Illumination of a Planet by a Mini-Black-Hole Moon as a Technological Signature
Are you concerned about your high electric bill as well as about having too much trash?
Imagine a magic trash-can that returns clean energy from any trash you throw into it, with a conversion efficiency of mass to energy of 10% — which is ten times higher than the most efficient nuclear fuel. Once the trash enters into this magic can, it disappears — never to be seen again. Sounds like a fantasy? Not so. Nature features all these qualities in a black hole. Once trash enters the event horizon, it becomes invisible to the outside world.
Advanced civilizations could satisfy their power needs by processing their trash through an accretion disk around a mini black hole that orbits their planet like a moon.
The technological challenge in producing a mini black hole involves the enormous mass density required to make it. If it is possible to manufacture a mini black hole and keep it as a luminous moon, then this artificial furnace could replace the Sun in illuminating and warming a rogue planet that is otherwise frozen and uninhabitable.
Cosmologists speculate that mini black holes might have been produced in the infant Universe when the radiation density was high enough. However, a sufficiently advanced technological civilization might have been able to manufacture a mini black hole in order to satisfy its energy needs.
Stephen Hawking realized in 1974 that a mini black hole would shine on its own, even without an external supply of fuel. The so-called Hawking radiation is brighter for smaller black holes, making them evaporate over a finite time. What would be the optimal black hole mass for providing the energy supply of an Earth-size planet over a year?
Imagine a mini black hole at a low Earth orbit with an altitude of 1,500 kilometers, about a quarter of the radius of the Earth. Such a black hole would supply the energy flux that Earth is currently receiving from the Sun if its luminosity (energy emitted per unit time) is ten billion times smaller than the solar luminosity.
For an Earth-like planet, the source would illuminate the ground under it with daytime alternating to nighttime and back to daytime every ~90 minutes. The duration of an artificial day would scale with the orbital radius of the source to the 1.5 power.
The required energy flux can be supplied by a mini black hole with a mass of a hundred thousand tons, which is thirty times larger than the mass of Starship and equivalent to the mass of an asteroid with a 60-meter diameter. The Hawking evaporation time for such a black hole is of order a year. In order to maintain the operation of the furnace for a longer period than a year, it would be necessary to fuel it with trash carrying its mass every year. This act would resemble the deposition of logs in a wood-burning fireplace. The civilization could automate that by steadily launching material from a twin-satellite of similar mass, orbiting in the vicinity of the black hole and feeding its accretion disk in a steady state to compensate for its Hawking radiation loss.
The Hawking radiation of such a mini black hole would include high-energy particles with a hundred times the rest mass of a proton. These particles would be reprocessed by the planet’s atmosphere into heat and optical light that could nurture life on the planet’s surface.
The technology to produce a mini black hole of this mass would require creating a mass density that is 61 orders of magnitude larger than that of water or 44 orders of magnitude larger than the density of an atomic nucleus. Whether such a technological feat was accomplished by an advanced civilization in the Milky-Way galaxy remains to be seen. Gamma-ray telescopes can search for moons made of mini-black-holes as a technological signature around exoplanets.
If we ever detect a rogue rocky planet which is illuminated by a bright gamma-ray moon with no stellar-mass companion, we would need to consider the possibility that the source was created or trapped by a highly-advanced technological civilization. Last night in Miami, Florida, I was honored with the “Space Anomaly Hunters” award for 2024, but I would have been far more thrilled to discover an anomalously bright moon.
There is no better marker of technological innovation than creating a furnace out of spacetime curvature in the form of a mini black hole.
Currently, there are a large number of startup companies aiming to make compact fusion reactors. A mini black hole would be far more efficient and environmentally friendly.
For the corresponding scientific paper, click here.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, was published in August 2023.
