In a world where travel is limited to the speed of light, reaching a different star, while not necessarily impossible, would be very time consuming. For an empire stretching across several solar systems, which has a centralized source of political power, this would pose a huge problem.

So here is my question, how could an empire maximize its size and how big could it possibly get without collapsing?

You may assume that this universe obeys to all of Einstein's laws (including time dilation while travelling, which is likely the most relevant effect) and that travelers can reach over 99% of c. There are a few factors that tie in here, what I would be interested in is an answer that answers how these various factors can be used to determine a likely maximum size.

  • $\begingroup$ I'd like an answer that looks back at history and to see how long it took to traverse ancient empires or kingdoms with the technology they had back then. Bonus points if you mention the Mongol Empire. $\endgroup$
    – overactor
    Commented Feb 10, 2015 at 14:05
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    $\begingroup$ Have a look at Isaac Asimov's Foundation Trilogy - It's not exactly about how big an empire can get, but about what happens to a huge, centrally governed empire inevitably. $\endgroup$
    – dot_Sp0T
    Commented Feb 13, 2015 at 13:23
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    $\begingroup$ One other thing to consider is to place the empire where the star density is higher. physics.stackexchange.com/questions/25706/… Probably not at the center of the galaxy but just another way to keep the physics real. $\endgroup$ Commented Feb 13, 2015 at 23:40
  • $\begingroup$ Are wormholes allowed in your story? $\endgroup$ Commented Oct 15, 2015 at 22:09
  • $\begingroup$ @AndersGustafson I'd say yes, depending on the answer you write around them. $\endgroup$
    – overactor
    Commented Oct 16, 2015 at 6:25

11 Answers 11


To me, it depends on these factors:

Lifespan of commoner - I mean, how long does common empire person live? They will track time differently, if they are humans (about 80 years to live) or million year living being. While one year for human is long time, for million year living being it is less than hour

Psychology of the empire itself How much do we love the Emperor? If we adore him/her (as God, preferably) then it is big factor in how the empire will hold together. On the other hand, if he/she is "that dude who will be replaced in 10 years" then there is not much holding you in staying in the Empire

How diverse the Empire actually is Look at Humans. There is seven billion of them and sometimes it feels that every single person has different religion, or at least religious viewpoint. If the empire has one religion which is followed by everyone you have chance for bigger empire than if the opposite


It's difficult to make space empires work within known physics

Travel times would be far longer than experienced by any earth-bound empire. In the days of the Roman empire, you could travel from London to Rome in a month under ideal conditions, no expenses spared [1]. The famous Silk Road trade route took two years for a round-trip from China to Rome and back [2]. Later, a year or two was plenty time for Columbus to discover a new continent.

Compare this to an Einstein-compliant round-trip of almost 9 years from Earth to Alpha Centauri and back and 16 years for the Sirius road. The nearest earth-like planets that we know of [3] would have a round-trip time of 20 to 80 years.

It would for instance take 40 years to make a round-trip from Earth to Gliese 581 d and back, but it only took 35 years from the day Genghis Khan was proclaimed ruler of the Mongols to the day their horde stood at the gates of Vienna. In modern times, it took 31 years for Earthlings to start and end two world wars (1914-1945). From this you can see that politics and warfare on-planet will be practically independent from a galactic empire

Even if we postulate that the travel times are balanced by long lifespans, there's the issue of how the empire will go about its "empire things". There's some evidence that empires emerge as a response to the threath of warfare [4], but how do the emperor protect his flock if it takes a couple decades from an alarm about an attack to when the cavalry arrives on the scene?

If we stick closely to known physics, there's also the issue of explaining trade and colonization by emigration. There's some back-of-the-envelope calculations showing that if we struck oil on Mars, there's no way we could run a profit shipping the oil back to earth. Nor do the human race have enough fuel on the planet to lift a sizable portion of its population into orbit, so you can't export populations to settle new land.

