It was pointed out here that the speed of light is a huge issue for any government that spans multiple stellar systems. The consensus seemed to be that it is very difficult to make large empires work.

However, I don't think that anyone said that such empires are impossible. . .

So, I figured I'd ask about galactic empires, a common sci-fi trope. There are some issues that stem from the speed-of-light problem:

  • Communication
  • Shuttling troops to battle
  • Sending diplomats/envoys to different star systems
  • Ensuring that civilizations survive long enough for any of the above to be remotely possible
  • Biological/sociological/technological diversity among different species.

These are but a few of the many problems that a galactic empire would face. There are, of course, many others.

So, how could a galactic empire stay stable for any significant amount of time (i.e. millions of years)?

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    $\begingroup$ Presumably, with technology we don't have access to and may not yet have theorized. I look forward to answers. $\endgroup$
    – Frostfyre
    Commented Aug 27, 2015 at 15:55
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    $\begingroup$ Millions of year? That's a long time considering that politicians think no more than 4 years in advance (for the reelection). $\endgroup$
    – Vincent
    Commented Aug 27, 2015 at 16:18
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    $\begingroup$ @Vincent Yes, politicians can be short-sighted. Well, considering that not all civilizations will form at the same time, and that establishing contact takes a while, it seems reasonable that stability requirements would be longer. If a governments survives for 1,000 years, then it's survived for a fraction of the time it takes to travel to one of the furthest systems in the galaxy. That's akin to a government surviving for roughly . . . perhaps one day on Earth. $\endgroup$
    – HDE 226868
    Commented Aug 27, 2015 at 16:21
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    $\begingroup$ Have you read the Foundation series by Isaac Asimov? The main plot is the decay and rebirth of a galactic (or intergalactic) empire. $\endgroup$ Commented Aug 27, 2015 at 18:17
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    $\begingroup$ @DeveloperWeeks I haven't, though I've been meaning to. $\endgroup$
    – HDE 226868
    Commented Aug 27, 2015 at 18:18

9 Answers 9


A galactic empire is not going to fit the mold we’re used to as a government. Even if we only look at the 25 or so closest stars, we’re looking at four years of travel time at best (assuming you’re traveling at the speed of light, which is highly unlikely) and 11+ years at worst. Planets are going to have a huge degree of autonomy. All communication and decision making is going to be stale, sometimes devastatingly so. What kind of structure would work here?

The United Galactic Systems

The modern-day United Nations isn’t so much a government as it is an organization with a collection of representatives from actual governments. It is, however, a means to facilitate diplomacy, peace, and trade. These three things will continue to have some importance, even as the “nations” begin spreading out across light years. More importantly, the loose influence is a natural fit for the physical and logistical infeasability of tightly controlling disparate worlds.

The Shape of Politics

The political reality of STL travel is going to influence everything. On a planet with individual nations (such as the Earth today) the concerns and problems of each nation are relevant on a global scale. Diplomacy and trade between nations often affects many other nations on that planet. When we expand to the interstellar scale, the concerns of an individual nation become much less important. To our friends in Alpha Centauri, the journey to Earth is going to be 4 years whether they’re coming to the aid of the United States or China. As the distances between planets get larger, this is going to naturally cause politics at the galactic level to care more about planets as a whole than the nations that divide them. In turn, this will require nations to act much more cooperatively if they are to be economically competitive and safe from attack. Wars and conflict will continue to rage at the planetary level, but the galactic empire isn’t going to have as large a stake in the outcomes (unless total destabilization or withdrawal from the union is likely).

The political situation gets even more complicated when you consider non-human civilizations. As soon as we run into the first one, a new threat becomes possible: racial annihilation. A human planet is unlikely to ever invade another human planet in an STL world. The logistics are a nightmare and there is nothing concrete to be gained. Once we encounter another space faring species, however, invasion with the intent of total destruction or colonization becomes a real possibility (in either direction). This is likely to be the most typical circumstance in which major wars are fought.


Communication is going to be a problem. Information is always going to be very, very stale. In most cases, the goals you were sent to pursue will have been dictated by officials that aren’t even still in power. When you arrive to invade a planet, your home world may very well have already agreed to peace. These problems are unavoidable, but they can be mitigated and will change our behavior across interstellar space. Every ship traveling between worlds, regardless of its purpose, is going to be a message carrier. Massive amounts of news and data will routinely be packed up and shipped out. With large numbers of ships traveling back and forth between worlds you may even get (stale) updates on planets weekly or monthly.

