# Gravity of an electrically charged body

 AstroNuclPhysics ® Nuclear Physics - Astrophysics - Cosmology - Philosophy Gravity, black holes and physics

3.5. Reissner-Nordström geometry

Previous spherically symmetric case can be generalized somewhat while maintaining the spherical symmetry, that the excitation body will consider the electrically charged with charge Q. In order to preserve symmetry must be distribution of electric charge also spherically symmetrical, because the gravitational field is generated not only source body material, but also by the energy-momentum tensor of the electromagnetic field. An asymmetrical charge distribution would lead to an asymmetric electric field, which would excite an asymmetric gravitational field.
The space-time interval element will again have the same spherically symmetric basic shape (3.10)

ds 2   =  g tt dt 2 + g rr dr 2 + r 2 (d J 2 + sin 2 J d j 2)   .

The difference from the previous Schwarzschild solution is now that the external solution will not be a solution of Einstein's equations without the right side, but the right side here will be the energy-momentum tensor of the electromagnetic field Tikelmag (1.118). In this case, it is a Coulomb centrally symmetric electrostatic field with intensity E = E(r). er , where er is the unit base vector of the radial direction. Because the Schwarzschild radial coordinate R retains the meaning that the surface of a sphere centered at the point of symmetry, having a radius r , is equal to 4p.r2, according to Coulomb's law (Gauss's theorem), E(r) = Q/r2. The energy-momentum tensor of this electric field is then (non-diagonal components are zero)

Tttelmag =  -Trr  =  TJJ =  Tjj =  E(r)2/8p  =  Q2/8p.r 4  ;

this we substitute into Einstein's equations Rik - (1/2) gik R = 8p Tik elmag and for spherically symmetric metrics (3.10) we get again two independent differential equations by a similar procedure as when deriving the Schwarzschild solution

dgtt/dr =  (1/r) gtt (1 + grr - grr.Q2/r2)  ,   dgrr/dr =  -(1/r) grr (1 + grr - grr.Q2/r2)  .

The solution to this system is

gtt = C1(1 - C2/r + Q2/r2)  ,   grr = (1 - C2/r + Q2/r2)-1  ,

where for the same reasons as in Schwarzschild's case the integration constants must be C1 = 1 and C2 = -2M. The final expression for the element of the space-time interval of a spherically symmetric gravitational field excited by a spherically symmetric body of total mass M and an electric charge Q thus has the form (3.32)

in geometrodynamic units; in common units it is (3.32 ')

This geometry is called Reissner-Nordström geometry (RN). Here, the parameter M again has the meaning of the total mass, the parameter Q the meaning of the total electric charge measured by a remote observer using the Gaussian integral fluxes of the vector E, resp. by analyzing the motion of charged test particles. In the case of 0 < Q2 < M2, the RN field could theoretically describe an electrically charged black hole (§4.4 "Rotating and electrically charged Kerr-Newman black holes").

Derivation of the gravitational-electric solution from the principle of least action
The general derivation of the space-time structure in the presence of a combined gravitational + electromagnetic field is sometimes performed using the variational principle of least action using the combined Lagrangian of gravitational L G and electrodynamic L EL :
L =
LG + LEL = (R - 2L) - FikFik ......,
where R is the Ricci scalar curvature,
L is the cosmological constant, Fik is an electromagnetic field tensor.
The variational principle of least action, then gives the Einstein-Maxwell equations of gravitational and electromagnetic fields :
Gik +
Lgik = 1/2 gik.................... .. ...........................
............... ........ ...........

