Other meanings of black hole
Astrophysics
A micro black hole (also called a quantum black hole or mini black hole) is a hypothetical black hole with a mass much smaller than that of a star, potentially as small as the Planck mass (~2.18×10−8 kg). Unlike stellar black holes, which form from gravitational collapse of massive stars, micro black holes are predicted by certain quantum gravity theories and could have formed in the early universe or be produced in high-energy particle collisions. Their existence remains unconfirmed, but they are a key testbed for theories of quantum gravity, extra dimensions, and Hawking radiation.
Micro black holes are black holes with masses far below the Chandrasekhar limit, typically in the range from the Planck mass up to about 1015 kg (the mass of a small mountain). They are predicted by theories that unify general relativity with quantum mechanics, such as string theory and loop quantum gravity. In these theories, the Heisenberg uncertainty principle implies that at very small scales, quantum fluctuations of spacetime become significant, potentially allowing the formation of black holes from concentrated energy densities without the need for stellar collapse.1
The concept dates back to the 1970s, when Stephen Hawking showed that black holes emit thermal radiation due to quantum effects near the event horizon. This led to the realization that small black holes would evaporate rapidly, and that primordial black holes (including micro ones) could have formed in the early universe from density fluctuations.2
Hawking radiation is the process by which a black hole loses mass and energy over time. For a micro black hole, the temperature is inversely proportional to its mass: a Planck-mass black hole would have a temperature of about 1032 K, emitting particles with energies up to the Planck scale. The lifetime of a black hole scales as the cube of its mass; a black hole of 1012 kg would evaporate in about the age of the universe, while a Planck-mass one would vanish in about 10−42 seconds.3
This evaporation process is a crucial prediction that distinguishes micro black holes from classical ones. If micro black holes exist, their final moments would produce a burst of high-energy gamma rays, which could be detectable by observatories like the Fermi Gamma-ray Space Telescope.4
Primordial black holes (PBHs) are hypothetical black holes that formed in the first second after the Big Bang, from regions of overdensity. Depending on the initial conditions, they could have masses ranging from the Planck mass to thousands of solar masses. Micro PBHs with masses below about 1015 kg would have already evaporated by now, but those with masses around 1015 kg would be evaporating today, potentially producing observable gamma-ray bursts.5
Observational constraints from gamma-ray background measurements and microlensing surveys have ruled out PBHs as the sole component of dark matter for many mass ranges, but they remain a candidate for a fraction of dark matter.6
Some theories with extra spatial dimensions, such as the Randall–Sundrum model, predict that the Planck scale could be effectively lowered to the TeV scale, making it possible for particle collisions at the Large Hadron Collider (LHC) to create micro black holes. If such black holes were produced, they would decay almost instantly via Hawking radiation, producing a distinctive signature of multiple high-energy jets and leptons.7
As of 2024, the LHC has found no evidence for micro black holes, placing lower limits on the fundamental Planck scale in extra-dimensional models. This absence of detection has constrained but not ruled out the existence of extra dimensions.
One lesser-known aspect is the possibility that micro black holes could be used as a source of energy or as a probe of quantum gravity. Some speculative proposals suggest that a micro black hole could be stabilized by adding electric charge, forming a 'charged black hole' that might be manipulated. However, these ideas remain purely theoretical.
Another niche topic is the concept of 'black hole analogues' in condensed matter systems, such as sonic black holes in fluids, which mimic the behavior of Hawking radiation and could provide experimental insights into micro black hole physics without actual black holes.8
Historically, the idea of micro black holes was also linked to the 'cosmic censorship hypothesis' and the information paradox, as their evaporation would involve the loss of information, a problem that remains unresolved.
Astronomical searches for micro black holes have focused on detecting the final explosive evaporation of primordial black holes. The Fermi-LAT instrument has searched for gamma-ray bursts from such events, and the lack of detections has placed upper limits on the abundance of PBHs in the mass range around 1015 kg.4
Additionally, the possibility that micro black holes could pass through Earth and leave detectable seismic or acoustic signatures has been considered, but no such events have been observed. The absence of evidence has led to tighter constraints on the density of PBHs in the solar neighborhood.6
This article focuses on the hypothetical micro black hole, distinct from stellar and supermassive black holes.
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