Brian Aldiss, in the foreword to the 1976 anthology Galactic Empires, remarked, “Galactic empires are the ultimate absurdity in science fiction.” That is debatable, but certainly every interstellar state in SF, whether Empire, Republic or Federation, seems to be predicated on a technology that is absurd based on what we currently know of physics: Faster-than-light travel. But I do recall that in Heinlein’s Have Space Suit, Will Travel, Kip Russell works it out that because the Wormfaces/Only People have a sublight constant-boost drive, they have a “star drive”; would that be enough to facilitate interstellar government of any kind?
I’d say it’s possible without FTL travel but you’d probably need FTL (preferably instantaneous) communication. That’s pretty much the basis behind the Starways Congress in the later Ender (Speaker for the Dead et al) books by Card. I think you’d want to be able to move at a substantial fraction of c in any case.
Even without FTL, there’s no physical law that says we can’t travel at 0.99C. That means that ships can travel between stars at a speed of roughly 1 light year per year (with the added bonus of much less time passing subjectively for the crew).
Now, that still means travel time of 1-2 years between systems, at the least. But remember that Britain managed to rule India before steam and the Suez Canal, when it took, what, 6 months of travel to reach? If people live long enough, and a society develops that operates slowly enough, there’s no reason to assume that a relatively decentralized empire can’t operate with a 1-2-year communications gap between satrapies.
Yeah, you’d have to have local governors with a good deal of autonomy, but a small interstellar empire is still plausible with merely light-speed communications.
In theory maybe. But the British Empire was really stretching at the limits of what could be controlled across time and distance. And they were operating at a maximum lag of six months (one way) to a year (round trip). An sub-light speed interstellar empire based on Earth would be looking a minimum lags of over four years (one way) and eight years (round trip). And that’s just to Alpha Centauri. A small interstellar empire of the ten closest stars to our system would include Lacaille 9352 and Ross 128, both of which are over ten light years away. You’re to the point now where you’ve got a twenty year lag time. I can’t see how you can exercise any meaningful government over that kind of distance. Any colonies you had would become essentially self-governing.
Yep, anykind of intersteller empire/commonwealth would consist of the home system and a bunch of totally self-governing colonies that maintain a “symbolic allegiance” to authorities back home.
And all of this assumes an essentially magical drive that works at .99C. If you can’t make galactic interaction realistic using magic, then there’s no hope.
TO: Imperial Headquarters, Earth
FROM: Planetary Capital, Epsilon Eridani
RE: Have arrived
DATE: 30 March 2510
Just a quick note to inform His Majesty that I have arrived and would like to thank you once again for appointing me Imperial Viceroy of Epsilon Eridani.
TO: Planetary Capital, Epsilon Eridani
FROM: Imperial Headquarters, Earth
RE: Your message of 30 March 2510
DATE: 10 June 2533
You’re most welcome. And congratulations on your retirement. Please forward report of how it went.
Maybe more than symbolic. One possibility that’s been used in science fiction is that you have two groups of “slow” people: people that spend most of their time in high-tau starflight, and people who spend most of their time in suspended animation. So for example a system could be ruled by a- dare I say it?- royal family that spends a few months out of hibernation every ten years leaving the day to day running of things to a class of trusted stewards. To enforce loyalty to the central Imperium, an imperial battlefleet with enough firepower and resources to conquer an entire rebellious system if necessary spends centuries touring between the systems and reenforcing imperial control.
Geez, this sounds like Star Wars without the hyperdrive, doesn’t it?
It’s remotely plausible that a larger ‘empire’ might be possible in the presence of some over-riding factor. External threat or widespread fanaticism seem possible candidates. Nothing like a good old-fashioned Burn-The-Heritics religion, for instance, to ensure unity and comonality of purpose - Especially if only one class controls interstallar transport, and that class happens to be the priesthood… Certainly, there’d be heresies and scisms, but he who holds the orbitals holds the planet - A religion fanatic enough to destroy entire planetary populations to enforce confomity could keep things limping along. For a while.
Seems to me you’re thinking of a fake fantasy civilization during the Middle Ages, not an advanced interstellar community where the culture changes ten times faster than ours.
Actually, no - My thoughts are prompted by fairly recent books by Walter Jon Williams; the “Dread Empire’s Fall” series.
If you have a sufficiently ruthless and fanatical select / elect group, willing to stop at nothing, and whom are motivated by an over-riding goal, you should be able to literally lock down a large number of systems. Think of it as empire by terror - No one dare step out of line, for if they do, the fanatics won’t just stop with killing you, or your family - they’ll kill everyone you know, and maybe even your entire planet. The Inquisition were a bunch of innocent flowerchildren by comparison. Lower-rank fanatics are kept in line by the same methods. Overwhelming firepower (C-fractional missiles charged with antimatter) keeps the line taut.
Of course, even that system will fail. Everything fails, eventually. But for a period of time, it can be made to work. More-or-less.
In fact I know Walter Jon Williams and he is a very good writer, though I haven’t read that particular series.
But you can’t seriously use a fictional example as proof of a real civilization. A writer can make anything possible in fiction, including FTL. In reality the odds that a civilization like that would occur or endure are exactly as the same as FTL. Zero.