[1] orbis.stanford.edu/

[2] http://www.theglobalist.com/a-silk-road-caravan/

[3] http://en.wikipedia.org/wiki/List_of_nearest_terrestrial_exoplanet_candidates

[4] http://www.newscientist.com/article/dn24262-realworld-civilisation-game-shows-impact-of-war.html#.VNop1UZ0xaQ

[5] http://en.wikipedia.org/wiki/Mongol_Empire

  • $\begingroup$ +1 for thoroughness and "... how do the emperor protect his flock if it takes a couple decades from an alarm about an attack to when the cavalry arrives on the scene?" $\endgroup$
    – Wayne
    Commented Feb 25, 2015 at 23:12
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    $\begingroup$ I guess if trade between stars is infeasible, taxation between stars becomes meaningless, since there's no way for those taxes to pay for a meaningful transfer of goods and resources from one place to another. Without trade to balance it out, a one-way flow of money would just result in inflation in the capital system and depression in the colonies. $\endgroup$
    – user867
    Commented Oct 15, 2015 at 6:23
  • $\begingroup$ Unless I misunderstanding time dilation (which is totally possible), isn't the lifespan of the travellers irrelevant? At the limit of the speed of light, aren't all trips instantaneous from the point of view of these travellers? $\endgroup$
    – Blackhole
    Commented Jan 24, 2016 at 0:26
  • $\begingroup$ @Blackhole No matter how long the travel takes for the travelers, lifespans would have to increase 120 times for a 40-year round trip to be as inconsequential as a month-long trip. An emperor could never leave his seat or he would be quickly deposed. $\endgroup$ Commented Jan 24, 2016 at 3:38
  • $\begingroup$ @Azor-Ahai My comment doesn't invalidate this answer at all, just this precise point. Such an empire will indeed be unmanageable if each planet is 20 light-years away from another; but a traveller will not age during the trip, that's what I mean :). $\endgroup$
    – Blackhole
    Commented Jan 24, 2016 at 13:54

The more lyrical version

The Empire of Earth

Listen to the Song of Earth, or you will surely wither and perish!

Earth is the One homeworld, the sacred core, the soul of humankind. Earth was blessed by the touch of the Prophets of the eternal Catholic Zensunni Scientific faith.

Countless thousands are the worlds hanging in the void, their faces changed by the great machines, their surface seeded with animal and human life alike, made in the likeness of that on Earth and given breath by the Great Undying Overseers in orbit, who stand ever vigilant and ready to cleanse heresy with sacred fire.

The Song of Earth tells its children of its many secrets, each a challenge for the seedlings to rise up to and uncover, each one more step towards Earth-like perfection.

The Earth demands but one tribute from its daughters. Every twenty years, in Earth-reckoning, five are chosen from among the greatest artists and the brightest minds. These lucky souls are Ascended and the great Bulk Matter Transmitter in the sky will send their patterns to be recreated back on Earth, so that they will see the World with new eyes, and tread upon sacred ground.

The more prosaic version

Earth keeps a clenched iron fist around its colonies

Earth sends out terraforming van Neumans, who later seed the worlds with human and animal life. The Song of Earth helps the new worlds ascend the technological, ecological and sociological ladder towards a presumed state of Earth-like transcendence. The von Neuman machines in the forbidden depths of Space harness stellar resources for further colonization and also serve to enforce (with Mongol-like ferocity if needed, down to planetary sterilization and reseeding) the will of Earth upon the colony, in particular ensuring the extraction of the bidecadal tribute: the very best and brightest of each world, in a galactic scale brain-drain, are turned into information and their patterns are transmitted back to Earth. With its vast empire, there are hundreds of thousands of such tributes arriving each year, further contributing to the glory and power of the Earth. The Earth's sphere is growing at about 0.1% of the speed of light, and will engulf the entire galaxy in about a million years.

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    $\begingroup$ Sounds like the stage for a great space opera :-) But you'd have to add a few details. Total control of all population and of interstellar communication would be required, else another world could do just the same, preferably completely outside Earth's current sphere of influence. $\endgroup$
    – MauganRa
    Commented Oct 12, 2016 at 19:49

My first answer would be : 1 solar system.

While any race could potentially spread across an entire galaxy, jumping beyond your own galaxy are just TOO long distances to be plausible. There are plenty of stars within 10 light years that are totally at reach within normal science boundaries. And one could just start a colony there and launch from there beyond.. slowly spreading out over the whole galaxy.

