By David Warner 
The terms micro black hole and primordial black hole (PBH) are often used in similar contexts, but it is helpful to know the difference. A primordial black hole is defined by when it formed: in the first second of the universe’s existence, before stars and galaxies even existed. These PBHs could have a huge range of sizes, from very small to very large. A micro black hole is a subcategory of PBH defined by its size—it is a primordial black hole that is exceptionally tiny, generally having a mass less than that of our moon, but possibly down to the mass of a small asteroid or even smaller. Since black holes formed from collapsing stars must be at least several times the mass of the Sun, only a primordial black hole can be considered truly “micro.”
The idea behind their formation is that, in the extremely dense and chaotic early universe, there may have been small areas with slightly higher density than their surroundings. In that environment, the pull of gravity in those tiny, dense patches was so strong that they immediately collapsed into a black hole. Because they weren’t formed from regular star matter, they don’t have the same minimum size limit as the black holes we see forming today. This makes the search for PBHs in the micro-mass range one of the key ways scientists are trying to solve the puzzle of Dark Matter.
One of the most mind-bending theories about black holes comes from the physicist Stephen Hawking. He proposed that black holes are not completely black, but actually emit a type of radiation, now called Hawking radiation. This happens due to quantum effects near the edge of the black hole, the event horizon. When a black hole loses energy through this radiation, it loses mass and slowly shrinks. This is where the size of the black hole becomes extremely important.
The smaller the black hole is, the more intensely it radiates, and the faster it evaporates. A black hole with the mass of our sun would take an incredibly long time to evaporate, much longer than the current age of the universe. However, a micro black hole—say, one with the mass of a large asteroid—would emit Hawking radiation so powerfully that it would evaporate almost instantly in a final burst of energy. According to the math, any micro black hole with a mass below about $10^{11}$ kilograms, which is still much heavier than any skyscraper on Earth, would have already completely evaporated away over the $13.8$ billion year history of the universe. This means that if we detect a micro black hole today, it must have been born heavy enough to have survived until now, or we would have to detect the final powerful burst of an evaporating one.
The theoretical minimum mass for a black hole is a fascinating concept that involves the most extreme limits of physics. This smallest possible size is known as the Planck mass, which is about $20$ micrograms (a microgram is one-millionth of a gram). This is incredibly tiny, but it is still heavier than a single atom. A black hole of the Planck mass would have an event horizon, or radius, equal to the Planck length, which is the smallest theoretical unit of space possible, about $10^{-35}$ meters. This size is so small that it is $100$ billion billion times smaller than a proton.
A black hole this small is only theoretical because, at the Planck scale, the physics we currently use, like general relativity and quantum mechanics, break down and must be replaced by a full theory of quantum gravity, which we do not have yet. If a black hole could form at this tiny size, it would evaporate almost instantly due to Hawking radiation. Therefore, scientists generally focus their search for surviving micro black holes on masses much larger than the Planck mass, where the black hole might be small, but still massive enough to have avoided total evaporation over the age of the universe.
The idea of creating a black hole in a laboratory sounds like science fiction, but it has been considered by physicists, particularly when discussing high-energy particle accelerators like the Large Hadron Collider (LHC). The possibility hinges on some highly speculative theories, particularly those that suggest there are extra, unseen dimensions of space. In a universe with extra dimensions, gravity could become much stronger over very short distances. If this is true, the energy needed to compact mass into a micro black hole might be lowered to a level that is achievable in a powerful particle accelerator.
When protons are smashed together at extremely high speeds, the collision releases vast amounts of energy in a tiny space. If extra dimensions exist, some theories suggest that these collisions could create tiny black holes. However, if such a micro black hole were formed, it would be incredibly small and would immediately lose its mass through Hawking radiation. It would decay or “fizzle out” so quickly—in far less than a trillionth of a second—that it would be completely harmless and essentially undetectable as a black hole, only registering as a complex pattern of decay particles. The consensus among scientists is that the LHC does not and cannot produce stable micro black holes.
Since micro black holes are invisible and cannot be found by simply pointing a telescope at them, scientists have to look for their unique gravitational or radiation signatures. One of the main ways to search for them is to look for the final burst of Hawking radiation from a micro black hole that is at the end of its life. This final explosion would release a powerful flash of gamma rays, which are the highest energy form of light. Scientists use specialized telescopes and detectors, like those on certain satellites, to scan the cosmos for these unique gamma ray bursts that could signal the evaporation of a micro black hole.
Another method is looking for their gravitational effects. If a micro black hole were to pass through our solar system, its intense gravity could cause subtle but measurable changes to the orbits of planets, moons, or even our GPS satellites. Detailed analysis of decades of planetary motion and satellite tracking data is one way researchers are trying to determine if one of these tiny, fast-moving, invisible objects has ever passed close by Earth. The search is challenging because the signature of a black hole could easily be mistaken for a regular, dark asteroid.
