Micro black hole

Micro black holes, also called quantum mechanical black holes or mini black holes, are hypothetical tiny black holes, for which quantum mechanics effects play an important role.^[1]

It is possible that such quantum primordial black holes were created in the high-density environment of the early Universe (or big bang), or possibly through subsequent phase transitions. They might be observed by astrophysicists in the near future, through the particles they are expected to emit by Hawking radiation.

Some hypotheses involving additional space dimensions predict that micro black holes could be formed at energies as low as the TeV range, which are available in particle accelerators such as the LHC (Large Hadron Collider). Popular concerns have then been raised over end-of-the-world scenarios (see Safety of particle collisions at the Large Hadron Collider). However, such quantum black holes would instantly evaporate, either totally or leaving only a very weakly interacting residue. Beside the theoretical arguments, the cosmic rays bombarding the Earth do not produce any damage, although they reach center of mass energies in the range of hundreds of TeV.

Minimum mass of a black hole

In principle, a black hole can have any mass equal to or above the Planck mass (about 22 micrograms). To make a black hole, one must concentrate mass or energy sufficiently that the escape velocity from the region in which it is concentrated exceeds the speed of light. This condition gives the Schwarzschild radius, , where G is the gravitational constant, c is the speed of light, and M the mass of the black hole. On the other hand, the Compton wavelength, , where h is Planck's constant, represents a limit on the minimum size of the region in which a mass M at rest can be localized. For sufficiently small M, the reduced Compton wavelength (, where ħ is Reduced planck constant) exceeds half the Schwarzschild radius, and no black hole description exists. This smallest mass for a black hole is thus approximately the Planck mass.

Some extensions of present physics posit the existence of extra dimensions of space. In higher-dimensional spacetime, the strength of gravity increases more rapidly with decreasing distance than in three dimensions. With certain special configurations of the extra dimensions, this effect can lower the Planck scale to the TeV range. Examples of such extensions include large extra dimensions, special cases of the Randall–Sundrum model, and string theory configurations like the GKP solutions. In such scenarios, black hole production could possibly be an important and observable effect at the large hadron collider (LHC).^{[1][2][3][4][5]} It would also be a common natural phenomenon induced by the cosmic rays.

All this assumes that the theory of general relativity remains valid at these small distances. If it does not, then other, presently unknown, effects will limit the minimum size of a black hole. Elementary particles are equipped with a quantum-mechanical, intrinsic angular momentum (spin). The correct conservation law for the total (orbital plus spin) angular momentum of matter in curved spacetime requires that spacetime is equipped with torsion. The simplest and most natural theory of gravity with torsion is the Einstein-Cartan theory.^[6][7] Torsion modifies the Dirac equation in the presence of the gravitational field and causes fermion particles to be spatially extended.^[8] The spatial extension of fermions limits the minimum mass of a black hole to be on the order of 10¹⁶ kg, showing that mini black holes may not exist. The energy necessary to produce such a black hole is 39 orders of magnitude greater than the energies available at the LHC, indicating that the LHC cannot produce mini black holes. But if black holes are produced, then the theory of general relativity is proven wrong and does not exist at these small distances. The rules of general relativity would be broken, as is consistent with theories of how matter, space, and time break down around the event horizon of a black hole. This would prove the spatial extensions of the fermions limits incorrect as well. The fermion limits assumes a minimum mass needed to sustain a black hole, as opposed to the opposite, the minimum mass needed to start a black hole, which in theory is achievable in the LHC.^[9]

Stability of a micro black hole

Hawking radiation

In 1974, Stephen Hawking argued that, due to quantum effects, black holes "evaporate" by a process now referred to as Hawking radiation in which elementary particles (photons, electrons, quarks, gluons, etc.) are emitted.^[10] His calculations show that the smaller the size of the black hole, the faster the evaporation rate, resulting in a sudden burst of particles as the micro black hole suddenly explodes.