But the long distanced between stars would make having a central center of government impractical, so it would heavy rely on local rules and local legions/fleets to govern those colonies, and send home the gains to the imperial home-planet.

With such long lines, I expect every planet to declare independence from the empire, and the empire being unable to do anything about it. As such a race would be spread out in the whole galaxy../ but it would not be 1 empire, but rather hundreds of thousands of them each comprising of the planets within 1 solar-system. Some might be at war with each other, some may trade with each other... some may try to form some form of federation of independent empires.

So in short a SPACE empire could just never exist... every trip would be a single trip.

First you would need energy. Since the energy needs need to be reasonable... (zero point energy is out of the question as is warp travel) also you cannot surpass 5g, or your crew would be tomato soup.

I also presume that the ship is magnetically shielded, so neither hard radiation nor tiny space particles would be a problem (you bend them around you, as the impact of even 1 atom at 0.5c on your ship would be impossible to armor plate against).

So I would think you accelerated constantly at a maximum of 5g for 6 months, towards 0.5c (before the energy needs outstrip the usefulness of going faster) than fly in 7.5 more years to the nearest star and than start decelerating for 6 months. This way you'd make the trip in 8 years.

*if you were to turn around and return you would have flown for 16 years, yet almost 19 would have passed on your home planet.

That's doable.

However even if you were to communicate with your home planet.. 4 years to get a reply is just TOO long to be of any use for any empire. (the roman empire collapsed and it had 2 WEEKS communication lines) and the roman empire also gave strong power to local leaders.. with local legions, and the imperial army being slow to respond if needed if regional leaders rebelled against Rome.

In this situation.. any news about any rebellion would take 4 years to arrive.. and responsive action would take 13 years to arrive. It would be pretty hard to keep order in that kind of a situation. And that's why I presume that no empire as such could exist.

But supposing that all colonies are loyal to the home world (they're not like humans that like to self govern, but LOVE to please their empire-king or something like that) that it comes down to travel time towards the center of the empire.

Are there any outside attacks? Then no space empire as such could exist as responding would just take too long.. you would have lost the battle decades before you could send a counterattack.

But suppose you had none of that either?

Than it bottles down to lifespan or energy. Every 1 light-years of distance, means 2.25 years of flight. It takes for humans at least 25 years to be properly trained, and they can reach 120 years (when we reach our biological optimum). If you were to swap them for new crew for the return flight.. this would give us close to 100 years of flight enough to cross 50 light-years (giving us the maximum size for a human space empire).

However biological immortality is theoretical possible.. and while it could take half a million year to fly across some galaxies even at 0.5c, if one does not age or the ship is generational, it is plausible.

But energy, even a ship that could continue flying, would need to refuel, at the very least to be able to maintain life-support and keep growing food. (even biodomes need HEAT, as space is cold and perfect isolation does not exist) also the magnetic shielding and artificial gravity would consume energy)

* there are 2 methods to acquire this extra fuel (A) bring it all along, meaning more fuel load, giving every less return on cargo. or..

*refuel at a colony every 4-10 light-years, jumping towards the home planet in steps (having the disadvantage of having to decelerate/accelerate more often, increasing travel times and fuel use).

But suppose they would just DROP the cargo at the second base, and return... that way bigger and bigger ships would move towards the capitol, with each only carrying their tax and that o the worlds that paid it to them, towards the capitol. Well THAT highly strange race could get a civilization to run as large as a galaxy.

But it would not be a galaxy that would wield large armies or could respond to you taking one of their worlds in any reasonable way, any more than just a lone planet could.

  • $\begingroup$ In orion's Arm Universe Project, there's some sort of beamriders network. Just massive ships that travel at 0.1-0.5c at a roughly circle route. They were propelled by beamed propulsion at first, but then using onboard fuel to navigate and slowly arching their flight path to roughly circular route. Fuels and payloads were accelerated toward passing ship to match the speed and then merge temporally. Then when the payload nearing the target, they'll detach and decelerate toward nearby station. That way they saving up energy to propel the big ship, and benefit from it later. $\endgroup$ Commented Jan 24, 2016 at 13:46
  • $\begingroup$ orionsarm.com/eg-article/460c3685cd4c4 for the link. I believe the route could be used to both supplying outside colonies and to transport colonist as required. The point is, you don't have to accelerate and decelerate so often with the large ships, instead you just have to accelerate and decelerate the payloads, which is smaller than the full blown cargo ship $\endgroup$ Commented Jan 24, 2016 at 13:49
  • $\begingroup$ This answer raises the question whether trade or war for economical reasons would even make economic sense. Even a war of destruction in another system would likely wreck the attacker's economy. Of course unless von Neumann machines are used, which open a completely different can of worms... $\endgroup$
    – MauganRa
    Commented Oct 12, 2016 at 19:57

