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Friday, 20 August 2021

Black hole could provide energy for advanced alien civilization

Are we the only technologically advanced species in the entire Universe? Scientists have been pondering this question for years, and it may never be answered. But assuming an alien intelligence exists, what would their technology look like? And above all, how to detect this technology which is unknown to us? Researchers at the Institute of Astronomy at National Tsing Hua University in Taiwan are trying to provide some answers, assuming that a potential advanced life form would rely on the exploitation of a Dyson sphere.

A Dyson sphere - named after physicist Freeman Dyson, who first described it in 1960 - is a hypothetical spherical-shaped structure, two to three meters thick, located around a star, which is said to be designed to capture all of its energy. For the physicist, such a sphere could be used by an advanced civilization to meet increased energy needs, which planetary resources alone can no longer satisfy; it could even shelter habitats.

In 1964, Russian astronomer Nikolai Kardachev established a classification of extraterrestrial civilizations according to how they harness the energy available in their environment. A Type I civilization is able to use all the energy available on its planet and store it for later use; a type II civilization must be able to collect all the energy of its star. Finally, a type III civilization can theoretically exploit all the energy emitted by the galaxy in which it is located. According to this classification, a Dyson sphere belongs to type II.

Several potential energy sources "around" the black hole

In his article, Dyson explains that infrared emissions produced by thermal energy could escape as the sphere captures and converts stellar energy. So this infrared signature, if we could detect it, would allow us to spot extraterrestrial civilizations. A team of researchers from Taiwan's National Tsing Hua University took the concept even further, imagining that this Dyson sphere could be located not around the star of a planetary system, but around a black hole . In particular, it was a question of whether in this case it would be possible to detect it from Earth.

“In this study, we consider an energy source of a well-developed Type II or Type III civilization. They need a source of energy that is more powerful than their own sun,” the researchers write in their article. According to their calculations, an accretion disk, corona, and relativistic jets could be potential power plants for a Type II civilization. In other words, a stellar mass black hole could be a good candidate for delivering much more energy than the main star in a system.

True, black holes are notorious for absorbing any surrounding material and not letting anything escape. However, many physical processes take place around black holes, and the energy produced by these processes could potentially be harvested and harnessed by an advanced civilization. In their article, the researchers mention in particular the thermal energy generated by the accretion disc: through friction, it can reach millions of degrees. Evaporation from black holes, which results in Hawking radiation , can also be the source of usable radiation. Relativistic plasma jets - which reach speeds close to that of light - just like the corona of magnetized plasma surrounding a black hole, could also be significant sources of energy.

Enough energy for a Type II civilization

Based on this principle, and on the basis of black hole models similar to Sagittarius A * - the supermassive black hole located at the heart of our galaxy - the researchers therefore attempted to determine whether a Dyson sphere surrounding a black hole could theoretically collect enough energy for the needs of an advanced civilization.

They estimate that an energy of 4.1026 W (equivalent to the luminosity of the Sun) is necessary for the development of future type II civilizations. However, an accretion disk alone is capable of emitting the equivalent of 105 solar luminosity; and if a Dyson sphere is capable of capturing all types of energy (and not just the energy provided by electromagnetic radiation), especially the kinetic energy of relativistic jets, the energy collected could be five times higher!

It turns out that such structures could be detected at several wavelengths. The results indeed show that a Dyson sphere around a stellar mass black hole located in the Milky Way would be detectable in the ultraviolet (100 to 400 nm), the visible (400 to 760 nm), the near infrared. (760 nm to 5 μm) and mid-infrared (5 to 40 μm) via waste heat radiation, using current telescopes, such as the Galaxy Evolution Explorer - a NASA telescope dedicated to observations in ultraviolet radiation .

The researchers stress, however, that the task is complex, because black holes already emit a lot of radiation in these different wavelength ranges and the radiation specific to the Dyson sphere, if it exists, would therefore be "drowned" in the mass. To work around the problem, the team suggests performing other measurements simultaneously, such as variability in brightness or radial velocity - as the black hole would in theory be subject to the gravity of the sphere. This is the only way, according to them, to ever detect these hypothetical man-made structures.


A Dyson sphere around a black hole by  Tiger Yu-Yang Hsiao, Tomotsugu Goto, Tetsuya Hashimoto, Daryl Joe D Santos, Alvina Y L On, Ece Kilerci-Eser, Yi Hang Valerie Wong, Seong Jin Kim, Cossas K-W Wu, Simon C-C Ho ...
Monthly Notices of the Royal Astronomical Society, Volume 506, Issue 2, September 2021, Pages 1723–1732,

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