Mach's Principle and a Variable Speed of Light
  Ernst Mach (1838-1916) suggested that the origin of gravitational interaction could depend on the presence of all masses in the universe. A corresponding hypothesis of Sciama (1953) on the gravitational constant, c^2/G = \sum m_i/r_i, is linked to Dicke's (1957) proposal of an electromagnetic origin of gravitation, a precursor of scalar-tensor-theories. In this an equivalent description in terms of a variable speed of light (VSL) is given, and the agreement with the four classical tests of general relativity is shown. Moreover, VSL opens the possibility to write the total energy of a particle as E=mc^2; this necessarily leads to the proportionality of inertial and gravitating mass, the equivalence principle. Furthermore, a formula for c depending on the mass distribution is given that reproduces Newton's law of gravitation. This mass distribution allows to calculate a slightly variable term that corresponds to the `constant' G. The present proposal may also supply an alternative explanation to the flatness problem and the horizon problem in cosmology. 
Alexander Unzicker