Black holes and unpredictability

When quantum effects are taken into account it turns out that black holes are not completely black but emit particles and radiation with a thermal spectrum. The radiation from a black hole can be thought of as coming from a singularity and having quantum mechanically tunnelled out of the black hole. Because the radiation comes from the singularity, it has an extra degree of randomness or unpredictability over and above that normally associated with quantum mechanics. This unpredictability implies that the black hole has an intrinsic entropy. The thermodynamic properties of black holes show that in attempting to quantise gravity it is not sufficient just to work with perturbation theory about flat space as is normally done. Instead one has to develop a perturbation theory which allows for metrics with all different possible topological structures. A start has been made in this direction and it is hoped to derive the entropy of a black hole from a quantum gravitational field theory treatment rather than the original semiclassical approximation. It is hoped that some extension of these methods may enable one to deal with the unrenormalisability of quantum gravity. (U.K.)