Many of the estimates of the possible existence of advanced non-human technological civilizations in our galaxy (see, for example, AV-212 in the May-June-2021 issue of Analog) conclude that there may be many of them and that our human civilization on Earth may be a rather remote from the locus of earlier civilizations and a late arrival on the scene.  There have been many millions of years during which other high-tech alien civilizations in the galaxy may have arisen and progressed.  So how can we tell if there are actually Elder Races out there in our galaxy, masters of super-advanced technologies that may require the entire energy output of a star for their implementation?

In 1964, the prominent Russian astrophysicist Nikolai Kardashev published a paper examining the possibility of detecting radio-frequency signals from such civilizations.  In the paper, he proposed a ranking of highly advanced civilizations on the basis of their energy use.  This has become known as the Kardashev scale:  a Type I Kardashev civilization would use all of the energy that its planet received as solar radiation from its parent star, about 10¹⁶ W (watts);  a Type II Kardashev civilization would use the entire energy output of its parent star, about 4×10²⁶ W;  a Type III Kardashev civilization would use the entire energy output of its galaxy, about 4×10³⁷ W.

A decade after the publication of Kardashev's paper, Carl Sagan in his book The Cosmic Connection suggested that Kardashev's civilization-ranking scheme should be generalized into a continuous "Kardashev index", which he defined as K ≡ [Log₁₀(P) - 6]/10, where P is the power in watts used by the civilization, so that the consumed power P = 10^(10K+6) W.  For civilizations with Kardashev indexes of 1.0, 2.0, and 3.0, this would correspond to P =10¹⁶ W, 10²⁶ W, and 10³⁶ W.  By this ranking, our present civilization has a small but increasing Kardashev index of only about K = 0.73.

The implication of what Kardashev would call a Type II civilization, as first suggested by Freeman Dyson in 1960, is that in order to intercept all of the available energy from its parent star, it would be necessary to construct structures that would almost completely surround the star, intercepting all of its light output and, as required by thermodynamics, re-radiating a considerable fraction of the received power as waste heat in the infrared region of the electromagnetic spectrum.   Dyson suggested that a star surrounded by such a "Dyson Sphere" might be almost invisible in the visible region of the electromagnetics spectrum but very bright in the infrared region, and thus might be detectable by infrared astronomy.  (At this writing, infrared astronomy has been going strong for many years, but there have been no such detections.)

Now, a group based in Taiwan , with lead author T. Y. Hsiao, has submitted a preprint in which they have suggested an alternative arrangement: why not place the equivalent of a Dyson sphere around a black hole instead of a main-sequence star?  At first glance, that idea sounds like a non-starter. After all, black holes are indeed black objects that gravitationally trap all light and radiation, while main-sequence stars are very luminous, actively generating energy by nuclear fusion and radiating it across the electromagnetic spectrum.

However, that description is not the whole story; around the black holes there are hot accretion disks, hot gas coronas, and in some cases ultra-relativistic jets, all of which are visible to astronomers and represent large amounts of potentially usable energy.  Or, to put it another way, a black hole provides a gravity-based mechanism for converting mass to energy that, in principle, is far more efficient and productive than either nuclear fusion or nuclear fission.

Long ago, John Wheeler described how a space station orbiting a black hole might convert its garbage into usable energy by dropping the unwanted garbage in just the right grazing orbit.  The Hsiao paper considers a much more elaborate scheme for using the gravitational mass-to-energy transformation capabilities of black holes.

There is, however, a fundamental problem with the original Dyson Sphere concept, which the Hsiao paper points out.  A Dyson sphere, subject to inward gravitational forces and outward pressure from light and solar wind from the star that it surrounds, has been shown to be dynamically unstable. A Dyson sphere could not actually be constructed, even by a K≥2 civilization.  The more stable alternatives are a "Dyson swarm", in which the star is surrounded by a very large number of independently orbiting energy collector-distributors, or a "Dyson bubble", employing a vast number of low-mass energy collector-distributors that are effectively solar sails, held in position by light pressure that offsets the pull of gravity.  For the purposes of the Hsiao paper, all of these black-hole-surrounding Dyson constructions are effectively equivalent and lead to the same result.

