If we didn't have the capability to use faster than light travel then how could we stay unified and are any realistic examples in science fiction?
If you have only lightspeed communications and STL transport, any organization larger than a single star system (including close binaries etc.) will be more a matter of cooperation than of actual government.
This is far, far worse than what the British Empire dealt with in the 18th and 19th centuries; instead of months for both travel and communication, you'll be seeing years, decades, or centuries even for a message from the outlying colony to reach the "capital world" -- and thousands to tens of thousands of years to send any kind of military force.
In general, this will mean any system will be on its own, left with advice that might be thought of as the English language library in Kolkata or Sidney during colonial times -- new books might well arrive continuously, beamed from the homeworld, but they'll be the ones that were requested anywhere from fifty to a thousand years ago (and that's still a tiny bubble in a big galaxy).
It's likely the "language of science" would remain reasonably constant, because of the library effect, but nothing else about the culture or government could reasonably maintain a connection with a land further separated in time than Columbus is from Donald Trump.
Medical Science Makes People Nearly Immortal
Let's say you are the governor of a planet 50 light years from Earth, there is no reason to care about the repercussions of ignoring an order from the home world since you will be dead before they can do anything about it. But... if medical science extends the life expectancy into the thousands of years, and planetary governors are lifetime appointees, then ignoring an order from 50 years ago that could result in major consequences for you 100 years from now suddenly seems like a very bad idea.
Why do planetary governors have to be lifetime appointees?
Historically, lifetime leaders and fixed-term leaders treat their rulership very differently. Politicians who are constantly vying for re-election have to think in the short term because being unpopular for a few years will end their career, but people who rule for life are more likely to be able to think in long term goals. Kings, Pharaohs, and Emperors would plan out building projects that could take decades to complete whereas in the modern world, any plan that takes more than 5 years typically falls apart as the opposing party works to undermine it.
Another more relatable example may be the supreme court justices in the United States. Justices tend to treat their jobs very differently than Senators do because they are life-term leaders. Where as Senators often write short-sighted bills because they are trying to stay in office, justices tend to show a lot more discretion for the long term effects of their decisions because their entire foreseeable future involves dealing with the aftermath of their choices as leaders.
A consequence 100 years from now may not seem like a big deal to us, but if you turn the math around and say that there is a consequence that you will have to live with for thousands of years... well that becomes a VERY big threat that modern man can not quite yet relate to, but a person who's already been a alive a few centuries will perfectly understand.
If we didn't have the capability to use faster than light travel then how could we stay unified
IMHO, based on the colonization episodes along the history, we can't stay under a "united government". As soon as a colony becomes self-sufficient, it declares independence or at least demands self-determination. And, nota bene, this assumes a FTL-communication.
And, at STL flight speeds, a colony needs to become self-sufficient pretty soon.
The best an Earth-based government can do is to admit upfront it will happen this way and carve in the "Charter of human colonization" stone that any colony has self-determination rights the moment their ship leaves the Solar System.
If neither comms nor travel can happen at FTL, the colony is on its own in any practical sense.
are any realistic examples in science fiction?
The first to pop into my mind:
- "Speaker for the dead", "Xenocide" and "Children of the Mind" by Orson Scott Card
- the Hainish Cycle by Ursula K Le Guin
On independence vs self-determination
I realized that I totally misused the "self-determination" term in respect with its meaning under international law.
What I intended for the meaning od self-determination was something along the line of "Yeah, OK, in name, you are the source of law in regards with human rights. But it is up to us to organize ourselves how we see fit and, apart for the respect of human rights, we have no obligation to you (even if it's likely we will assist you if you ask)".
Give the job of governance to Artificial Intelligence's. Or perhaps the AIs just decided to take over.
Given that the AIs have an overarching purpose, the AIs might have the structure that actually allows for a multi-stellar or even galactic empire. Note the the purpose need not make sense to the humans. The AIs could have a goal that requires continuous expansion thus spreading to galactic scale. Likewise AIs could also require a static society so that human civilization retains coherency -- if nothing else to prevent disruption that affects their true goal.
By controlling essentially all aspects of human life, this would be stable even on a very large scale.
