Notes on Relativity and Cosmology - Physics Department, UCSB

9.6. BLACK HOLE ODDS AND ENDS 275
Inner horizon
r = oo
t = oo
finite
separation
light wave
r = oo
t = -
oo
infinite
separation
However, inside the black hole, we can see that the description is different. Now
the two light rays have a finite separation. This means that that near the light
ray marked “inner horizon,” what was a long wavelength light ray outside is
now of very short wavelength, and so very high energy! In fact, the energy
created by any small disturbance will become infinite at the “inner horizon.” It
will come as no surprise that this infinite energy causes a large change in the
spacetime.
The result is that dropping even a small pebble into an RN black hole creates a
big enough effect at the inner horizon to radically change the Penrose diagram.
The Penrose diagram for the actual spacetime containing an RN black hole
together with even a small disturbance looks like this: