Inertial Mass = Gravitational Mass. Why?

Question

Okay, so the inertial mass of an object is always equal to the gravitational mass of the object. Conceptually, however, they seem different. Then what makes them identical? Is it because they are rooted in the same, more fundamental quantity? What's the complete explanation of their equivalence? Or is it simply a postulate in general relativity without any further explanation?

Answer 1

They are thought to be equivalent, in a theoretical sense, because (inertial) mass causes curvature in spacetime and it is that curvature that gives rise to gravitational forces. This is the essence of General Relativity, which was developed on the postulate that the two masses are equal.

That they are equal was one of the most important null experiments in the history of Physics, which was to demonstrate that objects fall at the same rate regardless of their mass or composition. That is to say, if the acceleration of an object subjected to a particular force depends on its inertial mass, and the the gravitational force exerted on an object depends on its gravitational mass, these two masses were shown experimentally to be equal to within experimental error.

Experiments were performed by Galileo, Newton, Lorand von Eotvos (1922), Braginsky (1971) and others that demonstrated this result with increasing precision over the years. The "Gravitation" book by Misner, Thorne, and Wheeler has a good explanation of these experiments that you may want to consider.

Your question is a bit analogous to asking, in the context of Special Relativity, "why is it that the maximum speed at which information is transmitted is the speed of light?" There was nothing special about the speed of light when it was used as a thought experiment by Einstein to come up with Special Relativity.

He pondered, if the laws of Physics are the same in every reference frame, then what happens when a moving train, for example, casts a beam of light as it is approaching the station?

This led to the Lorentz transformations (which were independently and previously derived in another context), predictions of time and space dilation that were then confirmed through experiment. Those equations reinforce the notion that a particle is massless in the limit of v = c (limit of velocity approaching speed of light).