Three Roads To Quantum Gravity

70 THREE ROADS TO QUANTUM GRAVITY
travellers of the far future will have to be careful to avoid
them. But beyond the fascination they hold for astronomers,
black holes are important to science for other reasons. They
are a central object of study for those of us who work on
quantum gravity. In a sense, black holes are microscopes of
in®nite power which make it possible for us to see the physics
that operates on the Planck scale.
Because they feature prominently in popular culture, almost
everyone knows roughly what a black hole is. It is a
place where gravity is so strong that the velocity required to
escape from it is greater than the speed of light. So no light
can emerge from it, and neither can anything else. We can
understand this in terms of the notion of causal structure we
introduced in the last chapter. A black hole contains a great
concentration of mass that causes the light cones to tip over so
far that the light moving away from the black hole actually
gets no farther from it (Figure 13). So the surface of a black
hole is like a one-way mirror: light moving towards it can pass
into it, but no light can escape from it. For this reason the
surface of a black hole is called the horizon. It is the limit of
what observers outside the black hole can see.
I should emphasize that the horizon is not the surface of
the object that formed the black hole. Rather it is the boundary
of the region that is capable of sending light out into the
universe. Light emitted by any body inside the horizon is
trapped and cannot get any farther than the horizon. The
object that formed the black hole is rapidly compressed, and
according to general relativity it quickly reaches in®nite
density.
Behind the horizon of a black hole is a part of the universe
made up of causal processes that go on, in spite of the fact that
we receive no information from them. Such a region is called
a hidden region. There are at least a billion billion black holes
in the universe, so there are quite a lot of hidden regions that
are invisible to us, or to any other observer. Whether a region
is hidden or not depends in part on the observer. An observer
who falls into a black hole will see things that her friends who
stay outside will never see. In Chapter 2 we found that
different observers may see different parts of the universe in