The Equivalence Principle Let’s consider an object on Earth (Fig. 1). The volume of this object causes a curvature of spacetime which exerts a gravitational force on it of g = 9.81 m.s-2 on the surface of Earth.
Let’s now consider the same object accelerated out of any gravitational field. We can represent this object in two different views (fig. 2a and 2b).
In both cases, acceleration a is identical to g, i.e. a = 9.81 m.s-2. Without any reference, a local observer can’t know if the curvature of spacetime is due to a pressure on the object (fig. 2a) or to its acceleration (fig. 2b). In fact, figures 2a and 2b are identical and depend on where the observer stands, as described in Special Relativity. Since,
We deduce that the "gravitational mass effect" (fig. 1) is identical to the "inertial mass effect" (fig. 2a or 2b):
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