Charged Black Holes: The Reissner-Nordström Geometry

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Reissner-Nordström geometry

Embedding diagram of the Reissner-Nordström geometry with Charge/Mass = 0.9.

The Reissner-Nordström geometry describes the geometry of empty space surrounding a charged black hole.

If the charge of the black hole is less than its mass (measured in geometric units \(G = c = 1\)), then the geometry contains two horizons, an outer horizon and an inner horizon. Between the two horizons space is like a waterfall, falling faster than the speed of light, carrying everything with it. Upstream and downstream of the waterfall, space moves slower than the speed of light, and relative calm prevails.

Fundamental charged particles like electrons and quarks are not black holes: their charge is much greater than their mass, and they do not contain horizons.

If the geometry is continued all the way to the centre of the black hole, then there is a gravitationally repulsive, negative-mass singularity there. Uncharged persons who fall into the charged black hole are repelled by the singularity, and do not fall into it.

The diagram at left is an embedding diagram of the Reissner-Nordström geometry, a 2-dimensional representation of the 3-dimensional spatial geometry at an instant of Reissner-Nordström time. Between the horizons, radial lines at fixed Reissner-Nordström time are time-like rather than space-like, which is to say that they are possible wordlines of radially infalling (albeit not freely falling) observers. The animated dashes follow the positions of such infalling observers as a function of their own proper time.

Caveats

The Universe at large appears to be electrically neutral, or close to it. Thus real black holes are unlikely to be charged. If a black hole did somehow become charged, it would quickly neutralize itself by accreting charge of the opposite sign.

It is not clear how a gravitationally repulsive, negative-mass singularity could form. If it did, it is likely that the singularity would spontaneously destroy itself by popping charged particle-antiparticle pairs out of the vacuum inside the inner horizon. By swallowing particles of charge opposite to itself, the singularity would tend to neutralize both its charge and its negative mass, redistributing the charge over space inside the inner horizon.

In these pages I have somewhat arbitrarily replaced the Reissner-Nordström geometry near the singularity with flat space. Specifically, the inward rush of space into the black hole slows to a halt at the turnaround point \(r_0\) inside the inner horizon (see the discussion in the section below on the Free-fall spacetime diagram), and I have replaced the space interior to \(r_0\) with flat space. This is equivalent to concentrating all the charge of the black hole into a thin shell at the turnaround point \(r_0\).

Reissner-Nordström metric

The Reissner-Nordström metric is \[ d s^2 = - \, B(r) d t^2 + {d r^2 \over B(r)} + r^2 d o^2 \] where the metric coefficient B(\(r\)) is \[ B(r) = 1 - {2 M \over r} + {Q^2 \over r^2} \ . \] This expression is in geometric units, works also for the Reissner-Nordström geometry. The mass \(M(r)\) at radial position \(r\) is the effective mass interior to \(r\) which is the total mass \(M\) at infinity, less the mass \(Q^2 / (2r)\) contained in the electromagnetic field outside \(r\): \[ M(r) = M - {Q^2 \over 2r} \ . \] The electromagnetic mass \(Q^2 / (2r)\) is the mass outside \(r\) associated with the energy density \(E / (8\pi)\) of the electric field \(E = Q / r^2\) surrounding a charge \(Q\).

The infall velocity \(v\) of space passes the speed of light \(c\) at the outer horizon \(r_+\), but slows back down to less than the speed of light at the inner horizon \(r_-\). The velocity slows all the way to zero at the turnaround point \(r_0\) inside the inner horizon, \[ r_0 = {Q^2 \over 2M} \ . \]

The free-fall metric for the Reissner-Nordström geometry takes the same form as for Schwarzschild, \[ d s^2 = - d t_\textrm{ff} + (d r - v \, d t_\textrm{ff})^2 + r^2 d o^2 \ , \] with free-fall velocity \[ v = - \sqrt{2 M(r) \over r} \ . \] The free-fall time coordinate \(t_\textrm{ff}\) is the proper time experienced by persons who free-fall at velocity \(d r / d t_\textrm{ff} = v\) from zero velocity at infinity: \[ t_\textrm{ff} = t + \sqrt{2 M} \left[ 2 \sqrt{x} - {r_+ \sqrt{x_+} \over r_+ - r_-} \ln \left( {\sqrt{x} + \sqrt{x_+} \over \sqrt{x} - \sqrt{x_+}} \right) + {r_- \sqrt{x_-} \over r_+ - r_-} \ln \left( {\sqrt{x} + \sqrt{x_-} \over \sqrt{x} - \sqrt{x_-}} \right) \right] \ , \] where the coordinate \(x\) is the radial position relative to the turnaround point \(r_0\), \[ x \equiv r - r_0 \ , \] and \(x_\pm \equiv r_\pm - r_0\) are the values of \(x\) at the horizons \(r_\pm\).

