Micro black holes
, are the tiny hypothetical black holes
also called quantum mechanical black holes
or mini black holes
, for which quantum mechanical
effects play an important role. In theory, a black hole can have any size or mass above the Planck mass
. Under some speculative theories, primordial black holes
were created during the big bang
at the earliest stages of the evolution of our universe. In 1974 Stephen Hawking
argued that due to quantum
effects, such primordial black holes could "evaporate" by a theoretical process now referred to as Hawking Radiation
in which particles of matter would be emitted. Under this theory, the smaller the size of the micro black hole, the faster the evaporation rate, resulting in a sudden burst of particles as the micro black hole suddenly explodes. Searches for such evaporating micro black holes are planned for the Fermi Gamma-ray Space Telescope
satellite launched in June of 2008, which will search for gamma ray bursts which should be associated with such evaporation.
It has been argued that the Large Hadron Collider (LHC) could produce one of these micro black holes. Although the Standard Model of particle physics predicts that LHC energies are far too low to create black holes, some extensions of the Standard Model posit the existence of extra spatial dimensions, in which it would be possible to create micro black holes at the LHC at a rate on the order of one per second. According to the standard calculations these are harmless because they would quickly decay by Hawking radiation.
Smallest possible black hole
It is believed that the smallest mass a black hole could have without quantum effects eliminating any such description is of the order of the Planck mass, which is about 2 × 10−8
kg or 1.1 × 1019 GeV
. At this scale the black hole thermodynamic
formulae predict the mini-black hole would have an entropy of only 4π nats
; a Hawking temperature
/ 8π (5.6×1032
K), requiring thermal energy quanta
comparable in energy to almost the mass of the entire mini black hole; and a Compton wavelength
equal to the black hole's Schwarzschild radius
(this distance being equal to the Planck length
). This is the point where a classical gravitational description of the object stops being retrievable with merely small quantum corrections, but in effect completely breaks down.
The existence of a small black hole of this mass is purely hypothetical but if primordial black holes exist, they might reach this condition as the final stage of runaway evaporation due to Hawking radiation. If Hawking Radiation is real, then small black holes would radiate away matter as pairs of virtual particles emerge from the vacuum near the event horizon, with one falling into the black hole, and the other wandering away, with the net result that the black hole loses mass [due to conservation of energy]. Under Hawking's theory, this "evaporation" rate would increase as the black hole lost mass, eventually resulting in a micro black hole that would suddenly explode in a burst of particles.
Creation of micro black holes
Under standard theories, such an energy to produce a micro black hole is orders of magnitude greater than that which can be produced on Earth in particle accelerators
such as the LHC (maximum about 1.15 × 106
GeV), or detected in cosmic ray
collisions in our atmosphere. It is estimated that to collide two aggregates of fermions
to within a distance of a Planck length with the currently achievable magnetic field strength would require a ring accelerator about 1000 light years
in diameter to keep the aggregates on track. Even if it were possible, any collision product would be immensely unstable, and almost immediately disintegrate.
Some string theorists have suggested that the multiple dimensions postulated by string theory might make the effective strength of gravity many orders of magnitude stronger at small distances (very high energies). This might effectively lower the Planck energy, and perhaps make black-hole-like descriptions valuable at even lower masses such as those which are reachable at the LHC. This higher-dimensional component to gravity is, however, purely theoretical as of 2008.
Stephen Hawking also said in chapter 6 of his Brief History of Time that physicist John Wheeler once calculated that a very powerful hydrogen bomb using all the deuterium in all the water on Earth could also generate such a black hole, but Hawking does not provide this calculation or any reference to it to support this assertion.
Stable micro black holes
Others have wondered about the basic assumptions of the quantum gravity
program, and whether there is really a compelling case to believe in Hawking radiation. It is only these quantum assumptions which lead to the crisis at the Planck mass: in classical general relativity, a black hole could in principle be arbitrarily small, once created. Accordingly, it remains a possibility that a stable micro black hole could be created at the LHC, or that they are created in nature by high-energy impacts, only to zip through earth at nearly the speed of light
. Chances of these tiny black holes devouring the earth are absurdly implausible
- In David Brin's novel Earth a manmade micro blackhole slips into the core of the earth.
- In the short story How We Lost the Moon, A True Story by Frank W. Allen, which is actually written by Paul J. McAuley, a micro black hole is accidentally created on the moon, which gradually consumes it.
- Larry Niven's Hugo Award-winning short story The Hole Man deals with a "quantum black hole".
- In the Guyver manga, it is one of the weapons used by Guyver Exceed.
- In John Reed's literary satire, "The Whole," or "Duh Whole" (published by Simon & Schuster/Pocket Books/MTV Books), a small black hole appears in the middle of the United States, proceeds to engulf four states, and is still growing, at novel's end.
- Quantum Mechanical Black Holes: Towards a Unification of Quantum Mechanics and General Relativity -- http://citebase.eprints.org/cgi-bin/citations?id=oai:arXiv.org:quant-ph/9808020
- S.W. Hawking, Commun.Math. Phys. 43 (1975) 199 : the article it all began with !
- D. Page, Phys. Rev. D13 (1976) 198 : first detailed studies of the evaporation mechanism
- B.J. Carr & S.W. Hawking, Mon. Not. Roy. Astron. Soc 168 (1974) 399 : links between primordial black holes and the early universe
- A. Barrau et al., Astron. Astrophys. 388 (2002) 676 , Astron. Astrophys. 398 (2003) 403 , Astrophys. J. 630 (2005) 1015 : experimental searches for primordial black holes thanks to the emitted antimatter
- A. Barrau & G. Boudoul, Review talk given at the International Conference on Theoretical Physics TH2002 : cosmology with primordial black holes
- A. Barrau & J. Grain, Phys. Lett. B 584 (2004) 114 : searches for new physics (quantum gravity) with primordial black holes
- P. Kanti, Int. J. Mod. Phys. A19 (2004) 4899 : evaporating black holes and extra-dimensions
- D. Ida, K.-y. Oda & S.C.Park, : determination of black hole's life and extra-dimensions
- Sabine Hossenfelder: What Black Holes Can Teach Us, hep-ph/0412265
- Jason Doukas, S. Rai Choudhury, G. C. Joshi: "Lepton number violation via intermediate black hole processes",