Mach's Principle and Extended Theories of Gravity

Dubravko Horvat and Zoran Narančić
Faculty of Electrical Engineering and Computing (FER), University of Zagreb

Abstract Mach's principle is regarded as one of the milestones which guided Einstein to his General Theory of Relativity (GTR). For theories of gravity which go beyond GTR and which will be called here extended theories of gravity (ETG) the Mach principle may play the same role. Here an elementary presentation of the Mach principle is given with some of the philosophical issues concerning absolute space, relative motion, inertial forces and structure of spacetime. Extended theories of gravity will be sketched for the case where curvature R is replaced by torsion T and the role of the Mach principle for matter-geometry coupling will be explained.