It is not an exaggeration to say that one of the most exciting
predictions of Einstein's theory of gravitation is that there may exist
"black holes" putative objects whose gravitational fields are so strong
that no physical bodies or signals can break free of their pull and
escape. The proof that black holes do exist, and an analysis of their
properties, would have a significance going far beyond astrophysics.
Indeed, what is involved is not just the discovery of yet another even
if extremely remarkable, astro- physical object, but a test of the
correctness of our understanding of the properties of space and time in
extremely strong gravitational fields. Theoretical research into the
properties of black holes, and into the possible corol- laries of the
hypothesis that they exist, has been carried out with special vigor
since the beginning of the 1970's. In addition to those specific
features of black holes that are important for the interpretation of
their possible astrophysical manifestations, the theory has revealed a
number of unexpected characteristics of physical interactions involving
black holes. By the middle of the 1980's a fairly detailed understanding
had been achieved of the properties of the black holes, their possible
astrophysical manifestations, and the specifics of the various physical
processes involved. Even though a completely reliable detection of a
black hole had not yet been made at that time, several objects among
those scrutinized by astrophysicists were considered as strong
candidates to be confirmed as being black holes.
