We're all already governed by the laws of nature and physics. So in that regard it has been that way for as long as the earth has existed.
As for human government more than likely any world government would be built on the basis of establishing basic human rights and then allowing "states" within that structure to set themselves up as they see fit. I would actually really enjoy seeing what happens when you let people choose between the major political ideologies they wish to live under.
It would be quite revealing to see which weaknesses appear and which philosophy was able to sustain itself the longest. 4 states - whenever a state collapses or loses X% of its population to other states over Z years it would be dissolved and reformed anew (ideally learning from mistakes) under whoever remained. If the state ever drops to less than 5% of world population it is dissolved and its land is split amongst the remaining states proportional to population.
This is of course far to cold to test on real people - but the concept of the experiment is fascinating.








