This 'elastic surface' thing, is actually what whole lot of physics is about. An 'inelastic' interaction is just one where the transfer of momentum includes some 'erasure' from the local system; the total system--the universe--just sees a change in state somewhere, and carries on. Inelastic interactions are locally observed events. I once read in an article (I saw the author was Ph.D-ed), that once the universe was 'big' enough it started ringing, like a bell, and these ancient 'tones' are still around--still ringing 'elastically'. This suggests that the spacetime is an elastic 'space', a medium. Or the three dimensional space is an elastic surface of the 4d-spacetime. Math, huh?