An empire can only be such as only if it can strongly influence the outlying colonies. Right now with our tech level, we would likely be able to have an empire the colonizes the inner solar system, out to the asteroid belt past mars.

As Pavel stated, general life expectancy will play a part with time travel between locations. Communication is also relevant. If Light is the the fastest form of communication then the nearest stars would be really to far away. Say we can travel 1/3 the speed of light, it would take about 12-15 years to get to the nearest star, once there fastest communication is almost 9 years round trip. They are effectively their own government.

The life expectancy comes in of course for how much of your life is wrapped up in travel. If it takes 25 years for you to shuttle colonists from earth to the nearest star and come back, you could make 2 trips in your life. (with relativistic speeds, you might get a 3rd. But that is a large investment, much more than the few months it took to cross oceans.


That's tricky. One way to manage the Empire is with decentralization, a lot of it. It is normally considered a bad thing if it goes too far but it's a necessary evil here. It is bad because the central government does not control the borders and they might ignore the centre or try to secede eventually. It is also a good thing because, as you mentioned, it's not possible to send reinforcements fast enough to defend the border. Therefore, the frontier needs to be able to defend itself with minimal help from the exterior. They need to have troops under their command but also the resources to maintain these forces. This means that they are benefiting form the Empire.

Weaker systems are dependent on exterior help. Alone, they cannot maintain enough troops. Other richer world will have to pay for that. Just like Russia is maintaining Transnistria, Lugansk and Donetsk People's Republic. Oh my!

The rich planets are in another game. They are wealthy and they are the ones maintaining far away lines of defence. Depending if the enemy is also limited in movement by the same technology, the frontier worlds might actually provide a strong system of defence, like a series of fortress. Let's suppose it is and that most enemies will attack only through certain lanes because they need to be resupplied and cannot afford to travel in an empty space for too long. In that case, rich planets have an advantage in staying with the others.

But not everyone will agree on the real cost of security. This is especially true as we get away form the frontier, away from the sources of danger. Planets get more reluctant to pay for a treat that does not exist in their eyes and they might challenge the authority of the Empire.

Alone some planets could secede but that seems unlikely considering they would be against the rest of the Empire. That's way they will more likely do it in group. There is security in numbers and with enough planets, they might be able to secure their independence.

The answer: My guess is that, passed a certain size, the Empire will become less stable. If you have a large numbers of well developed planets that are not too expose at the risk of external threats, the Empire might be in trouble. That is as precise as I can get.

Bonus: The second eastern Mongol Empire's campaign against Russian and other European states including Hungary, Poland and the Germans was planned to take around five years. From Mongolia, back to Mongolia for supper. (based on my memories form my readings)

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    $\begingroup$ You are vastly underestimating the size of space. Also there's a vast underestimation of the energies involved in crossing space. Space is really big. Saying something like "Weaker systems are dependent on exterior help. Alone, they cannot maintain enough troops." is are nonsense. It will take a decade to get word out even your closest neighbors. Then however long it takes to assemble a response force plus another decade to send the response force. If your forces aren't already in system when they are needed, it is already too late. $\endgroup$
    – Shane
    Commented Feb 16, 2015 at 22:38
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    $\begingroup$ Look at your Russian example. Russia is supporting those smaller nations, but in each case, they get returns on those investments. Whether it is natural resources, land, or ports and access to the world markets, Russia gets something. There is no equivalent analog in space. You don't need special geographic access to water to ship your exports, you just fly towards where you are sending them. There are more resources in a system's asteroid belts than there are on the terrestrial planets. $\endgroup$
    – Shane
    Commented Feb 16, 2015 at 22:43
  • $\begingroup$ It wouldn't make sense to prop up poor colonies for resources. Propping them up and shipping the resources take more than you could ever hope to get in return. Fortresses are set up in key strategic locations. At a mountain pass so there is no way to go around the fortress. In space, you fly directly to where you want to go. You would just go to the resource rich planets. $\endgroup$
    – Shane
    Commented Feb 16, 2015 at 22:45
  • $\begingroup$ If a faraway system can defy imperial authority, then imperial authority also can't defend the system in the first place. Von Neumann machines change the picture, but one would have to have them in place everywhere already, and at that point there would be no point in maintaining colonies at all. $\endgroup$
    – MauganRa
    Commented Oct 12, 2016 at 20:15

