Public interest in Albert Einstein's relativity theory had become so great in 1929 that, when he presented to the Prussian Academy his comprehensive theory fusing electromagnetism and gravitation in a single law, the New York Times urged him to prepare an explanation of his new work in terms as simple as the subject would allow. Einstein's article entitled, Field Theories, Old and New, appeared in the New York Times on February 3, 1929. In the article, Einstein distinguished three stages of the relativity theory in the following words:
The first, special theory of relativity, owes its origin principally to Maxwell's theory of the electromagnetic field. From this, combined with the empirical fact that there does not exist any physically distinguishable state of motion which may be called 'absolute rest' arose a new theory of space and time. It is well known that this theory discarded the absolute character of the conception of simultaneity of two spatially separated events ...
On the other hand, the services rendered by the special theory of relativity to its parent, Maxwell's theory of the electromagnetic field, are less adequately recognized. Up to that time, the electric field and the magnetic field were regarded as existing separately even if a close causal correlation between the two types of field was provided by Maxwell's field equations. But the special theory of relativity showed that this causal correlation corresponds to an essential identity of the two types of field. In fact, the same condition of space, which in one coordinate system appears as a pure magnetic field, appears simultaneously in another coordinate system in relative motion as an electric field and vice versa. Relationships of this kind displaying an identity between different conceptions, which therefore reduce the number of independent hypotheses and concepts of field theory and heighten its logical self-containedness are a characteristic feature of the theory of relativity. For instance, the special theory also indicated the essential identity of the conceptions, inertia mass and energy. This is all generally known and is only mentioned here in order to emphasize the unitary tendency which dominates the whole development of the theory.
The second stage in the development of the theory of relativity is the so-called general theory of relativity. This theory also starts from a fact of experience which till then had received no satisfactory interpretation: the equality of inertial and gravitational mass, or in other words, the fact known since the days of Galileo and Newton that all bodies fall with equal acceleration in the earth's gravitational field. The theory uses the special relativity theory as its basis and at the same time modifies it: the recognition that there is no state of motion whatever which is physically privileged - i.e., that not only velocity but also acceleration are without absolute significance - forms the starting point of the theory. It then compels a much more profound modification of the conception 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 the 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 and inertia were fused into an essential entity. The confirmation which this theory has received in recent years through the measurement of the deflection of light rays in a gravitational field and spectroscopic examination of binary stars is well known.
The characteristics which essentially distinguished the general theory of relativity and even more the new third stage of the theory, the unitary field theory, from other physical theories are the degree of formal speculation, the slender empirical basis, the boldness in the theoretical construction and, finally, the fundamental reliance on the uniformity of the secrets of natural law and their accessibility to the speculative intellect. It is this feature which appears as a weakness to physicists who incline toward realism or positivism, but is especially attractive, nay, fascinating, to the speculative mathematical mind. Meyers in his brilliant studies on the theory of knowledge justly draws a comparison of the intellectual attitude of the relativity theoretician with that of Descartes, or even Hegel, without thereby implying the censure which a physicist would read into this.