People who are sent to perform duties, such as diplomats and militaries, are going to have a large degree of autonomy and leeway to make decisions. We’ve already seen exactly how this is likely to work with diplomacy and conflict between the Americas and Europe. Diplomats could not phone home when a crisis occurred — they had to act. Militaries were given a mission to complete, and they engaged the enemy until new orders said otherwise. In this galactic empire militaries will not be dispatched lightly. Not only will you be recruiting soldiers and dispatching them for nearly the entirety of their lives, but once they have been sent they cannot be stopped. This will almost certainly make interplanetary conflict infrequent, except in existential circumstances as described earlier.


What this empire has going for it is that it’s not really an empire by traditional standards. By behaving similarly to today’s United Nations it is minimally intrusive and maximally useful for planets to belong to it. In a multi-species space faring situation is has the added benefit of being able to bring planets together to defend against existential threats. Any traditional government that attempts to control more than one world is going to be incapable of holding onto all of them -- it’s simply too difficult logistically in an STL world. By using a loose coalition like the United Nations you can maximize the chances of its continued existence while continuing to provide services useful to all planets, even multiple species.

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    $\begingroup$ How long did it take at the time of the Roman empire to go from Rome to the farthest point of the empire? $\endgroup$
    – celtschk
    Commented Sep 13, 2015 at 17:33
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    $\begingroup$ @celtschk Due to their advanced roads and light courier wet-navy ships, about 3 months at a forced march for armies, and about 2 months for single couriers. $\endgroup$ Commented Feb 27, 2020 at 1:39

There's one important precondition that needs to be understood before the question of a "galactic empire" can be addressed. So far, nobody but @user11599 has mentioned it (+1 for doing so.)

An empire is a predatory economic structure.

More precisely, one of the best definitions for present purposes comes from John Michael Greer's useful article The Nature of Empire:

An empire is an arrangement among nations, backed and usually imposed by military force, that extracts wealth from a periphery of subject nations and concentrates it in the imperial core. Put more simply, an empire is a wealth pump, a device to enrich one nation at the expense of others. [Emphasis added.]

From this perspective, it's easy to see that the traditions of Science Fictional worldbuilding have more often than not put the cart before the horse, concentrating on the trappings of imperial prosperity rather than the underlying wealth pump that provides the reason for the empire's existence, and that makes it all possible.

True, there are some thoughtful exceptions - Dune's commercial empire, for example, and the various interstellar states of Asimov's Foundation series - but usually we are shown mighty fleets, militaristic cultural tropes, dizzyingly wealthy stellar imperial capital cities, and so on. These interstellar empires are (especially in MilSF) often imagined as the pageantry and politics of some historical terrestrial empire (commonly British or Austro-Hungarian) transposed to the stars. The wealth pump is handwaved, or simply ignored.

However, @HDE 226868, you are doing something that none of your imperial forbears in the field has attempted: you are investigating the feasibility of a lightspeed-bounded interstellar empire. Your list of concerns (communications, troop deployments, longevity & stability of distant civilizations etc) is a good start.

However, you are ignoring the elephant in the room: sublight travel even to a nearby star such as Proxima Centauri is shockingly costly. (See Charlie Stross's The High Frontier, Redux for a brutal but very well-founded description of just how hard and costly sublight interstellar travel would be. Also, scan the comments, and read the shrieking denunciations by fans. Most enlightening. When people are saying to Charles Stross - Charles Stross - "And you call yourself a Science Fiction writer!" you can almost taste the tears through cyberspace. A serious endorsement of the strength of his article: these people, emotionally dependent on the narrative of human colonization of interstellar space, are (understandably) losing their shit... which underscores my present point.

In sublight travel, you don't get to build a reasonable ship and send it through a wormhole, or engage warp drive; the thing has to be big and expensive. Even fuel for the trip is costly. You're building something large and specialized, that will take a lot of fuel for the trip. It might be a relativistic time-dilation vessel, a huge generation ship, or a large and difficult coldsleep ship - and we don't yet even know how to do coldsleep.

And then, no matter which type you build, it's a significantly long voyage, with severe challenges at the destination. (See James P. Hogan's A Voyage from Yesteryear for a rather nice exploration of the social/military disadvantages facing a sub-c expedition upon arrival.)

Which brings us to the core question. What kind of wealth pump can be constructed, sustained, and defended under these conditions?

More importantly, what possible kind of wealth could such a pump possibly transfer, to make such a huge and costly effort worthwhile?

These questions are fascinating and powerful, and very possibly disturbing. Great stuff for a story.