Database error shut me out from completing my post, but now I’ll add:
Add in biological immortality, where people live for centuries or millennia realtime, on top of any extension caused by relativistic travel or hibernation. Then waiting a century for word from the taskforce you dispatched to inspect the loyalty of a distant system is more plausible. For that matter, if the Imperium and the ruling families keep the means of life extension as a privileged secret, then controlling the supply offered to a privileged class of stewards and viceroys keeps them loyal as well as not having to replace them in the relative blink of an eye.
So you have an imperial family ruling the whole shebang, noble houses running individual systems, a class of vassals given extended lifespans in return for their loyal service, and a peasantry of ordinary people who make due with their threescore and ten.
Geez, this sounds like Dune without the Navigator’s Guild, doesn’t it?
The time scale of the creatures and their society would be relevant.
To Galactic Headquarters:
It’s been almost 7000 years since my father died. Boy, I still miss him.
His experiments with the humanoids we called Neanderthals haven’t gone as well as he hoped, but I’ve been doing some more breeding experiments and expect fine things. They’ve had advanced speech for several thousand years already.
Won’t really know for sure till we see how they handle the interglacial which isn’t scheduled for another 22,000 years. Unless you’ve changed the mandatory retirement age, I’ll just miss it. :mad:
Not to worry about my replacement though. My son is in fine shape, and I have two wonderful grandkids! The 1800-year old can already beat me at superchess, and the 600-year old just said her first word!
Even having a mythical spaceship capable of .99C, you still have to accelerate to that speed. Acceleration much beyond 1G is not going to be healthy over any period of time.
Found here
Round-trip times assuming an acceleration of 1g
time as measured on time on Earth maximum range attainable target
board ship(years) (years) (light-years)
1 1 0.059 Oort Cloud
10 24 9.8 Sirius
20 270 137 Hyades
30 3,100 1,565 Orion Nebula
40 36,000 17,600 Globular cluster
50 420,000 209,000 Magellanic Clouds
60 5,000,000 2,480,000 Andromeda Galaxy
So while a system may be two or three light years distant, unless your mythical ship can go from 0 to .99C in 60 seconds, you’re still dealing with decades long transit times.
You don’t need FTL engines if you have wormholes.
You just need to send someone ahead at sublight speed to make the wormhole first.
Read it; It’s some of his best. And very cool that you know him! 
Since we’re dealing with fictional physics, I don’t see why not - and already fictional society types ahve been proposed. Anyway, seeing as we do have past and present historical examples of fanatical, oppressive, regressive or aggressively static civilizations here on earth (See Pol Pot, Kim Jong Il, et al), I have no problem swallowing WJW’s ideas as a potential template, albeit he takes his template far past the point where I think it’d fail.
In practice, the way I’d work it is to put surveliance satelites around each planet, and require taxes / tribute (building materials, hydrocarbons, fuel) to be launched into mid-planetary orbit. I’d have my fleets constantly in transit from system to system. In each system, they’d send a collier in to take a read off the satelites and collect the goods from orbit whilst the remainder of the fleet takes a more leisurely, higher-energy loop about the system. The collier can replace satelites / nuke the plannet / etc. as needed, and rejoin the fleet as needed, and then the fleet leaves the system for the next system. If the collier is destroyed, the remote fleet drops a lot of rocks into the gravity well, then re-colonizes the planet. In ten or fifteen years, the next fleet comes visiting… Send the fleets from central HQ, where orthidoxy is maintained, and require a tour of the empire as a prereq for advancement to high rank, and you’ve got a semi-stable, if incredibly repressive regieme… The tour of the empire need not even be a hardship, as much of it will be taken at substantial fractions of C, and time dialation means you’re only moderately aged when you get back to HQ.
No argumant there - The question was, “can you postulate even a temporary interstellar empire absent FTL?” And I can. 
Skip the drive, and go for transmission. transmit the information that a mind consists of to a receiver in another system, download it into a system that can run a mind and you have interstellar travel at lightspeed. You’d only need to send ahead a robot for the first trip to build a receiver, and habitats if you prefer biological existence.
A few ideas:
AI viceroys, programmed for absolute loyalty. Its primary orders can only be changed by transmission from central command, even if that takes years or decades.
The “Empire of One”, an interstellar government where the Imperial government, and each planetary government is composed of millions of copies of the same person. Since they agree on everything and have the same goals, they act as one. Even with slow communications, they are still all working in the same direction. You’d need some system for ensuring that the copies don’t drift too far apart of course. New ones being created from a master version, and old ones checked against it, for example.
And as said by others , a regime where the unifying factor is ideology, a plan and/or code of behavior. Fast communication is only necessary if you intend the regime to respond to change, not prevent it.
I’ve never understood why people think that there’s an advantage to repairing the problems caused by bad physics with sociology that’s even worse.
I’m an actual social scientist by academic training. These “societies” read to me the way Timecube must read to an actual physicist. They’re painful when they’re not hilarious.
If the only way you can salvage galactic empires is with a different science that’s ridiculous then you’re proving Aldiss right in saying that they’re “the ultimate absurdity.” It doesn’t matter which science you make absurd.