The free-fall metric shows that the spatial geometry is flat, having spatial metric \(d r^2 + r^2 d o^2\), on hypersurfaces of fixed free-fall time, \(d t_\textrm{ff} = 0\).

The colouring of lines in the free-fall spacetime diagram is as in the Reissner-Nordström spacetime diagram, with the addition of green lines which are worldlines of observers who free fall radially from zero velocity at infinity, and horizontal dark green lines which are lines of constant free-fall time \(t_\textrm{ff}\).

Watch Reissner-Nordström morph into free-fall (41K movie icon GIF); or same morph, double-size on screen (same 41K GIF).

Finkelstein spacetime diagram of the Reissner-Nordström geometry

Finkelstein spacetime diagram of the Reissner-Nordström geometry with Charge/Mass = 0.9.

As usual, the Finkelstein radial coordinate \(r\) is the circumferential radius, defined so that the proper circumference of a sphere at radius \(r\) is \(2\pi r\), while the Finkelstein time coordinate is defined so that radially infalling light rays (yellow lines) move at \(45^\circ\) in the spacetime diagram.

Finkelstein time \(t_\textrm{F}\) is related to Reissner-Nordström time \(r\) by \[ t_\textrm{F} = t + {1 \over 2 g_+} \ln \left( {r - r_+ \over r_0 - r_+} \right) + {1 \over 2 g_-} \ln \left( {r - r_- \over r_0 - r_-} \right) \ , \] where \(g_\pm \equiv g(r_\pm)\) are the surface gravities at the two horizons \[ g_\pm = \pm {r_+ - r_- \over 2 r_\pm^2} \ . \] The gravity \(g(r)\) at radial position \(r\) is the inward acceleration \[ g(r) = {d v \over d t_\textrm{ff}} = - v {d v \over d r} = {1 \over 2} {d B \over d r} \ . \]

The colouring of lines is as in the Schwarzschild case: the red line is the horizon, the cyan line at zero radius is the singularity, yellow and ochre lines are respectively the wordlines of radially infalling and outgoing light rays, while dark purple and blue lines are respectively lines of constant Schwarzschild time and constant circumferential radius.

Watch Reissner-Nordström morph into Finkelstein (37K movie icon GIF); or same morph, double-size on screen (same 37K GIF).

Watch Finkelstein morph into free-fall (38K movie icon GIF); or same morph, double-size on screen (same 38K GIF).

Penrose diagram of the Reissner-Nordström geometry

Penrose diagram of the Reissner-Nordström geometry with Charge/Mass = 0.9.

Coordinates of Penrose diagram constructed so that the metric is well-behaved across both outer and inner horizons. Given this restriction, it's impossible to make the zero radius part vertical.

Watch Finkelstein morph into Penrose (51K movie icon GIF); or same morph, double-size on screen (same 51K GIF).

Penrose diagram of the complete Reissner-Nordström geometry

Penrose diagram of the complete Reissner-Nordström geometry with Charge/Mass = 0.9.

Suppose that you fall into a charged black hole. At the moment that you cross the inner horizon, you see an infinitely blueshifted point of light appear directly ahead, in the direction of the black hole. This infinitely blueshifted point of light is a record of the entire past history of the Universe, condensed into an instant.

Inside the inner horizon, the gravitational repulsion of the central singularity slows you down and turns you around, accelerating you back out through the inner horizon of a white hole. As you approach the inner horizon of the white hole, this time looking outward directly away from the black hole, part of the image of the outside Universe seems to break away from the rest. As you pass through the inner horizon this breakaway image concentrates into another infinitely blueshifted point of light, which disappears in a blazing flash. This time the infinitely blueshifted point of light contains the entire future of the Universe, condensed into an instant.

The white hole spews you out into a new Universe. Since light cannot fall into the white hole from the new Universe, you do not see the new Universe until you pass through the outer horizon of the white hole. At the instant you pass through the outer horizon, you witness once again an infinitely blueshifted point of light appear directly ahead, away from the white hole. The infinitely blueshifted point of light contains the entire past of the new Universe concentrated into an instant. The point of light opens up to reveal the new Universe, which you join.

Looking back into the white hole, you can see the Universe from which you came, but to which you cannot return.


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Updated 19 Apr 2001; converted to mathjax 3 Feb 2018