If we discover a new particle that mediates inertial mass, let’s call it the Sheld-on.
Doesn’t Chronos check in about now and grumble about the whole rest-or-inertial mass terminology and complications in subsequent analysis?
No, I don’t grumble about that, because so far as anyone can tell, rest mass (the portion of a system’s energy which cannot be transformed away in any reference frame) is identically the same as inertial mass (the ratio of the net force on an object to its proper acceleration). In fact, they appear to be so identically equal, that it’s difficult to even see where to draw the lines between them: One could equally say, for instance, that Newton’s Second Law is the definition of force, not the definition of mass.
That was the grumble I was talking about.
I didn’t mean it to be a grumble. To be (hopefully) more clear, there are a number of ways one can define “mass”. If one wants to ask “are the different definitions of mass equivalent”, then one must specify exactly what definitions one means, which is hard work. But it’s pretty much irrelevant anyway, because for any reasonable set of definitions, the answer to that question is “yes, they are equivalent, so far as we know”.
In other words, while it’s true that precise definitions are very difficult, there’s no point being pedantic about it, because it doesn’t matter to the answer.
EDIT: And I wouldn’t even have said that much, except that you brought it up.
We have not really ever been able to observe a thing which is literally “at rest”. Everything we can detect is in some way in motion, rest is wholly relative to the frame of reference. Even spacetime itself seems to be in motion (expanding space).
And perhaps the inertness of gravitational flexing could be more easily understood in terms of speed = distance / time: it is spacetime that is flexing, and everything already seems to be stuck moving through at least one of those dimensions at a relatively constant rate, so why should things not be naturally inclined to move through the other three at a constant rate?
There’s no cause, it’s just always been that way.