Three Roads To Quantum Gravity

BLACK HOLES AND HIDDEN REGIONS
73
it, a black hole has a horizon ± the surface that divides the
region from which light cannot escape from the rest of the
universe. Light leaving a point just inside the horizon of the
black hole will be pulled inexorably into the interior; light
just outside the horizon will be able to escape (Figures 13
and 14). Although the horizon of a black hole is an observer
dependent concept, there are a large number of observers who
share that horizon: all those who are outside that black hole.
So the horizon of a black hole is an objective property. But it is
not a horizon for all observers, because any observer who falls
through it will be able to see inside. And an observer who
crosses the horizon of a black hole will become invisible to
observers who remain outside.
Light ray moving
away from singularity
from outside the
horizon
Light ray moving
away from singularity
just at the horizon
Light ray moving
away from singularity
inside the horizon
time
space
Horizon
Singularity
FIGURE 14
The paths of three light rays that move away from the singularity. They start
just inside, outside and just at the horizon.