Essentially, all metric theories satisfy the equivalence principle, and this paper is not an exception. We will not discuss equivalence principle with its details here, but let us examine its exact physical meaning at the smallest scale.

Our definition of mass (and energy) is a function of the tightness of the confinement volume. This tightness contracts more during free fall in a gravitational field. Extra tightening of the confinement volume has been discussed before; hence, tightness of two confinement volumes of the same kind (e.g. electrons) changes if there is velocity difference between them or if one is under the effect of a gravitational field.

E 7.11

The radius (r) variable (the tightness of the confinement volume) in our definition of mass involves both conditions, and this definition is the basis of the exact and comparable magnitude of mass for all relative observers.

Consequently, this generalized definition of mass indicates that both gravitational and inertial mass are the same in any relativistic case. Variation in the tightness of the confinement volume equivalently defines all relativistic effects like the decrease of clock-ticks for accelerating reference frames or those in a gravitational field.