Penrose (1968) first pointed out that a person crossing an
inner horizon would see the outside universe infinitely blueshifted.
Poisson & Israel (1990) clarified the full non-linear nature of the instability, which they called “mass inflation.”
Mass inflation is caused by relativistic counter-streaming between
ingoing and outgoing streams near the inner horizon.
To drop through the inner horizon, the streams must exceed the speed of light
relative to each other, which they cannot do.
11/16. How to make a baby universe?