Superior civilizations might use Dyson spheres to gather vitality from black holes. That is how we are able to acknowledge them

Black holes are more than just massive objects that swallow everything around them – they are also one of the largest and most stable sources of energy in the universe. That would make them invaluable to the kind of civilization that needs vast amounts of power, such as a Type II Kardashev civilization. But to use all that power, the civilization would have to orbit the entire black hole with something that could capture the energy it radiates.

One possible solution would be a Dyson sphere – a kind of stellar mega-engineering project that encapsulates an entire star (or, in this case, a black hole) in an artificial shell that captures all of the energy that the object emits at its center . But even if it could capture all of the black hole’s energy, the sphere itself would still be suffering from heat loss. And that heat loss would make it visible to us, according to a new study published by an international team led by researchers from National Tsing Hua University in Taiwan.

Obviously, no such structure has yet been discovered. Still, the paper proves that this is possible, even though no visible light passes the surface of the sphere and a black hole has a reputation for being more light sinks than light sources. To understand how we would recognize such a system, it would be helpful first to understand what this system was designed for.

The authors examine six different sources of energy that a potential Dyson sphere could collect around a black hole. They are the ubiquitous cosmic microwave background radiation (which would inundate the sphere no matter where it is), the black hole’s Hawking radiation, its accretion disk, its Bondi accretion, its corona, and its relativistic jets.

Composite image of Centaurus A, the central supermassive black hole of our galaxy, showing the exiting jets along with their associated gamma rays.
© ESO / WFI (optical); MPIfR / ESO / APEX / A. Weiss et al. (Submillimeter); NASA / CXC / CfA / R. Kraft et al. (X-ray), HESS collaboration (Gamma)

Some of these energy sources are much more powerful than others, with the energy from the accretion disk of the black hole leading the way in terms of potential energy capture. Other types of energy would require very different technical challenges, such as capturing the kinetic energy of the relativistic jets shooting out of the poles of the black hole. Obviously, size plays a big role in how much energy these black holes emit. The authors mainly focus on black holes with stellar mass as a good point of comparison with other potential energy sources. At this size, the accretion disk alone would provide a hundred times the energy output of a main sequence star.

With the materials currently known, it would be impossible to build a Dyson sphere around an object of this size. But the kind of civilization that would be interested in taking on such a technical challenge would most likely have much stronger materials than we do today. Alternatively, they could work with known materials to create a Dyson swarm or bubble that doesn’t require as much material strength but loses some of the energy that a full sphere would capture and multiple planes in coordinating orbits which adds complexity and other factors. Any such structure would have to be outside of the accretion disk to take full advantage of the energy that the black hole emits.

UT video showing Dyson swarm detection

Even a single sphere around a single black hole of stellar mass would be enough to force any civilization that created it into the Type II area, giving it a feat unimaginable with current technology. But even such a powerful civilization will most likely not be able to bend the laws of physics. Regardless of the power level, some of the heat is lost.

For astronomers, heat is just another form of light – infrared, to be precise. And according to the researchers, the heat emitted by a Dyson sphere around a black hole should be detectable by our current telescopes, like the Wide Field Infrared Survey Explorer and the Sloan Digital Sky Survey, at least up to a distance of about 10 kpc. That is about 1/3 of the distance across the entire Milky Way. No matter how close they were, they wouldn’t appear like traditional stars, but could be detected using the radial velocity method commonly used to find exoplanets.

Sloan Digital Sky Survey, one of the telescopes that could find a potential Dyson sphere around a black hole.
Source: SDSS team, Fermilab Visual Media Services

While this is useful theoretical work, there has certainly been no evidence for such a structure – Fermi’s paradox still holds. But given all the data we’re already collecting on these telescopes, it might be interesting to search them again to see if heat is emanating from a place where it would not be expected. It would be worth the time to look at at least such a fundamentally groundbreaking discovery.

Learn more:
arXiv – A Dyson sphere around a black hole
ScienceAlert – Dyson spheres around black holes could reveal extraterrestrial civilizations, scientists say
Vice – Black hole megastructures could power alien civilizations, scientists say

Mission statement:
Example of a partial Dyson sphere around a star.
Credit – Kevin Gill

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