For example, considering the of meeting another galactic AI similar to Fred Saberhagen's Beserkers). Why do the Berserkers expand and destroy life it is assumed to be the result of a programming error or replication error in their programming. Nonetheless, the Berserkers grow in size and complexity and retain their characteristic even though they adapt to changing circumstances.
Our AIs could still have STL or light-speed communication networks for the purpose of being able to summon the full strength of the galactic empire if needed to defend against the Berserkers. Sure response would be slow, but it is assumed that the attacker would also have similar communications limits since the same physics must be followed. The AIs could even have planned ahead or criteria for abandoning stellar systems consistently even in such harsh situations.
Hopefully, our AIs decide to keep the humans as pets or they feel responsible for taking care of them, or something else that only the AIs understand but that allows us to continue existence in a desirable state.
Following a billion, or a trillion or more lines of code can cover a lot of planned societal choices, and it would be able to be followed consistently.
Stable does not mean stagnant. A multi-stellar society controlled by AIs would not necessarily be stagnant in order to be stable. There are shared network update algorithms that allow distributed software updates to occur, even dealing with lossy and slow communications. Sure, they have never been tested, developed or even planned for such large scale, but the AIs will develop such if it serves their purposes.
The AI logic may be completely inscrutable to humans, but it could allow parallel human development, perhaps even demanding trans-human development - Why? No mortal knows.
True AI could well be as far beyond our ability to understand as a cockroach trying to understand string theory. We already do not really understand how some of the limited AI that we use works. As a race, we may not even perceive that we are being controlled by an AI, life could be pleasant, fulfilling, etc.
BTW, I am well aware of software development process (see my profile). Yes, true AI may not happen soon, or ever. But at least theoretically, it certainly seems there is no inherent reason it can't exist. Certainly the wetware we use seems to be a very strong indication that it should be possible. I am not trying to predict the future of AI, I am trying to posit a realistic affirmative answer to the question of how civilization could be unified even though multi-stellar without FTL.
One additional embellishment. If you are familiar with Asimov's Psychohistory in the Foundation series, I would conjecture that very intelligent AI would effectively incorporate that ability. Given the ability to accurately predict the future (in the mass, not the individual level), they could also accurately predict the effects of their planned changes. This also argues that humans could well be allowed / encouraged /coerced to develop under the AI's rule. In fact, AI Psychohistory would improve their ability to govern at the multi-stellar level.
I add this not merely to buttress my claim that this is a realistic solution. But, also remind that something fundamentally unpredictable can break the stability and unity of any society, in Foundation, The Mule story is that discontinuity. The Mule was a mutant with the ability to alter the minds of other people, resulting in a large variation from Psychohistory.
Catastrophe caused by a rogue black-hole that triggers a supernova killing or harming systems for hundreds of light years could easily break or otherwise alter a smaller multi-stellar civilization.
Karl Schroeder's excellent novel Lockstep has a solution for this, albeit on a far smaller scale (a few solar systems close to Earth and the many planetoids in between).
In Lockstep, colonies hibernate in lockstep, going under for e.g. 11 months of every year (there are several different cycles). This allows a person to travel 11 months while hibernating, spending 1 month at his goal, then travel 11 months back, and arriving 1 month of experienced time after he left (there has only been one awake period while he was away. During downtime months, robots and other machines collect resources that allow the colonies to be fully active for a month, even in marginally habitable areas.
On a truly interstellar scale, downtime would have to be far longer, e.g. being awake a few years in every thousand - and every colony would have to agree to this cycle.
Say that at the end of every four-year awake cycle, every solar system elects a representative for the central government. Representatives travel nearly a thousand years in hibernation and arrive at the Capital at the beginning of the next awake cycle. Then they spend four years deciding on every issue that needs to be dealt with, after which they are sent back along with a package of edicts.
For colonies on Earthlike planets, it might be difficult to persuade the population of this - even with the promise of immense wealth during the awake period. A handful of rebels that decide to stay awake could become a large population in a thousand years and threaten the local order of things, unless robots hunt them down and either kill them or forcefully hibernate them (story idea!).
There is also the danger that some aliens might appear during a downtime and settle a lot of the 'empty' colonies. Of course, there might be emergency systems in place that awaken people out of time if something unexpected happens.
This might not be the most practical way to have an interstellar government, but it is a possible one.
