General introduction

It is now over 60 years since Einstein first put forward his General theory of Relativity, providing what was, at that time, a daring new view of the geometry of the world and an extraordinary insight into the nature of the gravitational field - perhaps the single most strikingly original contribution to the scientific thought of recent times. But despite an initial period of active development both in the observational and theoretical sides of the subject, there followed a long period of comparative quiescence during which the subject had seemed to have little contact with the rest of physics and even less with feasible observations or experiments. In recent years, however, this situation has changed dramatically. On the theoretical side, for example, there has been much clarification of the nature of gravitational waves and of the fact that these waves carry positive mass-energy. Much more is known concerning exact solutions of Einstein’s equations. The space-time singularities that had been previously suspected as being features only of special symmetrical solutions are now known to be inevitable for wide classes of space-times. The theory of black holes has arisen and has led to a remarkably complete picture with a detailed and well-understood space-time geometry (the Kerr geometry). The relation to quantum theory, while still enigmatic, has made significant advances (e.g. Hawking radiation). The subject of General-Relativistic astrophysics has been born and has flourished.