# Conformastatic spacetimes

Conformastatic spacetimes refer to a special class of static solutions to Einstein's equation in general relativity.

## Introduction

The line element for the conformastatic class of solutions in Weyl's canonical coordinates reads[1][2][3][4][5][6]
${\displaystyle (1)\qquad ds^{2}=-e^{2\Psi (\rho ,\phi ,z)}dt^{2}+e^{-2\Psi (\rho ,\phi ,z)}{\Big (}d\rho ^{2}+dz^{2}+\rho ^{2}d\phi ^{2}{\Big )}\;,}$
as a solution to the field equation
${\displaystyle (2)\qquad R_{ab}-{\frac {1}{2}}Rg_{ab}=8\pi T_{ab}\;.}$
Eq(1) has only one metric function ${\displaystyle \Psi (\rho ,\phi ,z)}$ to be identified, and for each concrete ${\displaystyle \Psi (\rho ,\phi ,z)}$, Eq(1) would yields a specific conformastatic spacetime.

## Reduced electrovac field equations

In consistency with the conformastatic geometry Eq(1), the electrostatic field would arise from an electrostatic potential ${\displaystyle A_{a}}$ without spatial symmetry:[3][4][5]
${\displaystyle (3)\qquad A_{a}=\Phi (\rho ,z,\phi )[dt]_{a}\;,}$
which would yield the electromagnetic field tensor ${\displaystyle F_{ab}}$ by
${\displaystyle (4)\qquad F_{ab}=A_{b\,;a}-A_{a\,;b}\;,}$
as well as the corresponding stress–energy tensor by
${\displaystyle (5)\qquad T_{ab}^{(EM)}={\frac {1}{4\pi }}{\Big (}F_{ac}F_{b}^{\;\;c}-{\frac {1}{4}}g_{ab}F_{cd}F^{cd}{\Big )}\;.}$

Plug Eq(1) and Eqs(3)(4)(5) into "trace-free" (R=0) Einstein's field equation, and one could obtain the reduced field equations for the metric function ${\displaystyle \Psi (\rho ,\phi ,z)}$:[3][5]

${\displaystyle (6)\qquad \nabla ^{2}\Psi \,=\,e^{-2\Psi }\,\nabla \Phi \,\nabla \Phi }$
${\displaystyle (7)\qquad \Psi _{i}\Psi _{j}=e^{-2\Psi }\Phi _{i}\Phi _{j}}$

where ${\displaystyle \nabla ^{2}=\partial _{\rho \rho }+{\frac {1}{\rho }}\,\partial _{\rho }+{\frac {1}{\rho ^{2}}}\partial _{\phi \phi }+\partial _{zz}}$ and ${\displaystyle \nabla =\partial _{\rho }\,{\hat {e}}_{\rho }+{\frac {1}{\rho }}\partial _{\phi }\,{\hat {e}}_{\phi }+\partial _{z}\,{\hat {e}}_{z}}$ are respectively the generic Laplace and gradient operators. in Eq(7), ${\displaystyle i\,,j}$ run freely over the coordinates ${\displaystyle [\rho ,z,\phi ]}$.

## Examples

### Extremal Reissner–Nordström spacetime

The extremal Reissner–Nordström spacetime is a typical conformastatic solution. In this case, the metric function is identified as[4][5]

${\displaystyle (8)\qquad \Psi _{ERN}\,=\,\ln {\frac {L}{L+M}}\;,\quad L={\sqrt {\rho ^{2}+z^{2}}}\;,}$

which put Eq(1) into the concrete form

${\displaystyle (9)\qquad ds^{2}=-{\frac {L^{2}}{(L+M)^{2}}}dt^{2}+{\frac {(L+M)^{2}}{L^{2}}}\,{\big (}d\rho ^{2}+dz^{2}+\rho ^{2}d\varphi ^{2}{\big )}\;.}$

Applying the transformations

${\displaystyle (10)\;\;\quad L=r-M\;,\quad z=(r-M)\cos \theta \;,\quad \rho =(r-M)\sin \theta \;,}$

one obtains the usual form of the line element of extremal Reissner–Nordström solution,

${\displaystyle (11)\;\;\quad ds^{2}=-{\Big (}1-{\frac {M}{r}}{\Big )}^{2}dt^{2}+{\Big (}1-{\frac {M}{r}}{\Big )}^{-2}dr^{2}+r^{2}{\Big (}d\theta ^{2}+\sin ^{2}\theta \,d\phi ^{2}{\Big )}\;.}$

