In physics, there is a speculative hypothesis that if there were a black hole with the same mass, charge and angular momentum as an electron, it would share other properties of the electron. Most notably, Brandon Carter showed in 1968 that the magnetic moment of such an object would match that of an electron.[1] This is interesting because calculations ignoring special relativity and treating the electron as a small rotating sphere of charge give a magnetic moment that is off by roughly a factor of 2, the so-called gyromagnetic ratio.

However, Carter's calculations also show that a would-be black hole with these parameters would be 'super-extremal'. Thus, unlike a true black hole, this object would display a naked singularity, meaning a singularity in spacetime not hidden behind an event horizon. It would also give rise to closed timelike curves.

Standard quantum electrodynamics (QED), currently the most comprehensive theory of particles, treats the electron as a point particle. There is no evidence that the electron is a black hole (or naked singularity). Furthermore, since the electron is quantum mechanical in nature, any description purely in terms of general relativity is inadequate. Hence, the existence of a black hole electron remains strictly hypothetical.

Details

A paper published in 1938 by Albert Einstein, Leopold Infeld and Banesh Hoffmann showed that if elementary particles are treated as singularities in spacetime, it is unnecessary to postulate geodesic motion as part of general relativity.[2] The electron may be treated as such a singularity.

If one ignores the electron's angular momentum and charge, as well as the effects of quantum mechanics, one can treat the electron as a black hole and attempt to compute its radius. The Schwarzschild radius rs of a mass m is the radius of the event horizon for a non-rotating, uncharged black hole of that mass. It is given by

\( r_{s}={\frac {2Gm}{c^{2}}}, \)

where G is Newton's gravitational constant and c is the speed of light. For the electron,

m = 9.109×10−31 kg,

so

rs = 1.353×10−57 m.

Thus, if we ignore the electric charge and angular momentum of the electron, and naively apply general relativity on this very small length scale without taking quantum theory into account, a black hole of the electron's mass would have this radius.

In reality, physicists expect quantum gravity effects to become significant even at much larger length scales, comparable to the Planck length

\( {\displaystyle \ell _{P}={\sqrt {\frac {G\hbar }{c^{3}}}}=1.616\times 10^{-35}} m \)

So, the above purely classical calculation cannot be trusted. Furthermore, even classically, electric charge and angular momentum affect the properties of a black hole. To take them into account, while still ignoring quantum effects, one should use the Kerr–Newman metric. If we do, we find the angular momentum and charge of the electron are too large for a black hole of the electron's mass: a Kerr-Newman object with such a large angular momentum and charge would instead be 'super-extremal', displaying a naked singularity, meaning a singularity not shielded by an event horizon.

To see that this is so, it suffices to consider the electron's charge and neglect its angular momentum. In the Reissner–Nordström metric, which describes electrically charged but non-rotating black holes, there is a quantity rq, defined by

\( r_{{q}}={\sqrt {{\frac {q^{{2}}G}{4\pi \epsilon _{{0}}c^{{4}}}}}} \)

where q is the electron's charge and ε0 is the vacuum permittivity. For an electron with q = −e = −1.602×10−19 C, this gives a value

rq = 1.3807×10−36 m.

Since this (vastly) exceeds the Schwarzschild radius, the Reissner–Nordström metric has a naked singularity.

If we include the effects of the electron's rotation using the Kerr–Newman metric, there is still a naked singularity, which is now a ring singularity, and spacetime also has closed timelike curves. The size of this ring singularity is on the order of

\( {\displaystyle r_{a}={\frac {J}{mc}}}, \)

where as before m is the electron's mass and c is the speed of light, but J = \( \hbar /2 \) is the spin angular momentum of the electron. This gives

ra = 1.9295×10−13 m

which is much larger than the length scale rq associated to the electron's charge. As noted by Carter,[3] this length ra is on the order of the electron's Compton wavelength. Unlike the Compton wavelength, it is not quantum-mechanical in nature.

More recently, Alexander Burinskii has pursued the idea of treating the electron as a Kerr–Newman naked singularity.[4]

See also

Quantum gravity

Abraham–Lorentz force

Black hole thermodynamics

Entropic force

Hawking radiation

List of quantum gravity researchers

Entropic elasticity of an ideal chain

Gravitation

Induced gravity

Geon (physics)

Micro black hole

References

Carter, B. (25 October 1968). "Global structure of the Kerr family of gravitational fields". Physical Review. 174 (5): 1559–1571. Bibcode:1968PhRv..174.1559C. doi:10.1103/physrev.174.1559.

Einstein, A.; Infeld, L.; Hoffmann, B. (January 1938). "The gravitational equations and the problem of motion". Annals of Mathematics. Second Series. 39 (1): 65–100. Bibcode:1938AnMat..39...65E. doi:10.2307/1968714. JSTOR 1968714.

Carter, B. (25 October 1968). "Global structure of the Kerr family of gravitational fields". Physical Review. 174 (5): 1559–1571. Bibcode:1968PhRv..174.1559C. doi:10.1103/physrev.174.1559.

Burinskii, Alexander (April 2008). "The Dirac-Kerr-Newman electron". Gravitation and Cosmology. 14 (2): 109–122. arXiv:hep-th/0507109. Bibcode:2008GrCo...14..109B. doi:10.1134/S0202289308020011. S2CID 119084073.

Further reading

Duff, Michael (1994). Kaluza–Klein theory in perspective. arXiv:hep-th/9410046. Bibcode:1995okml.book...22D.

Hawking, Stephen (1971). "Gravitationally collapsed objects of very low mass". Monthly Notices of the Royal Astronomical Society. 152: 75. Bibcode:1971MNRAS.152...75H. doi:10.1093/mnras/152.1.75.

Penrose, Roger (2004). The Road to Reality: A Complete Guide to the Laws of the Universe. London: Jonathan Cape.

Salam, Abdus. "Impact of quantum gravity theory on particle physics". In Isham, C. J.; Penrose, Roger; Sciama, Dennis William (eds.). Quantum Gravity: an Oxford Symposium. Oxford University Press.

't Hooft, Gerard (1990). "The black hole interpretation of string theory". Nuclear Physics B. 335 (1): 138–154. Bibcode:1990NuPhB.335..138T. doi:10.1016/0550-3213(90)90174-C.

Popular literature

Brian Greene, The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory (1999), (See chapter 13)

John A. Wheeler, Geons, Black Holes & Quantum Foam (1998), (See chapter 10)

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