Any primordial black hole of sufficiently low mass will evaporate to near the Planck mass within the lifetime of the Universe. In this process, these small black holes radiate away matter. A rough picture of this is that pairs of virtual particles emerge from the vacuum near the event horizon, with one member of a pair being captured, and the other escaping the vicinity of the black hole. The net result is the black hole loses mass (due to conservation of energy). According to the formulae of black hole thermodynamics, the more the black hole loses mass, the hotter it becomes, and the faster it evaporates, until it approaches the Planck mass. At this stage, a black hole would have a Hawking temperature of T_P / 8π (5.6×10³² K), which means an emitted Hawking particle would have an energy comparable to the mass of the black hole. Thus, a thermodynamic description breaks down. Such a mini-black hole would also have an entropy of only 4π nats, approximately the minimum possible value. At this point then, the object can no longer be described as a classical black hole, and Hawking's calculations also break down.

While Hawking radiation is sometimes questioned,^[11] Leonard Susskind summarizes an expert perspective in his recent book:^[12] "Every so often, a physics paper will appear claiming that black holes don't evaporate. Such papers quickly disappear into the infinite junk heap of fringe ideas".

Conjectures for the final state

Conjectures for the final fate of the black hole include total evaporation and production of a Planck-mass-sized black hole remnant. Such Planck-mass black holes may in effect be stable objects if the quantised gaps between their allowed energy levels bar them from emitting Hawking particles or absorbing energy gravitationally like a classical black hole. In such case, they would be WIMPs (weakly interacting massive particles); this could explain dark matter.^[13]

Primordial black holes

Formation in the early Universe

Production of a black hole requires concentration of mass or energy within the corresponding Schwarzschild radius. It is hypothesized that, shortly after the Big Bang, the Universe was dense enough for any given region of space to fit within its own Schwarzschild radius. Even so, at that time, the Universe was not able to collapse into a singularity due to its uniform mass distribution and rapid growth. This, however, does not fully exclude the possibility that black holes of various sizes may have emerged locally. A black hole formed in this way is called a primordial black hole and is the most widely accepted hypothesis for the possible creation of micro black holes. Computer simulations suggest that the probability of formation of a primordial black hole is inversely proportional to its mass. Thus, the most likely outcome would be micro black holes.

Expected observable effects

A primordial black hole with an initial mass of around 7012100000000000000♠10¹² kg would be completing its evaporation today; a less massive primordial black hole would have already evaporated.^[1] In optimistic circumstances, the Fermi Gamma-ray Space Telescope satellite, launched in June 2008, might detect experimental evidence for evaporation of nearby black holes by observing gamma ray bursts.^[14][15][16] It is unlikely that a collision between a microscopic black hole and an object such as a star or a planet would be noticeable. The small radius and high density of the black hole would allow it to pass straight through any object consisting of normal atoms, interacting with only few of its atoms while doing so. It has, however, been suggested that a small black hole (of sufficient mass) passing through the Earth would produce a detectable acoustic or seismic signal.^{[17][18][19][lower-alpha 1]}

Man-made micro black holes

Feasibility of production

In familiar three-dimensional gravity, the minimum energy of a microscopic black hole is 10¹⁹ GeV, which would have to be condensed into a region on the order of the Planck length. This is far beyond the limits of any current technology. It is estimated that to collide two particles to within a distance of a Planck length with currently achievable magnetic field strengths would require a ring accelerator about 1000 light years in diameter to keep the particles on track. Stephen Hawking also said in chapter 6 of his Brief History of Time that physicist John Archibald 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.

However, in some scenarios involving extra dimensions of space, the Planck mass can be as low as the TeV range. The Large Hadron Collider (LHC) has a design energy of 14 TeV for proton–proton collisions and 1150 TeV for Pb–Pb collisions. It was argued in 2001 that, in these circumstances, black hole production could be an important and observable effect at the LHC^{[2][3][4][5][20]} or future higher-energy colliders. Such quantum black holes should decay emitting sprays of particles that could be seen by detectors at these facilities.^[2][3] A paper by Choptuik and Pretorius, published on March 17, 2010 in Physical Review Letters, presented a computer-generated proof that micro black holes must form from two colliding particles with sufficient energy, which might be allowable at the energies of the LHC if additional dimensions are present other than the customary four (three spatial, one temporal).^[21][22]

Safety arguments

Hawking's calculation^[10] and more general quantum mechanical arguments predict that micro black holes evaporate almost instantaneously. Additional safety arguments beyond those based on Hawking radiation were given in the paper,^[23][24] which showed that in hypothetical scenarios with stable black holes that could damage Earth, such black holes would have been produced by cosmic rays and would have already destroyed known astronomical objects such as the Earth, Sun, neutron stars, or white dwarfs.