Properties of the Reissner-Nordström field
RN geometry with space-time metric (3.32) is a static spherically symmetric solution of Einstein-Maxwell gravitational equations. This geometry is asymptotically planar and its horizon has a spherical topology. It is characterized by the mass
M and the electric charge Q and is singular at the beginning r = 0 of the radial coordinate.
According to the mutual ratio of M and Q values, we can distinguish four significant cases with different global geometric structure in Reissner-Nordström spacetime :
a
) At Q = 0, M> 0 we get the Schwarzschild geometry from the previous §3.4 ;
b ) 0 < Q2 < M2 ;
c ) 0 < Q2 = M2 ;
d ) Q2 > M2 > 0.
First, we analyze the case 0 <Q 2 <M 2 , which is the most physically interesting. The Reissner-Nordström metric (3.32) is very similar to the Schwarzschild metric (3.13), but differs in that the expression

1 - 2 M r + Q 2 / r 2

is in the case of 0 < |Q| <M equal to zero for two "roots" r = rg+ and r = rg- :

 r g + = M + Ö (M 2 - Q 2 )  ,   r g - = M - Ö (M 2 - Q 2 )   . (3.33)

In ordinary units, these are values

 rg+ = (G/c2) [M + Ö(M2 - Q2)] ,  rg- = (G/c2) [M - Ö(M2 - Q2)]   . (3.33 ')

Thus, there are two "horizons" in RN geometry where the metric (3.32) is not regular - " outer" horizon r = rg+ and "inner" horizon r = rg-. The outer horizon r = rg+ has a similar meaning as the Schwarzschild sphere in Schwarzschild spacetime - is the horizon of events, causally separating the inner region from the outer one; from (3.33) we see that in the presence of an electric charge the gravitational radius rg+ is less than rg = 2M in the Schwarzschild case. Under r = rg+ the light cones are turned inwards toward r = 0 (because gtt > 0) and it would seem that every object that gets there will necessarily end up in the singularity r = 0. However, on the inner horizon r = rg- the light cones begin to straighten again (the time component of the metric tensor changes the sign again: gtt <0) - it is therefore possible to move the particle so as to avoid singularity (Fig.3.20). However, it cannot get across the outer horizon (ie the horizon of events) back to the original space-time, but necessarily to the "other universe" that lies with the original in the absolute future (see below). Fig.3.20 Kerr space-time diagram of Reissner-Nordström geometry (the situation will look similar in Kerr and Kerr-Newman geometry - §3.6). The outer horizon r = rg+ is the event horizon (the light cones below it face are inwardly towards r = 0). Below the inner horizon r = rg-, however, the light cones begin to "straighten" again, so that the world-line of a body that has penetrated below the horizon r = rg+, does not necessarily end in the singularity r = 0.

The singular behavior of the metric (3.32) in the standard coordinates on these horizons is again only apparent and can be removed by transition to more suitable coordinates similar to Kruskal's. With the help of the modified coordinate r* (3.34)

similarly to the extension of Schwarzschild geometry, we introduce isotropic coordinates p = t + r*, q = t - r*. We then further transform these in order to remove the singular coefficient in the metric : (3.34 ')

When introducing new temporal and spatially coordinates of u = (q'- p')/2 and v = (q'+ p')/2 the Reissner-Nordström metric has form (3.35)

After a conformal transformation (analogous (3.5) in §3.2) in order to more clearly present the asymptotic structure, the Reissner-Nordström geometry will be described by the interval (3.35´) Fig.3.21. Penrose conformal space-time diagram of the complete extension of the Reissner-Nordström geometry for the case Q
2 < M2 .
a ) Coordinate network - hypersurfaces r = const. and t = const.
b ) Global geometric structure - infinitely many periodically recurring outer regions ("universes") A..., - 1,1,2, .., inner regions B..., - 1,1,2, .. and C..., - 1,1,2, .., horizons and singularities.

The space-time diagram of the conformal image of RN geometry for the most physically probable case 0 < |Q| <M is shown in Fig.3.21. The geometric structure of this complete extension of the Reissner-Nordström solution is unexpectedly complex. An infinite number of periodically recurring "universes" (separate asymptotically planar outer regions A…,-1,1,2,…), horizons and singularities appear here. In contrast to Schwarzschild geometry (Fig.3.19), where singularities are spatial type (and therefore inevitable for every object in area B), singularities of Reissner-Nordström geometry according to Fig.3.21 are of time type - they are, so to speak, "time-limited" and can be avoided. Fig.3.22. The O observer moving in the outer asymptotically planar region A1 of the Reissner-Nordström spacetime has three possibilities. Either it will constantly move in A1 (solid line), so it will reach infinity I+ or Á+ in the limit. However , if the observer penetrates below the horizon r = rg+ (dashed trajectory) into the inner region B1, he also passes through the horizon inner r = rg- into the region C1, where he has two possibilities: he either encounters singularity (dotted path) where it is absorbed and destroyed, or it can avoid singularity (dashed trajectory) and reach another asymptotically planar outer region A2. The situation in this next "universe" A2 is not completely determined by the initial conditions on the Cauchy hypersurface S , as can be seen, for example, at the point p Î A2 .

