How the special theory of relativity (see Note 4) led to the general
theory of relativity (which included gravitation) may now be briefly
traced.
When we speak of electrons, or negative particles of electricity,
in motion, we are speaking of energy in motion. Now these electrons
when in motion exhibit properties that are very similar to matter in
motion. Whatever deviations there are are due to the enormous velocity
of these electrons, and this velocity, as has already been pointed
out, is comparable to that of light; whereas before the advent of the
electron, the velocity of no particles comparable to that of light
had ever been measured.
According to present views "all inertia of matter consists only of the
inertia of the latent energy in it; ... everything that we know of the
inertia of energy holds without exception for the inertia of matter."
Now it is on the assumption that inertial mass and gravitational
"pull" are equivalent that the mass of a body is determined by its
weight. What is true of matter should be true of energy.
The special theory of relativity, however, takes into account only
inertia ("inertial mass") but not gravitation (gravitational pull or
weight) of energy. When a body absorbs energy equation 2 (see Note 4)
will record a gain in inertia but not in weight--which is contrary
to one of the fundamental facts in mechanics.
This means that a more general theory of relativity is required to
include gravitational phenomena. Hence Einstein's General Theory of
Relativity. Hence the approach to a new theory of gravitation. Hence
"the setting up of a differential equation which comprises the
motion of a body under the influence of both inertia and gravity,
and which symbolically expresses the relativity of motions.... The
differential law must always preserve the same form, irrespective of
the system of coordinates to which it is referred, so that no system of
coordinates enjoys a preference to any other." (For the general form
of the equation and for an excellent discussion of its significance,
see Freundlich's monograph, pages 27-33.)