As a power source

If a way to create artificial micro black holes were discovered, they could provide an abundant energy source by absorbing and converting their Hawking radiation. The process may occur with a smaller mass black hole evaporating as a gamma ray burst immediately after creation. It may also occur in a zero gravity environment, with a larger mass black hole, that may emit radiation for years before becoming unstable and needing replacement, such as in a black hole starship.

Mass may be added to a black hole power source as fuel during its existence. At the end of the black hole's service life, the energy released may be captured for re-use. A factor limiting power output is the rate at which mass can be moved into the black hole during its short duration of existence. As the fuel mass cannot travel faster than the speed of light, smaller masses of roughly 1 g to 1 kg added to the black hole could not travel fast enough to be absorbed in time (between 10^-25 to 10^-16 seconds) to create a viable energy source. Larger masses of 1000+ kg for each round of explosion (creation of black hole followed by its destruction) are required instead. When converted to energy (E=mc^2) the explosions have an energy level of 10+ TeraWattHours. The capturing of the explosion energy for re-use would be an engineering challenge.

Black holes in quantum theories of gravity

It is possible, in some theories of quantum gravity, to calculate the quantum corrections to ordinary, classical black holes. Contrarily to conventional black holes which are solutions of gravitational field equations of the general theory of relativity, quantum gravity black holes incorporate quantum gravity effects in the vicinity of the origin, where classically a curvature singularity occurs. According to the theory employed to model quantum gravity effects, there are different kinds of quantum gravity black holes, namely loop quantum black holes, non-commutative black holes, asymptotically safe black holes. In these approaches, black holes are singularity free.

Virtual-micro black holes (VMBH) have been proposed by Stephen Hawking in 1995,^[25] and by Fabio Scardigli in 1999 as part of a GUT which could be a quantum gravity candidate.^[26][27]

See also

• Black holes in fiction
• Planck particle
• Holeum
• Kugelblitz (astrophysics)
• Black hole starship
• Black hole electron

Notes

References

Bibliography

External links

• Astrophysical implications of hypothetical stable TeV-scale black holes
• A. Barrau & J. Grain, The Case for mini black holes : a review of the searches for new physics with micro black holes possibly formed at colliders
• Mini Black Holes Might Reveal 5th Dimension – Space.com
• Doomsday Machine Large Hadron Collider? – A scientific essay about energies, dimensions, black holes, and the associated public attention to CERN, by Norbert Frischauf (also available as Podcast)

Black holes
Types	Schwarzschild Rotating Charged Virtual Binary
Size	Micro Extremal Electron Stellar Intermediate-mass Supermassive Quasar Active galactic nucleus Blazar Large quasar group
Formation	Stellar evolution Gravitational collapse Neutron star Related links Compact star Quark Exotic Tolman–Oppenheimer–Volkoff limit White dwarf Related links Supernova Related links Hypernova Gamma-ray burst
Properties	Thermodynamics Schwarzschild radius M–sigma relation Event horizon Quasi-periodic oscillation Photon sphere Ergosphere Hawking radiation Penrose process Blandford–Znajek process Bondi accretion Spaghettification Gravitational lens
Models	Gravitational singularity Penrose–Hawking singularity theorems Primordial black hole Gravastar Dark star Dark-energy star Black star Eternally collapsing object Magnetospheric eternally collapsing object Fuzzball White hole Naked singularity Ring singularity Immirzi parameter Membrane paradigm Kugelblitz Wormhole Quasi-star
Issues	No-hair theorem Information paradox Cosmic censorship Alternative models Holographic principle Black hole complementarity firewall (physics) ER=EPR Final parsec problem
Metrics	Schwarzschild Kerr Reissner–Nordström Kerr–Newman
Lists	Black holes Most massive Quasars
Related	Timeline of black hole physics Rossi X-ray Timing Explorer Hypercompact stellar system Black hole starship