# Kerr–Newman metric

The Kerr–Newman metric is a solution of the Einstein–Maxwell equations in general relativity that describes the spacetime geometry in the region surrounding a charged, rotating mass. This solution has not been especially useful for describing astrophysical phenomena, because observed astronomical objects do not possess an appreciable net electric charge. The solution has instead been of primarily theoretical and mathematical interest. (It is assumed that the cosmological constant equals zero which is near enough to the truth.)

## History

In 1965, Ezra "Ted" Newman found the axisymmetric solution of Einstein's field equation for a black hole which is both rotating and electrically charged.[1][2] This formula for the metric tensor ${\displaystyle g_{\mu \nu }\!}$ is called the Kerr–Newman metric. It is a generalisation of the Kerr metric for an uncharged spinning point-mass, which had been discovered by Roy Kerr two years earlier.[3]

Four related solutions may be summarized by the following table:

 Non-rotating (J = 0) Rotating (J ≠ 0) Uncharged (Q = 0) Schwarzschild Kerr Charged (Q ≠ 0) Reissner–Nordström Kerr–Newman

where Q represents the body's electric charge and J represents its spin angular momentum.

## Mathematical form

The Kerr–Newman metric describes the geometry of spacetime for a rotating charged black hole with mass M, charge Q and angular momentum J. The formula for this metric depends upon what coordinates or coordinate conditions are selected. One way to express this metric is by writing down its line element in a particular set of spherical coordinates,[4] also called Boyer–Lindquist coordinates:

${\displaystyle c^{2}d\tau ^{2}=-\left({\frac {dr^{2}}{\Delta }}+d\theta ^{2}\right)\rho ^{2}+\left(c\,dt-a\sin ^{2}\theta \,d\phi \right)^{2}{\frac {\Delta }{\rho ^{2}}}-\left(\left(r^{2}+a^{2}\right)d\phi -ac\,dt\right)^{2}{\frac {\sin ^{2}\theta }{\rho ^{2}}}}$

where the coordinates (r, θ, ϕ) are standard spherical coordinate system, and the length-scales:

${\displaystyle a={\frac {J}{Mc}}\,,}$
${\displaystyle \ \rho ^{2}=r^{2}+a^{2}\cos ^{2}\theta \,,}$
${\displaystyle \ \Delta =r^{2}-r_{s}r+a^{2}+r_{Q}^{2}\,,}$

have been introduced for brevity. Here rs is the Schwarzschild radius (in meters) of the massive body, which is related to its mass M by

${\displaystyle r_{s}={\frac {2GM}{c^{2}}}}$

where G is the gravitational constant, and rQ is a length-scale corresponding to the electric charge Q of the mass

${\displaystyle r_{Q}^{2}={\frac {Q^{2}G}{4\pi \epsilon _{0}c^{4}}}}$

where 1/4πε0 is Coulomb's force constant.

### Component form

The components of the Kerr–Newman metric can be read off after a simple algebraic re-arrangement:

{\displaystyle {\begin{aligned}c^{2}d\tau ^{2}&={\frac {(\Delta -a^{2}\sin ^{2}\theta )}{\rho ^{2}}}\;c^{2}\;dt^{2}-\left({\frac {\rho ^{2}}{\Delta }}\right)dr^{2}\\&-\rho ^{2}d\theta ^{2}+(a^{2}\Delta \sin ^{2}\theta -r^{4}-2r^{2}a^{2}-a^{4}){\frac {\sin ^{2}\theta \;d\phi ^{2}}{\rho ^{2}}}\\&-(\Delta -r^{2}-a^{2}){\frac {2a\sin ^{2}\theta \;c\;dt\;d\phi }{\rho ^{2}}}\end{aligned}}}

### Alternative (Kerr–Schild) formulation

The Kerr–Newman metric can be expressed in "Kerr–Schild" form, using a particular set of Cartesian coordinates as follows.[5][6][7] These solutions were proposed by Kerr and Schild in 1965.

