One of the most fundamental mysteries facing humanity is whether we are alone in the universe. There are a multitude of ongoing efforts to answer this question, ranging from attempts to find microbial fossils on Mars to speculation about the technological traces that advanced alien civilizations might produce in the cosmic wilds.
Now, researchers led by Tiger Yu-Yang Hsiao, an astronomer at the National Tsing Hua University in Taiwan, have come up with an absolutely wild new idea to aid the search for extraterrestrial intelligence: looking for signs of Dyson spheres, a type of hypothetical alien megastructure, around black holes.
Dyson spheres are typically envisioned as advanced power plants that surround stars and harvest their immense energy for some purpose, for example to power an extremely powerful computer. But the novel hypothesis proposed by Hsiao and his colleagues turns “the idea of the Dyson Sphere inside out” by proposing an “Inverse Dyson Sphere,” or IDS, that feeds on the power of a “cold sun,” or black hole, according to a recent study published in the Monthly Notices of the Royal Astronomical Society.
“Overall, a black hole can be a promising source and is more efficient than harvesting from a main sequence star,” said Hsiao’s team in the study, noting that black holes could potentially produce 100,000 to one million times the energy of living stars. The researchers also added that waste heat from an IDS could be “be detected by our current telescopes,” including the Hubble Space Telescope.
The idea of harvesting light-based energy from objects that notoriously erase light—thus the name “black hole”—may seem counterintuitive. Nothing that passes beyond the event horizon of a black hole can ever escape again, including light, rendering the vast majority of these extreme objects invisible to light-based telescopes.
However, interactions that occur just outside of the event horizon can produce some of the most explosive and radiant light shows in the universe. For instance, gassy material that falls into the gravitational clutches of a black hole tends to accrete in a disk surrounding the event horizon, where tidal forces heat it up to extraordinary temperatures. Sometimes, a super-hot “corona” forms at the center of these disks, or the in-falling material gets shot back out into space in the form of plasma jets that travel at near light-speed.
Hsiao and his colleagues modeled the potential energy yields that an IDS could capture from each of those sources—an accretion disk, a corona, and relativistic jets—along with a few other radiative processes around black holes. The researchers also took into account the huge mass ranges of these objects by envisioning IDS-like structures around stellar-mass black holes that are about five to 20 times as massive as the Sun, all the way up to supermassive black holes like the one at the center of the Milky Way, which contains four million solar masses.
Black holes are dangerous and volatile objects, and the authors make clear that building an IDS would be a significant design and safety challenge. There’s no known metal that could withstand the high temperatures of disks and jets, so alien civilizations would have to make use of exotic technologies to harvest energy from these objects.
“In this study, we only discuss the civilisations which were born and raised from other stars,” Hsiao and his co-authors said in the study. “We speculate that this kind of civilization can collect the energy remotely or treat the energy source as a power station rather than living around a black hole with a harsh environment. Therefore, throughout the paper, we do not discuss whether the temperature and the gravity of our configurations are suitable for life.”
With that caveat, the team’s results suggest that an IDS built around these black hole structures could satisfy the needs of an advanced alien civilization in ways that far exceed a Dyson sphere around a regular star. To demonstrate the possible applications of these megastructures, the team uses the Kardashev scale, a metric developed by astronomer Nikolai Kardashev that divides potential civilizations into levels of energy mastery.
The study focuses in particular on civilizations of Type II, which can harness the energy of stars and operate on interstellar distances, and Type III, which can plug into the power of a full galaxy and may operate on intergalactic distances. Hsiao and his colleagues conclude that “an accretion disk, a corona, and relativistic jets” around a stellar-mass black hole “could be potential power stations for a Type II civilization” in the study. However, only a supermassive black hole could meet the energy needs of a Type III civilization.
As mind-boggling as it is to imagine such megastructures, it may be possible to detect the waste heat from them across a wide variety of wavelengths. The team found that “a hot Dyson Sphere around a stellar-mass black hole in the Milky Way”—which stretches across 30,000 light years— “is detectable in the UV, optical, near-infrared, and mid-infrared,” according to the study. Future modeling and observational efforts “will help us to identify these possible artificial structures,” the team concluded.
Perhaps one day, astronomers on Earth will capture a glimpse of a civilization that can mine a black hole. Even if that day never arrives, the trippy notion of an IDS is the kind of brain food that fuels new ways of looking at the universe, and the possible lifeforms that may inhabit it.