In 1960, physicist Freeman Dyson put forth a paper in Science that changed the way we talked about alien civilizations.
In it, Dyson conjectured that a sufficiently advanced civilization would harness the power of its home star to generate power on a massive scale. He also proposed that nascent search for extraterrestrial intelligence (SETI) programs should “search for sources of infrared radiation” to “accompany the recently initiated search for interstellar radio communications.”
These structures became known as Dyson spheres, and a handful of programs have sought them out by looking for weird infrared signatures around neighboring stars. But what if a more advanced extraterrestrial civilization is not only building megastructures that surround stars, but turning to the infamous beasts of the cosmos for energy: black holes.
A recent paper published in Monthly Notices of the Royal Astronomical Society suggests that scientists should start to look for Dyson spheres around black holes as an indication of an alien civilization.
What are Dyson spheres?
A Dyson sphere is a theoretical megastructure that surrounds a star to collect its energy output.
Dyson proposed the spheres as a solution to a growing alien civilization needing a large enough energy output to sustain its existence. They can accomplish this by using space-based stellar energy harvesting swarms.
Dyson’s based his proposal on the fact that a planet only receives a small fraction of the energy produced by its host star. About one one-billionth of the Sun’s total energy output actually reaches the Earth.
By creating a space-based structure, an alien civilization could meet the energy needs to push its civilization to the next level.
Can you build a Dyson sphere around a black hole?
The recent study suggests that instead of building a Dyson sphere around a star, a civilization can build the same megastructure around black holes.
A black hole is a region of space with a gravitational pull so strong that nothing can escape its influence, not even light itself. Scientists believe that every major galaxy has a black hole lurking at its center.
The researchers behind the new study suggest that building a Dyson sphere around a black hole would allow an alien civilization to garner energy from six sources:
- The cosmic microwave background, electromagnetic radiation left over from the Big Bang
- Hawking radiation, theoretical radiation released by the black hole
- The accretion disk, a disk of material that orbits around a black hole
- Bondi accretion, the spherical accretion of ionized gas around an object.
- A corona, a bright region of white light above a black hole that forms from hot gas particles.
- Powerful jets of radiation and particles emitted from the black hole at nearly the speed of light.
By not only relying on radiation as the main source of energy but a Dyson sphere around a black hole would also collect five times as much energy as one around a star, according to the study.
Can we detect alien Dyson spheres?
In 2015, astronomer Tabetha Boyajian spotted a mysteriously dim star, its light flickering irregularly in a way that scientists had never observed before.
Some scientists believed that the star, officially known as KIC 8462852 but later dubbed Boyajian’s Star, had a Dyson sphere built around it, which resulted in its strange dimming. The idea was later dismissed in favor of debris around the star, but it sparked a search for these megastructures.
“There are a few people who think about looking for them, they are looking for large structures that might pass in front of the star or anything else, but large things,” Jason Wright, a Penn State astronomer and director of the Penn State Extraterrestrial Intelligence Center, tells Inverse. “So if you think about (Boyajian’s) Star, and the way that it got dimmer and all of its weirdness [...] That's the sort of thing they're looking for, more of those.”
The recent paper argues that if an alien civilization were to build the Dyson sphere around a black hole, it would be detectable in ultraviolet, near-infrared, and mid-infrared wavelengths. But Wright suggests it might be exceedingly difficult.
“In general, that'd be harder to detect,” he says. “Or at least, harder to tell that it was a Dyson Sphere.”
Using current telescopes such as the Galaxy Evolution Explorer, an orbiting space telescope launched in 2003, the authors argue that one could detect such structure around a distant star.
The challenges of building a Dyson Sphere around a black hole
But constructing a sphere around a star sounds tricky enough — civilizations would need materials that would not tear themselves apart or gravitationally attach themselves around any nearby objects. There’s also a possibility that a meteor or asteroid could nudge one of the pieces out of place.
“Think of a Dyson Sphere is a very large building on a wall,” Wright says. “The basic component is like a little brick, and we know how to build bricks to build solar panels and load them into space. If you want to have so many of them that you block a significant amount of the sun's light, then we would need to build billions and billions more of them. And that's challenging.”
How challenging? You might need an entire planet’s worth of material, Wright says.
Combining those challenges with the properties of a black hole gets even more tricky. Black holes feed on surrounding material from stars or other objects in its vicinity, and some of that material then shoots out in the form of two powerful jets.
The black hole is surrounded by its event horizon, a boundary that outlines the black hole from which nothing that enters the black hole can be observed from outside.
If a structure were to be built around the black hole’s event horizon, it would constantly be disturbed by falling matter, and it would have to be built to survive temperatures up to 2,700 degrees Celsius.
On the other hand, pairs of particles, one particle and one antiparticle, separate due to the strong gravitational field whereby one gets inside the event horizon, and the other is emitted as radiation.
Using that radiation and the radiation of the accretion disc, there is a lot of potential energy swarming around a black hole. If a strong enough material is used, an alien civilization could potentially tame a black hole, feeding it matter to produce energy and harness its force.
Should a civilization this advanced exist, then they would probably already know of our existence by the time we discover the black hole Dyson sphere.
Abstract: The search for extraterrestrial intelligence (SETI) has been conducted for nearly 60 years. A Dyson Sphere, a spherical structure that surrounds a star and transports its radiative energy outward as an energy source for an advanced civilisation, is one of the main targets of SETI. In this study, we discuss whether building a Dyson Sphere around a black hole is effective. We consider six energy sources: (i) the cosmic microwave background, (ii) the Hawking radiation, (iii) an accretion disk, (iv) Bondi accretion, (v) a corona, and (vi) relativistic jets. To develop future civilisations (for example, a Type II civilisation), 4×1026W(1L⊙) is expected to be needed. Among (iii) to (vi), the largest luminosity can be collected from an accretion disk, reaching 105L⊙, enough to maintain a Type II civilisation. Moreover, if a Dyson Sphere collects not only the electromagnetic radiation but also other types of energy (e.g., kinetic energy) from the jets, the total collected energy would be approximately 5 times larger. Considering the emission from a Dyson Sphere, our results show that the Dyson Sphere around a stellar-mass black hole in the Milky Way (10kpc away from us) is detectable in the ultraviolet(10−400nm), optical(400−760nm), near-infrared(760nm−5μm), and mid-infrared(5−40μm) wavelengths via the waste heat radiation using current telescopes such as Galaxy Evolution Explorer Ultraviolet Sky Surveys. Performing model fitting to observed spectral energy distributions and measuring the variability of radial velocity may help us to identify these possible artificial structures.