${\displaystyle g_{\mu \nu }=\eta _{\mu \nu }+fk_{\mu }k_{\nu }\!}$
${\displaystyle f={\frac {Gr^{2}}{r^{4}+a^{2}z^{2}}}\left[2Mr-Q^{2}\right]}$
${\displaystyle \mathbf {k} =(k_{x},k_{y},k_{z})=\left({\frac {rx+ay}{r^{2}+a^{2}}},{\frac {ry-ax}{r^{2}+a^{2}}},{\frac {z}{r}}\right)}$
${\displaystyle k_{0}=1.\!}$

Notice that k is a unit vector. Here M is the constant mass of the spinning object, Q is the constant charge of the spinning object, η is the Minkowski tensor, and a is a constant rotational parameter of the spinning object. It is understood that the vector ${\displaystyle {\vec {a}}}$ is directed along the positive z-axis. The quantity r is not the radius, but rather is implicitly defined like this:

${\displaystyle 1={\frac {x^{2}+y^{2}}{r^{2}+a^{2}}}+{\frac {z^{2}}{r^{2}}}}$

Notice that the quantity r becomes the usual radius

${\displaystyle R={\sqrt {x^{2}+y^{2}+z^{2}}}}$

when the rotational parameter a approaches zero. In this form of solution, units are selected so that the speed of light is unity (c = 1). In order to provide a complete solution of the Einstein–Maxwell Equations, the Kerr–Newman solution not only includes a formula for the metric tensor, but also a formula for the electromagnetic potential:[5][8]

${\displaystyle A_{\mu }={\frac {Qr^{3}}{r^{4}+a^{2}z^{2}}}k_{\mu }}$

At large distances from the source (R >> a), these equations reduce to the Reissner–Nordström metric with:

${\displaystyle A_{\mu }={\frac {Q}{R}}k_{\mu }}$

In the Kerr–Schild form of the Kerr–Newman metric, the determinant of the metric tensor is everywhere equal to negative one, even near the source.[9]

## Special cases and generalizations

The Kerr–Newman metric is a generalization of other exact solutions in general relativity:

The Kerr–Newman solution (with cosmological constant equal to zero) is also a special case of more general exact solutions of the Einstein–Maxwell Equations.[9]

## Some aspects of the solution

Newman's result represents the simplest stationary, axisymmetric, asymptotically flat solution of Einstein's equations in the presence of an electromagnetic field in four dimensions. It is sometimes referred to as an "electrovacuum" solution of Einstein's equations.

Any Kerr–Newman source has its rotation axis aligned with its magnetic axis.[10] Thus, a Kerr–Newman source is different from commonly observed astronomical bodies, for which there is a substantial angle between the rotation axis and the magnetic moment.[11]

If the Kerr–Newman potential is considered as a model for a classical electron, it predicts an electron having not just a magnetic dipole moment, but also other multipole moments, such as an electric quadrupole moment.[12] An electron quadrupole moment has not been detected empirically yet.[12]

In the G=0 limit, the electromagnetic fields are those of a charged rotating disk inside a ring where the fields are infinite. The total field energy for this disk is infinite, and so this G=0 limit does not solve the problem of infinite self-energy.[13]

Like the Kerr metric for an uncharged rotating mass, the Kerr–Newman interior solution exists mathematically but is probably not representative of the actual metric of a physically realistic rotating black hole due to stability issues. Although it represents a generalization of the Kerr metric, it is not considered as very important for astrophysical purposes since one does not expect that realistic black holes have an important electric charge.

The Kerr–Newman metric defines a black hole with an event horizon only when the following relation is satisfied:

${\displaystyle a^{2}+Q^{2}\leq M^{2}.}$

An electron's a and Q (suitably specified in geometrized units) both exceed its mass M, in which case the metric has no event horizon and thus there can be no such thing as a black hole electron — only a naked spinning ring singularity.[14] Such a metric has several seemingly unphysical properties, such as the ring's violation of the cosmic censorship hypothesis, and also appearance of causality-violating closed timelike curves in the immediate vicinity of the ring.[15]

The Russian theorist Alexander Burinskii wrote in 2007: "In this work we obtain an exact correspondence between the wave function of the Dirac equation and the spinor (twistorial) structure of the Kerr geometry. It allows us to assume that the Kerr–Newman geometry reflects the specific space-time structure of electron, and electron contains really the Kerr-Newman circular string of Compton size". The Burinskii paper describes an electron as a gravitationally confined ring singularity without an event horizon. It has some, but not all of the predicted properties of a black hole.[16]

## The electromagnetic fields

The electric and magnetic fields can be obtained in the usual way by differentiating the four-potential to obtain the electromagnetic field strength tensor. It will be convenient to switch over to three-dimensional vector notation.

