One of the most exciting predictions of Einstein's theory of
gravitationisthat there may exist 'black holes': putative objects whose
gravitational fields are so strong that no physical bodies and signals
can break free of their pull and escape. Even though a completely
reliable discovery of a black hole has not yet been made, several
objects among those scrutinized by astrophysicists will very likely be
conformed as black holes. The proof that they 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, astrophysical object, but a test
of the correctness of our understanding the properties of space and time
in extremely strong gravitational fields. Theoretical research into the
properties of black holes and into the possible corollaries of the
hypothesis that they exist, has been carried out with special vigor
since the beginning of the 1970s. 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 nurober
of unexpected characteristics of physical interactions involving black
holes. By now, a fairly detailed understanding has been achieved of the
properties of the black holes, their possible astrophysical
manifestations, and the specifics of the various physical processes
involved. Furthermore, profound links were found between black-hole
theory and such seemingly very distant fields as thermodynamics,
information theory, and quantum theory.