Let us observe the fate of the observer (as indicated in Fig.3.22), who, during his movement in Reissner-Nordström spacetime, penetrated below the outer horizon r = rg+. Because it has fallen below the horizon of events, it can no longer return to the original outer space (area A1) and has basically two possibilities. First, to reach a singularity, where its world line (and thus its existence within the considered manifold) will definitely end. However, this is not (unlike Schwarzschild spacetime) unavoidable, the observer can avoid the singularity and move further until it appears in the second asymptotically planar region A2, in the second "universe", which lies with respect to the initial A1 in the absolute future. Thus, we see that a real material object moving in RN geometry within a light cone can in principle "travel" between individual "universes" *) without having to go through a singularity (unlike Schwarzschild geometry, where the Einstein-Rosen bridge could pass only at superlight speed).
*) It should be noted that this "travel between different universes" (and the very existence of these other universes) is possible only theoretically in an extremely idealized model of an asymptotically planar universe without other bodies and fields, with precise Reissner-Nordström or Kerr geometry. A critical assessment of similar possibilities will be in §4.4, section "Black holes - bridges to other universes?".
If we look at the Causal relationship of this second universe with respect to the original, we see that the inner horizons r = rg- are also Cauchy horizons. If we take some event P in the region A2 and observe how it can be influenced in principle, we see that although it can be influenced by geodesics (eg G1) coming from the region A1 (and given the initial conditions for suitable Cauchy hypersurface in A1), but new information can also come there "uncontrollably" by geodesics (eg G2 , G3 , G4) from areas of infinity of the past I- , Á - and from the singularity, that can be "seen" from here. This information may undermine any prediction made on the basis of the initial conditions in area A1. Thus, the observer emerged in the region of spacetime (another "universe"), which is not unambiguously determined by the initial conditions for any Cauchy hypersurface in the original region A1.
Let us confront this with the deterministic idea of classical (non-quantum) physics, which Laplace had already formulated: If at certain time moment we found all physical quantities in all parts of the universe (ie the instantaneous state of the universe - the complete set of initial conditions on Cauchy's hypersurface) and knew the physical laws by which all these quantities are governed,, we could indefinitely to predict the evolution of the universe, ie its state at any time in the future (or even the past). However, in the complete extension of the Reissner-Nordström geometry, this is not fulfilled, there are Cauchy horizons (and therefore not exist global Cauchy hypersurfaces) and there are therefore regions, whose state is not unambiguously determined by any set of initial conditions. Only in the outer asymptotically planar region can the future be unambiguously "predicted" from partial Cauchy hypersurfaces. Thus, even in the classical physics (on which the Reissner-Nordström geometry, as a solution to Einstein equation, is based) the possibility of predicting the future may be limited not only by the practical unavailability of physical quantities in all parts of the universe at a certain point in time, but in principle also by the global geometric-topological structure of spacetime...
In Fig.3.21, each point of area B between rg+ and rg- (where the areas r = const. are of the spatial type) represents a two-dimensional spherical surface, which is a closed absorbing (trapped) surface (see definition 3.10). Observer O, as he passes through the surface r = rg- (Fig.3.22), will see the whole further history of the asymptotically planar outer region A1, which he leaves, for the finite time. Therefore, each body from this area will be visible with an infinitely increasing purple shift. It follows that the Cauchy horizon r = rg- is unstable to perturbations of initial conditions on the initial spatial hypersurface S . It is clear that the "cosmological" questions of the evolution of the "universe" A1, which the observer is leaving, will be very important to him. If the universe A1 may collapse in the future, even observer O will not avoid this fate; on the horizon r = rg- it encounters an infinite density of matter ~ energy, that is, in the end, with a singularity. Fig.3.23. Penrose conformal space-time diagram of the complete extension of the Reissner-Nordström geometry for the case Q
2 = M2 .
a ) Hypersurfaces (coordinate lines) r = const. and t = const.
b ) Global geometric structure - infinitely many periodically repeating external areas A and the inner regions B .

