N ow that this book is done I can look back, and identify two purposes which
led me to begin writing, and which have guided the work to completion.
One good practical purpose was to bring together and assess the wealth of data
provided over the last decade by new techniques in experimental physics and in
optical, radio, radar, X-ray, and infrared astronomy. Of course, new data will
keep coming in even as the book is being printed, and I cannot hope that this work
will remain up to date forever. I do hope, however, that by giving a comprehensive
picture of the experimental tests of general relativity and observational cosmology,
I will help to prepare the reader (and myself) to understand the nm\' data as they
emerge. I have also tried to look a little way into the future, and to discuss what
may be the next generation of experiments, especially those based on artificial
satellites of the earth and sun.
There was another, more personal reason for my writing this book. In learning
general relativity, and then in teaching it to classes at Berkeley and M.LT., I
became dissatisfied with what seemed to be the usual approach to the subject.
I found that in most textbooks geometric ideas were given a starring role, so that a
student who asked why the gravitational field is represented by a metric tensor, or
why freely falling particles move on geodesics, or why the field equations are
generally covariant would come away with an impression that this had something
to do with the fact that space-time is a Riemannian manifold.