Preface

In 2015, we celebrated the 100th anniversary of the development of general relativity theory (GRT). Einstein presented his theory at the Prussian Academy of Science in Berlin on November 25th, 1915. In GRT, he replaced the absolute space and time of Newton in favor of a changing arena called “spacetime,” in which gravity appeared as curvature. The equivalence principle linked every acceleration locally with gravitation. In principle, GRT poses the possibility of understanding all forces in the world using geometry. Galileo Galilei expressed this thought nearly 400 years ago when he pronounced: He who understands geometry, understands anything in the world. Therefore it was logical that Einstein continued this program even after completing his GRT, with the development of proposals for a unified field theory. Carl H. Brans chose to investigate such theories for his undergraduate thesis at Loyola University in New Orleans. It was the beginning of a lifelong engagement with GRT. Even the mathematical beauty of GRT and the unified field theory attracted him. As a 10-year-old boy, he taught himself differential and integral calculus, and difficult books on mathematics and mathematical physics held a great appeal for him. His preference for GRT was a little bit unusual at the time. Since the 1920s, the GRT had lost its prominent role in theoretical physics. The advent of quantum mechanics and elementary particle physics, together with new work on quantum electrodynamics, inspired more interest among physicists in those days. In the 1950s, the situation began to change. John A. Wheeler, at Princeton University, started to develop geometrodynamics as a new representation of GRT. At the same time, Wheeler established his by-now famous working group, which focused on problems in GRT and on the foundations of quantum mechanics. In parallel, Robert Dicke, Wheeler’s colleague at Princeton, began to work on the experimental problems in GRT during his sabbatical year of 1954. He also became interested in Mach’s principle, which Einstein had used as a guide during the development of GRT. Dicke considered Mach’s principle to imply: The gravitational constant, κ, should be a function of the mass distribution in the universe. Paul A.M. Dirac had earlier conjectured that there is a relation between the coupling