Three Roads To Quantum Gravity

78 THREE ROADS TO QUANTUM GRAVITY
space
time
Horizon
Singularity
FIGURE 15
A rocket hovering just outside the horizon of a black hole. By keeping its
engines on, the rocket can hover a ®xed distance over the horizon.
speed, without accelerating) light will always catch up with
you. But if you continually accelerate, then light, if it starts
out from a point suf®ciently far behind you, will never be able
to catch you up. In fact this has nothing to do with a black
hole. Any observer who continually accelerates, anywhere in
the universe, will ®nd themself in a situation rather like that
of someone hovering just above the horizon of a black hole.
We can see this from Figure 16: given enough of a head start,
an accelerating observer can outrun photons. So an accelerating
observer has a hidden region simply by virtue of the fact
that photons cannot catch up with her. And she has a horizon,
which is the boundary of her hidden region. The boundary
separates those photons that will catch up with her from those