Hi, I'm currently writing a paper on Relativity, which mostly uses original papers of Einstein. For this reason, I have little idea what the ultimate fallout of all his upheaval is. I am aware that electromagnetic fields become "shadows" of the complex mathematical entity called the "Electromagnetic tensor," length and time become "shadows" of invariant "spacetime" intervals, gravitational mass becomes inertial mass following warped geodesic/inertia lines (expressed, again, by tensors), and inertial mass becomes another measure/property of energy. Since energy is not invariant, this implies that mass is not invariant, which implies (in my opinion) that "matter" is not invariant. It seems that--while the laws are becoming more and more invariant--the invariant, persisting subject which underlies all of this begins to disappear. Our only measure of matter has become a measure of energy content, and you can therefore not call "inertial mass" the same thing as "stuff," since you can increase the inertial mass of something by putting more energy into it-- without necessarily putting more "stuff" into it, as in the case of a tensed string having more mass than a loose string. Meanwhile, subatomic particles can pop in and out of existence. SO! I'm wondering if, like the invariant space-time interval, there's something analogous in relativistic physics for a persisting "object" that we can say does NOT pop in and out of existence depending on reference frames etc. Obviously, it would have to be at least four-dimensional. Is there anything like this, or do we really lose any applicability we had of our notion of "stuff/substance/substratum" in the metaphysical sense? Thanks!