However, it won't be trivial to come up with Worldbuilding.SE-grade answers. :-)


Might I suggest (not too helpfully) that the first step in this idea will be to figure out what the BENEFIT is to the central civilization in the empire. That will determine what means are required to achieve it. For instance, if there were a substance valuable enough to use as tribute, the central empire could just leave the peripheral regions to their own devices, except there would be a monitor in each that would check to see that they (1) keep sending tribute and (2) aren't getting too advanced technically. If the peripheral area gets close to how advanced the center is, or doesn't send tribute, the monitor sends a signal to the nearest "fortress" to have an attack launched on the offending client state. The benefit to the central empire is the flow of goods.

  • $\begingroup$ Of course another approach would be to plant a huge nuclear device inside each peripheral civilization's planets, to be initiated if the tribute isn't forthcoming. :) $\endgroup$
    – user11599
    Commented Aug 28, 2015 at 6:45

Possibility #1: Fast-timers and slow-timers

There was a clever novel (by Alastair Reynolds I think) in which there are two levels of hierarchy of galactic civilisation. The bottom level lives in realtime, contains nearly all (99.999 % or whatever of total sophont-mass) and they come and go on the 1000-100,000 year timeframe. These civilisations only move about and interact in their local sphere (one or maybe a few close by solar systems). These are the 'Butterfly civs'.

Then on top of that there is the upper level of society which employs more sophisticated space propulsion technology (still STL though) but spend most all their time asleep in deep freeze. You could say they are the 'guardians' who carry knowledge and trade between the base level civilisations, although in practice this cadre is factionalised and some are despotic. These are the slow-timers.

Members of the Butterfly civs may be lifted up to be included in slow-timer civilisation from time to time. There also various alien or post-human entities that play at this level.

So there you have it - simply have the 'international' players of the empire (the diplomats, sellers of information and embargoed tech, imperial fleets etc) operate in slowtime, and the hoi-polloi in real-time/fast-time.

Countless butterfly civilisations that evolve and live for the blink of a slow-time-eye and eldritch master civilisation that farm the former to fuel their deep time ships etc.

The only problem for the slow-timers is running out of time - most main sequence stars don't have that long to live on these timescales (which suggests a cassus-belli for conflict among the slow-time civs).

Possibility #2: Wormholes and 'empire-time'

This is taken from 'Traversable Wormholes, Some Implications' by Michael Clive Price.

Basically we assume that wormholes can be created but chronology protection conjecture (CPC) prevents them from generating FTL paradoxes.

So this means when a wormhole is created and one end of it transported at relativistic speeds to a system say 1000 light years away on an interstellar line-layer, the crew of the line-layer only perceive for example a 6 month elapsed time. Therefore to avoid a paradox of FTL we assume that when we traverse the wormhole we emerge into the remote system 1000 years in the future of the universe (this is referred to as co-moving time in the article). However the 'empire time' of the remote system is only 6 months in the future.

If some other imperial entity attempts to lay a wormhole line sufficiently close to this system (and from a sufficiently mis-matched 'empire-time' reference), then one or both of the holes are destroyed by CPC.

So the idea is that an empire establishing itself with relativistic wormhole linelayers 'stamps' its chronology (its own empire time) on that region of space. After which, travel between the nodes of the empire network is instantaneous, but without any FTL paradoxes.

However wormhole networks belonging to competing empire-times would not be able to approach very closely to one another and would need to resort to standard relativistic invasions in the no-mans-land (or no-mans-time) in between.

  • $\begingroup$ The book I am thinking of is 'House Of Suns' by Al Reynolds. $\endgroup$
    – rumguff
    Commented Aug 27, 2015 at 16:31
  • $\begingroup$ Invoking the CPC is a bit much. It's not too well established or accepted, and it could be falsified. $\endgroup$
    – HDE 226868
    Commented Aug 27, 2015 at 17:30
  • $\begingroup$ Its a fair point HDE but I claim that is is admissible if only because an energy source capable of powering any significant mass anywhere near C requires a similar level of hand-waving. There is certainly no 'accepted' way that near-C propulsion of a ship capable of carrying many human is possible. $\endgroup$
    – rumguff
    Commented Aug 27, 2015 at 18:49
  • $\begingroup$ Why not? Besides, there's no requirement that a ship travel at speeds near $c$. $\endgroup$
    – HDE 226868
    Commented Aug 27, 2015 at 18:51
  • $\begingroup$ I presumed you meant c-like speeds because you mentioned sending diplomats across the empire. If they are travelling << c then why not use electronic communications via intermediate relays? $\endgroup$
    – rumguff
    Commented Aug 27, 2015 at 19:06

Time Dilate All the Peoples!

