40
I've heard it said before that the limitation on empire size is about 2 weeks. That is to say, if it takes longer than 2 weeks to get a message from the capital to the frontier it causes instability. So, it's more about time than absolute physical size.
Assuming no FTL and centered at Earth (or the sun) that puts the max distance roughly a little past the low estimate for where the Oort cloud begins.
(Wolfram Alpha says 14 light days is 2424 AU and the cloud has inner edge estimates from 2000 to 5000 AU
As with Rome, the limitation is often communication and transportation.
You'd have a hard time even keeping Mars part of unified empire with Earth given our current technology level. We simply can't move things back and forth easily enough until we figure out fusion reactors (or some other power source) to a much higher level than we currently have.
Any sort of empire spanning more than a single solar system would require faster than light travel and communications.
Even with FTL travel, it could take months or years to reach very distant places and, afaik, we can't make methods of radio communication faster than light. So travel might be something possible, but what about communication? Earth could not possibly stay in live contact with another planet several hundred light years away.
Historically speaking, colonies were often months away from the main empire.
If the colony’s level of development improves but communication speed does not, the colony might declare independence.
It’s only in the last 150 years that information could travel any faster than a currier ship or rider. Before the telegraph and the radio, information traveled at roughly the same speed as goods or soldiers.
If you had FTL travel, you wouldn't use radio, but you could set up an FTL pony express. Or even something like a "conveyor belt" of microsats FTLing to a destination, dropping off their messages, and FTLing back with the replies. Whether that's practical or not and how fast it is depends on how exactly your FTL works, but it's hard to build a situation where you can FTL travel without having almost equivalently fast communication times.
Without FTL though, you're right, the communication situation gets real dicey real fast. Whether you're using generation ships or cryosleep or life extension to span the lightyears is irrelevant, by the time a colony ship reaches another system, both it and the humanity back on Earth will have diverged significantly due to lack of communication, and will continue to diverge as different events and pressures and developments affect each group differently. Imagine we started getting pen pals from the 1800s, but then to leave them a message, we have to wait for them to get to the 2020s to read it, and then by the time we get a reply maybe we'll be in the 2300s. What can we even try to meaningfully talk about? It's more like opening time capsules and studying history than anything resembling communication. It's interesting, but neither of our lives are at all relevant to each other. We are separated by too much time and without breaking light speed we can't break that.
Psychologically it is very difficult to imagine any two hypothetical civilizations not eventually separating and developing independently, because in all practical senses they will be completely independent of each other. And that's a manifestation of physical reality, there is really nothing we can do to avoid it. Not even an attempt at tyrannical oppression to make it otherwise can survive hundreds of years of delayed communication. Any "oppressors" sent will quickly end up having more in common with and more dependence on the colonists they are oppressing than they will with their "command" back in Earth-space. Even if they are successful, it's almost impossible to imagine they will not just end up as rulers of a separate independent colony. And then when Earth finds out hundreds of years later that their guys aren't quite following the orders they would've sent if they had been able to, what could they even do about it? Spend more hundreds of years sending more oppressors? Which will just end up doing the same thing? And why would they bother? If they want to loot their resources, is it still even going to be useful by the time they can get it back?
It's wild to think that after a few thousand years of independent development, we may not even recognize each other anymore, and we definitely won't think of each other as the "same" people. This will probably even be the case within the solar system, in some respects it already happens even here on Earth, nevermind once you start trying to stretch humanity across light years. Whoever those creatures that colonize other stars might be, it won't be long before we probably won't consider them "us", and they probably won't consider themselves "us" either.
Supposing Stargate technology exists, you could conceivably control an entire galaxy, but that is proposing the existence of technology incompatible with our best understanding of the universe. Realistically the solar system will probably be the limit of my empire.
I feel like a major limitation to a Stargate empire would be Stargate throughout.
A large enough military would need to be supplied at each Stargate to secure access. At a certain point, it is going to be difficult for the home world to supply all garrisons, even if some garrisons some self-sufficiency. Worse, if a garrison is self sufficient, how do you maintain loyalty to the home world.
42 is the max.
Sci-fi is delightfully circumspect on how an intergalactic empire would work. Maybe Herbert's Dune universe is clearest and he just took us back to the middle ages with sandworms and drugs, fiefdoms and nobility.
I think whatever area shares the same government is a country. It doesn't have to be contiguous or on the same body floating through space. It could be the size of the Vatican or half the universe.
I suspect the definition of the word will change once (if) we make it to the stars. We have gone from nomadic life to loosely defined borders to kingdoms to empires to multinational and intranational federations of sort. These terms may no longer be fit for purpose when we colonize Mars etc. And maybe that's why you struggle to comprehend how it would all work behind the scenes. We don't know for sure, sci-fi authors don't know (or don't want to be too specific and limit themselves in what stories they could tell in the future).
I imagine interstellar coalition governments probably function a lot more like the EU than a single country. Individual planets/solar systems would probably function under a somewhat independent governorship that is still subject to the overall laws of the federation/republic.
It's sort of like asking "how big can a city get?" before states and nation-states existed, when in reality at some point a higher tier governance inevitably takes over. An interstellar or galactic scale government is necessarily going to be massively stratified with many levels of hierarchy in between. I think this is why many sci-fi governments at this scale are also described as bloated, unintelligible bureaucracies.
The maximum size of a hemisphere is half of a planet…
Historically speaking, about 6.6m square miles.
this post was submitted on 30 May 2025
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