Interstellar relations based on logic of the Prisoner's Dilemma
This idea is explored in John C. Wright's stories beginning with Count To A Trillion. The Prisoner's Dilemma is a thought experiment often discussed in an introduction to game theory. It deals with a situation where two prisoners have a lot to gain by cooperating, but at any time either one can benefit from betraying the other's trust. Why don't they betray each other? Well, if the "game" is played only once, it makes sense to cheat, but if the game is played over and over again, it makes more sense to cooperate, because if you cheat the other player will retaliate and you'll both lose.
In an interstellar agreement, laws must be obeyed, treaties must be observed, contracts must be honored, over very long time spans, perhaps more than a lifetime. Imagine an astronaut returns from a distant star, bearing a treaty that George Washington sent his ancestor to negotiate. Would we honor the treaty? What if it was highly offensive to our present-day sensibilities? What if it included, for example, the right to import slaves to our markets?
If we broke the treaty, the other star wouldn't find out until long after our deaths. So there's a huge temptation to break or ignore the treaty. In a one-off case, no problem. "Cheating" is to our advantage.
In the long run, though, if we want to have a stable interstellar civilization, we would need to learn to honor those agreements, at least to tolerate them for the duration of a new round-trip for a new ambassador to renegotiate them. Because we would need to establish a reputation for honoring such agreements, or no other worlds would make any deals with us at all. The other worlds would have to do the same. "Cooperating" is to our long-run benefit.
EDIT: What would this actually look like? I'd say it needs:
Profound cultural reverence for the rule of law (and contracts, oaths, treaties, etc), to the point that citizens would lay down their lives to enforce laws that they don't even like.
Willingness to inflict painful retribution against violators. If any world proves to be a "pushover" that allows other worlds to cheat it, that world cannot cooperate as part of the network (or union, federation, whatever you call it).
Expectations made clear in advance. One of the ways to do well at the repeated prisoner's dilemma is to tell the other player in advance how you will reward their cooperation or punish their defection, and follow through accordingly. Your interstellar government would probably have a crystal-clear and unchanging constitution/compact/basiclaw that is its foundation.
When round trip communication times are measured in decades (at a minimum!) the notion of "unified" is a bit tenuous. Forming consensus on an issue could easily take longer than an unmodified human lifespan, at which point you really have to take a long hard look at what you're doing and wonder what the point of it all is.
The daughter colonies of the interstellar civilisation could get a continuous stream of updates via super-powerful communication lasers and remain more or less culturally cohesive with their parent civilisation, but that's a big investment of time and effort for the parent and there's no real way for them to directly benefit from the relationship, either economically or culturally, for an extremely long time. What seems like novel research to the colonists might already have been a solved problem for the parents. What seems like an exciting new style of music or other entertainment medium to the parents might already have been seen as an old and fleeting and slightly embarassing fad to the colonists who originally created it, by the time a request for "more, more!" arrives.
Offhand, I can think of two examples of (mostly) hard-scifi settings with both sublight space travel and communication that I thought were OK: Alastair Reynolds' Revelation Space universe, and Charles Stross' Neptune's Brood setting (even if the book itself left a little to be desired).
In the former case, there was no interstellar government but there was an interstellar culture of sorts. Near-lightspeed interstellar spacecraft travelled between colony worlds (and occasionally founded new ones) taking art, technology and people as they went. This slow diffusion of ideas meant that the rate at which the colonies grew apart was limited to some degree, and whilst leaving your own world for another would mean a massive culture shock when you arrived (and another almost as big if you ever wanted to return) you'd meet people at the other end who were more or less human, doing human things with human brains. The crews of the spacecraft spent so much time travelling at relativistic speeds and in hibernation that the passing of time during sublight travel wasn't an issue for them and their memories and experiences would have spanned much of starfaring human history. They might be seen as a sort of loosely associated society, though certainly not an intergrated one, or one capable of being governed.
In the latter case, the main driver of interstellar cohesion was debt finance. Colonies cost a lot to set up, and you had to borrow a lot to do so, and you had to give something useful back or you risk having your colony cut off from critical supplies of manpower and valuable and interesting new ideas if you decided to renege on your debts. There was a complex financial system set up around bank transactions that would take decades to resolve. The critical thing there was that the inhabitants of the civilisation had more or less human minds, but were in fact human-modelled artificial intelligences in robotic bodies and so were extremely long lived. The rate of cultural change was lower, and having your mind uploaded to a new body in a new world to live and work there for a few decades before returning wasn't seen as a particularly big deal.