### Charged dust disks

Some conformastatic solutions have been adopted to describe charged dust disks.[3]

## Comparison with Weyl spacetimes

Many solutions, such as the extremal Reissner–Nordström solution discussed above, can be treated as either a conformastatic metric or Weyl metric, so it would be helpful to make a comparison between them. The Weyl spacetimes refer to the static, axisymmetric class of solutions to Einstein's equation, whose line element takes the following form (still in Weyl's canonical coordinates):
${\displaystyle (12)\;\;\quad ds^{2}=-e^{2\psi (\rho ,z)}dt^{2}+e^{2\gamma (\rho ,z)-2\psi (\rho ,z)}(d\rho ^{2}+dz^{2})+e^{-2\psi (\rho ,z)}\rho ^{2}d\phi ^{2}\,.}$
Hence, a Weyl solution become conformastatic if the metric function ${\displaystyle \gamma (\rho ,z)}$ vanishes, and the other metric function ${\displaystyle \psi (\rho ,z)}$ drops the axial symmetry:
${\displaystyle (13)\;\;\quad \gamma (\rho ,z)\equiv 0\;,\quad \psi (\rho ,z)\mapsto \Psi (\rho ,\phi ,z)\,.}$
The Weyl electrovac field equations would reduce to the following ones with ${\displaystyle \gamma (\rho ,z)}$:

${\displaystyle (14.a)\quad \nabla ^{2}\psi =\,(\nabla \psi )^{2}}$
${\displaystyle (14.b)\quad \nabla ^{2}\psi =\,e^{-2\psi }(\nabla \Phi )^{2}}$
${\displaystyle (14.c)\quad \psi _{,\,\rho }^{2}-\psi _{,\,z}^{2}=e^{-2\psi }{\big (}\Phi _{,\,\rho }^{2}-\Phi _{,\,z}^{2}{\big )}}$
${\displaystyle (14.d)\quad 2\psi _{,\,\rho }\psi _{,\,z}=2e^{-2\psi }\Phi _{,\,\rho }\Phi _{,\,z}}$
${\displaystyle (14.e)\quad \nabla ^{2}\Phi =\,2\nabla \psi \nabla \Phi \,,}$

where ${\displaystyle \nabla ^{2}=\partial _{\rho \rho }+{\frac {1}{\rho }}\,\partial _{\rho }+\partial _{zz}}$ and ${\displaystyle \nabla =\partial _{\rho }\,{\hat {e}}_{\rho }+\partial _{z}\,{\hat {e}}_{z}}$ are respectively the reduced cylindrically symmetric Laplace and gradient operators.

It is also noticeable that, Eqs(14) for Weyl are consistent but not identical with the conformastatic Eqs(6)(7) above.

## References

1. ^ John Lighton Synge. Relativity: The General Theory, Chapter VIII. Amsterdam: North-Holland Publishing Company (Interscience), 1960.
2. ^ Hans Stephani, Dietrich Kramer, Malcolm MacCallum, Cornelius Hoenselaers, Eduard Herlt . Exact Solutions of Einstein's Field Equations (2nd Edition), Chapter 18. Cambridge: Cambridge University Press, 2003.
3. ^ a b c d Guillermo A Gonzalez, Antonio C Gutierrez-Pineres, Paolo A Ospina. Finite axisymmetric charged dust disks in conformastatic spacetimes. Physical Review D 78 (2008): 064058. arXiv:0806.4285[gr-qc]
4. ^ a b c F D Lora-Clavijo, P A Ospina-Henao, J F Pedraza. Charged annular disks and Reissner–Nordström type black holes from extremal dust. Physical Review D 82 (2010): 084005. arXiv:1009.1005[gr-qc]
5. ^ a b c d Ivan Booth, David Wenjie Tian. Some spacetimes containing non-rotating extremal isolated horizons. Accepted by Classical and Quantum Gravity. arXiv:1210.6889[gr-qc]
6. ^ Antonio C Gutierrez-Pineres, Guillermo A Gonzalez, Hernando Quevedo. Conformastatic disk-haloes in Einstein-Maxwell gravity. Physical Review D 87 (2013): 044010. arXiv:1211.4941[gr-qc]