I have a solution, that works completely within current, well understood physics, that makes the finite speed of level a non issue. In fact, we want it to be as finite as possible. What you do is time dilate everyone!

So, pick a efficient energy source. Let's say you use micro black holes. Now what you do, is use this energy to put people at very close to the speed of light. Now, even though it may take a millennium for your email to get to your grandma, from both your reference frames, that may only be a day! Indeed, getting places is also faster.

Now, you may ask, what if you don't want people having to fly everywhere all the time. How are supposed to settle if you are flying everywhere. The answer is you orbit. So you make some largish, black holes. You may have to sacrifice a sun* to get one that is big enough; I'm not sure. I choose a black hole because of their density. You can orbit a black hole much more closely than anything else of the same mass (but you still can't orbit to close.) Now, you get to go really fast for free! Time dilation for free!

Okay, no for a little bit of math (I only know a little bit of relativity.) So, the milky way is about 100,000 light years across. Let's say you want, in your empire, to be able to cross the milky way in a year. Note that this isn't just a year for our traveller, but everyone is being time dilated enough that it seems like a year. I used the wolfram alpha time dilation calculator how fast we would have to go, and it said ~1 c. Okay, we can't go c, but it is just rounding. Anyway, that means we need to be time dilated about 100,000 times. Luckily, the Lorentz factor makes calculations simplier:

$$\Delta t' = \gamma \Delta t$$ $$E_k=(\gamma-1)m_0 c^2$$

So we want $\gamma = 100,000$. This means for each kilogram of ship, we have $9 \times 10^{18}$ joules of kinetic energy. Okay, that is a lot of power. But at the end of the day, it is an engineering challenge, and no longer a fundamental limitation of nature. Also, gravitational dilation would help a lot. I would recommend conserving energy very closely. Namely, instead of slowing yourself down, aim for the photon spheres of black holes. This will trap you and you kinetic energy in orbit. You also probably won't start with $\gamma = 100,000$, but slowly work up to it. If you have a hard time finding energy, a smaller Lorentz factor could do for a small part of the galaxy.

The great thing is, sans engineering challenges, this is all . STL isn't the problem, its the solution!

As to how this affects various things:

  • Communication: From the frame of reference of the humans, communication isn't too slow (but slower than we are used to now.)
  • Shuttling Troops to Combat: Prohibitively Expensive. Let's waste resources on rail guns instead!
  • Diplomats/Envoys: Again, not to bad considering time dilation.
  • Ensuring Civilization Survives: An interesting problem is that if you time dilate too much (you get really efficient with energy), the heat death of the universe know becomes a problem for your civilization. Not much you can do about that.
  • Biological/Sociology/Technological diversity: Probably would be cause racial tension, especially if they aren't time dilated. (We could watch entire civilizations unfold before our eyes!)

*Ha Ha! Instead of sacrificing people to the sun, we sacrifice the sun to the people.

  • $\begingroup$ If everyone is dilated no one is dilated. Also I'm not sure which would kill you first; the searing heat of the accelerated CMB or the crushing pull of centrifugal force. Probably that latter. No wait, maybe the former. $\endgroup$
    – Skek Tek
    Commented Oct 15, 2018 at 20:20
  • $\begingroup$ "Now, even though it may take a millennium for your email to get to your grandma, from both your reference frames, that may only be a day!" I'm afraid I'll need to put a '[Citation needed]' for this one. $\endgroup$ Commented Feb 27, 2020 at 1:34
  • $\begingroup$ @AdrianColomitchi what do you mean? I calculated the lorentz factor later in the answer, which backs up that statement. $\endgroup$ Commented Feb 27, 2020 at 17:01

Bomb in a Vault

Take a large, multistage, thermonuclear bomb (a few thousand megatons), and put it in a vault on the governing planet of a star system, along with communication equipment to communicate with the nearest empire-controlled star. If communication is lost, the bomb explodes. If it detects tampering, it explodes. The vault is designed so that the time required to disable the bomb is greater than the communication delay with the nearest empire-controlled star. Keep the local government on that planet and make sure they don't try to leave.


A digital empire

The concept is quite simple. You have a completely digital civilisation. The empire would have a giant supercomputer built around each star that it controlled. The inhabitant's time could be slowed down so although the radio signal takes 12 years to get to it's destination the people living in each of the computers would experience that as instant. Due to the fact that everyone lives in a simulation time can be programmed. To move people around the empire they can be sent as radio transmissions due to the fact that they are no longer physical. Imagine that each person is a file and that you are e-mailing one of those files to another computer. This also uses the same slow personal time concept from before so the person(s) experience the as transit as instant. For war, since you have machines collecting virtually all the energy output of a star you can send of some of that energy as a large destructive beam. If soldiers are really needed you could put some digital people in a spaceship, send it of at almost the speed of light and slow down their time so that the journey feels instant. The society will not fall apart as communications feel instant.