This situation resolves the lack of FTL communications by stretching out the lifetimes of the people involved so that communication delay wasn't a problem.
There was a time when sending a message from Spain to Constantinople meant taking a trip that could take a few weeks, either by ship or on horseback. Still the pope reigned sovereign, because he was the one validating the rule of kings.
Religion will do wonders to unite people under a banner.
The Imperium of Man in the Warhammer 40,000 franchise controls more distant worlds by a system of regional governance, each world has a Planetary Governor who is empowered to do whatever they deem appropriate with their world, with the caveat that they pay taxes and give up soldiers for the empire on a regular basis.
To ensure this, the tax-collectors come in warships and are perfectly able to claim their tithes by force.
To make sure it doesn't come to that, most worlds have a branch of an organisation known as the Adeptus Arbites, essentially the FBI if the FBI had the equipment, look/feel and authority of the Judges from Judge Dredd.
This organisation generally stays hands-off, they deal with security and legal threats to the Empire at large, not local issues. They hunt interstellar criminals and assist the empire's trouble-shooters, The Inquisition, as problems come up.
Their real purpose is to be an utterly loyal cadre of trained paramilitary, they aren't local. They're brought in from offworld and serve to remind the governor of their duty, either by being visible, or if necessary by killing them and replacing them with someone more suitable if they turn rogue.
Essentially, the imperium holds itself together by a system of internal checks and balances and does not assume that any of its citizens actually want to be part of the empire.
With an extremely hierarchical division of powers
Different aspects of government would have to be decided semi-independently at different levels of the hierarchy.
Laws would be set, trade deals decided and space-based military directed by each star system government. Policing policies and a militia-like military might be the domain of a planetary government. Individual regions might set their own by-laws.
Only very high level aspects of governance would be decided centrally, such as the constitution and core principles of governance, such as political philosophy. If the galactic government broadcasts an amendment to the Constitution it will take centuries, but every star system will eventually be expected to update their own laws accordingly. Obviously these amendments couldn't be voted on by the planets as it would take too long, so the central galactic government would need to claim legitimacy from something other than a purely Democratic mandate.
My initial thought was the same as everyone else's: "Impossible"
But you asked for how, so I came to this:
It is the best government possible.
After a few millenia of trying out different government systems, humanity has found the one that actually works, and has scientifically proven that no better system exists and any change to this system will make it worse.
The star colonies adapt this system because it would be irrational if not outright stupid not to. Of course, this system takes the long communication delays into account and has the necessary autonomy for far-away colonies taken into account, so the whole "unified government" is mostly structural, rather than day-to-day - but it's not like any federated government today would be unfamiliar with such a setup.
To stay unified, you not only need to communicate; the government also needs to be able to project power over its domain.
Interstellar warfare is a logistical and economical nightmare with sublight travel, hence any colony which wishes to declare independence can do so with impunity. The New World managed to drive the Brits out even though these had much shorter logistic routes than any interstellar force would have to manage.
He who controls the spice controls the universe ~ Dune
The only way for this to work is by making colonies dependent on the central government.
Now, as others have mentioned, colonies basically need to be self-reliant if they want to survive, since the capital won't be able to help them in case of emergency. But that's just what the colonies need; the central government might have other priorities.
A forward-thinking central government may design the colonies such that they can be self-reliant for everything they need to survive... but no better.
More specifically, the future technology might require some foundational equipment or raw material which is only found on the capital system. This may be due to a fluke (Mars' red dust has some unique property not found anywhere else in the universe) or a deliberate choice by the government: matter/anti-matter containers are used in all future powerplants, but their development and manufacture is only allowed in Minerva station orbiting Jupiter and the Ares fablab on Mars.
The possibility of artificial control is eased by the fact that building the equipment is extremely difficult, requiring infrastructure which can take decades or even centuries to build. So any counterfeiting operation would need to be very secretive, or it'll be found and quickly vaporized.
The colonies then rely on a consistent supply line of antimatter containment chambers from the capital system. Thankfully autonomous freight ships come all the time with "maintenance" quantities of containers. In the case of a disaster at the colony which requires many replacement containers, each ship also has emergency stores which can be accessed, but not without broadcasting a signal.
