Micro black holes, also known asmini black holes andquantum mechanical black holes, are hypothetical tiny (<1M☉)black holes, for whichquantum mechanical effects play an important role.[1] The concept that black holes may exist that are smaller thanstellar mass was introduced in 1971 byStephen Hawking.[2]
It is possible that such black holes were created in the high-density environment of the early universe (orBig Bang), or possibly through subsequent phase transitions (referred to asprimordial black holes). They might be observed by astrophysicists through the particles they are expected to emit byHawking radiation.[3]
Some hypotheses involving additional spacedimensions predict that micro black holes could be formed at energies as low as theteraelectronvolt (TeV) range, which are available inparticle accelerators such as theLarge Hadron Collider. Popular concerns have then been raised over end-of-the-world scenarios (seeSafety 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,cosmic rays hitting the Earth do not produce any damage, although they reach energies in the range of hundreds of TeV.
In an early speculation,Stephen Hawking conjectured that ablack hole would not form with a mass below about10−8 kg (roughly thePlanck mass).[2] To make a black hole, one must concentrate mass or energy sufficiently that theescape velocity from the region in which it is concentrated exceeds thespeed of light.
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 includelarge extra dimensions, special cases of theRandall–Sundrum model, andstring theory configurations like the GKP solutions. In such scenarios, black hole production could possibly be an important and observable effect at theLarge Hadron Collider (LHC).[1][4][5][6][7]It would also be a common natural phenomenon induced bycosmic rays.
All this assumes that the theory ofgeneral relativity remains valid at these small distances. If it does not, then other, currently unknown, effects might limit the minimum size of a black hole. Elementary particles are equipped with a quantum-mechanical, intrinsicangular momentum (spin). The correct conservation law for the total (orbital plus spin) angular momentum of matter in curved spacetime requires that spacetime is equipped withtorsion. The simplest and most natural theory of gravity with torsion is theEinstein–Cartan theory.[8][9] Torsion modifies theDirac equation in the presence of the gravitational field and causesfermion particles to be spatially extended. In this case the spatial extension of fermions limits the minimum mass of a black hole to be on the order of1016 kg, showing that micro 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 Large Hadron Collider, 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 theevent horizon of a black hole. This would prove the spatial extensions of the fermion limits to be incorrect as well. The fermion limits assume 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 under some conditions.[10][11]
In 1975,Stephen Hawking argued that, due toquantum effects, black holes "evaporate" by a process now referred to asHawking radiation in which elementary particles (such asphotons,electrons,quarks andgluons) are emitted.[3] His calculations showed 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 willevaporate to near thePlanck 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 ofvirtual particles emerge from thevacuum near theevent 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 toconservation of energy). According to the formulae ofblack 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 aHawking temperature ofTP/8π (5.6×1030 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 micro 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,[12]Leonard Susskind summarizes an expert perspective in his bookThe Black Hole War: "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."[13]
Conjectures for the final fate of the black hole include total evaporation and production of aPlanck-mass-sized black hole remnant. Such Planck-mass black holes may in effect be stable objects if the quantized 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 beweakly interacting massive particles; this could explaindark matter.[14]
Production of a black hole requires concentration of mass or energy within the correspondingSchwarzschild radius. It was hypothesized by Zel'dovich and Novikov first and independently by Hawking that, shortly after theBig 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 asingularity 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 aprimordial 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.[citation needed]
A primordial black hole with an initial mass of around1012 kg would be completing its evaporation today; a less massive primordial black hole would have already evaporated.[1] Under optimal conditions, theFermi Gamma-ray Space Telescope satellite, launched in June 2008, might detect experimental evidence for evaporation of nearby black holes by observinggamma ray bursts.[15][16][17] 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 orseismic signal.[18][19][20][a]On the moon, it may leave a distinct type of crater, still visible after billions of years.[21]
In familiar three-dimensional gravity, the minimum energy of a microscopic black hole is1016 TeV (equivalent to 1.6GJ or 444kWh), which would have to be condensed into a region on the order of thePlanck 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 1,000light years in diameter to keep the particles on track.[citation needed]
However, in some scenarios involving extra dimensions of space, the Planck mass can be as low as theTeV range. TheLarge Hadron Collider (LHC) has a design energy of14 TeV forproton–proton collisions and 1,150 TeV forPb–Pb collisions. It was argued in 2001 that, in these circumstances, black hole production could be an important and observable effect at the LHC[4][5][6][7][22] or future higher-energy colliders. Such quantum black holes should decay emitting sprays of particles that could be seen by detectors at these facilities.[4][5] A paper by Choptuik and Pretorius, published in 2010 inPhysical 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 additionaldimensions are present other than the customary four (three spatial, one temporal).[23][24]
Hawking's calculation[2] and more generalquantum mechanical arguments predict that micro black holes evaporate almost instantaneously. Additional safety arguments beyond those based on Hawking radiation were given in the paper,[25][26] which showed that in hypothetical scenarios with stable micro black holes massive enough to destroy Earth, such black holes would have been produced bycosmic rays and would have likely already destroyed astronomical objects such as planets, stars, or stellar remnants such asneutron stars andwhite dwarfs.
It is possible, in some theories ofquantum gravity, to calculate the quantum corrections to ordinary, classical black holes. Contrarily to conventional black holes, which are solutions of gravitational field equations of thegeneral 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, and asymptotically safe black holes. In these approaches, black holes are singularity-free.[citation needed]
Virtual micro black holes were proposed byStephen Hawking in 1995[27] and byFabio Scardigli in 1999 as part of aGrand Unified Theory as aquantum gravity candidate.[28]
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