From the reviews: "This book differs from the many other books on the same topics in three essential points: 1st, it concentrates on the historical development of the question of the unification of quantum theory and gravity, 2nd, it uses the quaternion approach to describe the metric and other geometric objects, and 3rd, it is ... Read morea quite personal book. … a very readable monograph, containing also philosophical issues, considerations about electromagnetism, and unified field theory." (Hans-Jürgen Schmidt, Zentralblatt MATH, Vol. 1052, 2005) "The basic attempt of this book is to initiate a study of a single, coherent theory of matter applicable to all domains, from elementary particle physics to cosmology. … This very interesting and clearly written book is even readable by a final year’s student in physics. The presentation is very well got-up. … I highly recommend this excellent work to every theoretical physicist interested in general relativity and unified field theories." (Fernand Dhoore, Physicalia, Vol. 57 (3), 2005) "Sachs’ book is a fascinating exposition of how an approximation in general relativity brings out quantum mechanical features. Perhaps Einstein would have been greatly interested. … I think it should be read by anyone who is interested in quantum mechanics and gravity." (J. Hartley, Contemporary Physics, Vol. 45 (6), 2004) Show Less