[tsarchitect]

> how is it that we can have a whole population of systems (eg scientists) eventually coming to a consensus about some phenomenon (eg the value of some physical quantity?) It seems necessary to have a unified global system coordinating the whole thing.

Not necessarily. It's not a "global system coordination" thing, it is coming to a consensus that "we will use this reference frame as our starting point", which might look like a global system coordinating. I guess you can say that 'science' initiallizes a reference frame in which we can all participate, compare answers, reproduce results, etc.