It's the stress energy tensor all over again

And that one is one of the weirdest most difficult to understand properties of SpaceTime I know. And as you write it seems to have a lot to do with pressure.

Now this is just me wondering, and not directed to anyone in particular Pete

Then we have the proper invariant mass, defined as invariant through the mechanism of assuming something to be at rest relative something else. In a SpaceTime defined by no preferred frames of reference we still use one, although as a 'relation' between two frames. Where one frame can't be preferred we still find that using two frames of reference, at rest relative each other to work. To assume that we need a clear definition of what motion should be seen as. so do we have that? I'm not sure what motion is myself?

Take a uniform motion, define a 'speed' to it, now take another uniform motion and do the same. Are they the same? If not, are they still equivalent in all other forms of measurement, when measuring being at rest relative them?

If they are, what has speeds, as 'motion', to do with it? And with the stress energy tensor? Do a higher uniform motion, as defined by an arbitrarily made frame of reference, present a higher stress? on what, and where?

I keep getting back to this as it has to do with the question if SpaceTime can be seen as a 'whole indivisible continuum' where all invariant proper mass are like fishes in the ocean, and the stress energy tensor becomes a measure of tension in that ocean, well as I think of it