Originally published 2021-09-05.
Artist’s impression of a Dyson sphere enclosing a star. Image: Dotted Yeti / Shutterstock.
Could an advanced civilisation wrap a black hole in an energy-collecting shell, and could we spot it from Earth? A 2021 paper in the Monthly Notices of the Royal Astronomical Society says the idea is at least physically worth considering. Its authors calculated that the matter swirling into a black hole can release far more usable energy than an ordinary star, and that the waste heat from such a structure could, in principle, show up in existing sky surveys.
The short answer, then: yes in theory, not yet in practice. The study is a thought experiment that defines what astronomers might look for. It did not detect anything, and nobody has found an alien megastructure around a black hole or anywhere else.
What a Dyson sphere actually is
The concept has a literary ancestor. In his 1937 novel Star Maker, Olaf Stapledon imagined solar systems ringed with light traps that caught a star’s escaping energy. Physicist Freeman Dyson turned the image into a scientific proposal in 1960, in a short Science paper titled “Search for Artificial Stellar Sources of Infra-Red Radiation.”
Dyson’s reasoning was simple. If a technological species keeps growing, its energy demand will eventually outstrip what a single planet can supply. The biggest power source nearby is its own star, so a sufficiently ambitious civilisation might surround that star with collectors. Dyson himself did not picture a solid shell; he called a rigid sphere mechanically impossible and imagined instead a loose swarm of independent orbiting objects. Popular culture kept the solid-ball version anyway, and Dyson later admitted he was not thrilled that the idea carried his name.
The most useful part of his paper was the detection argument. Any structure that absorbs starlight must get rid of the energy it does not use, and it does so by glowing at longer wavelengths. A star hidden behind a swarm of collectors would therefore look dimmer in visible light and unusually bright in the infrared.
Why researchers looked at black holes
The 2021 study, titled “A Dyson sphere around a black hole,” was led by Tiger Yu-Yang Hsiao of the Institute of Astronomy at National Tsing Hua University in Taiwan, with co-authors from Taiwan, Turkey and the United Kingdom. The team framed the question with the Kardashev scale, a ranking Soviet astronomer Nikolai Kardashev proposed in 1964:
- Type I civilisations use all the energy available on their home planet.
- Type II civilisations harness the full output of their star, for example with a Dyson structure.
- Type III civilisations draw on the energy of an entire galaxy.
For scale, Carl Sagan estimated in the 1970s that humanity rated roughly 0.7 on this ladder. A Type II society, by the paper’s estimate, needs about 4 × 1026 watts, which is roughly the Sun’s entire luminosity.
A black hole might look like a strange place to find power, since nothing escapes it. The trick is that the energy comes from the surroundings. Gas falling towards a black hole forms a flattened, spinning accretion disc that heats to millions of degrees. Above the disc sits a hot corona, and many black holes also launch narrow jets of particles at close to light speed. The authors compared six possible sources: the cosmic microwave background, Hawking radiation, the accretion disc, spherical (Bondi) accretion, the corona and the jets.
What the study calculated
Two of the six options fail quickly. Hawking radiation from a stellar-mass black hole is vanishingly faint, and the cosmic microwave background only yields useful power around enormous black holes. The accretion disc is the clear winner. In the paper’s words, it “could provide hundreds of times more luminosity than a main sequence star,” even when the black hole is feeding slowly.
The authors modelled three examples: black holes of 5 and 20 solar masses feeding at modest rates, and a supermassive one of about four million solar masses, similar to the black hole at the centre of the Milky Way. For the stellar-mass cases, the disc alone could deliver up to around 100,000 times the Sun’s output. Capturing the corona and the kinetic energy of the jets as well could raise the total by roughly a factor of five. On the Kardashev scale, the three scenarios come out at about 2.5, 2.6 and 3.0.
The study also considered how large and how hot such a structure would need to be. Assuming building materials that can survive about 3,000 kelvin, the collecting surface would have to sit far out from the black hole, yet its total area would still be around a hundred times smaller than a sphere the size of Earth’s orbit around the Sun. That is enormous by human standards, but smaller than the classic star-wrapping version.
How astronomers could tell one apart
Like Dyson’s original proposal, this one relies on waste heat. The team estimated that a structure around a black hole about 10 kiloparsecs away (roughly 33,000 light-years) would glow in ultraviolet, optical, near-infrared and mid-infrared light at brightness levels within reach of past and current surveys, including NASA’s GALEX ultraviolet mission, Pan-STARRS, WISE and the Sloan Digital Sky Survey.
Brightness alone would prove nothing, because natural dust clouds also absorb light and re-emit it as heat. The authors therefore suggest two cross-checks. One is fitting the full spectrum of an object to see whether it matches a model of an artificial shell better than a natural one. The other is watching for radial-velocity wobbles that would reveal a black hole companion orbiting with a visible star. They also note a subtle side effect: a shell covering 90 percent of the sky around the black hole would make it look as if it were feeding ten times more slowly than it really is.
What other searches have found so far
Astronomers have been hunting for Dyson-style infrared signatures for decades. A Fermilab search led by Richard Carrigan, using data from the IRAS infrared satellite, reported 17 ambiguous sources in 2006 and called only four of them “amusing but still questionable.” In 2015, the strange dimming of KIC 8462852, often called Tabby’s Star, set off a wave of alien-megastructure headlines; follow-up observations published in 2018 found the pattern consistent with dust.
Most recently, the 2024 Project Hephaistos II study combed Gaia, 2MASS and WISE data and flagged seven red dwarf stars within about 1,000 light-years with excess infrared light. Its authors were careful not to claim artificial structures, and other astronomers quickly suggested more ordinary explanations such as distant dust-obscured galaxies lining up behind the stars. The pattern is consistent: odd signals appear, then natural causes explain them.
Common misunderstandings
“Scientists found alien Dyson spheres”
No. The 2021 paper is theoretical. It asks what a hypothetical structure would look like, not whether one exists.
“Aliens have probably built them”
Some headlines implied the authors thought advanced civilisations likely use black hole power plants. The paper itself makes no such probability claim. It works through the physics conditionally: if a civilisation wanted this much power, here is where it could get it and here is the signal it would leave.
“A Dyson sphere is a solid shell”
Dyson considered that version unworkable. Most scientific discussions picture swarms of separate collectors, which is also closer to how the black hole study treats the problem.
Why the thought experiment still matters
The search for extraterrestrial intelligence has long leaned on radio signals, which only reveal civilisations that choose to broadcast. Technosignatures such as waste heat are different: they would leak out whether anyone intended them or not. Papers like this one widen the list of places worth checking and, just as importantly, spell out the natural look-alikes that must be ruled out first.
For now, the honest position is that we have no evidence of anyone harvesting a black hole. What we do have is a clearer description of what that would look like, and survey data that can be re-examined with that description in mind.
Sources: Hsiao, T. Y.-Y. et al., “A Dyson sphere around a black hole,” MNRAS 506 (2021); Wikipedia: Dyson sphere; Wikipedia: Kardashev scale; first highlighted by ScienceAlert. Image: Dotted Yeti / Shutterstock.
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