It's unclear that continent-spanning governments can remain unified in the long run. There are sociological factors at work that make distance problematic.

Related Question. How could a galactic empire work

"stay unified"?

Depending on your definition of 'distant', it would take anything from generations to millennia to get a response to a query or situation from a central government. This is clearly intolerable.

You need to first achieve an easier step - an united goverment here right? Even that is not possible. Government is a dynamic result of war and fighting for limited resource, not a result of a wish.

Do you have some purpose in mind? Is a planet-spanning government desirable, much less an interstellar one? What are you trying to achieve in your world? And what counts as a "unified government"? Something like the UN? NATO? What kind of autonomy do you expect of the elements, what kind of centralization? There's probably little point in relying on violence to govern people if those responsible for an uprising are long dead by the time your punitive expedition arrives (especially if you only "hear" about it when a shipment fails to materialize).

It's not even clear if humans would stay united as a species.

Lock two humans in a room for a brief period and you'll end up with a minimum of two factions. :-)

I'd suggest changing "realistic examples in sci-fi" to "plausible examples in scf-fi" as the one thing we can't say is how realistic this is could be - STL to other solar systems is probably impractical to the point of impossibility and even with the solar system the challenges are daunting to reach colonization even of near (orbital) space to Earth.

The entire premise of the show 'the expanse' is that we couldn't even stay united while colonizing the inner solar system, which is probably pretty accurate.

Actually it's not even guaranteed that it would be easy with FTL.. I suggest reading "The Forever War", great book and has mindblowing considerations about it..

One interesting solution to this was suggested in "Lockstep" by Karl Schroeder, described here, where there's a society-wide agreement that everyone spends 359 months in suspended animation for every 1 month awake, all on the same schedule. So even if you can only do 50% light speed where time dilation is small, journeys between stars out to a radius of 720 light years seem to take under a year, not just for travelers but also those staying put in any given star system, making trading and such easier.

You'd have to find a quantum wormhole system which enables instant two atoms to be twinned across the unvierse. It can perhaps be based on subatomic particles like muons and tau and that stuff.

A good realistic sci-fi example of this is Greg Egan's "Amalgam" civilization, which appears in his novel Incandescence, and his short stories: Riding the Crocodile, Glory, and Hot Rock.

@Hypnosifl Overall you would need to have really similar mindsets to start out, and such a system as this seems ripe for non-like-minded individuals to try to out-game each other. Imagine the advantage over your rivals if you spent two months awake at a time!

There are over 150 stars within 20 light years of earth, of which 17 and counting may have inhabitable planets. How big of an 'empire' do you want? You do not specify the distance of the farthest system, therefore answers are just speculative. However, even an 'empire' within our own solar system, among planes, is speculative. Unless they are all 'Corporate Colonies' i.e. mega-corporation 'mining towns'.

stay unified : stagnate cultural evolution. That's usually not the intention of a funder of colonization; it's assumed. But it's a good reason why people become colonists, and another reason why not to assume anything. They don't, which is often precisely why they left.

Well, you could just say that they all follow the same laws, say a constitution, and that they're all the same civilization.

If you manage to find a solution, go back to 1776 and sell it to George III; he'll pay handsomely!

Possible duplicate of "how could a large empire like Rome stay united when they don't have faster-than-horse travel?

Well, put it like this: Staying united is not your biggest problem -- reaching the distant stars is! Compared to that, everything else is a piece of cake.

@Harper-ReinstateMonica I'd love to see that thread if it exists, but the obvious answer is they did have faster-than-horse travel and ships on the Mediterranean were much more important for keeping the empire together than couriers. Even as late as the founding of the US, it was much cheaper to transport goods across the Atlantic than 100 miles inland.