The relativity theories and the speed of light

From a positivist perspective Einstein's two relativity theories (the special, STR, and the general, GTR) have made very accurate predictions of observations. But from a realist point of view they have not provided satisfactory explanations of many phenomena or successfully related the two theories to each other or to quantum mechanics. It is our contention that practical experience with accurate clocks has provided a realistic interpretation and unification of the two relativity theories. In this interpretation the observed predictions of Einstein's two relativity theories lead logically to the conclusion that the primary effect of a gravitational field is a gradient in the speed of light and that all other observed effects of a gradient in the gravitational potential, including the curvature of light, the slowing of clocks, and the force of gravity, result either directly or indirectly from the gradient in the speed of light.