> In GR, the equivalence of inertial and gravitational masses is assumed No, it's not, it's a necessary part of the way GR models gravity as spacetime curvatur

> In GR, the equivalence of inertial and gravitational masses is assumed No, it's not, it's a necessary part of the way GR models gravity as spacetime curvature with the metric being the only "field" associated with gravity. You can't do that and have inertial and gravitational masses being different. I guess you could call modeling gravity as spacetime curvature with the metric being the only "field" associated with gravity an assumption, but it's an assumption that has a lot more consequences than just the equivalence of inertial and gravitational mass. So the latter is not usefully viewed as an "assumption" in GR by itself. It's one of many consequences of a much broader assumption that is the entire basis of the theory. It is true that there are other theories of gravity that use spacetime curvature, but they all have additional fields besides the spacetime metric that are associated with gravity. And all of them known so far are ruled out by experiment, or at least the only region of their parameter spaces that are compatible with experiment make the extra fields observationally meaningless and the theories become equivalent to GR in all their predictions.