Prior to the discovery of the Reissner-Nordström solution Schwarzschild had discovered the noncharged nonrotating black hole solution, the Schwarzschild metric
which is an exact vacuum solution to Einstein's field equations.
Given the Schwarzschild solution as known it is not as difficult as one might assume to arrive at the Reissner-Nordström solution. The Schwarzschild solution takes the equation form of
So it is only natural that one would start with that as a trial solution in looking for the exact point charge solution for the field equations. To do this one finds the Einstein tensor for that line element and then looks to see if there is an that yields the right Einstein tensor corresponding to the stress-energy tensor of the electric field from a point charge. The Einstein tensor one arrives at for that line element is
The stress-energy tensor for a field mediated by massless particles such as the electric field from a point charge corresponds to a zero Ricci-scalar. This means tells us that and that therefor . For this to be the case we must have
which gives us the following second order differential equation to solve
Inserting the trial solution one finds that yielding
We may recognize from the Schwarzschild solution that , but to find we insert the solution for into the equation for the Einstein tensor and look for what corresponds to the exact solution from the electric field. We get
And Einstein's field equations relate the two tensors by
where G is the gravitational constant of the universe.
From this we get
to arrive at
So we find that the exact solution to Einstein's field equations for the stress-energy tensor from the electric field of a spinless point charge is
It is common for authors to either use geometric-plank units where or to convert the charge to units of distance by and the mass to distance units by in which case you will more often find it written something like
Penrose diagram for coordinate extension of a charged or rotating black hole
Above we see a Penrose diagram representing a coordinate extension (1) for a charged or rotating black hole. The known charged rotating black hole solution is the Kerr–Newman metric.
The same way as mapping Schwarzschild coordinates onto Kruskal-Szekeres coordinate reveals two seperate external regions for the Schwarzschild black hole, such a mapping done for a charged or rotating hole reveals an even more multiply connected region for charged and rotating black holes.
Lets say region I represents our external region outside a charged black hole. In the same way that the other external region is inaccessible as the wormhole connection is not transversible, external region II is also not accessible from region I. The difference is that there are other external regions VII and VIII which are indeed accessible from region I by transversible paths at least one way. One should expect this as the radial movement case of geodesic motion for a neutral test particle written above leads back out of the hole without intersecting the physical singularity at r=0.