It is often said that modern theoretical physics began with Newton's law of gravitation. There is a good measure of truth in this remark, especially when we take into account the aims and methods of modern physics – to describe and explain the diverse and complex phenomena of nature in terms of a few basic laws.

Gravity is a basic force of the Universe. From the motions of ocean tides to the expansion of the Universe, a wide range of astronomical phenomena are controlled by gravity. Three centuries ago Newton summed up gravity in his simple inverse-square law. Yet, when asked to say why gravity follows such a law, he declined to hazard an opinion, saying ‘Non fingo hypotheses’ (I do not feign hypotheses). A radically new attempt to understand gravity was made in the early part of this century by Einstein, who saw in it something of deeper significance that linked it to space and time. The modern theoretical physicist is trying to accommodate it within a unified theory of all basic forces. Yet, gravity remains an enigma today.

In this book I have attempted to describe the diversity, pervasiveness, and importance of this enigmatic force. It is fitting that I have focused on astronomical phenomena, because astronomy is the subject that first provided and continues to provide a testing ground for the study of gravity. These phenomena include the motions of planets, comets, and satellites; the structure and evolution of stars; tidal effects on the Earth and in binary star systems; gigantic lenses in spaced highly dense objects, such as neutron stars, black holes, and white holes; and the origin and evolution of the Universe itself.