${\displaystyle A_{\mu }=\left(-\phi ,A_{x},A_{y},A_{z}\right)\,}$

The static electric and magnetic fields are derived from the vector potential and the scalar potential like this:

${\displaystyle {\vec {E}}=-{\vec {\nabla }}\phi \,}$
${\displaystyle {\vec {B}}={\vec {\nabla }}\times {\vec {A}}\,}$

Using the Kerr-Newman formula for the four-potential in the Kerr-Schild form yields the following concise complex formula for the fields:[17]

${\displaystyle {\vec {E}}+i{\vec {B}}=-{\vec {\nabla }}\Omega \,}$
${\displaystyle \Omega ={\frac {Q}{\sqrt {({\vec {R}}-i{\vec {a}})^{2}}}}\,}$

The quantity omega (${\displaystyle \Omega }$) in this last equation is similar to the Coulomb potential, except that the radius vector is shifted by an imaginary amount. This complex potential was discussed as early as the nineteenth century, by the French mathematician Paul Émile Appell.[18]

## References

1. ^ Newman, Ezra; Janis, Allen (1965). "Note on the Kerr Spinning-Particle Metric". Journal of Mathematical Physics. 6 (6): 915–917. Bibcode:1965JMP.....6..915N. doi:10.1063/1.1704350.
2. ^ Newman, Ezra; Couch, E.; Chinnapared, K.; Exton, A.; Prakash, A.; Torrence, R. (1965). "Metric of a Rotating, Charged Mass". Journal of Mathematical Physics. 6 (6): 918–919. Bibcode:1965JMP.....6..918N. doi:10.1063/1.1704351.
3. ^ Kerr, RP (1963). "Gravitational field of a spinning mass as an example of algebraically special metrics". Physical Review Letters. 11: 237–238. Bibcode:1963PhRvL..11..237K. doi:10.1103/PhysRevLett.11.237.
4. ^ Hajicek, Petr et al. An Introduction to the Relativistic Theory of Gravitation, page 243 (Springer 2008).
5. ^ a b Debney, G. et al. "Solutions of the Einstein and Einstein-Maxwell Equations," Journal of Mathematical Physics, Volume 10, page 1842 (1969). Especially see equations (7.10), (7.11) and (7.14).
6. ^ Balasin, Herbert and Nachbagauer, Herbert. “Distributional Energy-Momentum Tensor of the Kerr-Newman Space-Time Family.” (Arxiv.org 1993), subsequently published in Classical and Quantum Gravity, volume 11, pages 1453–1461, abstract (1994).
7. ^ Berman, Marcelo. “Energy of Black Holes and Hawking’s Universe” in Trends in Black Hole Research, page 148 (Kreitler ed., Nova Publishers 2006).
8. ^ Burinskii, A. “Kerr Geometry Beyond the Quantum Theory” in Beyond the Quantum, page 321 (Theo Nieuwenhuizen ed., World Scientific 2007). The formula for the vector potential of Burinskii differs from that of Debney et al. merely by a gradient which does not affect the fields.
9. ^ a b Stephani, Hans et al. Exact Solutions of Einstein's Field Equations (Cambridge University Press 2003). See page 485 regarding determinant of metric tensor. See page 325 regarding generalizations.
10. ^ Punsly, Brian (10 May 1998). "High‐Energy Gamma‐Ray Emission from Galactic Kerr‐Newman Black Holes. I. The Central Engine". The Astrophysical Journal. 498 (2): 646. Bibcode:1998ApJ...498..640P. doi:10.1086/305561. Retrieved 16 May 2013. All Kerr-Newman black holes have their rotation axis and magnetic axis aligned; they cannot pulse.
11. ^ Lang, Kenneth. The Cambridge Guide to the Solar System, page 96 (Cambridge University Press, 2003).
12. ^ a b Rosquist, Kjell. "Gravitationally Induced Electromagnetism at the Compton Scale," Arxiv.org (2006).
13. ^ Lynden-Bell, D. "Electromagnetic Magic: The Relativistically Rotating Disk," Physical Review D, Volume 70, 105017 (2004).
14. ^ Burinskii, Alexander. "The Dirac-Kerr electron," Arxiv.org (2005).
15. ^ Carter, Brandon. Global Structure of the Kerr Family of Gravitational Fields, Physical Review 174, page 1559 (1968).
16. ^ Burinskii, Alexander. "Kerr Geometry as Space-Time Structure of the Dirac Electron," Arxiv.org (2007).
17. ^ Gair, Jonathan. "Boundstates in a Massless Kerr-Newman Potential".
18. ^ Appell, Math. Ann. xxx (1887) pp. 155–156. Discussed by Whittaker, Edmund and Watson, George. A Course of Modern Analysis, page 400 (Cambridge University Press 1927).