It seems to me this thread is not about inertia, it is about, "how, or why, can you tell when you are in an accelerating reference frame?" Inertia has to do with how much you accelerate when a force is applied, so it requires not only be able to tell you are accelerating, but also how much you accelerate when a force that has somehow been callibrated is applied. But I agree the more interesting part of this is simply how you can tell you are accelerating in the first place, and that is what the posts have been about. Apparently, you are accelerating relative to the local vacuum, and you can tell because of the appearance of mysterious gravity effects that have no explainable source other than your acceleration. So here's my question for those more knowledgeable in GR: if you enter a rotating reference frame, and observe centrifugal forces that act very similarly to gravity, can those centifugal forces be treated as a gravity-like curvature of the rotating spacetime, or would that not work because they don't behave like real gravity, i.e., they are repulsive and unbounded?