Once inside the event horizon, radiation or matter cannot escape to the outside world. Furthermore, in the case of a Schwarzschild black hole, nothing can prevent a particle from hitting the singularity in a very short amount of time (as measured by an external observer) once it has entered the horizon. In this sense, the event horizon is a point of no return. The case of a Kerr black hole is different, however. Here there are two event horizons, an inner and an outer, and a ring singularity which, in principle, can be avoided.
If the material of the Earth were squashed hard enough to make a black hole, its event horizon would be about 2 cm in diameter. A star with 10 times the mass of the Sun would form a black hole with an event horizon about 64 km across. Compare with apparent horizon.
Related entry• ergosphere
Related categories• BLACK HOLES
• SPACE AND TIME
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