Other freighters heading towards the colony which receive the signal will know not to make their emergency stores available (to stop the colony from creating a stockpile), and the capital will send out an armada to the colony to help with the repairs (if such help is still needed 100 years later) and ensure the emergency supplies were used for their intended purpose and not to create a stockpile. If an illegal stockpile is discovered... well, that's why they sent an armada and not a civilian ship.
And obviously, if any rebellious behavior is observed in a colony, those freighters can just turn around. Good luck being independent and having to survive with primitive, pathetic, 21st-century technology.
Workable, but very fragile
This would arguably work (as long as the government keeps its monopoly on the resource), since colonies would need to choose between 30th-century technology by accepting the central government or losing the technology they can't think of living without.
However, it would give humanity a huge single point of failure. All it takes is a single failed science experiment making the Sun go supernova to destroy humanity's only source of this fundamental raw material.
If you can't go Faster Than Light, than the gap between worlds is just too much to handle. Trying to communicate over a several year time lag isn't an effective means of having an Empire. It's just not practical to have a united government if you need to okay everything through a central bureaucracy that could take multiple life times to respond.
Not to mention that sub-light speeds has it's own massive reach of problem when it comes to physical transport. Sure, going .99c would get you there almost as fast as light, but it would take you months of real time to get to that speed using most conventional means of acceleration. In other words, worlds separated by four light years, even moving at .99c, it would probably take anywhere from five to six years to get from place to place. Now, while trade is possible, it not great to order something and have it arrive in half a lifetime, it's more going to be the variety of 'We'll load all the best stuff we have and sell it when we get there and hope they want it."
No Empire. It's not practical, there are just too many problems. And, if you want a series which handled this realistically, I'd recommend The Unincorporated Series, which does technically deal with the concept of a multi-star empire at sub-light speeds, (specifically, how it is feasible or rewarding), but that's only at the very end of the last book. (Most of the series is economic warfare across the solar system until it just turns to regular warfare across the solar system.)
One may communicate instantly using quantum entangled particles, yet I seriously doubt instant communication will alleviate the difficulties of maintaining a unified government when physical travel requires hundreds or thousands of years between the "home planet" and the "colony"...IMHO
In the Takeshi Kovacs series they figured out a way to download the human consciousness to digital media and pair with FTL communications they effectively had interstellar travel by buying/renting a body on the other side. Extra bodies were made available because criminals (and debtors) didn't physically sit in a prison, they had their bodies taken and set in digital storage. This got even crazier by bionics and genetic engineering special made bodies. This let's the leadership be able to travel anywhere nearly instantly and also have the ability to project force as long as they had an ally in control of a "sleeving" facility. As a side effect the rich and powerful are effectively immortal
Have the same society replicated multiple times across the galaxy.
We have created what we believe to be the ideal stable, creative harmonious society. Cloning is the predominant method of reproduction. Clones undergo a 20 year "incubation" period: a carefully controlled time of education, training and developmental training. At the end of the incubation period a new copy of each individual is released into society and work begins on the next generation.
Everyone has learned to operate and co-operate in a society which contains a number of copies of him- or herself at different stages of development. Whenever the empire expands, it does so by sending out a new self-replicating batch of the same set of individuals. It is one empire because it is ruled and sustained by the "same" people.
Maybe some random variation is allowed. Maybe there is a certain amount of sexual reproduction. There is communication between neighboring solar systems to sync up or, if necessary, "reboot" failed variations.
But maybe no variation is permitted (or desired). Technologies may develop at different rates due to accidents of environment or experience but there is no serious conflict because the individuals feel a deep sense of belonging to their society and kinship with their "other selves" whether on the same planet or in more distant locations.
Until one day... when our story begins!
Robert Reed's Marrow universe has one way - humans (and all the other space-faring species) are immortal. Needless to say, this makes their perspective very different from ours, and worthwhile large-scale projects can take tens of thousands of years.
In a world like this, if you had an evil empire ruling over the galaxy, sure, it's going to take you hundreds of thousands of years to project power - but by the time you get there, the people responsible are probably still there. Of course, there's still the other tiny problem - why bother? What is on the other side of the galaxy that has any value whatsoever to you, as the God Emperor of a Unified Human Government?
