Given the incredible time lags between inhabited planets, I don't know if a real interstellar government is possible. This is assuming, of course, that the speed of light is the fastest you can go.

Within the solar system, though, a United Planets could work. The longest communication lag would be about twelve hours, and the longest spaceflight could be only a few weeks (Turnaround trajectory).

I have a feeling that, if we do start sending people to other stars, they will be essentially on their own. Free to start up their own (hopefully non-oppressive) government. It will be like the 17th century all over again. This time, though, literally a New World.

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"Critical thinking and skepticism form the cornerstone of intelligence."
-me