The paper goes on to consider several possible black hole energy emissions as possible energy sources: Hawking radiation, the accretion disk, Bondi accretion, the corona, and relativistic jets.  Of these possibilities, they conclude that the accretion disk, corona, and jets are the most significant potential sources of usable energy for a K≥2 civilization.  Further, they find that the radius of the Dyson sphere or its equivalent can be much smaller and the available energy much greater from a black hole than could possibly be obtained by surrounding a main-sequence star.  At the upper limit of energy collection, even a K=3 civilization could have its energy needs met by a sufficiently active Dyson-encased black hole, without the necessity of commandeering all the star-supplied energy from an entire galaxy.   They also speculate that, because the black-hole-surrounding Dyson structure is expected to be quite hot, the K≥2 civilization is not likely to be located on the structure, but rather located at some safe distance away, with the energy beamed to them as needed.

So, given that surrounding a black hole with a Dyson structure might efficiently meet the needs of K≥2 civilizations, are there really any of them out there in our galaxy?  The Hsiao paper considers electromagnetic signatures that might be detected.  Basically, for a hot accretion disc, the usual black-body "bump" in emission that peaks in the x-ray region of the electromagnetic spectrum would be modified by an extra waste-heat bump that peaks in the ultra violet or near infrared, depending on the details of the Dyson structure around the black hole.

They consider all the ground-based and space-based telescopes that are currently available to look for such an object, and conclude that with between 6.5 to 100 hours of exposure time, many of the available devices have the definite capability of detecting the object, if it is within 10 kilo-parsecs of the Earth, and the detector is pointed in the right direction and given enough observation time.

OK, let's suppose that one of the recommended astronomical observation devices was pointed in the right direction and actually produced convincing evidence for the existence of a Dyson structure surrounding a black hole in our galactic neighborhood.  What would we do?  How would we react?  How would our view of the cosmos be changed?

Such a discovery would be a partial answer to Fermi's Question: "Where are they?"  It would constitute proof that intelligent life is not unique to the Earth.  However, the only additional information that would be available from the observation would be details like the location and distance away, the temperature of the emitting surfaces, the fraction of the accretion disc surrounded, the variation with time, etc.  There would be no information about the civilization itself, and no immediate possibility of communicating with that civilization.

The main impact would be philosophical and theological.  Some religious beliefs about human uniqueness and "specialness" would be challenged. The discovery would become a beacon for future possibility.  We would know that we are not alone in the universe and that existence and activity away from the protective atmosphere and gravitational pull of a comfortable planet is possible.  We would also know that another civilization has accomplished monumental engineering feats that we can barely imagine, and that we have a very long way to go on the precarious path of scientific and technical progress.  And we would know that there is a whole wonderful universe out there, awaiting our eventual arrival.

John G. Cramer's 2016 nonfiction book (Amazon gives it 5 stars) describing his transactional interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available online as a hardcover or eBook at:  http://www.springer.com/gp/book/9783319246406 or https://www.amazon.com/dp/3319246402.

SF Novels by John Cramer:  Printed editions of John's hard SF novels Twistor and Einstein's Bridge are available from Amazon at https://www.amazon.com/Twistor-John-Cramer/dp/048680450X and https://www.amazon.com/EINSTEINS-BRIDGE-H-John-Cramer/dp/0380975106 .  His new novel, Fermi's Question is coming soon from Baen Books.

Alternate View Columns Online: Electronic reprints of 212 or more "The Alternate View" columns by John G. Cramer published in Analog between 1984 and the present are currently available online at: http://www.npl.washington.edu/av .

References:

Kardashev Civilizations:

N. S. Kardashev, "Transmission of Information by Extraterrestrial Civilizations", (translation) Soviet Astronomy-AJ  v8#2, 217-221 (1964).

Dyson Sphere:

Freeman J. Dyson, "Search for Artificial Stellar Sources of Infrared Radiation", Science 131, 1667 (1960) and "Artificial Biosphere", Science 132, 252 (1960).

Dyson Structure for Black Hole:

Tiger Yu-Yang Hsiao, et al., "A Dyson Sphere Around a Black Hole", ArXiv 2106.15181v2 [astro-ph.HE].

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