This book is devoted to general relativity, i.e. to the synthesis of special relativity and gravitation. This Relativistic Gravitation, as it is sometimes called, appears to be of uppermost importance in all those astronomical phenomena that involve velocities close to that of light or intense gravitational fields. The study of the latter constitutes a new subject, Relativistic Astrophysics, an expression due to Alfred Schild (1967).

The content of this book is the minimum minimorum needed to approach this relatively recent domain.

It may be interesting at this point to recall how and why this new subject started. Once the classical tests of general relativity were performed (bending of light rays by the Sun (1919), gravitational redshift (in white dwarfs) [W.S. Adams (1925)]; perihelion advance of Mercury), the subject became very formal, as current technology did not provide contact with experiment or astronomical observation. Although much research had great conceptual interest (unified theories of gravity and electromagnetism, for example), general relativity became rather arid [see J. Eisenstaedt (1986)] because of the lack of laboratory experiments or observations of relativistic objects, which were in any case unknown to theory before the 1930s, and even then ignored in the 1940s and 1950s. Thus, cosmology was regarded more as a “free area for thinking about relativity” [J. Eisenstaedt (1989)] than a field for astronomical verifications of general relativity, or even the “Science of the Universe” [E.R. Harrison (1981)].