In the quest for advanced civilizations, the universe is not often very forthcoming with its answers. But the recent suggestion by Harvard astrophysicist Avi Loeb has set astrophysics and SETI communities abuzz: what if a planet’s source of energy is not a star at all, but a tiny black hole orbiting in as a moon, emitting energy with a signature that is not in nature?
Loeb’s hypothesis, which was published in the AAS Research Notes, takes decades of theoretical foundation, most significantly Roger Penrose’s 1971 suggestion for the extraction of energy from the accretion disk of a rotating black hole a scheme now referred to as the Penrose Process. This process harnesses the ergosphere, an area where spacetime is pulled along by the black hole’s rotation, enabling particles to be emitted with higher energy than with which they arrived, essentially harvesting the black hole’s rotational energy. Advances since then have indicated that, particularly when magnetic fields are involved, the efficiency of this process can be as high as 150 percent a rate that exceeds any power plant on Earth.
However, Loeb’s plan diverges from the Penrose Process. Instead, he suggests a “black hole moon” based on Hawking radiation the quantum mechanical discharge hypothesized by Stephen Hawking in 1974. In contrast to accretion-based engines, this method would depend on the evaporation of a small black hole, with a mass of around 100,000 tons (10¹¹ grams), which, if not refueled, would disappear in about a year and a half. The secret, Loeb says, is to supply the black hole with a constant stream of matter some 2.2 kilograms per second thus sustaining its mass and supporting a steady, gargantuan production of energy: 40 quadrillion watts, or roughly 10,000 times the world’s present energy use (Loeb, Medium).
The engine is, according to Loeb, “the most efficient engine that I ever thought about. The fuel is converted to energy with the perfect efficiency of 100%, because the mass falling into the black hole is ultimately coming out as Hawking radiation.” He adds further, “The only other method for converting mass to radiation with 100% efficiency is matter-antimatter annihilation,” but the technical hurdles to creating antimatter on a large scale are gigantic. Since 1995, CERN has only been able to produce less than ten nanograms of antimatter just enough to light a 60-watt bulb for four hours.
The engineering challenges, however, are daunting. It would take that much matter to compress to a density 60 orders of magnitude greater than solid iron, something the universe alone did in the first femtoseconds of the Big Bang (Loeb, Medium). Even neutron stars, one of the most dense objects, are 45 orders of magnitude short. But Loeb argues, “it is much easier to produce such a black hole than a baby universe,” mentioning his previous conjecture on quantum tunneling as a means of creating universes.
For SETI scientists, the most intriguing part is detectability. A black hole moon would produce a gamma-ray signature a signal that could light up an unaccompanied rogue planet. “The black hole engine could be discovered as a rogue rocky planet that is illuminated by a gamma-ray moon with no stellar-mass companion,” Loeb explained to Universe Today. This technosignature would be different from any natural astrophysical phenomenon, providing a possible sign of advanced engineering.
Hawking radiation itself is still a phantom that cannot be directly observed. Models predict that Hawking radiation is produced by the curvature of spacetime around the event horizon, rather than by particle-antiparticle popping as is so widely popularized. The intensity of Hawking radiation rises as the black hole becomes smaller, so small artificial black holes are vastly brighter than stellar-mass black holes. Laboratory analogues have started to simulate some features of this radiation, but astrophysical mini black holes have yet to be discovered.
The effect resonates much deeper than simple power production. A black hole moon can be used as a cosmic waste recycler, turning any type of matter garbage included into clean energy. For an advanced civilization, this would not only cure energy scarcity but also waste elimination, and with the added advantage of a technosignature detectable over interstellar distances.
As gamma-ray observatories and telescopes survey the heavens, the hunt for such signatures goes on. Chances of discovering a planet shining bright with the radiance of a black hole moon remain in the realm of speculation, but it’s one of the best symbols of human imagination and hubris and the extent of physics when considering the ultimate signposts of technological capability.
