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A micro black hole in physics is a black hole of small mass around which quantum mechanical effects play an important role.
It is also referred to as a quantum mechanical black hole, miniature black hole or mini black hole. The existence of micro black holes is hypothetical. They may eventually be produced on Earth in particle accelerators such as the Large Hadron Collider or they may be detected in cosmic ray collisions in our atmosphere. Such observations would give an empirical input to the development of a theory on quantum gravity and would be of dramatic importance to understand the world at small distances. For example, they would reveal the structure of spacetime and provide a means of actually observing the number of spatial dimensions (which could on a small scale be larger than three).
A black hole evaporates due to thermal radiation called Hawking radiation and gradually loses mass. The temperature of the radiation is inversely proportional to the mass. It is very low for massive black holes but extremely large for tiny black holes. The evaporation of astronomical black holes (such as stellar black holes and supermassive black holes) is entirely negligible, and astronomical black holes are indeed effectively black. Small black holes radiate a lot, lose mass and get hotter in the process. Their radiation keeps increasing, until they end their life in an explosive event.
It is such explosive events that scientists hope to observe in the future.
The Planck mass
The mass of a micro black holes in which quantum mechanical effects play a dominant role is called the Planck mass. This can be estimated as follows. The size of a black hole (the Schwarzschild radius) is proportional to its mass. The size at which quantum mechanical effects play a role can be taken from Heisenberg's Uncertainty principle and it goes the other way: the larger the momentum of the object (and thus also its total mass-energy), the smaller this distance is.
The mass for which these two distances are equal is called the
Planck mass. It is about 2 × 10-8 kilogram (see natural units), small by the standards of every day life, but extremely large as compared to the mass of elementary particles. The corresponding radius of a black hole is the Planck length (about 1.6 × 10-35 meter). Its mass-energy is the Planck energy (1019 GeV).
Nowadays, these Planck scales are also called four dimensional Planck scales, because they are computed in the traditional 4-dimensional spacetime with three spatial dimensions and one time dimension.
If two aggregates of particles (quarks and gluons) with a total mass-energy of the size of a Planck mass would collide in a particle accelerator within a distance of a Planck length, they would form a micro black hole. However, with the values above, there is no hope of ever producing one in the near future. The Planck energy is too high by many orders of magnitude.
The predictions given by standard theories of general relativity and quantum mechanics for the behaviour of a black hole with a mass less than the Planck mass are inconsistent.
Extra-dimensional black holes
The Planck energy is so high because gravitation is so weak, much weaker than the three other fundamental interactions in nature. This problem is not understood and known as the hierarchy problem in particle physics. In several independent approaches to this and other problems, it is assumed that maybe the physical world is not four-dimensional after all, but has in fact (many) more spatial dimensions. At the macroscopic level we don't observe these extra dimensions, because they are "curled up" to a small size. Compare for example the surface of a thin cylinder (such as a human hair). It is essentially a one dimensional world, only under the microscope is it revealed that the other dimension of the surface is the short circumference of the cylinder, rolled up to the small radius of the thickness of the hair.
Gravity then is so weak because (up to the size of the new dimensions) it is "diluted". The gravitational potential at a distance r for example (which in 4-dimensional ordinary spacetime goes as 1/r)
is quickly "diluted" in a world with n more spatial dimensions as
<math>1/r^{n+1}<math> up to the maximum size R of these rolled up extra dimensions. There after, in the observable world it continues to
decrease as 1/r. In a microscopic world with a larger number of dimensions, micro black holes would form much easier and at a larger mass and larger radii.
Taking this into account, the fundamental Planck length may be
much larger and the fundamental Planck energy much lower.
If it would be as low as one Tera electron Volt (1 TeV or <math>10^{12}<math> eV) one can expect micro black holes to be produced with a sufficiently large rate at Earth in particle accelerators and possibly by energetic cosmic rays in the Earth atmosphere.
Physicist Brian Greene has suggested that the electron may be a micro black hole. Small black holes would look like elementary particles because they would be completely defined by their mass, charge and spin. Since the electron is not observed to evaporate and lose mass, an explanation for electron stability is needed. An explanation is also needed for the extreme time dilation required at the micro black hole photon capture circumference (approximately 1.025 × 10-22 seconds per second). A ring singularity at the photon capture region could explain extreme time dilation, but the question of stability remains. The electron would be stable if it has the lowest energy allowed (without annihilation by its antiparticle) due to its minimum charge, spin and mass. If the micro black hole suggestion is correct, a more complete explanation for stability should be developed.
See Rotating black hole.
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