First, an aside: the OP's comment "which has never been taken seriously" about this topic is also entirely mistaken. Heck, I've heard that issue come up in seri

First, an aside: the OP's comment "which has never been taken seriously" about this topic is also entirely mistaken. Heck, I've heard that issue come up in serious conversation within the past year. I've got to concur with the other people you've heard from: simultaneity in classical (Galilean) physics is the same for every observer, simultaneity in Special Relativity is different for observers moving with different velocities, and simultaneity in General Relativity often isn't even globally well-defined for one observer at a time. That's not too surprising given other features of GR. You've no doubt heard about "gravitational lensing", where a large mass curves space so that "straight" light rays from the same distant object appear to come from two different directions. Now imagine the same thing happening to a "line of simultaneity", and you'll start to understand the issue. I suspect that one concept that might satisfy a part of what you're looking for as a "generalization of simultaneity" is a Cauchy surface or Cauchy hypersurface: http://en.wikipedia.org/wiki/Cauchy_surface http://en.wikipedia.org/wiki/Cauchy_surface This idea, essentially, is an arbitrary (possibly curved but everywhere spacelike) plane that divides time into "future" and "past", and that can serve as an "initial value surface". Have a look at that article for more information, and see if it's something like what you're looking for. [Also: I don't like the Copenhagen interpretation, either. I've become more of a "many worlds"/Everett person ever since a fantastic and highly mathematical quantum course in grad school, despite the somewhat uncomfortable philosophical issues with it.]