I now see how that could work. I imagine creating two particle trajectories so that the distance between the particles is Lorentz invariant, i.e. it looks the s

I now see how that could work. I imagine creating two particle trajectories so that the distance between the particles is Lorentz invariant, i.e. it looks the same for all observers. Once you managed that you can kind of fill space with many copies making sure that the relation holds for all possible pairs. I never thought about that and it I would probably naively have dismissed the idea that you can even have two trajectories that are Lorentz invariant let alone fill space that way. And it is even more unintuitive to me that you end up with something as well behaved as a fluid and not some totally unphysical mess. Now I would really like to see a visualization of two or three such trajectories, assuming I am picturing that at least somewhat right.