Extreme R-N geometry
For the case Q = M, the corresponding conformal space-time diagram of the complete extension of the Reissner-Nordström geometry has the shape shown in Fig.3.23. Again, an infinite number of periodically recurring "outer" regions
A (M <r < ¥) and inner regions B (0 <r <M) can be seen. The outer and inner horizons coincide (rg- = rg+ = M), this is a special case of an extreme black hole (see §4.4). Fig.3.24. Penrose conformal space-time diagram of Reissner-Nordström geometry in the case of Q
2 > M2 .
a ) Coordinate lines - hypersurfaces r = const. and t = const.
b ) Global geometric structure - shapes of light cones and radial motion of bodies and photons. There are no horizons, the singularity r = 0 is "naked" (it is visible from any point) .

R-N naked singularity
In the case of Q
2 > M2, no extension needs to be sought, because spacetime is non-extensible already in the original coordinates; is regular everywhere except the point r = 0 - irremovable physical singularities of spacetime. A conformal space-time diagram for this case is shown in Fig.3.24. The horizon of events is not here, it is a naked singularity (see §3.9 and §4.4).

Generalized Reissner-Nordström solution in nonlinear electrodynamics
In §1.6, the final passage "
Nonlinear electrodynamics", the theoretical possibility was discussed that the electric field would not behave exactly linearly at extremely high intensities according to the laws of standard Maxwell electrodynamics, but there could be fundamental boundaries - maximum possible electric field strength Emax. This alternative option could significantly (with interesting consequences) applied in Reissner-Nordström solutions, as well as the Kerr-Newman geometry (§3.6) - and generally black holes (event. also in other compact objects such as neutron stars and their special cases magnetars) with very strong electric and magnetic fields (§4 ..., § .....) .......
If we substitute Reissner-Nordstr
öm electro -gravitation solution (3.32), to which this §3.5 is devoted, standard Maxwell's electrodynamics by nonlinear Born-Infeld electrodynamics (1.120) - either using the Lagrangian (1.119) or the momentum energy tensor electromag. field, we get a generalized Reissner-Nordström solution : (3.36)

The function f(r, Q) expresses the effect of the electric field on the metric. The intensity of the electric field in BN electrodynamics depends on the radial coordinate r according to the law E(r) = Q/Ö(r4 + Q2.b2). At b = 0, ie without electrical nonlinearity, f(r, Q) = Q2/r2 - we get the basic RN geometry (3.32) with Coulomb electric intensity E(r) = Q/r2. The integral contained in the function f(r, Q) can be explicitly expressed using two kinds of special functions (3.36´)

where "F (...)" is the Legender elliptic function of the 1st kind and "2F1(...)" is the so-called hypergeometric function (more complex power series from Q2b2/r4). Specific values of these complex functions can be found for physical calculations in special tables, recently there are also computer programs for them.
This spacetime of the generalized RN solution with metric (3.36), which is a solution of Einstein-Born-Infeld equations, is characterized by three parameters: mass M, charge Q and Born-Infeld nonlinearity parameter "b" (whose inverse value indicates the final value of the electric field intensity at the origin of the coordinates r = 0). Even with b> 0 at larger distances r> 0, it asymptotically transitions into an RN solution.
......................
It is interesting that in the solution of nonlinear electrodynamics, zero (isotropic) geodesics represent only the orbits of
(hypothetical-model) gravitons, but generally not photons ! The paths of photons here can be influenced by "self-interaction" with a nonlinear electromagnetic field ...
........... ........... ....... .. .. ................   3.4. Schwarzschild geometry 3.6. Kerr and Kerr-Newman geometry

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