New examine suggests {that a} black gap in a planet's orbit may very well be an indication of a complicated civilization.

In 1971, English mathematician and Nobel Prize winner Roger Penrose proposed how to harvest energy from a rotating black hole. He argued that this could be done by building a harness around the black hole's accretion disk in which incoming matter is accelerated to near the speed of light, triggering the release of energy in multiple wavelengths. Since then, several researchers have suggested that advanced civilizations could use this method (the Penrose process) to harvest energy and that this represents a technosignature we should be on the lookout for.

Examples include John M. Smart's transcendence hypothesis, a proposed solution to the Fermi Paradox, in which he theorized that advanced intelligences could migrate to the region around black holes to harness the energy found there. The most recent theory comes from Harvard professor Avi Loeb, who proposed in a recent paper how advanced civilizations could rely on a “black hole moon” to power their home planet indefinitely. The way this black hole would illuminate the planet it orbits, he argues, could provide a potential technosignature for future SETI investigations.

Professor Loeb is the Frank B. Baird Jr. Professor of Science at Harvard University, Director of the Institute for Theory and Computation at the Harvard-Smithsonian Center of Astrophysics (CfA), Founding Director of the Black Hole Initiative (BHI), and leader of the Galileo Project. His most recent article, “Illumination of a Planet by a Black Hole Moon as a Technological Signature,” recently appeared in the Research Notes of the American Astronomical Society (RNAAS).

In 1975, Stephen Hawking theorized that black holes emit photons, neutrinos, and some larger particles – later called “Hawking radiation.” Since then, proposals to use black holes as an energy source have generally fallen into one of two camps. One is the possibility of using the angular momentum of their accretion disks (the “Penrose process”) or capturing the heat and energy generated by their hypervelocity jets (possibly in the form of a Dyson sphere). The other is the possibility of funneling matter into the black hole and harnessing the resulting Hawking radiation.

In his paper, Loeb suggests how an advanced civilization could resort to the latter process by constructing a black hole orbiting its home planet. This black hole would be very small, weighing only one hundred thousand tons (1011 g). If left uncontained, this black hole would evaporate in just a year and a half by emitting Hawking radiation. But as Loeb told Universe Today via email, it could be sustained by accreting relatively small amounts of matter (2.2 kg; 4.85 lbs) per second onto it. In return, it would provide an endless supply of energy:

“This system of black holes is the most efficient engine I have ever seen. The fuel is converted to energy with the perfect efficiency of 100%, because the mass that falls into the black hole eventually comes out as Hawking radiation. I have never seen this idea discussed before and had a 'eureka moment' when I realized this a few weeks ago. The only other method of converting mass into radiation with 100% efficiency is matter-antimatter annihilation.”

As Loeb suggests, the amount of antimatter required exceeds anything that humanity can currently achieve. Since 1995, particle accelerators at CERN have managed to produce less than ten nanograms of antimatter, enough to power a 60-watt lightbulb for four hours. By comparison, Loeb's proposed 1011-g black hole could provide 40 quadrillion (4015) watts continuously. “Global energy consumption is a few terawatts, ten thousand times less than the power supply of this black hole,” Loeb added. “The other advantage of this black hole engine is that it can use any form of matter as fuel. It could be garbage. There is no better way to recycle garbage than to convert it into clean energy with 100 percent efficiency.”

Another advantage is that a black hole can use any form of matter as fuel, including the waste produced by civilization. In this respect, a black hole engine would solve the garbage problems of a highly advanced civilization and in return provide an inexhaustible energy supply. Globally, humans produce about 1.92 billion tons (2.12 US tons) of waste annually, which has a serious impact on our environment. This would be enough to power a 1011 g black hole engine for over 437 million years!

As to how such a feat could be achieved, Loeb points to a previous opinion piece in which he theorized that a sufficiently advanced civilization could create a “baby universe” through quantum tunneling. While such a feat could only be achieved by a Type III (or even more advanced) civilization, a black hole engine would be much simpler and perhaps something a Type II civilization could construct:

“That's the big challenge. The good news is that it's much easier to create such a black hole than a baby universe. But any production line of a 1011 g black hole requires compressing matter or radiation to a mass density 60 orders of magnitude higher than the density of solid iron. The density of atomic nuclei or neutron stars is only 15 orders of magnitude higher than the density of solid iron. This was possible in the cosmic radiation density less than a femtosecond after the Big Bang.”

This was the subject of another recent paper by Loeb, in which he argued that black holes can be created from light based on general relativity. But what is most interesting about this proposed black hole engine is the fact that it would be detectable light years away, making it a viable technosignature that would indicate the existence of an advanced civilization. Like many proposed technosignatures, particularly Dyson spheres and other megastructures, the existence of a black hole engine is speculative and theoretical. But as Freeman Dyson himself once said, whatever we can imagine (and if the physics are correct), a sufficiently advanced civilization may already have been created. Loeb said:

“The black hole engine could be discovered as a rocky single planet illuminated by a gamma-ray moon and lacking a stellar-mass companion. If we ever find evidence of such an engine, we would have to consider the possibility that the source was created or captured as a primordial black hole by a highly advanced technological civilization. There is no better indicator of technological innovation than the creation of a furnace from the curvature of spacetime in the form of a mini black hole.”

Further reading: arXiv

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