# G From Special to General Theory of Relativity

Starting from the puzzling fact that inertial mass and gravitational mass are not experimentally distinguishable, we encounter Einstein's key insight to the integration of gravity in the relativity theory: the equivalence principle. We then explain for which simple but very important case we will describe the quantitative effects. In the fourth section we study the influence of gravity on ‘clocks and yardsticks’, just as in

**B**, where we studied the influence of velocity on length and time. For relatively weak gravitational fields, we can derive correct formulas. These formulas apply to even strong fields on the outer edge of non-rotating spherical masses, but this we can not prove. Next, we can deduce how velocities are transformed between observers, who are ‘immersed to different depths in a gravitational field’. This leads to the realization that for a distant observer, the speed of light in a vacuum is no longer the same everywhere!