Three Roads To Quantum Gravity

CHAPTER 5
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BLACK HOLES AND HIDDEN REGIONS
In the cultural iconography of our time, black holes have
become mythic objects. In science ®ction novels and ®lms
they often evoke images of death and transcendence, recalling
the irreversibility of certain passages and the promise of our
eventual emergence into a new universe. I am not a very good
actor, but I was once asked by a friend, the director Madeline
Schwartzman, to act in one of her ®lms. Luckily I got to play a
physics professor giving a lecture on black holes. In the ®lm,
called Soma Sema, the myth of Orpheus is merged with two
major scienti®c and technological themes of our time: total
nuclear war and black holes. Orpheus, my student, seeks
through her music to be an exception to all three versions of
the irreversible.
Among those of us who think about space and time
professionally, black holes play a central role. A whole
subculture of astronomers is devoted to understanding how
they form and how to ®nd them. By now, dozens of candidate
black holes have been observed. But what is most exciting is
that there are probably vast numbers of them out there. Many
if not most galaxies, including our own, seem to have an
enormous black hole at their centre, with a mass millions of
times that of our Sun. And there is evidence, both observational
and theoretical, that a small fraction of stars end their
lives as black holes. A typical galaxy such as ours could well
contain tens or even hundreds of millions of these stellar
black holes. So black holes are out there, and interstellar