Quote Originally Posted by from the internet
Einstein used the fact that gravitational and inertial mass were equal to begin his Theory of General Relativity in which he postulated that gravitational mass was the same as inertial mass and that the acceleration of gravity is a result of a 'valley' or slope in the space-time continuum that masses 'fell down' much as pennies spiral around a hole in the common donation toy at your favorite chain store.
This kinda makes some sense. Where spacetime is curved, an object travelling in a straight line will resist such curvature. An object's mass is still its resistance to a change of state of motion, it emerges as it encounters curvature. The more resistance it has, the more it distorts spacetime to maintain its straight path, and the more it influences other bodies. Rather than change course, a body curves spacetime by a sufficient amount to maintain equlibirium between its intertia and motion. This creates an emergent force as other bodies resist a change in motion that the curvature is demanding.

So gravitational mass and inertial mass, it makes sense for them to be syonymous. Gravity is merely curved spacetime, inertia is why spacetime is curved, and mass is either a measure of resistance (intertia) or curvature (gravity), which both amount to the same thing because the curvature emerges as a result of the resistance.

Quote Originally Posted by mojo
...as yet unexplained...
Depends who you're listening to!