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    $\begingroup$ Can you explain how to "slow down the inhabitant's time" and how to "send person around as radio transmission"? It is not so simple to me. $\endgroup$
    – L.Dutch
    Commented Aug 16, 2017 at 15:26
  • $\begingroup$ This is a digital variant on Karl Schroeder's Lockstep concept in his SF novel Lockstep (2014). The concept is discussed here: centauri-dreams.org/?p=30584 $\endgroup$
    – a4android
    Commented Aug 17, 2017 at 2:02

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. Likewise AIs could also require a static society so that the 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 and destroying them (in a way similar to Fred Saberhagen's Beserkers). The 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. The AIs could still have STL or lightspeed communication networks for the purpose of being able to summon the full strength of the galactic empire if needed to destroy a foreign civilization, sure response would be slow, but it is assumed that the attacker would also have similar communications limits. The AIs could even have planned ahead or criteria for abandoning stellar systems consistently in such situations.

Following a billion, or a trillion lines or more of code can cover a lot of planned societal choices, and it would be able to be followed consistently.


Hub and Spoke scientific hegemony

As other answers point out, the key to an empire is that a central power is able to extract resources from a periphery. Given the time and cost of moving physical goods across light-years in an inter-stellar empire without FTL, the resources being extracted and methods of control would all ideally be informational.

The way this would work would be a central planet or solar system (the "capital") would have settled colonies on a number of different other planets, but the location of these colonies and method of contact with them would be extremely tightly controlled. The goal of this is that any new colonies are created without any knowledge of or ability to contact other colonies; they only have contact with the capital, creating a hub and spoke model (with only a single hub).

Once this is set up, the capital can then engage in pair-wise exchanges of scientific knowledge with each of the colonies. Ideally, each colony would specialize in a particular area of science or technology, but even without this will likely be able to make progress that the capital hasn't just due to random chance involved in discovery (discoveries such as penicillin or Viagra involved a good bit of chance). The capital gets an asymmetric advantage in this exchange because of the asymmetry of the communication graph. It gets technological advances from 10+ colonies, and can trade one colony worth of information back to each of them in exchange for their progress.

On top of that, you get to have your colonies test the planetary impact of their technologies for you. Have a colony developing genetically engineered life-forms or nanobots? Let them try it out for a few years before you use it yourself so that you can dodge all those nasty invasive species and grey goo events. Want to run a planet-wide clinical trial? With enough colonies, now you can; you just have to wait ~10 years for the communication round-trip.

Necessary Technology

To make this work, you'd need a communication technology and protocol that is:

  • Sufficiently high bandwidth to carry enough information. If you're sending around genomes of engineered organisms, you'll want to be able to send terabytes per day.
  • Sufficiently targeted that other colonies don't find out about each other by accident. You can pitch this to the colonies as a feature; sending targeted transmissions can help with energy cost of the transmission and bandwidth.
  • Asynchronous: you want something much more philosophically like UDP than TCP. Transmissions will just get sent. Important ones might get sent a few times, but if you want to get the data being sent to you, you better be listening because there's no connection handshake or retries.

You also need some way to colonize the planets in the first place.


The big risk here is that colonies discover each other and try to "rebel" by embargoing the central power and trading exclusively amongst themselves. Here are a few ideas for how to solve this:

  • Have some reason (such as predatory aliens) that it's very dangerous for colonies to be sending out undirected broadcasts. Given this, planets can only safely communicate if they know where each other are (and the celestial mechanics of their orbits) with sufficient precision that they can send a directed message.
  • Have agents on-site on each of the colonies that are equipped with capital super technology to suss out and eliminate anti-capital sentiment.
  • If need-be, the capital can send an invasion fleet; it just might take 30 years to get there.

There is also a risk that your system will be subverted by colony-sympathizers in the capital. If they can gain control of your communication infrastructure even briefly, they can send the colony location list to all the other colonies and you would be dethroned. It is likely you would need heavy information restriction and secret police within the capital to tamp down on any such revolutionary sentiment. Potentially, only the capital ruling class even knows that the colonies exist. If colonies are founded in secret, then colony ships would be the mechanism for "disappearing" political dissidents, dangerous intellectuals, and criminals (like if the Soviets colonized Australia rather than the British).


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