Using Historical Precedent: One Solar System (sorry!)

If you want to use historical precedents, as you recommended in your question, we can compare empire sizes as long as travel was as fast as communication, because that will be your situation. So if travel and "mail" was by foot, horse, boat or train. Once telegraphy came about, communication became faster than travel, which will not be your case with speed of light travel.

Mongol Empire

The largest pre-telegraphy empire by land was the Mongol Empire, and a land-route could measure about 5,000km from a central point. With centralization, that means communication and travel (by good horse at 60km) per day would take eleven or twelve weeks. Double that to go from end-to-end.

Spanish Empire

The largest pre-telegraphy empire by sea distance was the Spanish and Portugese, although you could argue that the Dutch had to go further distance to southeast Asia. In the Spanish empire you could get across the Atlantic in about 10 weeks and then another few weeks to get to its destination over land.

Your Empire

IF you wanted to use historical precedent, where your travel is the same speed as communication (Light Speed), I would say 10 to 15 weeks of travel, or rather, a radius of 10 to 15 light weeks away, which unfortunately limits you to this solar system, as our nearest (non-sun) star would be 4.5 years of travel and communication.


Interplanetary empire in Sci-Fi seems to always be based on some sort of warp speed for good reason, there are few reasons to have an empire except to exploit others for the riches that can profitably be brought home. The technological limits of light speed would make empire consisting of other worlds across the vast distances of space simply impossible.

A conquered Empire like the Roman Empire would be quite impossible to hold together by threat of force. However a settled Empire like the English Empire could have some sustainability much like the modern day Commonwealth. To be sure there would be a couple of pesky colonists like The Americans that much could not be done about if they were determined to be completely independent and hell bent on some set of principles that called for total self independence. If the colony was on a planet were there were indigenous people the colonist might hold some control for awhile like the Spaniards in America or the English in Asia, but that would end just like it did for the Spaniards in most places, with the original empire with little influence and much hostility. About all the Empire could do was cut them off from communication of new technology.

There would be very few reasons for the Empire to even exist. The exchange would be limited to data and to a friendly port in the wilderness of space for the occasional space traveler. Even if there was an outside threat such as Borg or a competing Empire mutual defense would be minimal, consisting of the colonies being hostile cannon fodder that would slow down an enemy advance. No one is coming from the home world soon enough to save anything.


Let's obey to Relativistic Physics and try to make something consistent here.


Let's assume FTL is impossible and traveling at 99% c needs a huge amount of energy. A most economic way to achieve it is no to send people. Send frozen embrios. Make some IA defrozeen and baby-sit it to a small group of adult colonist just before it arrives destiny. The initial small group ill build (using the starship parts) and install power plants, terraformin devices and cloning centers the colony ill need to thrive. Start the growing stage and everything ill scale up from here using the planet resources. with a bit of luck you got a full developed colony just a few centuries after arriving.


Hard to control a colony if is not viable sending troops. Instead you can build the colony from the start with a failsafe system. It can be some kind of device: deadly spores, mind command, behaviour chips, military fascist minded cast, etc. Once any colonist or group of colonist try to "break free" failsafes automatic triggers.


To settle a colony is a great risk (and long, long term) investment. The central government ill not do it for free your know, they expect something in exchange. Since shipping goods between stars is not that viable (think of the cost of sending a tomato to Mars) what can the Metropolis possible get back? Well the only thing cheap enough to send to space are radio signals (wait! they travel at light speed too, sweet!) How radio signal can be worth? If it brings great news like... news techs.