Most real world "proto-empires" failed for much the same reason, and indeed, it is rare to see a colony providing benefits to the "parent" country - much more often, they are net resource sinks and the only real justification for having them tend to be along the lines of prestige, cultural extension and such. Even if a colony is beneficial to people at home (e.g. Rome's imports of grains from North Africa and exports of tools), it only lasts for a short time, historically speaking, and tends to cause more trouble home than it solves. It's hard to imagine how a colony 300 years away would ever be any benefit to someone back home.
Use an AI gouvrnement: The home world writes its government strategy into a computer code and builds multiple identical computers that are given to the colons when they leave. The computers then acts as a governors in the different colonies.
If the code is written well enough and the computer is smart enough, then it can govern exactly in the way that the home world would if it were able to make the decisions. Moreover, all the computers have the same code, share a common objective and know it. They can work out what the other colonies will do without communicating. For example, if a threat appears and a coordinated response is necessary, then each colony will take the most logical action based on the fact that all the others will do the same.
If you can keep travel to about a century round-trip it's doable, but hard. A century is about as long as people can realistically plan for (my son won't live to see it but my granddaughter will. I know my granddaughter and want her to do well.) So you start with that as a base.
Secondly you'd need the capital planet to have exclusive access to interstellar travel, or at least interstellar warships.
Thirdly you'd need the colonies to be dependent on the Capital for something other than interstellar flight. For example in Finity's End there are almost 0 habitable worlds, so the colonies are all Stations orbiting gas giants with valuable minerals and the the like that require a certain amount of specialist material from earth no matter how efficient they are.
With these parameters you essentially end up with an Imperial hegemony, regardless of the overall rulership of the colony planet. Something similar to the Imperium of Man in 40k (minus the FTL and demons). Each colony could be a democracy or theocracy or dictatorship, but as long as they paid tithes to the throneworld they're allowed to do as they wish. You couldn't get direct rule, as others have pointed out the distances involved just don't allow for it. But if the only guys with starships and Big Mac Sauce demand a 10% tithe once every X amount of years or you get cut off and slowly die, you'll pay attention and toe the line. Mostly. Of course you'd need enough Imperial traffic to make sure the colony isn't building their own shipping, and have dangerous enough transports/transport escorts to blow up anything they might try. And the colonies would likely send representatives to the Throneworld to work out super-long-term things like using colony world's people for future empire expansion and discuss inter-planetary trade.
In the end I don't think it would last for eternity. Invariably somebody would figure out how to make The Thing Only The Throneworld Has, or builds a secret fleet to overthrow the Imperial Oppressors. Or even just expansion beyond the 100-year limit resulting in the creation of Satraps that eventually grow into separate star nations by peaceful or nonpeaceful means.
First, you are going to have to assume some form of instantaneous communications through a very malleable space/time configuration. Not such a stretch, because all of the limitations on information traveling faster then light assume that time is actually a 'thing', and that this 'thing' somehow passes. In a dimension (thick enough physics textbook) in which time does not exist as anything meaningful to the physics (and it is beginning to appear as if the quantum world really doesn't care about time), then limitations on information travel time are irrelevant. Information, after all, is neither energy nor matter. It is the medium or mechanism of information transfer that is governed by energy and matter restrictions.
Second, you are going to have to assume an empire-wide economy, with one and only one empire-wide bank. All financial transactions have to go through this one bank. Perhaps it owns the patents and the intellectual property rights of the instantaneous communications system (Sort of like the Elizabeth Moon Vatta's War series and spin-offs, and the ansibles being controlled by only one company. However, in the Vatta's War series, there were apparently a few system-wide banks but I don't recall if it was a universal currency or if each system had their own).
Thirdly, consider that a bank account balance, or the 'cost of goods'. or the 'stock market price', although technically 'information', is not really 'information that causes something to happen' in the classical physical sense. Your bank balance is the same, everywhere in the universe. Revealing what it is, anywhere, is not really 'transferring information', it is just 'revealing existing information'. Compound interest increases a bank balance at the same rate, and the numbers are the same, no matter where you are. 'Instantaneous communications' is the ultimate block chain, available everywhere.
