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Scientific Record

• Loeb correctly explains tidal forces and black-hole accretion, but repeatedly turns simplified concepts into sweeping conclusions.
• His proposed “backyard” black hole is a mathematical thought experiment, not an engineering pathway.
• No observational evidence supports the suggestion that supermassive black holes are extraterrestrial construction projects.

By Samuel López | USA Herald

Avi Loeb has spent years urging the public to keep an open mind about the possibility that advanced civilizations may leave technological debris in space. In a 2024 interview with Live Science, Loeb said he wanted to search interstellar material for possible “space trash” or functional technology.

That premise, at least in principle, is testable. A physical object can be found, measured, independently analyzed and compared against natural explanations.

But Loeb’s recent Medium essay, “How to Build a Black Hole in Your Backyard,” moves far beyond that evidence-based framework. It begins with legitimate general-relativity principles, then builds toward an extraordinary and unsupported suggestion that some 100-million-solar-mass black holes could be “cosmic engineering projects” created by advanced civilizations.

The issue is not whether scientists should ask unconventional questions. They should. The issue is whether the question is presented with the evidentiary discipline that separates a provocative hypothesis from an alluring narrative.

Loeb is on firm ground when describing the Apollo 15 hammer-and-feather experiment, free fall, tidal acceleration and the fact that black holes can convert some infalling matter’s gravitational energy into radiation. NASA likewise explains that accretion around a black hole can radiate roughly 10% of a falling object’s rest-mass energy under certain conditions. But that is not the same thing as a black hole simply converting “trash” into clean energy with a guaranteed 10% efficiency. The efficiency depends on the accretion flow, black-hole spin, orbital dynamics, radiative losses and whether the energy can actually be captured and used. NASA’s black-hole overview

That distinction is central. Matter crossing an event horizon does not send usable energy back out. The observable energy is generated outside the horizon, in extraordinarily hot and violent material spiraling inward. Any civilization attempting to use such a process would still face the unsolved problems of safely feeding matter into the system, containing lethal high-energy radiation, converting that radiation into useful power and preventing the device from becoming a catastrophic hazard.

“The main challenge in producing artificial black holes is the required mass density. The recipe for making a black hole out of rocks is as follows: Fill a big enough volume with rocks, so that it matches the event horizon of the corresponding black hole.” - Avi Loeb, Medium

Loeb’s “recipe” also oversimplifies black-hole formation. He writes that one could make a black hole by filling a sufficiently large volume with rocks until the material matches the event horizon of the corresponding mass. As a compactness thought experiment, the statement gestures toward the Schwarzschild-radius condition. But it is not an engineering blueprint.

Assembling 100 million Suns’ worth of rock within an Earth-orbit-scale region would require controlling more mass than the central black hole in the Milky Way. It would require overcoming angular momentum, orbital motion, heat, fragmentation, radiation pressure and gravitational instability. Calling that a “recipe” obscures every physically decisive obstacle.

The essay also treats average density as if it were the controlling explanation for spacetime curvature. That is an incomplete and potentially misleading presentation of general relativity. Tidal effects are described by spacetime curvature, but curvature near a black hole is not merely a local reading of average mass density inside a chosen volume. Indeed, the space outside a black hole is vacuum, yet it still has powerful tidal curvature. The “mass divided by horizon volume” framing can be a useful classroom analogy, but it cannot bear the argumentative weight Loeb gives it.

There is a similar problem with his statement that no astrophysical process can form black holes below a solar mass because no modern astrophysical environment reaches the necessary density. The formation of a black hole is not determined by locating a pre-existing environment with an average density above a threshold. It is a dynamical collapse problem governed by gravity, pressure, angular momentum, energy transport and the properties of matter under extreme conditions. Primordial black holes remain hypothetical, but their possible existence does not validate an artificial-black-hole narrative.

Loeb’s discussion of tidal disruption events is more conventional. His 1997 work with Elihu Ulmer modeled radiation from stars torn apart by massive black holes, and tidal-disruption flares are a well-established area of astrophysical research. Loeb and Ulmer’s 1997 paper addressed a real physical process. Yet the valid existence of tidal disruption events does not create evidence that the black holes involved were built by nonhuman intelligence.

The same evidentiary problem appears when the essay asks whether any 100-million-solar-mass black holes represent alien engineering. It offers no observation that distinguishes an engineered black hole from one formed through ordinary galactic evolution. It identifies no artificial signature, no measurable anomaly, no proposed falsification test and no data point that cannot be explained by known astrophysics.

That is the fundamental reporting flaw. Loeb’s article begins with settled science, shifts into highly compressed and sometimes misleading explanations, and ends with an extraterrestrial implication that has no evidentiary bridge connecting it to the facts presented.

Speculation can be useful when it is clearly labeled and tightly bounded. But when “could be” becomes the headline implication without supporting evidence, readers are left with the impression that astrophysical mystery itself is evidence of extraterrestrial engineering. It is not.

About the Author

Samuel López is a Senior Legal Analyst, investigative journalist and legal researcher with more than two decades of experience evaluating complex factual records, competing expert claims and evidentiary standards in high-stakes litigation. His reporting applies that same disciplined framework to science, technology, national-security and public-interest controversies, separating documented observations from inference, theory and unsupported assertion.

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