At the end the metropolis can be simply using colonies as research centers, collecting breakdowns in all fields: Biology, Social, Quantic, Engineering, Computing, etc. this way it not only is ahead any colony it eventually ill achieve it's ultimately goal breaks the grips of relativistic physics and finally improve space travel to a cheap and reliable ftl.

And all in just a few hundred millenia.

  • $\begingroup$ Most research can already be done within our own solar system. Even if you establish colonies/research outposts in the Kuiper belt, the costs would be magnitudes lower. Also, it takes less time to build them up because brains can be afforded to be sent to Eris, unlike other raw materials. And if they try to secede, just don't supply them anymore, send a nuke or deviate a comet. $\endgroup$
    – MauganRa
    Commented Oct 12, 2016 at 20:12
  • $\begingroup$ @MauganRa I agree, but in the sake of the question we can also say it's hard to populate the Kuiper belt with dozens billions colonists and keep the logistics to supply them. Also if you do that a dozen billions brains can learn how to survive without supplies and organize to achieve independence. Finally there are all that sweet xeno flora and fauna no to say planetology, terraform and astrophysycs technolgy can leap generations $\endgroup$
    – jean
    Commented Oct 13, 2016 at 10:41

I think quite large, under the right circumstances.

With sufficiently good propulsion (high thrust, high efficiency, enough to carry significant cargo and personnel interstellar distances), time dilation would make trips appear to take very little time from the standpoint of the travelers, even for large interstellar distances. The trips themselves would take years from the standpoint of everyone else. Communications between worlds would also take years. Double that for a response. But that doesn't necessarily mean instability. These aren't unsolvable problems.

For colonies to exist in the first place, they'd have to produce something of great value for the homeworld. Otherwise, why put in all the effort? Even though travel times might be years from beginning to end, you could easily arrange for ships to regularly drop off supplies and fresh workers and fill their holds up with valuables to take back (along with workers who've finished their tours). To accomplish this, you'd have a lot of ships in transit all the time, which isn't that huge a deal if what you're transporting is valuable enough to travel to other stars and collect it.

In fact, having dozens of ships en route at all times allows them to respond relatively quickly to situations without consulting with the homeworld. While a 10ly distant colony might have to wait 20 years for a response from the homeworld, they might only have to wait months or weeks to hear from the nearest starship. If something goes wrong (alien invaders, local fauna overthrows colonial government, revolution) you can just have the shuttles stop decelerating and perform relativistic strikes on the offending parties. Presumably each ship en route (and back) would be carrying large numbers of people (miners, for example) rotating in and out of the colony to provide labor, security, administration, etc. These people could be used to quickly replenish population in the face of attacks or other misfortune, and mining would resume.

Paying volunteers for such a project would be easy in comparison to the cost (and presumably rewards) of such an undertaking. Extreme hazard pay and all that. High pay would also ensure loyalty.

  • $\begingroup$ Tl; dr: like good ol' colonialism, where there just has to be some kind of ROI. Right? $\endgroup$
    – MauganRa
    Commented Oct 12, 2016 at 20:17

A space empire would probably be limited to a handful of close-by solar systems. It would likely look like the British Empire in the 1700s. In that time period it could take up to 8 months to travel to the more remote regions of the empire. The space empire would consist of a densely populated homeworld and moderately populated colonies within the same solar supported by sparsely populated colonies that are primarily focused on resource extraction for the benefit of the homeworld. Of course, these colonies would not be administered directly by the homeworld government. They would instead be run by governors loyal to the homeworld government but given a large degree of autonomy. As such laws in the colonies would be different to the homeworld. The colonies in the home system would be more focused on manufacturing and supporting a large fleet of warships, which would be used to keep control over the interstellar colonies. What would the point of rebelling be if you knew a fleet and army larger than you could hope to match would be at your door in a 5 or 6 years? The fleet would be normally kept in the home system, with enough at the interstellar colonies to prevent a militia from taking over. The fleet at the homeworld would be in case whoever was in charge of the colonial fleet decided to take over. However, once you get past a few light years, there's enough time there to build up a defense before the fleet gets there.


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