For an interconnected economy to work, financial transactions would have to be made pretty much instantaneously. Money comes out of one account as soon as it goes into another. Money can not appear to exist in two different worlds, at the same time. Either instantaneous accounting exists, or otherwise, money would have to be physical, and would have to be physically carried from one system to another, like the boat loads of gold moved around the world in the 17th and 18th century. Totally impractical in a modern economy because this 'wealth' is taken out of the economy while it is being transported. Can you imagine the chaos if a corporation could be bankrupt on one planet, and no one knows it on another planet and so it is still conducting business and taking on debt? Completely unworkable.
Just like the Roman Empire, individual systems would have a great deal of autonomy in local governance, social laws and cultural conventions, and local courts and penalties. Rome allowed a lot of individual local autonomy, but the thing that mattered - the financial empire and the financial integration (the only real reason to have an empire) - was closely controlled because Rome controlled the vehicle for international trade - the Roman denarius. By controlling the currency, and centralizing trade, they controlled the empire.
The advantage of your empire over the Roman Empire, is that it is expanding and being created at the same time. It is not like an existing society being taken over by the Romans. Thus, the common currency and connected financial empire would exist from the get-go. Only one currency would ever have been used in interstellar trade. The colonists would have brought their cloud computing currency with them, as they built more outposts. Corporate towns would be financially tied very closely to the head office. Same accounting system, same books, same currency.
Thus, the empire could be controlled simply by the threat of shutting a node off from this system of instantaneous communication, universal financial economy, cloud information system, and block chain technology. Either the colony co-operated, or it was essentially cut off completely from all cloud financing, cloud banking, cloud computing, cloud data, and essentially civilization. Any renegade colony would become a backwater and descend into stagnation, its only source of contact being slower-than-light physical transfer.
And, really, if it takes ten years or more for a renegade colony to get any weapon to you try to attack you, what kind of threat is that? Methinks a financial and data embargo would suffice to keep any colony in check, without worries that they might be any kind of threat to you. 'Give us our freedom or in 100 years we will blow up your cities.'
Slow down the people
If you're a society of uploaded people, you think as fast as your resources allow. The usual assumption is that people will want to keep their perception of time the same, but with no more physical humans, why burn through the energy stores quickly? If everybody runs at 1/100th speed, the speed of light is effectively 100 times faster.
One problem is that this is only a solution for a very advanced society. It only makes sense if not only everybody is uploaded, but also if you've already disassembled all the stars so that their energy isn't wasted.
The American model of Representative Democracy works just fine on an intergalactic scale. Indeed, it was first designed in a time where travel was slow and communication was difficult.
To get around the problems of slow travel and difficult communication, there are 4 tiers of government: The municipality, the County, the State and the Nation. On an intergalactic level, there would be two more levels: The planet and the Empire.
The local levels are designed for more direct citizen participation. For example, a town voting on a proposition will collect yay or nay votes from every citizen, while the State, the citizens vote for representatives, and only those representatives vote on the state or national laws.
On the national level (or in your case, an intergalactic level), the state residents would vote for electors - people they trust to make an informed decision, and those electors would choose the president.
My question to you is, "Why do you want all communications to go through Earth?" A central, authoritative government only works quickly because it is close by. Additionally, by the time a new colony gets started and is self-sufficient enough to rebel, the originating world is probably dead.
The British Empire, in its heyday, did not route all requests through the monarchy - it relied on regional governors to maintain order directly, then it flowed up the chain of command as the problems grew.
The United States government also does this - problems are dealt with at the lowest possible level. When a problem arises that requires the help of a higher level, the two levels have to be on good terms. These mutual requirements start chaining and cumulating, meaning that your fringe planets have to be on good terms with your middle planets have to be on good terms with your near planets have to be on good terms with Earth. By going to regional leaders, communications time is cut down to only a few years.
There eventually becomes the problem of a colony planet becoming self-sustaining, but since they're not even human anymore, you can safely blast them away. Earth has probably gone extinct anyways.
A nice book with an example of the limitations of no-FTL travel (i.e., hard science-fiction) is Existence by David Brin, where their method of colonizing planets is to upload their consciousness into a hard format and fling millions of them out into space like a virus.
Government is, in many ways, a fiction that we collectively pretend into existence; the objective difference between a dictator and a crime boss is pretty flimsy.
So a hundred planets are under the same government if they collectively pretend that their governments are in fact the same government, even if there are no governmental structures above the level of the planet.
