[troymc]

Yes, Second Life has one persistent world ("one shard") but it's composed of a bunch of "sims" glued together at sim boundaries. (A "sim" is a 256 m x 256 m x notsure m volume of space.) The states of the things in that sim are the responsibility of one server; if an object crosses a sim boundary, the new sim may be handled by a different server. The sim-crossing hand-off can cause object motions to look non-physical.

As I understand it, Improbable also simulates one big world, but they map the various simulation calculations to servers in a different way. How exactly? That's their secret sauce.

Philip Rosedale's new startup, High Fidelity, also takes a different approach. See https://highfidelity.com/