The general theory of relativity starts from a fact of experience which admits the equality of inertial and gravitational mass, or, in other the words, the fact known since the days of Galileo and Newton that all bodies fall with equal acceleration in the earth's gravitational field. (To me this indicated that both are electro-magnetic in nature or else we would have to deal with two different natures of space. Its like saying we need two different sensory units: one for hearing and one in order to keep our balance when in effect the ear system covers both sending messages to the brain.)

The theory uses a special theory as its basis but claims there is no state of motion whatever which is physically privileged - that is, that not only velocity but also acceleration are without absolute significance. It then compels a much more profound modification of the conceptions of space and time than were involved in the special theory. For even if the special theory forced us to fuse space and time together to an invisible four-dimensional continuum, yet the Euclidean character of the continuum remained essentially intact in this theory.

In the general theory of relativity, this hypothesis regarding the Euclidean character of our space-time continuum had to be abandoned and the latter given the structure of a so-called Riemannian space. Before we attempt to understand what these terms mean, let us recall what this theory accomplished.

It furnished an exact field theory of gravitation and brought the latter into a fully determinate relationship to the metrical properties of the continuum. The theory of gravitation, which until then had not advanced beyond Newton, was thus brought within Faraday's conception of the field in a necessary manner; that is, without any essential arbitrariness in the selection of the field laws. At the same time gravitation and inertia were fused into an essential identity. The confirmation which this theory has received in recent years through the measurement of the deflection of light rays in a gravitational field and the spectroscopic examination of binary stars is well known.