If the central government officially grants legitimacy to the branches of the government on every other planet, and those branches all claim to derive their legitimacy from that central government, then it doesn't matter how different the branches end up being in practice; their initial charters can all give their people the right to democratically modify their charters as needed, such that each planetary branch grows a completely different constitution over time.
Even if a given planetary branch is overthrown in a coup, the new people in charge may still decide to claim the mantle of legitimacy from the central government. (This has plenty of historical precedent: non-Egyptian pharaohs ruling as pharaohs, non-Chinese dynasties ruling as dynasties, conquering kings claiming the same divine right to rule as the ousted kings.) That wouldn't be guaranteed to happen, of course. But it could happen.
Note that this approach probably doesn't work if the central government has nontrivial demands of the planetary branches; for example, this system won't allow the central government to extract resources and labor from other planets, because they won't stand for it. But if the central government represents benign democracy and stability, and only sends peaceful envoys that don't have the authority to try to interfere with things, then the legitimacy that they confer can be useful enough to justify claiming it.
This can only happen when the planet is supervised by machines with sort of super power that is very hard to detroyed by the clever and want-to-be-free beings supervied.
(1) The machine supervisor system (MSS) constantly receive commands from the central government system (CGS). The commands send by the CGS may travel billions of years at light speed to reach the MSS.
(2) The MSS have enough pattern recognition capabilities to checkout whether the supervised society work as the expected rules, if not, then punishment will be excuted, like bombing. The expected rules is updated constantly by the command stream from CGS.
(3) The MSS have super power that is very hard to detroyed by the clever and want-to-be-free beings supervied. For example, it is space based + land based. If the land based part is being attacked, then the space based part will lauch nuclear weapons to punish. If any space technology (to attack the space based part) is find to be being developed, then the facility will be destroyed immediately.
(4) Being supervised by a super power doesn't mean slavary or bad life. The commands send to the MSS may just to keep the society under control, not to force them to work for the CGS. Another example is "no war" command. If any war is detected, then nuclear weapon is launched to destroy both side. Another example is "pupulate control" command - then over pupulation will never happen.
(5) Any system have bugs that can be exploited. The MSS is very hard to destroy doesn't mean it will never be destroyed. Once it is destroyed, this planet goes free which may not be a good thing, like war will come back and pupulation will explode. If you want your control over the planet to be longer, then you'd better not to use this system as a slavary system. Once those folks realize the MSS is infact good for them, the majority will not want to destroy it - do you want't to detroy a system that can prevent war/over population/over consumption of resoruce?
(6) Machine may have bug and fail, it doesn't have the "final underlying judgement" human beings have. Then it is possible a bug or an attack in the machin trigger a "destroy all" action which destroy the society on the planet.This is something you have to pay to get the galaxy government. This is why even we need such superpower very much, such MSS can't be used on earth - you need more backup planet in case some planet is occasionlly destroyed by the system for your race to live on.
If it is not machine super power based, then the being in the local government will soon find out there is no need to be governed by the central government.
Post-scarcity Economy, combined with a high value and respect on intellectual property and art and culture
Although the post-scarcity economy concept is sometimes used as a 'catch-all' solution, in this regard it would remove most of the reasons why conflicts happen - battling over resources such as 'land', 'money' or 'power'.
If technology exists where robots could build anything we wish, removing concepts of ownership, hunger, and even production, then humanity can concentrate on what it does best: intellectual pursuits, art and cultural development.
If the following are achieved:
- The ability to have as much food as we want
- The ability to build anything we want, no matter how large (or how small)
- The ability to have as much space as we want (artificial habitats, or even make new planets)
Then we can exist fine, without any 'needs'. The issue then comes in mammalian emotional feelings of dominance which can be tempered by trade. Trade what? Well, although FTL is not possible, we can still communicate with each other at lightspeed, and therefore transmit and receive virtual 'items'.
These could be:
- Blueprints for new items to build or food to have
- New technological methods, programmes, VR worlds etc.
- Original works, such as music and art, which are not available on your own world
- News (even though many years or decades old) and very old and late communication between individuals
Cultural exchange is still possible at light speed - in a post-economy society. It would still be valuable therefore to remain at least loosely unified to gain new knowledge available on other planets.