The search for micro black holes is deeply connected to the biggest mystery in cosmology: Dark Matter. Dark Matter is the invisible material that makes up most of the mass in the universe, and we know it is there because of its immense gravitational pull. We have ruled out most ordinary forms of matter, like stars, gas, and dust, as the main components of Dark Matter. If there are enough primordial micro black holes scattered throughout the galaxy, they would perfectly fit the requirements for Dark Matter.
They are dark because they don’t shine. They provide the necessary gravity to hold galaxies together. And if they formed in the early universe, they could be non-baryonic (not made of normal protons and neutrons) in the same way the mysterious Dark Matter particle is thought to be. While current gravitational wave and microlensing observations have placed strong limits on some mass ranges of PBHs, the very light, micro-mass range is still largely unexplored and remains a viable, albeit increasingly constrained, possibility for being a component of the universe’s Dark Matter.
The concept of micro black holes forces us to think about gravity and matter on the most extreme scales imaginable, blurring the line between the quantum world and the cosmic world. While the existence of these objects is still hypothetical, their potential formation in the early universe and their eventual evaporation via Hawking radiation are crucial concepts in modern physics. We have ruled out that the universe is currently filled with rapidly evaporating, tiny black holes because we have not seen the expected gamma-ray bursts, but the search continues for any that might have survived and are passing through our solar system. Finding even one micro black hole would not only solve a part of the Dark Matter problem but also provide a monumental, much-needed clue toward a unified theory of quantum gravity.
If a micro black hole were to pass harmlessly through the Earth, how could we distinguish its effect from a natural earthquake or a meteoroid impact?
FAQs – People Also Ask
No, an atom-sized black hole cannot destroy the Earth. While it would be incredibly dense, a black hole the size of an atom would still only have the mass of a large mountain. Its gravitational pull would be very weak at the distance of the Earth’s surface. Furthermore, any black hole that small would have evaporated almost instantly due to Hawking radiation billions of years ago.
There is no strict scientific line, but a micro black hole is generally considered to be any black hole significantly smaller than the minimum stellar black hole mass, which is about three times the mass of the Sun. For primordial black holes, the “micro” range is often considered to be up to about the mass of the Moon or a very large asteroid, as any larger ones would be categorized differently when studying their formation.
The event horizon of a black hole is the boundary from which nothing can escape. For a micro black hole with the mass of a large mountain, its event horizon would be incredibly small, likely similar in size to a proton or an atomic nucleus. The smaller the mass of the black hole, the smaller its event horizon radius.
Yes, Stephen Hawking developed the theoretical framework for micro black holes, which he called mini black holes, in the early 1970s. He suggested that they could have formed in the chaotic, high-density environment of the early universe, and his later work introduced the concept of Hawking radiation, which predicts their eventual evaporation.
The temperature of a black hole, and therefore the rate at which it emits Hawking radiation and evaporates, is inversely related to its mass. This means that smaller black holes are much hotter and ‘burn’ their mass away much faster than larger, cooler black holes. A stellar-mass black hole radiates almost nothing, but a micro black hole radiates intensely.
In its final moments, a micro black hole would be shrinking rapidly, causing its temperature and radiation emission to increase dramatically. It would ultimately explode in a huge, final burst of energy, releasing high-energy particles and an intense flash of gamma rays as its remaining mass is converted into energy.
Micro black holes that formed in the early universe are not considered to be made of ordinary (baryonic) matter in the traditional sense, as they formed before atoms or even protons could stabilize. They are simply immense concentrations of mass-energy that instantly collapsed under gravity, and they are therefore generally considered non-baryonic, which is why they are candidates for Dark Matter.
If a micro black hole evaporates today, the flash of gamma rays it produces would be incredibly powerful. However, the flash would be very short-lived and, depending on how far away it happened, it could be difficult to detect against the background noise of the universe. Scientists have search teams constantly monitoring for these unique gamma ray signatures.
If a micro black hole with the mass of an asteroid were to pass through the Earth, its interaction would be purely gravitational. It would pass straight through the planet with little to no effect, much like a neutrino, leaving a very small, measurable gravitational disturbance. The chance of a catastrophic collision with an atomic nucleus is virtually zero due to its tiny size.
The Planck mass ($2.176 \times 10^{-8}$ kilograms) is a fundamental unit derived from fundamental constants in physics. It is believed to represent the minimum theoretical mass a black hole could have before quantum effects become completely dominant, defining the boundary where our current theories of gravity stop working.