even exceeds, the total energy available to our planet. So argued physicist Freeman Dyson way back in 1960. Borrowing from British sci-fi author Olaf Stapledon, Dyson proposed that any sufficiently advanced civilization that wanted to survive would need to build massive structures around stars that could harness their energy.

Most of these Dyson spheres involve numerous satellites orbiting or sitting motionlessly around a star. (A solid shell totally encasing a solar body—as envisioned in a Star Trek: The Next Generation episode—is considered mechanically impossible, because of the gravity and pressure from the central star.) Such megastructures would have to transform that solar energy into usable energy, a process that creates waste heat. This heat shows up in the midinfrared spectrum, and stars with an excess infrared signal have become a key target in the search for extraterrestrial life.

But astronomer Tiger Hsiao of National Tsing Hua University says we might be looking for the wrong thing. In a new study, he and colleagues set out to calculate whether it would also be possible to use a Dyson sphere around a black hole. They analyzed black holes of three different sizes: those five, 20, and 4 million times the mass of our Sun. These, respectively, reflect the lower and upper limits of black holes known to have formed from the collapse of massive stars—and the even more enormous mass of Sagittarius A*, the supermassive massive black hole thought to lurk at the center of the Milky Way.

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